vibrolux cuts out after long use

[fester gomez]

I have a silverface vibrolux that after a set of use cuts out.It sounds like a tremelo,but only once....the volumn drops then comes right back up,any help would be appriciated thanks fester g

[MWJB]

If it's always OK when you fire the amp up, it's probable that something is getting hot & failing. Volume returns when things have cooled down.

Do you see any red/orange glow in the grey/black metal plates inside the power tubes? I don't mean yellow points of light, or blue/purple gassy glow. Knowing what bias current the tubes are set to would be helpful. Has the amp been service to your knowledge.

[loudthud]

Does smacking it with your fist have any affect?

[fester gomez]

thanks for the replies,the volumn returns almost immediately after it drops,it has to be hot for a extended time before it acts up

[MWJB]

Too much plate current on the power tubes will make it hot quicker than anything else, so any answers to my previous questions?

[fester gomez]

tubes seem fine,just biased, can't recall what they're set at,caps were replaced in past couple years.

[pdf64]

Does the sound fade smoothly out over a few seconds, or just stop dead?

[fester gomez]

i just got it to do it again, it cuts out tonally,there is still volumn but it sounds thin.Also if you hit the top of the amp it will act up too.Had a scope on it all looked good then as you turned it up it would start to clip,then the signal would get really scrambled,and stayed like that.

[MWJB]

Is this on both channels?

I'm not a big fan of hitting amps, but this suggests a loose/broken component/pot, something shorting in certain conditions. Thorough visual inspection might help.

"Had a scope on it all looked good then as you turned it up it would start to clip,then the signal would get really scrambled,and stayed like that." this is a bit vague, at what sort of W output does this scrambling start, what poin in the circuit does this scrambling show up?

[fester gomez]

yes it is on both channels,all caps,filter & coupling are recent.I changed out power tubes,phase inverter tube and rectifier tube,also swapped around pre's.At first i was running through a bandmaster cab,closed back,and it was not as noticable as through the open back cab,were it sounds more pronounced.When a chord is struck and sustains you can hear the signal over distort as it rings out.The pots seem good when measured against each other.There aren't any components that even look suspect.

[MWJB]

If it's on both channels it's at the phase inverter/power section. I am assuming there is no Master vol, added or, otherwise. Check dc voltages at power tube (pins 3, 4 & 5) & V6 (pins 1, 6, 8) sockets.

Check speakers & wiring.