Peavey 6505 Low power issue

[Lee_ranaldo]

Hello folks

i have this amp that seems to be constipated/congested for some reason: I assumed it had a low power output based on the Q7 transistor issue so i replaced the J174 JFET and , of course the problem remains and still sounds weak and distorted. Every signal out (EFFECT SEND & PRE OUT) work flawlessly and the pre-amp section seems
to work as it should. However the power section does not have a beefy output and it can be heard barely when you set the post gain controls around 2. (on both channels)

I changed the tubes, checked the connections, cleaned the output jacks and resoldered every suspicious solder joint i could find
with my magnifying googles.I Checked screen and grid resistors and they seem within the specified values. I've traced the signal with my scope , I fed a 1khz 1 Vrms input (Sine wave btw) and i could measure the output from the PI tube being above 10-20 Volts rms (and up if i crank the post gain control).

What seems odd is that i can hear the tubes rattling and vibrating mechanically along with the signal input. I could assume there
might be something wrong with the OT itself. Removed the flyback diodes just in case somehow they suffered from a wrong behaviour and were limiting the output= no changes.

I measured the DC resistance of the OT primary windings and they seem a bit mismatched (might be on purpose)
30 ohms and 39 ohms . Checked the ground connection on the output jacks and everything seems to be fine. Connection
seems solid from the OT secondary winding to the output impedance switch and then the output jacks.

Measure the plate voltages w/o power tubes installed and they are all around the 500 Volts DC
Bias voltage -46.9 volts DC

Anyone in this forum had a problem similar to this?

[Jazz P Bass]

Is that a misprint? 1 Vac signal input?

Or is that into the Power Amp in jack?

At any rate, you should see a maximum signal amplitude at the grids of the output tubes of something like 90 Vac. (pre & post controls max)

If that is true & the output tubes are good, then that leaves the OT.

[Lee_ranaldo]

Is that a misprint? 1 Vac signal input?

Or is that into the Power Amp in jack?

At any rate, you should see a maximum signal amplitude at the grids of the output tubes of something like 90 Vac. (pre & post controls max)

If that is true & the output tubes are good, then that leaves the OT.

Yea its a 1 vrms sine wave at the HIGH GAIN input. just to trace every stage and see if its working correctly.

I'll check some values and post them here.

[Enzo]

You replaced Q7? OK, but did you try the amp with Q7 removed in the process? Q7 could have been bad, but it also could have been good but stick in the ON condition by the things that control it. With the amp running, go to Q7, and measure resistance from source to drain. ON resistance is something near 200 ohms, while OFF - the desired state - should measure whatever the post control selected is set for.

You can hear the tubes/transformer singing to the music. That tends to tell me the amp is working. Turn it off, plug a cord into the speaker jack. Now go to the free end of the cord, and measure resistance from tip to sleeve on the plug there. You SHOULD see the VERY low resistance of the output winding, do you? Or is there some higher reading?

And the other side of the coin: unplug the OT wires from the speaker jack board and connect a speaker directly to those wires. ANy change?

[Lee_ranaldo]

Enzo i did what you just suggested. I Completely removed the Q7 JFET and the issue remains the same. Although it seems a tiny bit louder and distorted.

Also i measured both outputs and the resistance is very low. Although there is one issue
one of the output jacks seems to be louder than the other (in terms of sound). Since they are both parallel jacks this might be the issue. I'll clean these now and if it doesn't work i'll
hook the OT directly into the cab without the output jacks. I'll post my findings here.

Update:

Low power seems constant even if i plugged the speaker cab directly into the molex connector.

[g1]

At the power tubes you have plate voltage and grid voltage, but how about the screen voltage (pin4) ?

[Lee_ranaldo]

I finally found the issue. And it was a kinda "stupid issue". The cabinet had a short on its output jack . So i finally fixed that issue and now the amp works fine. Previously it had some Issues with volume drop outs and that was caused mostly by the Q7 JFET transistor that i've replaced. Along with general maintenance and servicing i did this amp is good to go.

I'm so surprised with how much gain this amp has, even on the CLEAN/CRUNCH channel. way to go EVH!