Peavey 6505+ "singing" mains XFMR

[Le Basseur]

Hello to all,
Got this "hi-gainiac" on repair.The owner accuses a sudden loss of sound after approx.1/2 hour of playing.I opened the amp,went directly to the output stage and replaced the 100 Ohm/5W "white coffins" (one of them was showing no DC resistance) with 470 Ohm/2W as it's said to be "that's more like it for a 6L6".
Replaced the output tubes with a fresh JJ Tesla paired quartet.Checked the OPT,looks fine when DC resistance is measured across all the windings.Verified all the fuses,they're OK.
The problem appears when I power up the amp:the output signal is very low and I can clearly hear the mains XFMR "singing",depending on the input (guitar or jack/finger source) level.
What should I look for?
(I'm aware on the several steps that should be taken in such situations for nailing down a presumable power supply issue but I hope for someone's punctual answer since this specific amp isn't repairing-friendly at all:big PCB,etc,etc.)
TIA!
Regards,
LB

[Wilder Amplification]

Hello to all,
Got this "hi-gainiac" on repair.The owner accuses a sudden loss of sound after approx.1/2 hour of playing.I opened the amp,went directly to the output stage and replaced the 100 Ohm/5W "white coffins" (one of them was showing no DC resistance) with 470 Ohm/2W as it's said to be "that's more like it for a 6L6".
Replaced the output tubes with a fresh JJ Tesla paired quartet.Checked the OPT,looks fine when DC resistance is measured across all the windings.Verified all the fuses,they're OK.
The problem appears when I power up the amp:the output signal is very low and I can clearly hear the mains XFMR "singing",depending on the input (guitar or jack/finger source) level.
What should I look for?
(I'm aware on the several steps that should be taken in such situations for nailing down a presumable power supply issue but I hope for someone's punctual answer since this specific amp isn't repairing-friendly at all:big PCB,etc,etc.)
TIA!
Regards,
LB

Not that I'm doubting your knowledge or ability, but just so we're crystal clear, are you 100% positive that it is the mains XFMR that's singing and not the output XFMR?

[Le Basseur]

Not that I'm doubting your knowledge or ability,

You're free to suspect my knowledge because I'm not a repairman but a pro musician.

but just so we're crystal clear, are you 100% positive that it is the mains XFMR that's singing and not the output XFMR?

Just because of what I said before,ears don't fool me:it was the mains tranny.
...but...a few minutes ago I went back to my workbench,put a thick foam piece under each XFMR just to isolate them from the wooden table and for a better identification,put back the little PCB containing the output jacks and the impedance selector,hooked the connectors (as they were before,the same pin order,no mistake here) and powered it....big surprise,the amp isn't manifesting that tranny singing anymore but plays normally.
Well,this is really annoying...I'm not giving a damn to some "voodoo" thing in this amp and the logic assumption is that the issue could appear whenever....what possibly makes a mains XFMR singing???

[Steve Conner]

Well, the most likely scenario is a loose connection to the speaker, because it would also explain the destroyed power tubes and blown screen resistor. Unfortunately this requires that it's the OT you hear singing, not the PT.

If the speaker wiring were shorted, then the heavy current draw might cause the PT to sing, or rather buzz in a way that vaguely resembles the music signal.

Either way, if fiddling with the output jack board and its wiring fixed the problem, there's a hint.

[Le Basseur]

That was it!

It looks like I really need a serious ear wash!
After reading some more documentation,it became clear that the "singing mains" was,in fact,the OPT!
In other words,a mains XFMR has NO CHANCE to "sing" like that.
My mistake and sincere appologies for the confusion.
In the remote past,I had two or three "singing OPTs" and I recalled the simptom...what did I had in mind?!?
Since yesterday,the amp is running continuosly (idle) with no signs of malfunction.
However,I have a couple more questions about the 6505+ output stage:
1.Like I said before,I replaced the original 100 Ohm/5W screen resistors with 470 Ohm/2W,having in mind two things:
a."The textbook" where the screen resistor for 6L6s have this value....well,in most configs.
b.If anything goes wrong again,better sacrifice a couple of resistors than a tube quartet.
(I'm also aware about the virtues of lowering the value of the 6L6 screen resistor as I experimented with some other amps and lower values,as low as 220 Ohms...but we're talking "tubes and tubes" here)
My main worry in this moment is that with the new values,the amp might loose some of his "character" (also here,I'm aware that such amps get the tone mainly from the preamp and in a lesser extent from the output stage)
What's your oppinion about this?

2.As stated countless times,these amps are biased VERY cold....the one I have on my workbench goes up to 17 mA and nothing more.
(I installed a 1 Ohm resistor instead of a cathode jumper on the output PCB so now I can measure the current,not only the negative voltage on those factory-provided test points on the rear pannel).
The question is:17 mA is really normal for this kind of amp?
(...again,sorry for my lack of knowledge,it's my first hi-gain)
If not,do you suggest a bias mod in order to raise it a bit?For how much?

Thank you for your time!
Regards,

[bluesfreak]

17mA is normal for this kind of amp, I occasionally had old 5150s (which are similar) running as cold as 12mA. Unfortunately this had been done as a design as they intentionally ttried to get all the sound from the PreAmp and have a pretty linear power stage but in my ears peavey failed here. The good thing is that if you mod this amp to have an adjustable bias you can convert it into a really fire breathing monster which sounds really ouststanding, even with the some kind of whimpy OT (which would be the next stage for a proper mod).

just my 2c

[Enzo]

What he said, all the character of this amp is from the preamp, so those screen resistors ought not to make any audible difference. And yes, that cold is normal.

One can argue they should have made to power amp run hotter, but the amp sounds good enough to buy as stock, so to me such arguments are like I should take my moms car and improve it by adding a hot cam, racing suspension and 6-speed transmission. The power amp is loud and clean as is. If you are not happy with it, heat it up, if you are happy with it, leave it alone.