Peavey Heritage VTX - Terrible sound

[lowell]

I saw another post on this sound. The thread ended without any kind of diagnosis. It sounds like a bad speaker, but this is a head and my speakers are good.

Here's a schematic http://bmamps.com/Schematics/Peavey/...tx-sch+pcb.pdf

Preamp out signal is fine.
Into power amp sound terrible.
Line Out sounds terrible.

Using my signal tracer signal is good up to the power amp in 3.3k R166. At the junction of the cap C66 and that resistor there is very little signal. Then at U9B pin 7 it is that trashy sound.

+/-15v are there at pins 4/8. No DC offset on pin7.

I replaced the cap C66 and U9, still trashy. Here's what I've confirmed:

CR47/48 test good and there is 0v at their junction.
All components around U9 test good.
U9 pins 3/5 are grounded.
CR39 anode 1.8v
CR42 anode 1.2v
CR34/46 1.08v
Q7/9 Vb is 550mv
The power tubes are drawing 7ma per PAIR. I read this is normal and these run conservatively in this grounded grid arrangement.
+15v on all 6L6 grids
Continuity from all pins of U9 to surrounding components is good.

I'm baffled at this point.

[nickb]

Had a re-think so re-worded:

I'm really not sure what 'trashy; means. Could it be crossover distortion? 7mA total bias is just within the acceptable range. 7mA per side is ideal.

So see what is going on, disconnect the feedback, inject a sine wave into the power amp into and scope U9 pin 7, R155, R159, and output. I think it should sound pretty awful without the feedback due to crossover distortion. Lack of feedback (r164 open?) may be your problem.

[lowell]

Trashy like voice coil rub. At all volumes.

Checked those on the scope. Signal is there with an odd noise riding on top of it. As I was doing that I realized I missed a very important step. Whats the B+ voltage?

It read 460v. Snipped the top cap and clipped in a sub...FIXED. Amp sounds good. However the B+ is 720v!!

Ve on the drivers is 24mv and tube cathodes are 81v. Both within spec.

The wall at rhe PT primary is 125vac. About 4.6% high. No biggie. The secondary is 254vac on the lo setting which translates to 355vDC rectified. 1.4 x 254. I'm getting 360vDC. Seems about right mathematically but still high.

I get 0vAC from Red to fuse on the seconday. Not sure why this is. Any input here?

Aside from the high voltages it works and sounds real good.

[Enzo]

720v? I cannot imagine how it could do that. UI can imagine a volt meter set for AC volts readinf a DC voltage as funny.

[lowell]

I just double checked. 720vDC.

[g1]

How about AC volts, and have you tried with both of those totem pole caps replaced?

[lowell]

I did replace both. AC is 0v ish.

[g1]

Checked your meter battery lately .

[lowell]

Yes. Brand new Duracell that stings the tongue.

[g1]

What do you read for AC on that winding of the transformer?

[lowell]

254vac on red/yel. 0vac on Red.

[lowell]

Despite the oddly high voltage. It's only for the power tubes, and current is what really matters right? So 24mv/3 = 8ma /2 = 4ma per tube. 4ma X 720v = 2.9watts. Still absotootalutely fine.

Any concerns here?

[lowell]

I borrowed a friends meter. Indeed my meter is not reading right. The amp is fine, my meter is bad. Bummer. I always have 2 meters and my OTHER meter just fried yesterday. Sparked when reading voltage. Ah well. Thanks for the help dudes.