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Hi guys,
Looked around on the net for ideas on how to install a variable bias on a Hiwatt DR-504, and didn't see any circuits I wanted to use (I may have missed a good one, though...); I didn't want anything intrusive, so that you could restore it to stock in a flash (didn't want to mess with the underside jumpers), and didn't want to just hang a pot in mid-air. So this is what I came up with. All you have to do is unsolder the ground end of the existing 27K bias R, and attach it to the new 25K pot, which attaches to chassis via a U-bracket that you have to make up. U-bracket attaches to chassis via existing mounting hole for not-installed extra output tube. No drilling required. Works great, easy to install or remove. 25K pot and 10K R just happened to have... can use other values.
I've only shown the pertinent components of the amp. Hope it's of some use to someone.
Chevy
Looked around on the net for ideas on how to install a variable bias on a Hiwatt DR-504, and didn't see any circuits I wanted to use (I may have missed a good one, though...); I didn't want anything intrusive, so that you could restore it to stock in a flash (didn't want to mess with the underside jumpers), and didn't want to just hang a pot in mid-air. So this is what I came up with. All you have to do is unsolder the ground end of the existing 27K bias R, and attach it to the new 25K pot, which attaches to chassis via a U-bracket that you have to make up. U-bracket attaches to chassis via existing mounting hole for not-installed extra output tube. No drilling required. Works great, easy to install or remove. 25K pot and 10K R just happened to have... can use other values.
I've only shown the pertinent components of the amp. Hope it's of some use to someone.
Chevy
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