High-gain 36 watt, stablity problems
Posted: Mon 04/15/13 4:33 pm
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?
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?