|
OTL Asylum OTL, Output Transformerless Amplifier User Group. |
|
|
|
OTL Asylum OTL, Output Transformerless Amplifier User Group. |
|
In Reply to: RE: Oscillation posted by AJ on June 9, 2020 at 13:58:48:
I don't disagree with you about not changing the grid resistors, at least not yet, as I also responded to Ralph below.
Here is some more info that I communicated to Ralph privately: "I found that one of the wires to the anode cap of one tube was worn from flexing where it solders into the PCB; it broke off as I was trying to insert a replacement tube. It has been my past experience that worn solder joints or wires that have been flexed one time too many do give rise to oscillations in these amplifiers. I stripped some insulation to access fresh wire and re-soldered it and also replaced all 4 tubes. (The tubes I'd been using already were new ones, but it has been my past experience that some tube types are just more prone to oscillate than others. RCA brand seem to be the most trouble-free, and I replaced a mixture of new Sylvania and GE with all new RCA, now.) This leads to better measurements as regards oscillation, but I am not sure I won't have an audible problem when I re-connect the amp to the speaker. On each of the stator outputs I now see very low ACV, about 5-10 mV where DCV = 1530V. My Fluke says that this ACV does have a high frequency (~170kHz), however. On the screen output, I see zero VDC, as expected, and about 60mV of AC. I have never measured ACV on any of these nodes before, so I do not know whether this is normal or not. The frequency of the ACV on the screen output is around 30 kHz. Previously (before the empirical tube swap to RCA), I also was seeing about the same 60mVAC on the screen output,as now, but with a frequency of 150kHz or so, which I took to be the manifestation of an oscillation. The AC voltage and its high frequency can be detected coming off the 7.5M resistor that goes to the cathode of the top tube in one totem pole and then connects on to the screen output. I am wondering whether to just give it a try at this point on the notion that maybe the AC will be inaudible and was always there.
Yes, it would be helpful to know the ACV values seen in a functional amplifier, on my other speaker. I have been trying to avoid the labor involved in separating that amp from its speaker, due to back problems, but maybe that has to be done."
AJ, I tried to send email to you privately but you are not receiving via Audio Asylum. Which is fine. However, you can contact me via the Asylum. I greatly value the advice of anyone else who has worked on Beveridge amplifiers in the past. These amps have been thoroughly gone through not only by me but also by Bill Thalmann of Music Technologies in Springfield, VA. Pretty much the only original parts remaining are the coupling capacitors (all 3), those 2.2M resistors (Caddock, so originals may have been replaced by someone else.), the anode caps and wires to the anode caps. Bill installed 4 new tube sockets back right after I bought the system. I've cleaned all soldered areas with acetone on cotton swabs. Power transformers are dated 1978, so are probably OEM.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- RE: Oscillation - Lew 16:52:06 06/09/20 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
