Ampeg V4 wierd channel 1 & 2 interacting

[g1]

Also, without touching the sensitivity switch, what resistance do you measure from V1 pin 8 to ground.

[g1]

Cleaner version of schematic:

[Helmholtz]

Also, without touching the sensitivity switch, what resistance do you measure from V1 pin 8 to ground.

6.8K from post #11. Cathode and plate voltage are consistent, so I don't suspect cathode resistance.

[g1]

What happens/changes if you swap V1 and V2?

Just want to make sure this has been done. Boroman, I know you said you "checked/changed" preamp tubes, I want to verify they are not being checked on a tube tester and put back in the same spots.

[boroman]

Just want to make sure this has been done. Boroman, I know you said you "checked/changed" preamp tubes, I want to verify they are not being checked on a tube tester and put back in the same spots.

They all have >90% emission, but yes, I know the actual microphonics and sound can't be measured there. I do have a lot of tubes and put my 60s NOS Mullards there which I always do when testing something that is suspicious. Not the tube for sure. I will start now to remove potentiometer panel to see what's going on there...

[Helmholtz]

They all have >90% emission, but yes, I know the actual microphonics and sound can't be measured there. I do have a lot of tubes and put my 60s NOS Mullards there which I always do when testing something that is suspicious. Not the tube for sure. I will start now to remove potentiometer panel to see what's going on there...

There seems to be a misunderstanding.

My intention was to find out what changes regarding DC voltages and amp behaviour with V1 in V2 socket and vice versa.

[Helmholtz]

I was wondering what could cause such asymmetrical channel vol interaction.

I think it could work like this:

Suppose something causes a short between the input of the channel 1 vol pot (where it connects to C3) and the wiper of the channel 2 vol pot.
This would load the CH2 signal at the vol 2 wiper with another 100k (being the estimated circuit impedance at C3). While this would reduce the max available volume of CH2 by around 50%, it might go unnoticed.

But for CH1 the signal at C3 would be shorted at zero setting of vol 2.

[boroman]

I was wondering what could cause such asymmetrical channel vol interaction.

I think it could work like this:

Suppose something causes a short between the input of the channel 1 vol pot (where it connects to C3) and the wiper of the channel 2 vol pot.
This would load the CH2 signal at the vol 2 wiper with another 100k (being the estimated circuit impedance at C3). While this would reduce the max available volume of CH2 by around 50%, it might go unnoticed.

But for CH1 the signal at C3 would be shorted at zero setting of vol 2.

Helmholtz - I will continue with my two new points today. I figured out why I can't measure 5,6M resistor in the input1 jack. Because the jack itself has an open connection - each of the lugs is shorting to eveything else (so, signal is shorted with ground). I desoldered it, desoldered the resistor and it measures okay, but the input jack is trash. Just ordered new one. So this is the first issue.

Second - I noticed (having at the moment only CH2 soldered to pcb) that sound is loud but not super clear. Even at super low volume you hear distortion. I mean, you can have the level low and you can have it super high, but the "hint" of distortion is there all the time (so not volume dependent). This is actually VERY confusing. Should it be because nothing is connected to CH1 pcb points? I'm trying to understand why

By the way, the potentiometer board, assembly, solder joints, pots and the boards' components are all okay.

[Helmholtz]

By the way, the potentiometer board, assembly, solder joints, pots and the boards' components are all okay.

Did you check and clean the "bright" switches? Not sure if the bad input jack explains everything. It would require 3 fault conditions:

1) CH1 input signal is not connecting to V1.
2) CH1 connects to CH2 input instead.
3) CH2 jack is not shorting to ground.

[g1]

I noticed (having at the moment only CH2 soldered to pcb) that sound is loud but not super clear. Even at super low volume you hear distortion. I mean, you can have the level low and you can have it super high, but the "hint" of distortion is there all the time (so not volume dependent).

Not sure if this is the problem but if you have the sensitivity switch set on 0db it is fairly easy to overload the first stage which is pre volume control. Someone who likes a real clean sound may not be able to get it without that switch on the -9db setting. A friend of mine using my amp complained about this even with single coils.
So it will depend on the signal level you are using at the input.

[boroman]

Yes, the switches are clean, they all work well beside one thing. the ULTRA HI switch has 3 settings: (-) then (0) then (+). It does not go to the (+) setting. Switch does, but it sounds like it's back to minus, so like - / 0 / - rather than - / 0/ +

1) CH1 input signal is not connecting to V1.
2) CH1 connects to CH2 input instead.
3) CH2 jack is not shorting to ground.

1) As I see on the amp, the positive wire from input jack CH1 goes from its place on pcb to PIN7 of V1. I checked it now and there is a flow between them, so no need for the jumper.
2) I have only one working jack now, when I solder it to CH1 pcb and leave CH2 unsoldered I have a lot of strange buzz in the amp - like something is wrong with the transformer. That buzz you want to shut the amp down in a second.
3) It's shorting to ground (checked now). The PCB point of CH1 also shorts to ground.

Not sure if this is the problem but if you have the sensitivity switch set on 0db it is fairly easy to overload the first stage which is pre volume control. Someone who likes a real clean sound may not be able to get it without that switch on the -9db setting. A friend of mine using my amp complained about this even with single coils.
So it will depend on the signal level you are using at the input.

I know these amps, I've been playing through V4B (the other one) for about 10 years now. they are overloading on about 11o'clock, but not a hint of distortion on lower volumes. This behaves like it has that 10% overdrive on each volume setting you play. Like its an added from dist. pedal and mixed with the clean. The same proportion, not matter if you're on 1 or 10 in the vol. scale. I might record it but waiting for the new input jack first.


and BTW, R4 measures like 2-3 Ohm (desoldered is ok at 33k). Is it other components of circuit interacting?

[doombass]

1) As I see on the amp, the positive wire from input jack CH1 goes from its place on pcb to PIN7 of V1. I checked it now and there is a flow between them, so no need for the jumper.
2) I have only one working jack now, when I solder it to CH1 pcb and leave CH2 unsoldered I have a lot of strange buzz in the amp - like something is wrong with the transformer. That buzz you want to shut the amp down in a second.
3) It's shorting to ground (checked now). The PCB point of CH1 also shorts to ground.

Looking at the schematics and your picture of the input jacks I'd presume you've lost R8 5.6M resistor from the ch 2 input circuit since it's located on the input jack.

[Helmholtz]

R4 measures like 2-3 Ohm (desoldered is ok at 33k). Is it other components of circuit interacting?

Yes, it's supposed to be shorted by the sensitivity switch at max setting.

[Helmholtz]

Yes, the switches are clean, they all work well beside one thing. the ULTRA HI switch has 3 settings: (-) then (0) then (+). It does not go to the (+) setting. Switch does, but it sounds like it's back to minus, so like - / 0 / - rather than - / 0/ +

.

That makes the Ultra Hi switch suspicious. The unwanted coupling between the channels I indicated in post #22 might actually be happening inside this defective switch.

If the new input jack doesn't solve your problem, I would inspect/disassembly the Ultra Hi switch.

[boroman]

Looking at the schematics and your picture of the input jacks I'd presume you've lost R8 5.6M resistor from the ch 2 input circuit since it's located on the input jack.

It's there. And measures okay. Today I'm going to solder new jack for the V1 and we'll see again what's going on. If it does not help, I'm lost with ideas...