Hey everyone
Ive got a randall RG100G3 valve dynamic combo amp. The amp uses a single 12AT7 valve in the power amp, my problem is that the valve gets microphonic in a very short period. I have 2 questions, does anyone know the amount of volts that gets sent to the 12AT7 valve in these randall G3 amps (valve dynamic power amp) ? i would like to replace the the 12AT7 with a retrovalves solidstate valve from jet city amplification. Ive sent randall numerous emails but they never reply. My other question, would it be possible to modify the amp power amp so that i can get rid of the valve.
Any help/advice would be greatly appreciated.
Thanks
Andrei
In your schematic it looks like 375 v less the drop through the 4k7 res and the 100k plate res. Figure your 12AT7 current draw from the data sheet and you can calculate the voltage at the plate.
You have a working amp, aside from microphonics. MEASURE the voltages in the circuit.
1) I'm not so sure he's asking for DC voltages.
What *I* understood from that question is that "sent to" means "with how many volts (audio) it is driven"
May be wrong, of course.
*If* this is actually the question, it's not easy to answer because it's within an NFB loop, plus the first triode gets *heavily* loaded by said NFB network, but I think it's between 1 and 2 V RMS at mid frequencies.
There's a bass boost below 300Hz (often known as "resonance" and a highs cut above 5KHz, probably to reduce shrillness.
2) I'm not that comfortable with that "microphonism" either:
Why does the OP say so?
What are the symptoms?
How does he know it's the tube?
At first I "saw" that stage as 2 cascaded triodes, which means *a lot* of gain, that would make it very sensitive to microphonics, but then I saw how it's implemented and perceived gain is *MUCH* lower than expected, it shouldn't be more microphonic than a regular 12AT7 PI, to compare it to a full tube power stage.
Quote from: J M Fahey on July 02, 2013, 09:03:52 PM
1) I'm not so sure he's asking for DC voltages.
What *I* understood from that question is that "sent to" means "with how many volts (audio) it is driven"
May be wrong, of course.
*If* this is actually the question, it's not easy to answer because it's within an NFB loop, plus the first triode gets *heavily* loaded by said NFB network, but I think it's between 1 and 2 V RMS at mid frequencies.
There's a bass boost below 300Hz (often known as "resonance" and a highs cut above 5KHz, probably to reduce shrillness.
2) I'm not that comfortable with that "microphonism" either:
Why does the OP say so?
What are the symptoms?
How does he know it's the tube?
At first I "saw" that stage as 2 cascaded triodes, which means *a lot* of gain, that would make it very sensitive to microphonics, but then I saw how it's implemented and perceived gain is *MUCH* lower than expected, it shouldn't be more microphonic than a regular 12AT7 PI, to compare it to a full tube power stage.
Thanks for the reply
Im a noob when it comes to electronics
These Randall G3 valve dynamic amps are very problematic. Ive had a big problem with the FETs overheating, i took it to a tech and he was able to fix that for me, he said it was caused by bad design that caused oscillation if i remember correctly, he did a simple mod and fixed tjhat for me. According to him everything else in the amp is 100%
My reason for the question about the voltage sent to the 12AT7 : Jet City amplification sell solidstate alternatives called retrovalves, i would like to use one of these "valves" because they dont get microphonic and last a lifetime. I emailed Daughlas White from Jet City amplification and he wanted the know the amount of volts that get sent to the 12AT7 in my amp. Retrovalves need atleast 100 volts to do their job he told me.
For the microphonics, ive went through 3 12AT7 valves in a very short period, each time with a new 12AT7 the microphonics would disappear and the amp would sound great. I stay in South Africa and its difficult to get hold of valves here, the last 12AT7 i had, i got from guitar centre in the US.
There is a level control on the gain channel, when i turn it up past 1 the microphonics just get worst. Seems like the valve gets pushed harder.
Quote from: belleraphon88 on July 03, 2013, 03:31:11 AM
There is a level control on the gain channel, when i turn it up past 1 the microphonics just get worst. Seems like the valve gets pushed harder.
Hi, What exactly happens past one?
In what way does it negitivly impact your sound?
Does it feedback?
Phil.
Quote from: phatt on July 03, 2013, 07:00:45 AM
Quote from: belleraphon88 on July 03, 2013, 03:31:11 AM
There is a level control on the gain channel, when i turn it up past 1 the microphonics just get worst. Seems like the valve gets pushed harder.
Hi, What exactly happens past one?
In what way does it negitivly impact your sound?
Does it feedback?
Phil.
Hi
If i play say my high E, on any position on the fret board, there will be popping sounds and it sounds like theres extra notes being played. The tone also gets rather mooshy and ugly sounding. Then i replace the valve and all is good, i can turn up the level and the tone only gets better.
Agree, you are not describing what happens.
Don't "diagnose" (as in "it's the 12AT7" .... ""it happens because the tube gets pushed" .... etc.) , just tell us:
"I raise volume past 1 and then _________________________ "
Fill in the blanks.
An MP3 or a You Tube showing it *on yours* will help a lot.
EDIT: we posted at the same time. :o
Sorry but your description is confusing, post some MP3 or Video.
You may have a problem, but I wouldn't describe it as Microphonics.
Hi and
welcome.
Let's dispose of one thing at the outset - your overheating output FET's have nothing to do with your microphonic valve problem.
Quote from: belleraphon88the amount of volts that gets sent to the 12AT7 valve in these randall G3 amps
The supply voltage is given on the circuit as 375V. From looking at the
RetroValve specs they specify 120-400V, so it would appear that they are intended to work in amps circuits just like this one, and if you get the correct
RetroValve to replace a 12AT7 (gain wise) then I can see no obvious reason why it shouldn't work.
I do however share
JM's curiosity about your string of microphonic valves.
