Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pompeiisneaks on May 29, 2016, 11:08:11 am
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Hey all, I'm recapping my old '95 Fender Blues Deluxe (One owner, me :). I thought, while I was at it, I'd do a couple mods. One is the reverb mod where you remove the cap at the recovery stage that's s upposed to make the reverb sound nicer, but the bigger question I have is how I would mod the circuit to allow adjustable bias. I've got the schematic section here, I think if I just replace R87, the 27k Resistor with a 50k pot for bias instead, then I have the 27k minimum as well as higher and lower to adjust the bias. Does that sound right? I've got the section attached as an image, or do I want to keep the 27k and just add the pot?
~Phil
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That's a bit dangerous because you could turn the 50K pot down to zero ohms which would send zero volts negative bias to the output tubes and burn them up. A better way would be to replace R87 with a 25K pot wired in series with a 15K resistor. Then you can safely dial in 15K up to 40K, eliminating the danger of zero volt bias.
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ahh excellent, I was kinda thinking I should keep that resistor in, but then I thought I couldn't go below the voltage at the 27k so yeah, that sounds great. Thanks!
~Phil
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Oh Doug's only got 10k or 50k. Should I go for 50k and a 15k resistor? It would go a lot higher than would ever be needed I'm sure, but g ets the job done?
~Phil
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Yes, that will work.
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Excellent, thanks!
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I just wanted to update this thread, I had someone recently ask me about what the final schematic for the bias mod looked like so I drew it up. If anyone needs to do a bias mod, you can see the attached schematic change from that first one I showed.
~Phil