Carvin Belair 2x12 Gain Stage

[bobloblaws]

Hey guys, I could use a bit of advice on a problem I've encountered with a Carvin Belair combo amp (schematic below). On channel 2 there is significant hiss when the "Soak" (drive/gain) control is turned up past a specific point (in the neighborhood of between 6 and 7 on the dial). I'm scoping a test signal at pin 7 of V2:B (with the channel volume all the way down) and as I increase the drive pot I can see the amplitude increasing on a relatively linear curve until it jumps up significantly at the point where the hiss starts and then continues from that point in a typical fashion. The hiss is not present on the other side of C13. According to the schematic I should see about 300V at the plates on V2. In fact, the actuals are 147V on pin1 and 246V on pin 6. Based on that information can anyone point me in the right direction as to what the issue might be?

Edit: I see now that there is a couple of threads on here dealing with similar problems with this amp. I'll start by looking through those.


Carvin-Belair-Schematic.pdf

[J M Fahey]

What happens to hiss when you remove your test probe from V2b-7?

Hiss might be "what you hear" from an oscillation and probe could be an interference injecting antenna.

What do you see when scoping V2b-6 and you rise/lower volume pot?

At least, anything you see on pin 7 will be much more visible on pin 6 .

Also, what do you hear?

It should match what you see on screen.

I´m not much worried at schematic indicated plate volotages, if anything I find *them* high, go figure.

What you measured sounds more usual to me.

[g1]

Also, how does signal react to the pot, the same 'jump' or different?
As far as the DC voltages, agree with JM, what you measured sounds right, what's shown on the schematic looks wrong. The 2 halves of the tube have the same plate R's but different cathode R's, so they can't be the same. Is voltage at 'D' 385V? V2 pin3 around 3V and pin8 around 1V ?

[bobloblaws]

What happens to hiss when you remove your test probe from V2b-7?

In this scenario I have the output connected to a dummy load and I'm using an audio probe to listen for the hiss at various points in the circuit. So if I remove the probe I don't hear anything. I probably should have clarified that I'm probing with the scope at times and with the audio probe at other times.

Hiss might be "what you hear" from an oscillation and probe could be an interference injecting antenna.

But I noticed the hiss present via the output before I ever started probing so we can rule that out.

What do you see when scoping V2b-6 and you rise/lower volume pot?

Same as pin 6, just louder, as you would expect.

[bobloblaws]

Also, how does signal react to the pot, the same 'jump' or different?

Yes, same.

Is voltage at 'D' 385V?

Actual is 350V.

V2 pin3 around 3V and pin8 around 1V ?

pin 3 -> 0.7V pin 8 -> 2V

[g1]

So could it just be a glitchy pot then? Or something about the taper that makes it act that way? If the signal is reacting the same way, then I would think the pot is the cause.
As for the V2 cathode voltages, I got my pins 3 & 8 voltages reversed. For 350V at point D, your V2 pin3 at .7V and pin8 at 2V look correct. The schematic voltages shown for V2 plates are incorrect.

[bobloblaws]

So could it just be a glitchy pot then? Or something about the taper that makes it act that way? If the signal is reacting the same way, then I would think the pot is the cause.
As for the V2 cathode voltages, I got my pins 3 & 8 voltages reversed. For 350V at point D, your V2 pin3 at .7V and pin8 at 2V look correct. The schematic voltages shown for V2 plates are incorrect.

Just to make sure I understand what you mean, are you suggesting the pot is the source of the hiss? Typically with a master volume type amp you should be able to turn the gain control all the way up to get desired amount of saturation and control overall volume with the volume pot. I'm not sure I understand what you mean by "the signal is reacting the same way". The same way as what? To reiterate, there is a specific point on the turn of the pot where the hiss kicks in. If I probe a test signal with my scope at V2 pins 6 or 7 I can see the sine wave increase significantly in amplitude at the same point. If I recall correctly the jump in amplitude is apparent at the output as well (I would expect that in any case). Anyway, I guess my point is I don't see how the pot taper is relevant since there should not be that amount of noise regardless of where the pot is.

Out of curiosity, how do you determine what voltages to expect on the cathodes? Also, based on what you said about expecting different plate voltages based on different cathode resistor values, are the plate resistor and cathode resistor the only factors determining what the plate voltage will be (given the same B+ and same tube type?).

[J M Fahey]

Please replace pot with a new one and post results.

A pot with worn/dirty track leaving momentarily next tube grid "open" may account for all of your symptoms and then some, so let´s remove that out of the way first.

[bobloblaws]

Please replace pot with a new one and post results.

A pot with worn/dirty track leaving momentarily next tube grid "open" may account for all of your symptoms and then some, so let´s remove that out of the way first.

OK, I'll try that, fingers crossed.

[bobloblaws]

Please replace pot with a new one and post results.

A pot with worn/dirty track leaving momentarily next tube grid "open" may account for all of your symptoms and then some, so let´s remove that out of the way first.

On second thought, wouldn't it be feasible to first test that theory by simply bypassing the pot with a jumper, in effect having the pot wide open, but without the pot? Replacing the pot requires first disconnecting a slew of ribbon cables and wires with slide on connectors, unfastening all the pots, channel switch and input jack, removing the preamp tubes and then lifting the circuit board out. Seems like a lot of trouble if we are only guessing and there might be another way to possibly rule out the pot being bad.

[Jazz P Bass]

I agree.
Jumper it out.

[g1]

You were not clear on my question about 'does signal act the same way'.
What I meant was, is there a big jump in level when you are turning the pot with a signal applied.
Lets forget about the noise level with nothing plugged in for now. What I was getting at was the noise level relative to the signal. If there is an abnormal change in the signal level when adjusting the control, then there must be something wrong with the control, so that must be fixed first before being concerned with noise levels. (cleaning or replacement of the pot)
If the pot is good, then you will get no abnormal jump turning it while you have signal applied.

[g1]

I agree.
Jumper it out.

The pot serves as the grid leak for the V2. I'd suggest resistance checks from wiper to ground through it's rotation. You could jumper the pot and still lose the grid reference to ground.

[bobloblaws]

You were not clear on my question about 'does signal act the same way'.
What I meant was, is there a big jump in level when you are turning the pot with a signal applied.
Lets forget about the noise level with nothing plugged in for now. What I was getting at was the noise level relative to the signal. If there is an abnormal change in the signal level when adjusting the control, then there must be something wrong with the control, so that must be fixed first before being concerned with noise levels. (cleaning or replacement of the pot)
If the pot is good, then you will get no abnormal jump turning it while you have signal applied.

Maybe I can shed a little light from a slightly different perspective. While I did see an abnormal jump in sine wave amplitude scoping at V2, when I instead listened to the same test signal through the speaker it was more noticeable as a change in frequency response rather than simply a bump in volume. For that reason I kind of leaned toward it being something other than a wonky pot. Anyway, I'm gonna try jumpering the pot here shortly.

[g1]

Ok, but make sure the grid (pin7) has 500K or less to ground at all pot settings when measured with your meter.
Even with your jumper in place.