All valves tend to be a bit microphonic to some small extent, even brand new, but it sounds like you have either had a run of bad luck, or this particular circuit is sensitive to slightly microphonic valves, 'tho I can't see any obvious reason why that should be.
To be quite specific; a "microphonic" valve produces a glassy tinkling sound a bit like wind chimes when tapped with a chopstick or similar, not crackles or loud blurts, these normally being cause by poor socket contacts -
are we addressing the right problem here? Apart from the
character of the sound produced it should also be very
localised to the valve. There are other components (and dodgy connections) that can also produce strange sounds, but they are normally of a different character and not localised to tapping (gently) on the valve glass.
If indeed your problem is microphonic valves then a solid state replacement would seem to be a good idea to try, and this
RetroValve product seems suitable.
Quote from: belleraphon88would it be possible to modify the amp power amp so that i can get rid of the valve.
It is, but is a different order of complexity. Even as a tech I would be inclined to try the
RetroValve path first as the simplest solution to your problem.
The
RetroValve page does make a point about their product changing the tonality of an amp, mainly because FET's don't have an identical transfer function to the valve they replace, however they are still similar, but it's something to keep in mind - it may not "crunch" or distort in quite the same way you have become used to with a real valve.
If you do decide to try these out I'm sure we would all be interested in your report on how you find the replacement.
Postscript - a "popping" sound isn't microphonics; sounds more like a base connection problem. As
JM says, a sound sample of the problem would be helpful. Also, does wiggling the valve in its socket help, make the problem go away for a while?
Quote from: Roly on July 03, 2013, 07:20:46 AM
Hi and welcome.
Let's dispose of one thing at the outset - your overheating output FET's have nothing to do with your microphonic valve problem.
Quote from: belleraphon88the amount of volts that gets sent to the 12AT7 valve in these randall G3 amps
The supply voltage is given on the circuit as 375V. From looking at the RetroValve specs they specify 120-400V, so it would appear that they are intended to work in amps circuits just like this one, and if you get the correct RetroValve to replace a 12AT7 (gain wise) then I can see no obvious reason why it shouldn't work.
I do however share JM's curiosity about your string of microphonic valves. All valves tend to be a bit microphonic to some small extent, even brand new, but it sounds like you have either had a run of bad luck, or this particular circuit is sensitive to slightly microphonic valves, 'tho I can't see any obvious reason why that should be.
To be quite specific; a "microphonic" valve produces a glassy tinkling sound a bit like wind chimes when tapped with a chopstick or similar, not crackles or loud blurts, these normally being cause by poor socket contacts - are we addressing the right problem here? Apart from the character of the sound produced it should also be very localised to the valve. There are other components (and dodgy connections) that can also produce strange sounds, but they are normally of a different character and not localised to tapping (gently) on the valve glass.
If indeed your problem is microphonic valves then a solid state replacement would seem to be a good idea to try, and this RetroValve product seems suitable.
Quote from: belleraphon88would it be possible to modify the amp power amp so that i can get rid of the valve.
It is, but is a different order of complexity. Even as a tech I would be inclined to try the RetroValve path first as the simplest solution to your problem.
The RetroValve page does make a point about their product changing the tonality of an amp, mainly because FET's don't have an identical transfer function to the valve they replace, however they are still similar, but it's something to keep in mind - it may not "crunch" or distort in quite the same way you have become used to with a real valve.
If you do decide to try these out I'm sure we would all be interested in your report on how you find the replacement.
Postscript - a "popping" sound isn't microphonics; sounds more like a base connection problem. As JM says, a sound sample of the problem would be helpful. Also, does wiggling the valve in its socket help, make the problem go away for a while?
Hi, thanks for the reply
maybe popping wasnt the best way to describe the sound.
i have recorded a short clip, i hope it helps, not he best sound, but hopefully it gives you an idea
http://www.youtube.com/watch?v=Onolh61F_Ao&feature=youtu.be
Wiggling the valve doesn't change anything
I will order the retrovalve tonight, i will certainly let you know how the the retrovalve worked out for me
Andrei
Quote from: J M Fahey on July 03, 2013, 07:12:33 AM
Agree, you are not describing what happens.
Don't "diagnose" (as in "it's the 12AT7" .... ""it happens because the tube gets pushed" .... etc.) , just tell us:
"I raise volume past 1 and then _________________________ "
Fill in the blanks.
An MP3 or a You Tube showing it *on yours* will help a lot.
EDIT: we posted at the same time. :o
Sorry but your description is confusing, post some MP3 or Video.
You may have a problem, but I wouldn't describe it as Microphonics.
Hi, sorry if its confusing
here is a video clip i recorded
http://www.youtube.com/watch?v=Onolh61F_Ao&feature=youtu.be
Yeah that is more like blocking distortion,, nothing to do with microphonics.
The Amp works it's just got some issues so in time clever people here might nail down the offending part. Unlikely to be 3 bad Valves in a row so maybe a dying Capacitor or something in the design is prone to problems.
Phil.
Unfortunately, the PDF doesn't give the schematics for the entire amp, just back end sections.
It sounds like the signal is experiencing short volume dips or cutouts, which repeat at a low frequency rate of several times per second.
This suggests itself to be a motorboating oscillation in some circuit that affects the bias of a stage.
There is no outright reason to suspect that it is in the power amplifier. Even if it happens in every channel (which I suspect is not the case from a comment given), there are other common circuits that the signal goes through besides the power amp.
Does the amp have a pre-amp out jack (or effects loop send) and can you hear the artifact in that output? If so, that tends to eliminate the power amp.
Does the artifact occur only in one channel? That tends to blame that channel.
