Randall RG100G3 power amp help

[Roly]

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.

[belleraphon88]

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.

[stormbringer]

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).

[Kaz Kylheku]

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:

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.

[belleraphon88]

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:

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

[stormbringer]

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?

[belleraphon88]

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

[Roly]

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.

[belleraphon88]

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

[Kaz Kylheku]

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!

[Roly]

so 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:

like 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. }

[belleraphon88]

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

[belleraphon88]

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.