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Randall RG100G3 power amp help

Started by belleraphon88, July 02, 2013, 11:26:01 AM

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belleraphon88

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

JHow

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.

Enzo

You have a working amp, aside from microphonics.   MEASURE the voltages in the circuit.

J M Fahey

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.

belleraphon88

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.




   

phatt

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.

belleraphon88

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.

J M Fahey

#7
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.

Roly

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?
If you say theory and practice don't agree you haven't applied enough theory.

belleraphon88

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

belleraphon88

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

phatt

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.

Kaz Kylheku

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.







   
   
ADA MP-1 Mailing ListMusic DIY Mailing List
http://www.kylheku.com/mp1http://www.kylheku.com/diy

belleraphon88

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.

belleraphon88

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