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Tube DIY Asylum Do It Yourself (DIY) paradise for tube and SET project builders. |
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In Reply to: RE: feedback/balance control on a Magnavox pp el84 posted by Tom Bavis on May 7, 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.
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Follow Ups
- I believe Tom is correct... - JJ Triode 05/7/1420:23:45 05/7/14 (1)
- RE: I believe Tom is correct... - DAK 16:46:53 05/9/14 (0)
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