adjustable bias 4 peavey bravo.

[Grrrg]

Thanks again, wow that was a fast answer. I just see your also in Germany.. So I found a method of checking the bias measuring the voltage over the resistance of the output transformer. Does that work with different resistances? I know some electronics stuff, but tubes were pretty much gone when I did my training.

[Helmholtz]

Does that work with different resistances?

Sure, that's why you need to measure DCR at both halves.

...und Beste Grüße!

[Grrrg]

I have not the best multimeter at home, but I measured the transformer and the voltages and calculate 31 and 32 mA through the EL84. According to an article about bias adjustment the EL 84 at 450V should be between 13 and 19 mA. Is that too high or still ok?

Viele Grüße zurück und vielen Dank!

And the humming is coming from the reverb circuit. So I will probably change the 4558. That might also be the reason why the reverb is feedbacking so hard.

[Helmholtz]

I have not the best multimeter at home, but I measured the transformer and the voltages and calculate 31 and 32 mA through the EL84. According to an article about bias adjustment the EL 84 at 450V should be between 13 and 19 mA. Is that too high or still ok?

450V?? Are you sure?
Datasheet limit is 300V. Anything above 360V at EL84 plates makes me feel uncomfortable, though I've heard of amps using up to 420V.
What is your heater voltage?

And the humming is coming from the reverb circuit. So I will probably change the 4558. That might also be the reason why the reverb is feedbacking so hard

I doubt that the opamp causes the hum, as long as you have reverb. Check all ground connections/contacts. Do you have a scope?

[Enzo]

The 4558 wwill neither cause hum nor cause feedback in the reverb. Feedback is generally acoustic, and hum comes from a thousand things.

[Grrrg]

OK:, i got a better multimeter and set up my scope.

-when the tubes are in the voltage is 413V. Didn´t measure without them. but that probably is higher. So the question remains if 31mA is to high a idle current?

-it is only humming when I turn up the reverb. If I turn up the volume in each channel all the way there is a light hum, but I consider that normal and it will probably not be heard when playing at full volume.
The -27V for the 4558 have a slight hum, but if I turn the scope on DC it is too small to be seen in the line. If I take out the 4558 the hum is gone. Unfortunately the local electronics shop didn´t have any 4558 and no substitute either. The humming is there with or without the reverb tank

- The feedback does happen when I turn up the reverb. That´s why I thought the hum is causing it. It is not feedbacking when I put the tank outside the cabinet but I read that that is also a common issue with the Bravo

Next week I will get an 4558 and since it is so easy to change (it is not soldered but in a holder) this is gonna be the first thing I am gonna try. Then the caps which I wanted to change anyway.

[tedmich]

12w at idle does seem a bit high, but the 413VDC B+ is typical, see discussion here:
https://music-electronics-forum.com/...-plate-voltage

Those crappy 47uF PS caps don't last forever and this was the 1991 "Amp of the Year" so the electrolytics could definitely use a refresh!
You do have the schematic I assume? if not
https://www.audioservicemanuals.com/...-112-schematic

cleaning the FX loop often pays dividends in these old PVs, also you want to do split bias adjustment to the EL84s? What do you hope to gain with this?
A high quality speaker will make a TON more difference IMHO.

[Enzo]

If removing the reverb pan from the cab stops the feedback, then it is an acoustic problem.

[Grrrg]

If removing the reverb pan from the cab stops the feedback, then it is an acoustic problem.

True, but I am hoping that getting rid of that hum will improve the tendency to feedback without a signal.

[Grrrg]

tedmich
I will only put in an adjustable bias. I was just curious, if the split could be done by the two resistors mentioned earlier. And then other questions came up which probably are at the wrong place here.

[g1]

Have you tried shorting out the return line from the tank? That will help you narrow down if the hum is on the send or return side of the reverb circuit. Also make sure the connectors at the tank are clean.

[Grrrg]

I cleaned the effect loop's connectors and it got better. But now I am not sure if the loud humming only occurred, when I disconnected the reverb tank. But it is also better both ways now. Thanks tedmich! The cap in the - 27V line measures at 2200uF but I changed it anyway. As I will change the other caps. The only question left would be at what idle current those EL 84 should be running.?

[Helmholtz]

A plate dissipation of 12W (100%) is way too high with fixed bias. I would not go above 8W with this amp.
As you must have the export PT, I would use the 240V mains setting. I often do this when I see excessive voltages in the 220V position.

[Grrrg]

After i switched to 240V the plate voltage dropped, but is still above 400V. I will put in a trimmer to set the bias as described above and then listen.

[Grrrg]

So finally I was able to do some stuff. I put in a 10k resistor with a 25k pot but I wasn´t able to bring the bias voltage down below -4.5V. I ended up putting the 18k resistor I bought for a possible dual bias in row with the former and got the bias voltage down to -4.3 and -3.5 Volts with a plate Voltage of 385. I had to switch the tubes for one another(so the same tubes but in the opposite connector) because when I had them in the other way round they couldn´t be adjusted to a similar bias power. After that they are now at 8.2W and 8.75W if my calculations are correct.
I also changed all the electrolytics although they measured pretty good but while I was at it I thought it couldn´t hurt. The reverb humming is still there but the weird thing is: it starts to hum when i turn up the reverb, but the hum doesn´t get louder with turning up the volume. Any ideas on that? It is now acceptable but still, how can it not get louder with the volume turned up? The amp sounds fine, the reverb has a nice surfy drip, but as mentioned before and in many other forums changing the speaker will probably make a difference. As soon as that covid thing is done and I will go back to rehearsing with my band, I will take it to the room and try some speakers I have there.
Thanks everyone for taking part and I hope my experience (especially about the resistor values) might help some other person when working on this amp. I had been playing a Peavey Bandit 75 for quite a while when I started as a kid and also in my first band, but I gave that one away a while ago. Maybe I should get it back just for comparison between tubes and transistors.