I don't think you're hitting the tube hard just because you turn up the gain on the gain channel a little bit. What can hit the tubes hard is volume. The tube is just the differential summing stage for the amplifier's feedback, generating the error signal that is amplified. FET TR7 provides the voltage gain after that and drives the output stage. The gain of the amplifier is fixed, so the tube stage has to have reasonable clean headroom to pass through signal when the amplifier is played at the maximum volume. I.e. it is overall volume that will drive the tube, not gain on the gain channel.
Quote from: Kaz Kylheku on July 03, 2013, 11:04:34 AM
Unfortunately, the PDF doesn't give the schematics for the entire amp, just back end sections.
It sounds like the signal is experiencing short volume dips or cutouts, which repeat at a low frequency rate of several times per second.
This suggests itself to be a motorboating oscillation in some circuit that affects the bias of a stage.
There is no outright reason to suspect that it is in the power amplifier. Even if it happens in every channel (which I suspect is not the case from a comment given), there are other common circuits that the signal goes through besides the power amp.
Does the amp have a pre-amp out jack (or effects loop send) and can you hear the artifact in that output? If so, that tends to eliminate the power amp.
Does the artifact occur only in one channel? That tends to blame that channel.
I don't think you're hitting the tube hard just because you turn up the gain on the gain channel a little bit. What can hit the tubes hard is volume. The tube is just the differential summing stage for the amplifier's feedback, generating the error signal that is amplified. FET TR7 provides the voltage gain after that and drives the output stage. The gain of the amplifier is fixed, so the tube stage has to have reasonable clean headroom to pass through signal when the amplifier is played at the maximum volume. I.e. it is overall volume that will drive the tube, not gain on the gain channel.
Ive attached the pre amp and power supply pdfs
It does have a effects loop. I dont have a external cab to test it with, is there any way i can test it.
Its only the dirty channel that gets effected, the clean channel is fine, but if i use a OD infront of the clean channel it does the same thing.
Yip, the gain can be on max, its the level control on the gain channel that causes it to go all crazy.
Quote from: phatt on July 03, 2013, 10:05:31 AM
Yeah that is more like blocking distortion,, nothing to do with microphonics.
The Amp works it's just got some issues so in time clever people here might nail down the offending part. Unlikely to be 3 bad Valves in a row so maybe a dying Capacitor or something in the design is prone to problems.
Phil.
I did some reading up on blocking distortion, it does sound like the problem im experiencing
Quote from: phattthat is more like blocking distortion,, nothing to do with microphonics.
Totally agree,
that's not "microphonics" in any sense I understand the term. In fact it sounds almost like a beat note because it changes it's period depending on what note is being played.
Quote from: Roly on July 03, 2013, 03:37:42 PM
Quote from: phattthat is more like blocking distortion,, nothing to do with microphonics.
Totally agree, that's not "microphonics" in any sense I understand the term. In fact it sounds almost like a beat note because it changes it's period depending on what note is being played.
When i play chords, especially when palm muting, it like farts out.
You have no *tube*, nor even *amp* problem.
You are using way too much gain and your *guitar* is feeding back in an uncontrolled way.
1) you pluck your guitar 13 times.
Count them because it's more precise than referring so such and such second, the video is short, but enough.
2) video can be divided in:
silence/7 plucks/silence/4 plucks/silence/2 plucks/silence
"silence" is a joke, your guitar is
howling all the time, in an
uncontrolled way, normally at a relatively low frequency, more precisely what the open strings are tuned to.
When you push a string against the fretboard, it will howl at a different, higher frequency.
That higher frequency produced an unplucked but howling/oscillating string will beat with the frequency of the actual plucked one, and since the amp is *heavily* distorting, it will produce 2 beat frequencies, one the sum (it will sound as a dissonant octave) and one at the difference, which will be a low frequency which to boot will modulate volume, like a crazy tremolo.
That's what you hear as "chirps" or "bursts" because thanks to the unstability the howling string starts and stops at random.
You can hear it *clearly* on plucks nº:
end of 2 , beginning of 3, low frequency modulation, sounds similar to a tremolo/vibrato/phaser/chorus
8/9/10 : higher frequency modulation, sounds like "chirping"
That it's a
guitar feedback problem and
not an amp one (and
even less a *tube* problem, as confirmed by:
Quote
Its only the dirty channel that gets effected, the clean channel is fine, but if i use a OD infront of the clean channel it does the same thing.
Solutions?
a) use somewhat less gain
b) play further away from the amp
It will drive you crazy in the bedroom or garage, won't bother you onstage at an open air festival.
c) keep playing a long time and apparently "it willl disappear on its own" , which really means that it won't "disappear" but when you become an experienced guy instintively you'll avoid it, one way or the other.
d) don't waste time or money on substi-tubes (specially in *this* amp), use the prescribed 12AT7.
That's not a "gain stage" which offers some flexibility but a current buffered speaker driver.
Quote from: J M Fahey on July 03, 2013, 04:22:39 PM
You have no *tube*, nor even *amp* problem.
You are using way too much gain and your *guitar* is feeding back in an uncontrolled way.
1) you pluck your guitar 13 times.
Count them because it's more precise than referring so such and such second, the video is short, but enough.
2) video can be divided in:
silence/7 plucks/silence/4 plucks/silence/2 plucks/silence
"silence" is a joke, your guitar is howling all the time, in an uncontrolled way, normally at a relatively low frequency, more precisely what the open strings are tuned to.
When you push a string against the fretboard, it will howl at a different, higher frequency.
That higher frequency produced an unplucked but howling/oscillating string will beat with the frequency of the actual plucked one, and since the amp is *heavily* distorting, it will produce 2 beat frequencies, one the sum (it will sound as a dissonant octave) and one at the difference, which will be a low frequency which to boot will modulate volume, like a crazy tremolo.
That's what you hear as "chirps" or "bursts" because thanks to the unstability the howling string starts and stops at random.
