6505 212 Combo Bias Mod Question

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AndyInBaltimore
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6505 212 Combo Bias Mod Question

Post by AndyInBaltimore » Thu Sep 09, 2021 5:32 pm

I recently acquired a 6505 212 combo that I’m reviving (replace tubes, bad caps, fuse, etc) and I figured while I have it open I might as well add a bias pot. Based on the schematic, it looks like R114 (47k) should be replaced. My question is; if I replace R114 with a 20k pot in series with a 27k resistor, will this work? Is R114 the correct resistor?
I’ve attached a segment of the schematic showing R114 in the bias area.
Any input/suggestions appreciated!
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Enzo
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Re: 6505 212 Combo Bias Mod Question

Post by Enzo » Tue Sep 14, 2021 5:45 pm

Nope.

You are trying to create a variable voltage divider. Try calculating the resulting bias voltage when it changes. Stock says -50v, yours may be a little different, but you can check. The divider starts at the other end of R112. What is there? Maybe -51v? -50.5v? Not much different from -50v. And that is with the stock 47k R114. (ooh ooh, I just figured it out, to get 50v we need 50.5 at the start) So 47k/47.47k x 50-5 = 50

Now assume R114 is 27k instead. (your 20k pot dialed to zero) 27k/27.47k x 50.5 = 49.6 SO we cut R114 in half - ore or less - and the bias supply went down less than half a volt. Not gonna work.

You need to make R112 larger. I usually start with about 3k. 3.3k (3300 ohm) would be a close standard value.

Now using 3k, 27k/30k x 50-5 = 45. Well, that got us 5v range anyway

What if I make it 4.7k at R112? With 47k at R114: 47k/51.7k x 50.5v = 46v SO as I raise R112, the resulting bias does start to lower right away. And if I make R114 27k? 27k/31.7k x 50.5 = 43v Now we have a 3v range, but it is a trifle lower. ( I assume you want hotter bias, no one modifies the amp to run cooler)

What range of bias are you aiming for?

AndyInBaltimore
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Re: 6505 212 Combo Bias Mod Question

Post by AndyInBaltimore » Wed Sep 15, 2021 11:42 am

Thanks for the reply, Enzo! It looks like it is a bit more involved than I initially thought. You’re post makes sense, but I’m not great with math, so the equations are throwing me a little bit, but I’m going to give it some thought and see what I can come up with.
To answer you’re question: I’m looking to bias around 60-70%, maybe between 35-40ma, but I haven’t checked plate voltage or current bias yet. I need to order parts to get it back to 100% before I can do that, was hoping I could order all the parts (bias mod included) at the same time. Thanks again for you help!

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