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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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OTL Asylum OTL, Output Transformerless Amplifier User Group. |
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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:
although I have always thought of you as a knowledgable guy, I didn't realize until now that your hands on experience with these amplifiers was in the presence of RAM and with access to his ideas and judgement.In the amplifier that was oscillating (or at least was very noisy,whether or not it was truly oscillating), I found that ALL of the eight 470K ohm carbon comp resistors that are used to shunt or as bleeders, were "bad", in that the measured R was either "OL" or > 1M on my Fluke. Knowing that it is the fate of CC resistors to increase in value over time, unless they are made by Allen Bradley apparently, I first replaced them with Roederstein metal film types. To make matters worse, and because the 2W Roedersteins are physically large, I mounted them on the "bottom" side of the output board, the side with the tracings, facing the input board. That turned out to be a real no-no. The oscillation got worse. These amplifiers do not like any parts on the side between the output and input boards, and there is apparently a good reason why RAM or HB used CC types in the first place. I eventually had to go back to CC types, and they had to be mounted as per original. (CC resistors in "no-man's land" were also not tolerated.) One of many lessons I learned. I have to think that the shunt resistors were way over value for quite a while before I discovered the problem, yet most of that time the amplifier played music, albeit it was always much more unstable than its mate. I've got the "good" amplifier also on my workbench, so I can compare the two during the process of quieting down the noisy one; I need to check the CCs on that one too. In contrast, the 1K CC resistors that seem to be grid-stoppers are perfectly OK even after 40 years.
Five years ago, after I purchased this system, Bill Thalmann in Springfield, VA, worked on the amps to do some things I didn't feel skilled enough to do, given the voltages involved, like replace all the tube sockets. At that time, Bill installed all new HV rectifiers, so we are OK there. Except, during my early trouble-shooting, I de-soldered one end of one of the rectifiers in order to check it. When I tried to re-install it, I boogered the PCB tracing for the lead that goes to ground (cathode I think). I first got around that by soldering the end of the rectifier to the tracing but in a way that it was not secured to the board. But it looked just fine. Three or so months later, after feeling a sense of defeat over and over again, and in desperation really, I decided to do a better job of securing the rectifier to the board, for no good reason. THAT turned out to be the final step to making the amplifier quiet. Although the solder joint I first created, direct to the tracing, looked totally OK, it must have been "cold".
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Follow Ups
- We should talk.... - Lew 10/14/2016:16:04 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)
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