You can hear it *clearly* on plucks nº:
end of 2 , beginning of 3, low frequency modulation, sounds similar to a tremolo/vibrato/phaser/chorus
8/9/10 : higher frequency modulation, sounds like "chirping"
That it's a guitar feedback problem and not an amp one (and even less a *tube* problem, as confirmed by:
Quote
Its only the dirty channel that gets effected, the clean channel is fine, but if i use a OD infront of the clean channel it does the same thing.
Solutions?
a) use somewhat less gain
b) play further away from the amp
It will drive you crazy in the bedroom or garage, won't bother you onstage at an open air festival.
c) keep playing a long time and apparently "it willl disappear on its own" , which really means that it won't "disappear" but when you become an experienced guy instintively you'll avoid it, one way or the other.
d) don't waste time or money on substi-tubes (specially in *this* amp), use the prescribed 12AT7.
That's not a "gain stage" which offers some flexibility but a current buffered speaker driver.
ill make another vid with the gain on 1, it does the same thing.
Ive been playing guitar for a few years. My problem is definitively not feedback and too much gain.
QuoteMy problem is definitively not feedback and too much gain.
Both go hand in hand.
What would you call the
permanent howling between plucked notes?
Please reread my comments while listening to the video, time and again. :)
Plus, others are noticing the same as I do, maybe using different words but the general meaning is the same.
Quote from: J M Fahey on July 03, 2013, 05:16:12 PM
QuoteMy problem is definitively not feedback and too much gain.
Both go hand in hand.
What would you call the permanent howling between plucked notes?
Please reread my comments while listening to the video, time and again. :)
Plus, others are noticing the same as I do, maybe using different words but the general meaning is the same.
Phatt said it could be blocking distortion. what ive read about blocking distortion sounds more like what im experiencing. Ill record a better vid in the morning, i usually use a boss ns-2 and a boss sd-1, i put the amp gain on 5 and boost it with the boss sd-1.
The howling betwenn notes, do you mean that repeated notes?
what that is do not know. This is the first amp i have that does this when the gain channels level gets turned up.
What are your settings on the sd-1? Normally to boost, you go with lowest possible gain, and max volume. if you run drive full on the pedal and then distort again through the preamp, squealing is more or less inevitable.
If you bypass the sd-1, does it still sound that way?
Here is a simple experiment. Stand away from the amp, and have it set up to play, and presumably the noise will result. Now turn 90 degrees and face to the side. ANy change? Now another 90 degrees, and face away from the amp (well the speakers really). Any change, and finally another 90 degrees so you are facing the other side. ANy change? If the direction you aim the guitar changes the sound in any way, it is feedback. Feedback in a high gain amp can happen even at low volumes you can talk over.
Quote from: stormbringer on July 04, 2013, 02:55:53 AM
What are your settings on the sd-1? Normally to boost, you go with lowest possible gain, and max volume. if you run drive full on the pedal and then distort again through the preamp, squealing is more or less inevitable.
If you bypass the sd-1, does it still sound that way?
Hi
My settings on the sd-1 are, drive: 2 and level on 8. sometimes ill max the the level to 10.
If i dont use this method, it sounds the same, just with lots of noise and feedback.
Quote from: Enzo on July 04, 2013, 03:11:52 AM
Here is a simple experiment. Stand away from the amp, and have it set up to play, and presumably the noise will result. Now turn 90 degrees and face to the side. ANy change? Now another 90 degrees, and face away from the amp (well the speakers really). Any change, and finally another 90 degrees so you are facing the other side. ANy change? If the direction you aim the guitar changes the sound in any way, it is feedback. Feedback in a high gain amp can happen even at low volumes you can talk over.
That only changes the feedback to some degree. But the amp still makes those extra notes. What i found was that chords on the Low strings sound fine, only the high strings are making that ping crackling noise. I tested it with 2 different guitars just to make sure its not my guitar.
QuoteWhat i found was that chords on the Low strings sound fine, only the high strings are making that ping crackling noise. I tested it with 2 different guitars just to make sure its not my guitar.
Yeah, that's a good idea, the high E string tends to "oscillate" when it gets worn.
edit:
Never mind, i was listening through lousy speakers at work, i hear the problem properly now at home. you still need to change that string though ;)
QuoteThis is the first amp i have that does this when the gain channels level gets turned up.
Agree.
That amp has **incredible** gain, way above anything you used before.
To put it in perspective:
a classic Overdrive pedal (Tube Screamer) has gain 100X (500K/4K7) before hitting the diodes.
a classic Distortion pedal (MXR Dist+) has gain 200X (1M/4K7).
Your DS1 is also around 200X gain.
a classic Randall amps (RG100) had 2 20X Fet gain stages cascaded in the Red channel, for a gain of 20x20=400
This was Dimebag's amp, you won't call his sound "puny" or similar. 8)
Your amplifier, in the "Gain 1" channel has (cascaded) the following stages:
IC1A (3X) + IC1B (367X) for a whopping 1100X gain before hitting clipping diodes D5/D6 .
And that's the "mild" channel . :loco
In the "hot" Gain 2 channel, gain stages are again IC1A (3X) + IC2A (41X) + IC2B (142X). Do the Math.
Ok, to save some time, here it is: 3*41*142=
17466 X xP :duh
With that amount of gain, squealing is
guaranteed, with
any guitar on this Planet, specially if you are within the same room.
As I said before, such a monstrous amount of gain is somewhat controllable on a big , Rock Festival stage, because you are:
a) typically 10/15/20 yards away from the speakers.
Or more: you'll probably use a wireless transmitter .
b) you are in open air, or if "inside", the roof will probably be 25 yards above and the Stadium walls will be 30 to 80 yards away, same thing.
to which I should add :
c) usually the amps shown onstage are off or just props, and the actual amp miked is **backstage** .
