carbon composition shunt resistors - Lew - OTL Asylum

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carbon composition shunt resistors

Lew

In Reply to: RE: I think I fixed the problem with my Beveridge OTL DD amplifier posted by 6bq5 on October 13, 2020 at 14:20:04:

I already mentioned this to 6bq5 via private email, but I thank him for pointing me to check the CC shunt resistors in the "good" amplifier. What I found when first I paid attention to the DC voltage readings was that ALL of the CC resistors were either about 20% above their nominal resistance value or had completely gone short circuit, meaning the meter could read their value as "OL". One other read about 1.1M ohms. This was causing aberrant current sharing across the PS capacitors. (The Beveridge PS has to develop 3200V in two 1600V sections. It uses 8 X 450V electrolytic capacitors in series, between ground and the 3200V output, in order to do that. Hence the necessity for the shunt resistors to promote current sharing.) I noticed that some of the capacitors were not seeing their fair share of the voltage, and others were seeing way more than their 450V limit. Further, as a result the voltage was drifting under my observation. So, if I had just put this amplifier back into service, it would probably have failed catastrophically at some point, soon. In fairness, the CC resistors are ~40 years old, as my amplifiers are dated "1979". I tossed them, and I tossed the filter capacitors too, although they had been changed about 4-5 years ago after I bought the system. Only about 4 out of 8 were grossly defective, but I don't trust the remaining 4 to hold up. It was folly to have changed the caps without also changing the shunt resistors, but live and learn. Now I will rebuild with new parts.

Edits: 10/18/20

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Follow Ups Full Thread

Topic - I think I fixed the problem with my Beveridge OTL DD amplifier - Lew 11:50:29 10/13/20 (11)

Question about capacitor ratings for the cognoscenti - Lew 11:31:50 10/20/20 (2)

When that thing was built - Ralph 13:27:27 10/20/20 (1)

RE: When that thing was built - Lew 14:32:02 10/20/20 (0)

RE: I think I fixed the problem with my Beveridge OTL DD amplifier - airheadair 22:21:05 10/13/20 (2)

RE: I think I fixed the problem with my Beveridge OTL DD amplifier - Lew 16:22:01 10/14/20 (1)

RE: I think I fixed the problem with my Beveridge OTL DD amplifier - 6bq5 23:09:32 10/14/20 (0)

RE: I think I fixed the problem with my Beveridge OTL DD amplifier - 6bq5 14:20:04 10/13/20 (4)

carbon composition shunt resistors - Lew 09:07:58 10/18/20 (0)

We should talk.... - Lew 16:16:04 10/14/20 (2)

470K resistors - Ralph 08:50:25 10/15/20 (1)

RE: 470K resistors - Lew 09:18:55 10/15/20 (0)

Follow Ups

carbon composition shunt resistors - Lew 09:07:58 10/18/20 (0)

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