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In Reply to: RE: feedback/balance control on a Magnavox pp el84 posted by Tom Bavis on May 07, 2014 at 13:34:41
and in fact the 390R resistor (or +/- half of 750R) is actually the cathode resistor for the 6EU7 in the Magnavox circuit. When you got the oscillation, your first stage cathode had no ground reference except through the feedback resistor (probably a 3.9K) and the OPT secondary. Also all the voltage across the OPT, instead of only about a tenth of it, was being fed back to the input. With Tom's fix, the feedback should come in to the top of the 390R and the circuit should behave normally.
Som of the Mag PP amps had a shared cathode resistor for both channels (all 4 output tubes) and some had a shared resistor for each PP pair separately. The latter sounds better IMO.
Try replacing as many electrolytics as possible with film caps.
Follow Ups:
Hi JJ, I am not going to hang on to this amp as I have too many el84 amps. I just wanted to get it running properly. The fix to the fb seems to be good as I have no oscillation and the sound is decent. About the coupling caps I did upgrade them some. I went with .006 and .06 uf for each side. I think after playing the amp for about 4 hours that the bass is a little better. The sound is nice and lively with lots of sparkle. I upgraded the power supply with a CLCLC configuration which dropped the B+ down to 330 vdc with a 5u4 and 360vdc with a 5ar4. cheers, Dak
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