I have a triode el84 se amplifier and I've noticed that the voltages on the plates of the el84s do not like it when I add cathode bypass capacitors to the driver stage. The voltages, usually on one tube get really high. As soon as I remove the bypass capacitors the voltages go back to normal. What's happening and how can I prevent this?
Need a diagram of the driver section, at least. You may be assuming that you are working at the cathode with proper connections but really have some pin misidentification, or terminals and grounding misidentification. Cap polarity inversion...?
The amp works fine and measures fine without the cathode bypass caps on the input triode's cathodes. Don't have a schematic. It's pretty typical input stage. The amp is two stage with one 12ax7 driving el84s in triode mode. I was leaning toward somehow the higher gain caused by the bypass caps is causing oscillations or some kind of malfunction in the output stage.
Can you sketch a schematic of that part of the circuit by inspection? Are you trying to use electrolytic bypass caps? If so, which side is connected to the cathode + or - ?
Also, so you have a scope? That would be much better than a DVM if you suspect oscillation.
Is there any global feedback?
Why do you want bypass caps there anyway?
Also, so you have a scope? That would be much better than a DVM if you suspect oscillation.
Is there any global feedback?
Why do you want bypass caps there anyway?
Is it a commercial amp? PCB or hand-wired? We can't help with the info you've given. Take some pics of the guts if you can't finish a schematic.
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Probably you won’t like the sound with the capacitors after all. In most situations the sound without the capacitors will be better.
Regards, Gerrit
I think the problem is oscillation caused by insufficient decoupling of the power supply with respect to the driver tubes. This can happen when the anode resistors of the ECC 83 halves are connected with the same smoothing cap. If that's the case separate the two. Two 4k7 resistors and two 47mu caps will do the job.
Then, give every tube a 1k grid stopper especially when you use long wires from the anodes and/or the grids. And, and this may seem far fetched, oscillation can sometimes be caused by not grounding the filaments.
Then, give every tube a 1k grid stopper especially when you use long wires from the anodes and/or the grids. And, and this may seem far fetched, oscillation can sometimes be caused by not grounding the filaments.
Thanks for all the responses. Yeah, I guess I was just curious and just wanted to try them but the amp sounds good without and I actually prefer the output stage cathodes unbypassed. KeesB, the driver tube is on a different filter section. The filaments are not grounded. Hmmm. They are all running on non-center tapped A/C. A little twist I discovered: If I remove just one cap from one cathode the voltages go back to normal. But when I hook the second cathode bypass cap to ground the amp voltages go haywire. This is all at idle, no signal going through the amp, not hooked up to any other components.
Not every amplifier circuit is the same as every other amplifier. Gain, negative feedback, signal ground loops that go crazy with more gain, etc.
Lots of gain, and floating filament supplies is not a good idea.
A complete and accurate schematic is required for proper troubleshooting.
Anything less than that, is like a blindfolded archer trying to hit the bullseye.
$0.03
Adjusted for inflation.
Lots of gain, and floating filament supplies is not a good idea.
A complete and accurate schematic is required for proper troubleshooting.
Anything less than that, is like a blindfolded archer trying to hit the bullseye.
$0.03
Adjusted for inflation.
I think that it is somehow gain related as you say. I've been messing around with different driver tubes. I've noticed the problem doesn't happen using a 12au7, 12bh7, 6sn7. It only happens on 12at7, 12ax7, 6sl7. Perhaps my amp is getting negative feedback through the triode strapping resistor from plate to screen on the el84.
Try creating a virtual center tap for the filaments using a pair of 100 ohm 2W resistors and ground the center of the two resistors in series across the 6.3V.
It's quite unusual for a stereo 2 stage amp to oscillate with these type of tubes. The fact that the oscillations stop when using one amp only it seems something is wrong with the grounding scheme. Have you forgotten to ground the input plug? And, if you use gridstoppers (1k) these have to be mounted close to the sockets otherwise they don't do what they are meant to do. The negative feedback caused by the EL84 g2 grid stoppers(100 ohms I guess) have nothing to do with the problem. But a gridstopper at g1 might be important here.
Another thing that I just thought of - do you use negative feedback to the output transformer? If you have the wrong polarity the amp plays louder than without it . And as long as the total gain isn't too high the amp simply plays louder without going into oscillation, but as soon as you use these higher gain tubes problems arise. If this is the case leaving the overall feedback out should then stop the oscillation.
Another thing that I just thought of - do you use negative feedback to the output transformer? If you have the wrong polarity the amp plays louder than without it . And as long as the total gain isn't too high the amp simply plays louder without going into oscillation, but as soon as you use these higher gain tubes problems arise. If this is the case leaving the overall feedback out should then stop the oscillation.