Trouble with a 350B

[Oli]

Hi,

I had some trouble with my 350B last night. It runs o.k. for app. 10 minutes then it went noisy. After that the memrane(s) of the speakers were just gonig foreward and backwars with a constant frequency (app 5Hz(?)). So I swiched the amp off for 10 seconds, on again, played for another ten minutes and than the same happend again. So I took off all the floor stuff (tuner etc.), took off the bass cable, same happend again after ten minutes. Did anyone had the same effect?  My first idea is, that the 6AN8A (phase inverter) is gone.

I bought the amp two years ago (its from 1973), so it would be good idea to chage the caps aswell. Though they do look all right to me - I have to check it with a meter.

Any ideas are welcome!

Thanks & Greetings

Oli

[mckinnon audio]

   Hi Oli,you've got a LFO,low frequency oscillation.Might be comming from the preamp or phase inverter,maybe the feedback loop.If you have a scope,try to track where it's coming from,should't be too hard,good luck,Mel.

[Oli]

Thanks for quick answer. I'll try to change the two tubes first and see whats happening. If it doesn't slove the problem I'll open it up and check it with a scope. I'm just doing some repair in my cellar to get a proper room for some tools - two projects at the same time are too much, so I hope the easy way will work...

Greetings,

Oli