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I am rebuilding a Magnavox console amp the one that is very popular now, and ran across this problem with feedback balance control. This pot is noticeably "sticky" in operation with a definite spot in the center where perhaps the wiper and carbon track have made a ridge. Anyway, I would like to remove this feature/pot and have straight fb into the 6EU7 cathode. The pot has a resistor on the side legs going to each 6eu7 while the center leg is grounded. I tried removing the center leg but that started the amp oscillating. Anyone know of a good fix to remove the pot from the picture? thanx for the help , Dak
Follow Ups:
Replace the (750 Ohm) pot with two 390 Ohm resistors to ground - this will give the proper amount of feedback. Low frequency oscillation (motorboating) is a result of low-frequency phase shift and too much feedback.
Hi Tom, thanx for the info. I changed out the pot with 2 390 ohm resistors and installed a 2 place solder lug to hold the new parts. The results are much improved over the original. I am listening to the amp now on my JBLs and they sound decent maybe a little deficient in the bass, but that is not unexpected. Probably the amp is needing some break in time before the new parts settle in. Cheers! Dak
…consider replacing the very small Magnavox coupling caps with more normal values like 0.1 or 0.22 µF. Even Orange Drops (716P) will be an improvement in the bass.
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.
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
Thank you for the info Tom. I will give that a try and let you know how that works. cheers, Dak
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