High-gain 36 watt, stablity problems

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AcePepper
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High-gain 36 watt, stablity problems

Post by AcePepper »

I've just built a couple 36 watt amps w/JCM800-style preamps. I've built some 18 watters like this in the past and had no problems. This new 36 watt amp is having issues.

Power section is very much like the 36 watt TMB. Problem is: when I hit a loud chord with volume up (basicly pulling a lot of power) the amp freaks out and makes a sound like the fabric of space/time ripping apart. It will keep making this sound until I turn the volume down. Some sort of oscillation going on, maybe? Power tubes going runaway?

I'm using the 18 watt PI values, no negative feedback. 2.7k screen resistors on each power tube. Classic Tone Vox AC30 tranny set, including choke. GZ34 rectifier tube. Main B+ voltage is right at 350v. Screen voltage is 333v. I'm using one 75 ohm cathode resistor and 100uf bypass cap for all four EL84s.

A while back I had a large, fixed bias amp that made this same horrible noise when volume was up. In that case it was too small a cap in the bias supply. Larger cap/more uf fixed that. This 36 watt is cathode biased, so that fix doesn't apply here. But I'm wondering what else in this amp could mess with the bias?
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myker
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Re: High-gain 36 watt, stablity problems

Post by myker »

I have had an issue that may be the same or similar (depending on how I interpret your description) with a recent 18 watt build with a solda*o SLO preamp. I think it's an endless feedback loop/oscillation type thing and it went away for me when i found the right voltage divider from the master pot output. Actually I used a divider coming from the treble pot, then another leaving the master pot.

Maybe check the values of the resistors that you used in this area???
sorry if my answer is way off the mark, but I have learned that all you need to send a high gain amplifier into space/time warp is one little oscillation.
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Re: High-gain 36 watt, stablity problems

Post by AcePepper »

That's actually pretty right on. I do have a fairly gainy preamp and I do have a voltage divider between the treble pot and master volume (much like Soldano Atomic 16) so all that gain wouldn't freak-out the PI. I'll try juggling those divider values.

Could it also have something to do with filter cap values? I'm using 16uf caps for PI filter and preamp stages. May try raising those to 22uf. Whadda y'all think?
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myker
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Re: High-gain 36 watt, stablity problems

Post by myker »

I used 16uf as well, but I'm barely getting away with it, maybe I'll bump up to 22uf as well. Many of the guys building 800-ish 18's and 36's have switched over to 6V6's. I switched over to 6BW6's because they don't overdrive so easy and sound very much like a 6V6.
let us know how it goes...
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Re: High-gain 36 watt, stablity problems

Post by AcePepper »

Well, tried different values for the voltage divider. Kinda killed the tone and took the volume down, but didn't really help the ugly distortion much. Also tried 22uf filter caps. Actually made it worse. May try some different decoupling resistors in the filter cap section. May try a smaller filter cap for the preamp. Will try to get a schematic drawn up and posted soon. This thing is sucking my will to live. Thanks very much for the suggestions!

Myker, can you tell me what values you used in your voltage dividers?
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Re: High-gain 36 watt, stablity problems

Post by myker »

From the treble pot 220k/220k and from the master i used a 500k and used a lin pot to ground to find the right blend. I had a ton of gain to dump off here...I'd like to see your schematic to see what is going on, not that I am a super guitar amp wizard like many of the guys on here...
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Re: High-gain 36 watt, stablity problems

Post by AcePepper »

Hmmm...
Very interesting. I used 470k/150k off the treble pot. Got that idea from the Soldano Atomic 16. I had a 1M inline with the PI input, but that seemed to be causing some hum in the PI. Got rid of that 1M, no hum.

I'm working on a schematic I can post here with measured voltages and stuff. Hopefully that'll give an idea what I'm up to. Folks might dig it too. The 18 watt version of this amp is pretty sweet! Anyway, gimme a couple days to get a drawing together and I'll get back to y'all.
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