For one real example from a Forum member here, look at this:
http://youtu.be/3QEjiJ56Zac
You'll see our friend onebaldbloke there (I guess you'll recognize him ;) from his nickname), playing in a big stadium, with excellent sustain.
Oh, the wall of Marshalls? Look nice, huh?
He was *actually* using a little Squire SS amp, plus a Rat or Big Muff pedal:
QuoteI've always had a fondness for SS stuff. The biggest selling album I ever recorded (multi Platinum here in Oz) was done with a $50 SS Squire with an 8" speaker, though everyone seems to think I used valve big stacks - ah well.
QuoteThat live stuff is the same little Squire out the back in a road case with a Sennheiser 421 mic on it.
Fun setup!
QuoteAll those Marshalls on stage and what we're hearing is a miked up Fender Squier 15 hidden in back of a road case! I love it. Sounds good;
The full thread:
http://www.ssguitar.com/index.php?topic=2560.0
PS: in a nutshell: at those gain levels, *any* guitar will squeal, unless you get far away from the speakers.
EDIT: I forgot to mention (although it should be real evident): they are Australians !!!!
@ belleraphon88,
Re blocking distortion.
Some insight into this can be found here at Roly's wonderful Site. :tu:
http://www.ozvalveamps.org/choking.htm
This may or may not be the exact same thing but I have experienced this effect with both Valves and SS Builds.
FWIW I've just spent over a year trying to get it through to a young lad's passionate but vain attempt to extract a million watts from a 10 watt amp that gain does not go on forever and sky hooks just don't exist. :lmao:
Even the simple opamp Odrive pedals I build can suffer from this sort of spitting problem if you over do it.
In my limited understanding the signal is getting way too big to fast and the next stage may well momentarily cut off on big peaks.
Either way the end result is *Exactly* as J M Fahey has already explained.
It's possible it may just need to be backed off at some point in the signal path but wait and see.
Food for thought;
If you have trouble understanding what these chaps are saying then my advice is to throw out what you think is wrong and let these clever minds help you and guide you through the issue. They are well versed in fault finding.
Do the tests that are asked of you and reply to best of your ability.
Once you go off on tangents then it becomes so much harder to help.
As JMF has noted already,,Your Amp is super high gain and like a lot of them won't take pedals well.
So when you read a mag review of the latest BS mega uber hyper drive bragg rig and the writer says it does not take pedals well,,, It simply means it's got way too much inbuilt gain and if you use pedals then expect problems like you are experiencing.
Phil.
Quote from: J M Fahey on July 04, 2013, 07:51:08 AM
QuoteThis is the first amp i have that does this when the gain channels level gets turned up.
Agree.
That amp has **incredible** gain, way above anything you used before.
To put it in perspective:
a classic Overdrive pedal (Tube Screamer) has gain 100X (500K/4K7) before hitting the diodes.
a classic Distortion pedal (MXR Dist+) has gain 200X (1M/4K7).
Your DS1 is also around 200X gain.
a classic Randall amps (RG100) had 2 20X Fet gain stages cascaded in the Red channel, for a gain of 20x20=400
This was Dimebag's amp, you won't call his sound "puny" or similar. 8)
Your amplifier, in the "Gain 1" channel has (cascaded) the following stages:
IC1A (3X) + IC1B (367X) for a whopping 1100X gain before hitting clipping diodes D5/D6 .
And that's the "mild" channel . :loco
In the "hot" Gain 2 channel, gain stages are again IC1A (3X) + IC2A (41X) + IC2B (142X). Do the Math.
Ok, to save some time, here it is: 3*41*142=17466 X xP :duh
With that amount of gain, squealing is guaranteed, with any guitar on this Planet, specially if you are within the same room.
As I said before, such a monstrous amount of gain is somewhat controllable on a big , Rock Festival stage, because you are:
a) typically 10/15/20 yards away from the speakers.
Or more: you'll probably use a wireless transmitter .
b) you are in open air, or if "inside", the roof will probably be 25 yards above and the Stadium walls will be 30 to 80 yards away, same thing.
to which I should add :
c) usually the amps shown onstage are off or just props, and the actual amp miked is **backstage** .
For one real example from a Forum member here, look at this:
http://youtu.be/3QEjiJ56Zac
You'll see our friend onebaldbloke there (I guess you'll recognize him ;) from his nickname), playing in a big stadium, with excellent sustain.
Oh, the wall of Marshalls? Look nice, huh?
He was *actually* using a little Squire SS amp, plus a Rat or Big Muff pedal:
QuoteI've always had a fondness for SS stuff. The biggest selling album I ever recorded (multi Platinum here in Oz) was done with a $50 SS Squire with an 8" speaker, though everyone seems to think I used valve big stacks - ah well.
QuoteThat live stuff is the same little Squire out the back in a road case with a Sennheiser 421 mic on it.
Fun setup!
QuoteAll those Marshalls on stage and what we're hearing is a miked up Fender Squier 15 hidden in back of a road case! I love it. Sounds good;
The full thread:
http://www.ssguitar.com/index.php?topic=2560.0
PS: in a nutshell: at those gain levels, *any* guitar will squeal, unless you get far away from the speakers.
EDIT: I forgot to mention (although it should be real evident): they are Australians !!!!
I did another vid, with less gain, you should hear it better now, especially at 00:14. no pedals, just the amp
http://www.youtube.com/watch?v=Hd14djOlLu4
I'm really no expert here. Experienced player, kinda new builder. so better wait for the "big guys" here to come with more qualified analysis. (mr Fahey, Roly, hint hint).
But just a thought.. are you sure that the speaker itself is ok? any chance to test the send out to Another amp? and Another preamp or hell even the DS-1 into the return? or disconnect the speaker, and connect to an external cab (Watch the impedance). As somewhat of an amateur, that's what i would try just to find which of those 3 is causing the issue, and move on from there.
