Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: eahudimac on December 05, 2019, 09:35:18 am
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Hi everyone,
I am building a Hiwatt DR504 clone and have a couple of questions about adding adjustable bias to each tube. First of all, is it even worth it to add adjustable bias to each tube? I will always use matching tubes, so I assume that it is not necessary, but I like the idea of it. I am going by this picture from a dual biased 504 mod and wiring diagram by Mark Huss, which I have attached. For the life of me, I cannot determine what value resistor is connected to the wiper. I know the resistor to ground is 22K. Is it 220K? I'm not sure from looking at the schematic. I intend to install bias test jacks to make this procedure easy in the future.
Any advice on this would be greatly appreciated. Thank you!
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Is it 220K?
yes
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Thanks!
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First of all, is it even worth it to add adjustable bias to each tube? I will always use matching tubes, so I assume that it is not necessary,
The problem lies in as matched tubes age they can age differently and drift apart. As they drift, bass end becomes less and less, and noise rejection inherent in a PP power stage becomes less and less.
But, a single adjust pot has been the standard, if they have adjustable bias.
I like separate bias pots a lot.
Not much more to add, but do you have room for 2 pots?
intend to install bias test jacks to make this procedure easy in the future.
I've done that in amps I've built, and like it, but you still need to pull the chassis to get B+ readings as you rebias.
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The problem lies in as matched tubes age they can age differently and drift apart. As they drift, bass end becomes less and less, and noise rejection inherent in a PP power stage becomes less and less.
Not much more to add, but do you have room for 2 pots?
I've done that in amps I've built, and like it, but you still need to pull the chassis to get B+ readings as you rebias.
Thank you! I appreciate the advice. I do have room. The chassis is drilled for 4 power tubes. I will only be using two so I have the other to fit bias pots into.