Troubleshooting an homebrew amp: Stanby off = sound fades away

[Jonny toetags]

Hi guys,

I've been trying to figure out an issue with a homebrew amp a client dropped off.
Here is what is he told me: sound faded out while while playing one day.
So after trying the amp, here is what i have noticed:
-Amp on....wait a few minutes then turn the standby off (amp in play mode)
-You can here sound for a few seconds then it quickly fades out.
-When I switch standby off...sound works fully.

It's basically running 'backwards'

I did notice that the 4 6V6 tubes were really burning hot and blue.
I checked the bias and they were at ~46mA. There is no bias adjustment on the amp.
Swapped all preamp tubes, amp still acts strangely.

[stokes]

I would assume the standby "off" means the amp is in play mode,the "standby" is off.Standby is just what it sounds like,the amp is in standby mode.As to the bias measurement,is that 46ma's per tube or 46ma's for a pair of tubes.If it is per tube,it is quite high.Even at a modest 300 plate volts you only want to see about 28ma's per tube,that would be 70% max idle in class AB.Is the amp fixed or cathode bias?

[Enzo]

Think of the switches as a mains power switch and a high voltage power switch. For the amp to operate, both switches have to be closed. In most amps that means both are facing the same direction. In most amps if the standby switch is "ON" that means the amp is ready to play, NOT that it is in standby mode.

[stokes]

Okay,thats assuming the switch is marked ON/OFF.I was going on the assumption,for no real reason,it is marked Standby/Off.So assuming it is,as you say.marked ON/OFF then it seems it has been reversed,so just spin it around.It seems to me the real problem with the amp is that 46ma's his 6V6's are running at.

[Enzo]

Oh I wasn't aiming that at you stokes, I know you understand it. I got the impression the OP might be a little confused. I see the terms used backwards by a certain percentage of posters. And perhaps I was not clear either, by "ON" I meant the switch electrically being on or closed, not really thinking in terms of the panel label, though I really should I guess.

I suppose the question then might be is that 46ma per tube or 46ma for the whole quartet? 11ma or so per tube.

What if we are all off base, and the thing is wired right, and it reaqlly does fade out ofter a moment of sound. Might it have a grid resistor open? Then a grid would charge itself up.

[stokes]

Well,he does say "When I switch standby off...sound works fully." So I have to go with the switch is operating,just not matching however it is marked.When an amp is put into standby mode its not unusual for the sound to fade as the caps drain,so I am going with the switch either is backwards or it isnt even marked as it is a homebrew.To me the bigger issue is he said the 6V6's were burning hot and if that 46ma's is per tube.........