Sovtek MIG 50 Losing Sound After Standy Off

[Coop]

You have heater voltage on the other 2 tubes?

I have 7.3 vac heater voltage on V4, and V5 (6L6GC's) Measuring across pins 2 &7.

[Coop]

Capacitor cans 1 and two should be 200mf each. Can 1 measues 203mf, but Can 2 measures 108uf. Do you guys think Can 2 is the issue?

[Helmholtz]

Both cans should be 200µF (not mF, as 1mF = 1000 µF).

How did you measure? Out of circuit, what meter?

If can 2 is low capacitance it should be replaced.
But I don' think that's your main problem.

Were the voltage measurements taken with the amp in failure mode?
The issue seems to be triggered by temperature. You may try freeze spray to localize bad components or contacts (don't spray on hot tubes).

BTW, 550V at point C (R34/35) would be way too high if measured with all tubes in.

In your opening post you mention a touch sensitivity of C2 (not clear what C2 is, schem. shows 22µ/25V for heater elevation).
That kind of effect may also indicate an oscillation problem.

[Coop]

Both cans should be 200µF (not mF, as 1mF = 1000 µF).

How did you measure? Out of circuit, what meter?

If can 2 is low capacitance it should be replaced.
But I don' think that's your main problem.

Were the voltage measurements taken with the amp in failure mode?
The issue seems to be triggered by temperature. You may try freeze spray to localize bad components or contacts (don't spray on hot tubes).

BTW, 550V at point C (R34/35) would be way too high if measured with all tubes in.

In your opening post you mention a touch sensitivity of C2 (not clear what C2 is, schem. shows 22µ/25V for heater elevation).
That kind of effect may also indicate an oscillation problem.

Yes, Both cans should be 200uf. Sorry about that. I unsoldered the cans before measuring. So, can 2 is way off at 108uf.

The measurements I gave to Enzo were in play mode. Failure mode. With all tubes in. I just got the amp. This is how it was upon my aquirring it.

As for the touch sensitivity to C2, With negative meter lead to chassis ground, When i touch my positive meter to C2 lead, Sound would come back and then quick;y fade away. Just like initial power on. I'm using a Fluke 115 meter. I'll have to get some freeze spray.

[Helmholtz]

Failure mode.

Did you verify?

Sometimes a heat induced problem doesn't show with the chassis out of the cabinet.

Please clarify which cap you mean with C2.

Please post all supply node voltages at points A, B, C, D and E (bias supply).
Also measure V4/V5 screen voltages (pins 4).

Did you try changing tubes?

Do you have a scope?

[Coop]

Helmholtz Here’s the measurements that you requested

Power supply nodes voltages
Node A is: 555 vdc
Node B is: 551 vdc
Node C is: 506 vdc
Node D is: 294 vdc
Node E is: -78 vdc

What I previously referenced as C2 is actually C9. I think? Pic below. 2nd pic of the 47uf-450v axial capacitor. There are differences between my board screen print, and the schematic provided by Jon Snell.

Screen voltages:
V4 (pin 4) 549 vdc
V5 (pin 4) 549 vdc

I have tried swapping all tubes with known good tubes, with no different results.

I do have a scope, but not an expert at using it.

[Helmholtz]

Thanks.

What stands out is the very large negative bias voltage at point E of -78V. A typical value would be closer to -50V.
As a result power tube plate and screen currents will be low, causing high positive supply voltages.

What is the lowest negative voltage at point E you can adjust via PR 1?
Turn PR 1 slowly and watch for discontinuities/voltage jumps while turning.

Leave between -50V to -60V and measure power tube idle currents.
Do you have a bias probe?

[g1]

12ax7's
V1 / Pin 1 is 288 vdc
Pin 6 is 181 vdc

V2 / Pin 1 is 170 vdc
Pin 6 is 291 vdc

V2 is the cathode follower, so V2 voltages look good.
For V1 pin1 to be 288V sounds like that side of V1 is not conducting. What is V1 pin3 voltage?

What stands out is the very large negative bias voltage at point E of -78V.

In post #4, bias was reported to be -57V.

[Coop]

V2 is the cathode follower, so V2 voltages look good.
For V1 pin1 to be 288V sounds like that side of V1 is not conducting. What is V1 pin3 voltage?


In post #4, bias was reported to be -57V.

V1 pin 3 is 5.8 vdc

[Coop]

Thanks.

What stands out is the very large negative bias voltage at point E of -78V. A typical value would be closer to -50V.
As a result power tube plate and screen currents will be low, causing high positive supply voltages.

What is the lowest negative voltage at point E you can adjust via PR 1?
Turn PR 1 slowly and watch for discontinuities/voltage jumps while turning.

Leave between -50V to -60V and measure power tube idle currents.
Do you have a bias probe?

The lowest bias adjustment I can get is -49.5 vdc. I set it at -55 vdc. I'm taking that voltage reading with positive meter lead to middle of bias pot (PR1). and negative meter lead to chassis ground. I do have one bias probe socket, if you want me to measure bias current.

[Helmholtz]

V1 pin 3 is 5.8 vdc

Cathode voltage is much too high.

Measure resistance from pin 3 to ground (should be 1k).

There must be something wrong with the 1k cathode resistor. Either drifted high or more likely a bad connection.

BTW, does input II work?

[Helmholtz]

I do have one bias probe socket, if you want me to measure bias current.

Yes you should measure power tube idle currents.
Adjust for 35mA cathode current via PR 1.
Then measure node A voltage again.

[Coop]

Cathode voltage is much too high.

Measure resistance from pin 3 to ground (should be 1k).

There must be something wrong with the 1k cathode resistor. Either drifted high or more likely a bad connection.

BTW, does input II work?

Which tube? V1 or the output tubes?

[Helmholtz]

Which tube? V1 or the output tubes?

V1 pin 3.

[Coop]

V1 pin 3.

V1 pin 3 resistance to chassis ground is 4.6 kOhms.