6V6 1987 build sounds like a dying fuzz...

[MHProd]

Finished a new build which is essentially a 1987 circuit with 6V6s. Only change is PI 820/22K.

The (fixed) bias is fine and stable. No volume issue. B+ is around 390V on the 6V6s plates and screens.

When I turn up the volume, the sound breaks up very early and gets into some nasty distortion... think fuzz where the transistor is biased way too cold. The note decays quickly in a bit of a choppy way...

Nodes: Pi 290V, V2 247V, V1 230V.

I think the voltages are fine:
Plates: V1 P1 140V - P6 160V / V2 P1 151V - P6 247V - V3 P1 199V - P6 188V
Also V2 P8 151V, V3 P2/7 28V - P3/8 53V

Heater voltage is good on all tubes.

I thought I found the issue when I noticed I wired the tone stack wrong, V2 P8 was connected to the junction of 33K/.022uf caps instead of 33K/500pf. I fixed that but it sounds similar...

Used a chopstick everywhere, didn't find anything... Checked ground continuity, all good. Swapped tubes to a different set, same.

Thanks for your suggestions!

[MHProd]

I attached a clip of the noise it makes.

Lifted the coupling caps and measured for DC leakage and it's all good.

No DC on pots.

Ugh...

[Littlewyan]

Start simple by checking continuity of all connections, particularly on the signal path. It sounds like oscillation, but could just be a bad joint or connection.
Does the amp clean up if you roll the guitar volume off? If so, do you get a good clean sound?

[pdf64]

Do voltage survey results (at idle) differ according to whether the controls are all zeroed or set to put the amp in fault mode?

[alkuz1961]

If there is a FX loop in this amp, you can try connecting a preamp to another power amp, and also connect another preamp to this power amp. It will help you identify the culprit of this sound.
Sometimes similar effect may occur due to problems with the OT primary winding.
Can you check the output transformer?

[MHProd]

So...

-Traced the whole circuit.

-Checked continuity everywhere.

-A couple of joint looked so so, redid them.

-Moved/wiggled every pins

Sounds exactly the same.

With the guitar volume rolled down, the sound cleans up a tad but I still get that frying sound. It almost has a high octave quality to it. I noticed the volume drops really quickly after the initial note.

Whether the amp volume is on 2 or 6, there isn't much difference in sound/fuzz.

Initial measurements were done at idle with controls on 0. With everything on 5, nothing changes.

HOWEVER I noticed something strange with the PI today: cathode dropped from 53V to 42V but the plates went from 199V/188V to 221V/214V. I still get the same reading as yesterday with B+: 395V on the plates and 290V at the PI node.

No FX loop. How would I go about checking the OT?

[pompeiisneaks]

This may sound odd, but does the sound go away with the negative feedback disconnected? Often the negative feedback, into positive feedback, if wrong creates a massive oscillation/screeching noise. But sometimes it's a bit subtle. If you can temporarily disconnect the NFB it may go away? If so try swapping the OT leads or the PI leads to the output tubes and reconnect the NFB.

~Phil

[xtian]

Makes me think poor grid-to-ground reference. Measure the resistance of each preamp stages' grid (pins 2 and 7 [edited]) to ground.

[MHProd]

OK, I disconnected the NFB wire from the output jack and there is an improvement. It's not as choppy and the decay is not as strong. But it's still not sounding right. Again the sound almost has a fuzz/octave quality to it. Should I still swap the OT wires?

I also noticed the mid and bass pots have little effect on the sound but it looks like everything is wired properly. I checked the grounds again and it's all good...

[pompeiisneaks]

Did you also check what xtian suggested?

If it got better, it can't hurt to try, just desolder the OT leads and swap them to the opposite sides and reconnect NFB. Does that suddenly start howling, or does it sound better?

As for xtian's point, make sure that the grids have good ground reference (or is that what you meant by the pots having ground? ) The input to the amp also should have a good 1M reference on those (no pots there, just the grid reference of the input jack itself.

~Phil

[MHProd]

I meant I checked the grounding of the pots.

Didn't see Xtian's post...

You mean pins 2 and 7 (not 6)?

Sorry, I haven't done this one before...

With the amp off and controls on 0:
Pin2 30K/235K/1M
Pin 7 30K/530K/1M

"The input to the amp also should have a good 1M reference on those (no pots there, just the grid reference of the input jack itself."

Where do I measure this? I'm using Cliff jacks. If I go from pin7 to the jack I get 30K... am I doing this right?

[pompeiisneaks]

I meant I checked the grounding of the pots.

Didn't see Xtian's post...

You mean pins 2 and 7 (not 6)?

Sorry, I haven't done this one before...

With the amp off and controls on 0:
Pin2 30K/235K/1M
Pin 7 30K/530K/1M

"The input to the amp also should have a good 1M reference on those (no pots there, just the grid reference of the input jack itself."

Where do I measure this? I'm using Cliff jacks. If I go from pin7 to the jack I get 30K... am I doing this right?

Yeah I'm guessing that's a typo, pin 6 would be the anode of the second half, 7 is the grid. Are those three measurements for the three preamp tubes? Which one is which? Usually it's good to specify via the V1,V2,V3,V4,V5 method, where for your amp, the first triode pair is V1, second V2, third V3, and the power tubes would be V4,V5 (V6/V7 if a 100W)

then you'd do

V1.2 = x
V1.7 = y
V2.2 = z
etc.

Having only 30k for 2 and 7 of one of the tubes seems very low. 1m should be there for pin 2 and 6 of the first tube (V1) 1.01M for V3 and it can be variable for V2 due to the volume/tone stack being involved.

Ensure the volume is at max on all pots to get the 'highest' impedance in the measurements.

~Phil

[sluckey]

Having only 30k for 2 and 7 of one of the tubes seems very low. 1m should be there for pin 2 and 6 of the first tube (V1) 1.01M for V3 and it can be variable for V2 due to the volume/tone stack being involved.

That's normal for an amp with switching jacks wired correctly. The switching jacks will have the 1M shorted out. Plug in a phone jack to open the switch contacts and allow you to read the 1M.

[MHProd]

With all controls maxed out and jack in channel 1, measuring pin to ground:

V1.2 30K V2.2 715K V3.2 1M
V1.7 1M V2.7 230K V3.7 1M

If the OT primary were backwards, wouldn't I have gotten a howling sound first time I fired up the amp?

[pompeiisneaks]

Thanks sluckey, didn't think of that. You got 30k on one because that's likely the other channel you didn't put a jack into.

The howling happens with NFB and the phase wrong with the OT. This usually is a pretty horrible racket, but sometimes it can be more subtle and just make the amp sound 'bad'.

removing the NFB seemed to 'help' a little but not resolve it, so maybe not the case. As I mentioned, it's an easy test, if you swap them, and reconnect NFB and get a howl, just swap the leads back. If it's resolved, it was a phase issue.

Looks to me like you've got good grid reference to ground on all those readings.

Not sure exactly where else to look for the noise.

~Phil