Edit: By the way, wasnt that the amp that was modded by some Tech to get cooler? If that is the case, it would really help knowing exactly what was done. Maybe the mod has something to do with your problem? Did it behave like this Before the mod?
Well it's absolutely not valve microphonics.
"Blocking or "choking" essentially occurs under extreme drive conditions and is caused by some stage being driven so hard that it starts to rectify the signal instead of/as well as amplifying it. This results in a shift of the bias point of the stage, generally until it stop amplifying, therefore stops rectifying, the overbias leaks away, and the previous overdrive state restored.
How this turns out in practice depends on a number of factors such as which stage is being overbiased, how it is being restored, and how long this takes. In output stages this normally take a form that sounds like excessive compression with slow recovery, but this does sound more like preamp blocking of some sort. I can't tell you the exact mechanism in this case without putting a CRO on it, but given the extreme gain or x17k that JM points out I would say the basic cause here is extreme overdrive.
@Stormbringer makes a couple of good points; in the first vid it sounded like beat notes, but in the second it was more like dirty connectors or a speaker with a loose voice coil, so it's worthwhile doing the tests he suggests, if only to eliminate those possibilities.
Quote from: stormbringer on July 04, 2013, 03:06:24 PM
I'm really no expert here. Experienced player, kinda new builder. so better wait for the "big guys" here to come with more qualified analysis. (mr Fahey, Roly, hint hint).
But just a thought.. are you sure that the speaker itself is ok? any chance to test the send out to Another amp? and Another preamp or hell even the DS-1 into the return? or disconnect the speaker, and connect to an external cab (Watch the impedance). As somewhat of an amateur, that's what i would try just to find which of those 3 is causing the issue, and move on from there.
Edit: By the way, wasnt that the amp that was modded by some Tech to get cooler? If that is the case, it would really help knowing exactly what was done. Maybe the mod has something to do with your problem? Did it behave like this Before the mod?
Hii
The guy that did the mod told me he went over all the components and they are all 100%, the amp had a stupid buzzing as well that alot of people with these G3 models complained about, he somehow got rid of that as well. The speakers were tested as well, they sound fine. If you can hear the amp in person ull be able to hear the problem much better, my cam doesnt pick up all the weird noises i get when playing the high strings.
Ive been thinking of just taking it back to that tech, he designed mosfet amps for a few years, hes good but he takes forever, he had my amp for 4 months.
The amp did behave like this before the mod, it was even worst back then, the only way i ever got the amp to sound like it should was when i replaced the tube in the amp.
Alright. Read up a Little more on the amp.
Doing the tests i suggested would probably help alot to rule out the possible culprits.
1. The preamp
there is a level knob for your send jack. start with the level on the overdrive channel at 0, and the send level knob all the way down. connect the send jack to some kind of sound system. even the sound card line in should work as long as you're careful with the level buttons. it will sound like crap due to the frequency response, but you should be able to hear if there are any "dropouts".
Does it sound ok?
Yes
the preamp is probably not the issue.
2. so the preamp sounded fine?
Power amp
Get your DS-1 or a multi effect, Another preamp, a distortion pedal etc
connect your guitar to it, and into the return jack. start with the level/volume on the pedal low, and master volume low. adjust to non ear-bleed levels and listen.
Still doesnt sound right?
3. get/borrow an external speaker cab of 4 or 8 ohms, connect to your external output jack matching the cab impedance.
If this is where it works, the speaker(s) are probably broken.
4. Also, there seems to be quite some jack switching on this amp.
the external speaker jacks seems to disconnect the internal speaker.
the send/return jacks are also without switches, which probably mean that the jacks handle the switching there too.
Unused jacks (i've been told gold plated are less prone to this) tend to get bad Connections just sitting there. oxidize? cleaning or replacing them might do the trick?
Roly gave me this tip while i was working on a colleagues rocktron amp, and he sure was right. i Went nuts with the jacks, and it solved the problem.
WARNING:
I suppose that you COULD also disconnect the internal speakers and try them one at a time. but i really dont know what their impedance is and how they are connected, i strongly advice that you wait for one of the guys that actually know stuff here to comment on that Before you try. Speakers in combos are usually connected using spade lugs. which comes off quite easily
:WARNING
This is atleast what i would do. Once you know what part is causing the problem it gets easier troubleshooting (i work as a computer Tech/programmer, and this is more or less how i check a pc for errors. try one part at a time until i find the culprit).
So, it goes away after a tube swap and then comes back? That doesn't mean that it's the newness of the tube that fixes it, but simply that the tube socket was exercised. Try confirming your theory by pulling the tube and popping it back in a few times. (Power off, of course; I don't mean live).
Maybe the tube socket needs cleaning. (I use a toothpick with a very small amount of toothpaste to polish inside tube sockets; then another toothpick and lots of 99% pure isopropyl alcohol to clean up after the toothpaste.)
Maybe the tube socket or something else nearby has a cracked solder joint. Inspect all solder joints and repair if necessary.
Clean all jacks and potentiometers. Pots can make weird volume changes.
Check all hookup wires, like to the speakers.
You can test whether this problem is related to vibration by taking the speaker out of the amplifier (while keeping it connected) so that you can play test notes without rattling the enclosure.
You said:
Quote
It does have a effects loop. I dont have a external cab to test it with, is there any way i can test it.
You really have to do this test! Plug the effect send into a pair of computer speakers. Get an adapter plug for this: quarter inch plug, with 1/8" socket.
Knowing whether the problem is before the power amp or not is an important piece of information: divide (the signal chain) and conquer.
