Fender Bassman 6G6A Intermittent HT

[oc disorder]

Hi this is almost embarrassing
Have a very good condition blackface bassman with the presence
control and a solid state rectifier I guess its the 6G6A which works fine
about 98% of the time.
However occasionally and I mean frustratingly occasionally it drops out and
distorts like I'm transmitting on sideband and receiving the splatter
through a domestic Hi-Fi !
I have had it on the bench for hours and hours with a tone through it
just under clipping and it performs perfectly.
The fault will only show when abusing it by playing guitar at high levels
into a speaker.. doesn't seem to show when driving a dummy load.
Its not vibration - amp on bench speaker on carpet 2 meters away.
Had the amp before and couldn't find it .
Had it approx 5 weeks and failed twice enough to measure that the pre-amp voltage had dropped from approx 22Ov to about 90v.
The second time was able to determine that the entire ht was dropping
but my digital meter was auto ranging all over the place so the figures are a rough guess of the average. (Clue ?) I am guessing it may be one of the tiny elderly diodes as there are no hum issues even though some of the caps are Astron. The H.T. is normally 430/440 volts and seems to drop to half or 3/4 of this.
I'm measuring under the capacitor cover so the choke could also be suspect. Hard to flip it over to check the other side of the choke.There seems to be no way I can induce the fault.
It does correct itself sometimes while playing through it and the owner says turning it off for a short while and on again clears it.
I have used up my entire can of freeze to no avail although it's only 6 deg C outside at the moment so I could rug up and set up in the driveway !
Any clues anyone ? I only see a great Bassman like this once in a blue moon.
Also The owner would like to keep it as original as possible so I don't want to start replacing everything in sight.
He figures an unborn executive will probably buy it for $500,000 in thirty years to put in a glass case ...well considering there is a younger Plexi 50 watt Marshall Head on ebay in Aust for approx $14,000 who am I to argue...
Again any clues gratefully received.

[Enzo]

How about ball up your fist and whack the top of the amp hard - it should NOT respond, but does it?

Meter autoranging? Turn the range hold on. Use a scope.


SO clip a jumper across the standby switch. ANy help?

[Chuck H]

What Enzo said. And...

I've seen this before with the same amp and it was a parasitic oscillation. The amp was fine until it the temp, OD level and other parameters came into play and then it would just go sputtery, weak and the plate voltages would tank. The amp was drawing so much current trying to reproduce the inaudible frequency that it couldn't accurately reproduce anything else and the voltages tanked. This was early on in my amp experiences and it was someone elses problem, so I'm not sure how they fixed it. But I hope this helps anyway.

Does it happen on both channels? The way you describe the amp I think it's probably more "original" than it should be for it's age. You may aslo want to check the bias supply under hard use to make sure it's doing it's job. Something like a bad ground R or it's solder joint in the bias supplies voltage divider could cause what you desctribe.

Do like Enzo said and bang on it when the problem starts. If that fixes it (even temporarily) you may have corroded socket holes or tube pins that only get funny when things get loud. A little contact cleaner can fix that.

I also had very similar symptoms with a microphonic disc cap once. Once the amp was loud enought it would start ringing at a higher pitch than the speakers could reproduce. But the amp would sputter whenever you hit a note. Probably no voltage issues though. I didn't check voltage for that one. But hey, If it's a microphonic cap or a parasitic some poking around inside with a wooden stick can't hurt.

HTH

Chuck.