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Darrell Zero
Oct 14, 2023
In Questions & Answers
Awhile back I had a bias pot go bad, and I lost my left channel. I replaced all the original 20k (measured value) pots with some higher quality 10k Czech pots (changing the resistors in the 1.2k slots to 5.1k, and 27k in the 22k slots as per a previous discussion here). While I was at it, I also upgraded all the resistors on the output tubes. I then powered up without issue. So far, so good. Two problems, however: First, I still don't have a left channel. Or possibly one that is very, very quiet. (Edited to add: I am getting some sound in the left channel, about half or 1/3 that in the right) Right channel works fine. I checked the plate voltage with the bias pots zeroed out, and I got 473v across the board at initial power on. That decreased as the delayed start approached, but with a variance in the left channel (466v) vs the right (460v). Not sure how to account for the difference, and if that is indicative of the problem? At any rate, clearly the tubes are getting power. What could be the issue then with the left channel? Power but no signal? A preamp section issue...? Second problem: with the new 10k bias pots I can't get the the tubes to bias properly. By my calculation, to bias the el34s to 70% at that plate voltage, the settings should be in the neighborhood of 37ma. The most I could get out of the new pots was around 22-25ma, cranked all the way. Not sure what the problem is. Need different value resistors? Any help or suggestions as to proceed with diagnosis of the problem and/or potential fixes would much appreciated. PS: I have eliminated all potential problems with cables, tubes, inputs, etc. Thank you, and apologies if this too long 😁
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Darrell Zero
Oct 01, 2023
In Questions & Answers
Per adventure I have to replace the original 22k bias pots on my R8 with the higher quality, 10k Vishay pots. The SD Muzishare X7 video "Replacing the Tube Bias Potentiometers" (https://m.youtube.com/watch?v=CuCeE2EdMi4)(https://m.youtube.com/watch?v=CuCeE2EdMi4) describes a solution by which 5k and 18k resistors are implemented to allow for the use of 10k pots. Excellent video! As I understand it, the same thing can be implemented on my R8, but I have no idea how to do it. The bias boards are completely different. So, I asking for help from any and all who can point out where those 5k and 18k pots would go on the R8 bias board and which wire(s) I would then need to hook up to those respective resistor arrays? Assume I am a complete idiot when it comes to this because, you know, I am. 🙂 By way of reference, here is a pic of the V3 & V4 bias board from my R8.... Thanks for any help you can give! And special thanks to Skunkie Designs for her inspirational videos!
WR8: Implementing the M7 Resistor hack for 10k Bias Pots content media
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Darrell Zero

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