Quote from: Kaz Kylheku on July 04, 2013, 11:11:49 PM
So, it goes away after a tube swap and then comes back? That doesn't mean that it's the newness of the tube that fixes it, but simply that the tube socket was exercised. Try confirming your theory by pulling the tube and popping it back in a few times. (Power off, of course; I don't mean live).
Maybe the tube socket needs cleaning. (I use a toothpick with a very small amount of toothpaste to polish inside tube sockets; then another toothpick and lots of 99% pure isopropyl alcohol to clean up after the toothpaste.)
Maybe the tube socket or something else nearby has a cracked solder joint. Inspect all solder joints and repair if necessary.
Clean all jacks and potentiometers. Pots can make weird volume changes.
Check all hookup wires, like to the speakers.
You can test whether this problem is related to vibration by taking the speaker out of the amplifier (while keeping it connected) so that you can play test notes without rattling the enclosure.
You said:
Quote
It does have a effects loop. I dont have a external cab to test it with, is there any way i can test it.
You really have to do this test! Plug the effect send into a pair of computer speakers. Get an adapter plug for this: quarter inch plug, with 1/8" socket.
Knowing whether the problem is before the power amp or not is an important piece of information: divide (the signal chain) and conquer.
So here is what i did.
Did the tests with the send into my pc speakers, used 3 different tubes. It didnt change anything.
Next i took the tube out and did the same test, still sounded the same.
So cant be the power amp then or tube
That means it should be the preamp that's the issue.
What i would do first is check for electrolytic capacitors with any kind of bulge. (although they can still be bad without it showing). Do the preamp caps look ok?
Quote from: stormbringer on July 05, 2013, 05:50:33 AM
That means it should be the preamp that's the issue.
What i would do first is check for electrolytic capacitors with any kind of bulge. (although they can still be bad without it showing). Do the preamp caps look ok?
They all seem fine, no buldges
We seem to have eliminated the power amp and the valve.
It does it in both channels, the drive channel, and the normal channel when driven hard externally.
Well there are only two bits that are common to both, the first stage IC , and the last three IC4b, 5a, and 11b. I rather doubt that its the first stage because it would do it all the time, but the last stages only gets driven hard when internal drive is selected, or when the normal channel is externally driven hard by a stomp.
So I'm on RG_RH G3 Preamp.pdf, ref: C7, IC4b preceded by FET switch Q6.
Now my intuition says, an enlightened guess is, that we are not chasing a faulty component. I think everything is working as it should, but it's the conditions that it is being exposed to that is causing the blocking problem. I'm struck by the switching FET Q6 and the diode in its Gate circuit.
The function of this FET switch seems to be to mute the preamp unless something is actually plugged into the input.
We also have a number of connections at this point, a three conductor cable that apparently connects the Fx Send and Return sub assembly, and the contacts in the Return socket itself, all in the signal path.
A known good lead patched between Fx Send and Return would at least eliminate the return socket contacts.
Quote from: Roly on July 05, 2013, 07:56:13 AM
We seem to have eliminated the power amp and the valve.
It does it in both channels, the drive channel, and the normal channel when driven hard externally.
Well there are only two bits that are common to both, the first stage IC , and the last three IC4b, 5a, and 11b. I rather doubt that its the first stage because it would do it all the time, but the last stages only gets driven hard when internal drive is selected, or when the normal channel is externally driven hard by a stomp.
So I'm on RG_RH G3 Preamp.pdf, ref: C7, IC4b preceded by FET switch Q6.
Now my intuition says, an enlightened guess is, that we are not chasing a faulty component. I think everything is working as it should, but it's the conditions that it is being exposed to that is causing the blocking problem. I'm struck by the switching FET Q6 and the diode in its Gate circuit.
The function of this FET switch seems to be to mute the preamp unless something is actually plugged into the input.
We also have a number of connections at this point, a three conductor cable that apparently connects the Fx Send and Return sub assembly, and the contacts in the Return socket itself, all in the signal path.
A known good lead patched between Fx Send and Return would at least eliminate the return socket contacts.
Ive used a patch cable between send and return, didnt notice any difference if that is what u asked
Quote from: belleraphon88 on July 05, 2013, 05:09:04 AM
So here is what i did.
Did the tests with the send into my pc speakers, used 3 different tubes. It didnt change anything.
Next i took the tube out and did the same test, still sounded the same.
So cant be the power amp then or tube
Yes; the tube stage is after the preamp and effect send, so it makes no difference.
This is lucky for you because you don't have to mess around in the power amp where you have higher voltages (especially given the tube).
The thing to do now is to make yourself some kind of adapter so that you can use those same PC speakers as an audio probe. A simple setup would be to have a cable which has an alligator clip for ground, and a multimeter probe for signal. Clip the ground somewhere to a ground point in the amp, and then use the probe to pick up the audio from various sections of the preamp based on following the schematic. You can find out the first stage where it is happening. It would really help you if you reproduce the issue with some signal source other than your guitar, like using your PC's audio line out to play some continuous tones into the amp's input.
You said earlier that the issue occurs whether or not you use the gain channel; that you can hear it on a distorted tone that is produced by a pedal going into the clean channel.
Assuming this is really the case, the problem is really narrowed down. There is very little circuitry between the input and the effect send that is common to both channels. What is common to both channels is the IC1A op-amp input stage, and and the channel switching and muting FETs Q4, Q5 and Q6. (Well, Q4 and Q5 are specific to their respective channels, but the circuits could interact in some way since channel switching is supposed to be mutually exclusive). I can't find the logic that drives the channel switching in that schematic, by the way.
If the voltage at the base of Q6 oscillates for some reason, then you will get volume fluctuations or periodic cutouts. It's not obvious why that would be the case, since the Q6 gate is driven by a voltage divider that is straight off the power supply. Still, if you had an oscilloscope you could easily put a probe on the bases of these transistors to see what the voltage is doing. If there is such a thing happening, it is probably a little too fast to register on a multimeter, but a scope trace will react instantly.
