help with a VOX ac30

[JC@]

Hi, I'm working on one of these amps, it is a reissue. I believe it is a KORG one, made in China but looks very nice. Rectifier tube and such. It is a top boost model with reverb, tremolo and master. And bias preset to 50 ohms or 82 ohms choice.

It came in because it was making loud explosion like noises and I not even tested it. Now it has new tubes in (the original ones were really weak). With nothing plugged to it there are no explosions, but I saw that current draw for each of the EL84s is swinging up and down: 5mA to 20mA to 50mA to 70mA then back to 5mA and up again. Quite quickly.
The current draw for the whole amp stays steady though maybe high.

Both cathode resistors are ok, now I will check the cathode bypass electrolytic cap.

And please excuse me but I just thought I could take the easy way and ask over here for some opinions about where to look and what to check...
Thank you

[JC@]

Maybe I found the "problem". The swinging in the power tubes' current draw only show up when I turn the tremolo depth knob up. Fully closed there's no swing at all.
Is this ok and I have been spending part of my afternoon for nothing?

edit: I just looked at the schem and indeed the tremolo depth pot is supposed to do that. Am I correct?

[Enzo]

Korg is just the distributor for Vox products in the USA, they didn't make it. They also distribut Marshall in the USA.

[JC@]

Korg is just the distributor for Vox products in the USA, they didn't make it. They also distribut Marshall in the USA.

Oh, a complex world this is.

[trevorus]

I have the same amp, but I haven't had this sort of problem. The only problem that I had is a bad rectifier. Sorry I really couldn't help with it.

[flyingtele]

yes, the current draw swing is from the trem system. To get the tremolo sound i think the circuit just moves the bias up and down. other designs raise and lower the grid voltage.

[Robert M. Martinelli]

bias is constant! Check plate voltages/resistors.

Hi,
Bias on AC30s is set by means of fixed resistors, so there is no way it can change, at least not under normal conditions.... Get a schematic, you' ll see the HT ( or B+ ) for the preamp' s tubes reaches the preamp tubes' plates through resistors, so you can check the voltages and the voltages drop across them to figure out if something is wrong - be very careful though, as the final stage HT is some 320 VDC ( less, but still dangerous in the preamp - on the ECC82-12AU7 ( TREM MOD tube ) plates ( anodes ) you should find approx. 70-75 VDC, the ECC82 plate resistors on AC30s, if memory serves me well, should be 10K ones.

You can also check all the plate resistors' value switching off the amp, waiting some minutes ( 15 to be safe ) to allow caps to discharge, then remove the tubes, this way the plate resistors are open on one side ( the plate side, of course ) and can be easily measured, but it is better to use the above voltage / voltage drop check, because you' re under working conditions and the fault is more likely to be tracked down ( i.e. in case of thermal breakdowns ).

Hope this helps

Regards

Bob

[JC@]

Bob, I can't remember how I fixed it (this was about four months ago), but if I recall right it was a cathode (bias) resistor problem going open now and then... Anyway I am not completely sure about it, as I usually forget any particular amp as soon as they leave my bench
In fact I think it had two different problems but the current swing was because of the cathode resistor I mentioned above.
Thank you.

[tinchom23]

Feedback problem

hello people, I have an AC30 that has a problem of constant feedback as if a valve trembled, but cannot I fix it, could somebody help me please?