Mesa Boogie F-30 Hum Problem After Repair

[bobloblaws]

Here's a few traces. These are with all the tubes in except V3. First is the amp output. Next is V4 pin 1 and next is V4 pin 6. I'm still hearing a relatively small amount of hum in the headphones from the recording output even though I lifted the the .002 and .047 caps to isolate the recording circuit from the V4B grid. This is likely from the +15 supply at "H", but the problem at V4 is more significant. I don't know if the hum at V4 and the hum at "H" are manifestations of the same issue or whether they are separate issues.

[Helmholtz]

Please scope the bias voltage at point F using AC coupling.

[bobloblaws]

Please scope the bias voltage at point F using AC coupling.

OK, I was able to grab this trace at point F with scope set to AC coupling and trigger set to CH1.

[bobloblaws]

Crap! I just discovered one of my problems. With all the jostling and moving stuff around, one of the power tube plate/OT wires broke loose from the power tube PCB. I'm gonna fix that ASAP and see where I'm at then.

[bobloblaws]

Unfortunately, re-attaching that wire did nothing to help with the issues I'm dealing with.

[Helmholtz]

Your scope pic shows a significant 60Hz ripple on the bias supply and assumingly on the PI cathodes as well.
I don't have a reference, but 0.2Vpp doesn't look right to me.
Does the 1000µ cap measure good (capacitance and ESR) and is it mounted with correct polarity (positive to ground)?
A new cap isn't always a good cap.

[bobloblaws]

Your scope pic shows a significant 60Hz ripple on the bias supply and assumingly on the PI cathodes as well.
I don't have a reference, but 0.2Vpp doesn't look right to me.
Does the 1000µ cap measure good (capacitance and ESR) and is it mounted with correct polarity (positive to ground)?
A new cap isn't always a good cap.

The polarity is correct. The only way I have to test ESR is with the tester shown below. On that cap I measure 2.3 ohm ESR and 1.1% Vloss. Capitance is 950uF.

Something I noticed is that I'm only measuring 2V at "E" while the drawing indicates it should be 3V.The resistor is exactly 15K out of circuit. In any case, probably not relevant to my main issue.

[bobloblaws]

For the record, I tried swapping out the reversed diode and the 120K in the bias circuit, no difference. Is is worth a try swapping out the 0.047 PI coupling caps?

[Chuck H]

Bob. Please tell me that the reason for the scope trace shown for the plate at V4B (last image post 122) and the scope trace shown for point F bias supply (post 124) are identical is an accident. At 129 posts in I would hate to think you're f#&!ng with us.

[bobloblaws]

Bob. Please tell me that the reason for the scope trace shown for the plate at V4B (last image post 122) and the scope trace shown for point F bias supply (post 124) are identical is an accident. At 129 posts in I would hate to think you're f#&!ng with us.

Good catch. I'm very sorry about that. I corrected post #124.

[Helmholtz]

Now that's a lot less bias ripple.

[Chuck H]

Now that's a lot less bias ripple.

Does that mean the ripple seen there probably isn't the problem? I've never had to quantify how much bias ripple is a problem so I don't know. If that's the case I only have a couple more ideas to test for.

[Helmholtz]

Something I noticed is that I'm only measuring 2V at "E" while the drawing indicates it should be 3V.The resistor is exactly 15K out of circuit. In any case, probably not relevant to my main issue.

What are V4 plate voltages?

[Helmholtz]

Does that mean the ripple seen there probably isn't the problem? I've never had to quantify how much bias ripple is a problem so I don't know.

I'm not familiar with this amp, but a bit above 10mVrms doesn't seem much.
Also both bias and PI tail ripple are common mode signals and should not appear at the output - if PI and power tubes are balanced.