There is the possibility that it's an issue with the preamp power supply: that basically the entire preamp cuts out due to a periodic voltage brown-out. Your tech would almost certainly have picked up on such a glaring thing!
Quote from: Kaz Kylhekuso that you can use those same PC speakers as an audio probe.
Funny, I was discussing this with a technofriend not an hour ago and he made a very similar suggestion. :dbtu:
Quote from: Kaz Kylhekulike using your PC's audio line out to play some continuous tones into the amp's input.
It will also be interesting to see if a pure single tones produces the same effect as the more complex tones from multiple strings (because it does have a curious "beating" effect which I wouldn't expect with simple blocking).
Technofriend also commented that the muting FET Q6 could be aging or on the way out for some reason.
{Gotta say I'm a bit baffled by this one. (http://emoticoner.com/files/emoticons/smiley_faces/aussie-smiley-face.gif) (http://emoticoner.com) }
Quote from: Kaz Kylheku on July 05, 2013, 03:04:50 PM
Quote from: belleraphon88 on July 05, 2013, 05:09:04 AM
So here is what i did.
Did the tests with the send into my pc speakers, used 3 different tubes. It didnt change anything.
Next i took the tube out and did the same test, still sounded the same.
So cant be the power amp then or tube
Yes; the tube stage is after the preamp and effect send, so it makes no difference.
This is lucky for you because you don't have to mess around in the power amp where you have higher voltages (especially given the tube).
The thing to do now is to make yourself some kind of adapter so that you can use those same PC speakers as an audio probe. A simple setup would be to have a cable which has an alligator clip for ground, and a multimeter probe for signal. Clip the ground somewhere to a ground point in the amp, and then use the probe to pick up the audio from various sections of the preamp based on following the schematic. You can find out the first stage where it is happening. It would really help you if you reproduce the issue with some signal source other than your guitar, like using your PC's audio line out to play some continuous tones into the amp's input.
You said earlier that the issue occurs whether or not you use the gain channel; that you can hear it on a distorted tone that is produced by a pedal going into the clean channel. Assuming this is really the case, the problem is really narrowed down. There is very little circuitry between the input and the effect send that is common to both channels. What is common to both channels is the IC1A op-amp input stage, and and the channel switching and muting FETs Q4, Q5 and Q6. (Well, Q4 and Q5 are specific to their respective channels, but the circuits could interact in some way since channel switching is supposed to be mutually exclusive). I can't find the logic that drives the channel switching in that schematic, by the way.
If the voltage at the base of Q6 oscillates for some reason, then you will get volume fluctuations or periodic cutouts. It's not obvious why that would be the case, since the Q6 gate is driven by a voltage divider that is straight off the power supply. Still, if you had an oscilloscope you could easily put a probe on the bases of these transistors to see what the voltage is doing. If there is such a thing happening, it is probably a little too fast to register on a multimeter, but a scope trace will react instantly.
There is the possibility that it's an issue with the preamp power supply: that basically the entire preamp cuts out due to a periodic voltage brown-out. Your tech would almost certainly have picked up on such a glaring thing!
I will make me that audio probe during this week, just gota buy some parts.
Will update asap
thanks for all the help so far
Andrei
Quote from: Kaz Kylheku on July 05, 2013, 03:04:50 PM
Quote from: belleraphon88 on July 05, 2013, 05:09:04 AM
So here is what i did.
Did the tests with the send into my pc speakers, used 3 different tubes. It didnt change anything.
Next i took the tube out and did the same test, still sounded the same.
So cant be the power amp then or tube
Yes; the tube stage is after the preamp and effect send, so it makes no difference.
This is lucky for you because you don't have to mess around in the power amp where you have higher voltages (especially given the tube).
The thing to do now is to make yourself some kind of adapter so that you can use those same PC speakers as an audio probe. A simple setup would be to have a cable which has an alligator clip for ground, and a multimeter probe for signal. Clip the ground somewhere to a ground point in the amp, and then use the probe to pick up the audio from various sections of the preamp based on following the schematic. You can find out the first stage where it is happening. It would really help you if you reproduce the issue with some signal source other than your guitar, like using your PC's audio line out to play some continuous tones into the amp's input.
You said earlier that the issue occurs whether or not you use the gain channel; that you can hear it on a distorted tone that is produced by a pedal going into the clean channel. Assuming this is really the case, the problem is really narrowed down. There is very little circuitry between the input and the effect send that is common to both channels. What is common to both channels is the IC1A op-amp input stage, and and the channel switching and muting FETs Q4, Q5 and Q6. (Well, Q4 and Q5 are specific to their respective channels, but the circuits could interact in some way since channel switching is supposed to be mutually exclusive). I can't find the logic that drives the channel switching in that schematic, by the way.
If the voltage at the base of Q6 oscillates for some reason, then you will get volume fluctuations or periodic cutouts. It's not obvious why that would be the case, since the Q6 gate is driven by a voltage divider that is straight off the power supply. Still, if you had an oscilloscope you could easily put a probe on the bases of these transistors to see what the voltage is doing. If there is such a thing happening, it is probably a little too fast to register on a multimeter, but a scope trace will react instantly.
There is the possibility that it's an issue with the preamp power supply: that basically the entire preamp cuts out due to a periodic voltage brown-out. Your tech would almost certainly have picked up on such a glaring thing!
I made that audio probe!
just took a while, had to drive pretty far just to buy that capacitor for the probe.
I will do the testing tomorrow.
Ive noticed 2day that if i scoop the mids the tone is pretty nuch normal when i play riffs and palmute, as soon as i turn the mids up i do get blocking distortion.
Just thought id bring this up.