Peavey VT Series Deuce - doesn't seem loud enough

[Delta362]

I'm working on a Peavey Deuce VT with a low volume issue that a local tech asked me to help him solve. He had already replaced the electrolytics, balance resistors (R2-5), power tube screen and grid resistors, and the two phase inverter transistors. Overall volume just doesn't seem loud enough.

I put a 150mV input signal, cranked the post gain knobs to max, tones at noon, and adjusted pre gain pot. I'm able to get up to 2.7VAC on the preamp out jack, however the output peak to peak voltage is hard clipped at 30V (only about 12Vrms) for the 8 ohm jack with 8 ohm load. For a 100W amp, I'd expect something closer to 28V so this is about half.

For the below, numbers in parentheses are schematic values. Numbers before that are my measured values.

Starting at the power section:
B+ 490 (500V)
Post standby switch 485 (490)
322 (300V)
-62.13 (-60V)
25.5 and 16.2 (+26 and 15)
-25.5, -16.57 (-26 and -15)

At the phase inverter,
323 (300)
140 (145) for IQ1 collector
136 (145) for IQ2 collector
16.84 on emitters
17.31 on the base of those (+16)

16.41 at the junction of IR17 and IR18
16.26 at the other side of IR18

On the power tube board, plates are 490 with screens at 485.
Bias -60.46 pin 5s of each power tube

I replaced the two coupling caps IC22 and IC23 thinking they may be leaky, but no improvement noticed.

Thoughts on where to check next? Maybe suggestions on why IQ1 and IQ2 collectors have about 4 volts off between them while their bases and emitters are the same voltages between them? Other things to check? Thanks!

[Enzo]

When you replaced the electrolytics, did you just replace the filter caps as most do? Or did you replace ALL of them? Like those 2uf/35v ones? They did not age well and whenever I encounter a PV amp from the late 70s early 80s I can count on all those small e-caps being dried out.

Um, 30v peak is about 21vRMS.

$v difference on collectors that average 145v means they are off by maybe 2%? Close enough for rock and roll.

You have a power amp in jack, feed it a strong sine wave, how large can you drive it before clipping?

[glebert]

IIRC I worked on a Deuce once where IR15 and IR20 had gone way out of spec. From your voltages your 300V supply is high but the collector voltages on the PI transistors are low it seems like there is something weird going on there.

[Helmholtz]

30Vpp (or 15Vp) corresponds to 10.7Vrms or 14W into 8 Ohm.
What is the power tube grid voltage at the onset of output clipping?

[Delta362]

All electrolytic caps were replaced.

IR15 and IR20 both are pretty spot on for 100k

When clipping occurs, I noticed the following:
Plate voltage pulled down to 427
Screen voltage at 422
Pin 5 input measured 46Vrms and -60.5VDC

The phase inverter 300 pulled down to 269, with the Q1/Q2 collectors pulled down to 94V and 90V.

[Enzo]

Normal for a tube amp B+ to drop when developing high power.

[g1]

Was the low volume the initial fault, or something that was introduced when the work was done?

[Delta362]

Initial complaint was that the volume was low and distorts early.

I'm able to easily get 2VAC clean waveform at the preamp out jack. I tried sending that to a second power amp in and it gets loud and clear so the preamp circuit appears to be working properly.

The first waveform attached (with each scale shown as 2V on the scope) is using 10x on the two probes and the output of the phase inverter at the phase inverter side of the IC22 and IC23 where they both look clean and strong going to the power tube section.

Next waveform channel 1 (yellow) is the same as first waveform (10x probe, scale set at 2V on scope), and the blue is the 8 ohm speaker output load on 1x. There's crossover distortion on it, with the bias at -60VDC. Playing with the bias (reducing IR9) to clean up that waveform, my pin 5 voltage has to get down to around -38VDC before it seems that it's looking sinusoidal again. I didn't measure dissipation for that, but for 6L6 tubes, that's a pretty hot plate current setting. I didn't leave it there for long.

Third waveform is with the input pots maxed, scoping the phase inverter output and speaker output with same settings as second waveform, with bias back at -60VDC.

Next steps?

[glebert]

Is the first picture the same condition where you are getting 2VAC at the preamp out? If so the gain in the PI seems too low to me. It looks to me like you have 4.5VAC or so there, and for full power-ish grid signal to a 6L6GC I think it should be several times that.

On a different note, the signal at the speaker looks assymetric to me, do we know the tubes are good? All the plate and screen voltages are right? Sockets tensioned up?

[Delta362]

glebert, the preamp out probes are at 10x, so 20V per division.

Edit: phase inverter output probes, not preamp out probes.

[Helmholtz]

Up to and including the power tube grid signals nothing looks suspicious.
Of course an output of only 14W is ridiculous.

Are all powertubes good and working (measuring idle currents would tell)?
Also check/clean the speaker jack switching contacts.
Check CR16/CR17 diodes for shorts.

If the above doesn't help, I'd suspect the OT.

[Delta362]

Thanks for confirming the signal up to the power tube board is good.

Power tubes are brand new matched quad. Idle current is adjusted to 32mA (was originally 13mA). CR16 and CR17 were replaced with R3000 diodes. Speaker jacks are cleaned.

If it's the OT, got any suggestions for a suitable one?

[glebert]

If it's the OT, got any suggestions for a suitable one?

I think there are lots of OTs that could work, but may not be a perfect fit, and unfortunately replacing the OT will probably start to approach the value of the amp unless you have a suitable pull sitting around.

[Helmholtz]

Idle current is adjusted to 32mA.

So all 4 tubes have similar idle currents, say within 10%?

Are you familiar with testing an OT?

[Delta362]

Understood on the cost aspect of an OT.

All four power tubes currents are within 2mA of each other.

I'm open to learning how to test the OT.