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In Reply to: RE: The lm317 is a constant voltage source posted by DAK on May 09, 2016 at 11:03:17
OK, that sounds like you have it wired as a current source. The problem is, 1.25/19.6 is only 64 mA. You need more than that to center the EL34s in Class A. Does it motorboat even at low levels (without the cap), or just above a certain threshold? How much voltage is on the cathodes at idle? What mode is this amp - pentode, UL or triode? Are there grid stoppers on the output tubes?
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Buy Chinese. Bury freedom.
Follow Ups:
Hi, JKt, yes, in its cathode bias w/resistor, the amp was running 70ma at the cathodes. I thought that was too high so i dialed it back a little. The amp is running UL mode but there are no grid stoppers on the suppressor grids. There are 1K resistors on the input grids. As soon as the amp received a signal, it started motorboating.
The LM317 shouldn't cause that. It's likely there's another issue in the mix. See JKT's post below. If you have a scope, you can look at the stages ahead of the outputs to see if unwanted signal is leaking back through the B+ lines.
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Buy Chinese. Bury freedom.
Yeah, i did not think it was the lm317 but removing the bypass cap exposed some other situation which has now come to light. I will check the the voltages of the front end tubes and see if something jumps out there. cheers, dak
He'll need to disconnect the GNFB first.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
With motorboating I would also look for the possibility of low frequency coupling within the B+ supplies to the different stages in the amp. Relying on the caps in PS RC filters to shunt noise and signals to ground thus isolating different stages becomes more problematic the lower the frequency of the offending signal.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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