Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shaun on June 13, 2026, 11:54:35 pm
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Heyo,
I've built a pp 6L6GC amp using a toroidal PT that has no secondary center tap and uses full wave bridge diode rectification. As far as I can tell from my research to date, the lack of the toroidal CT may change how typical -52v grid biasing would work as it does, say, in a typical Fender 50w amplifier. Actually, I tried installing bias circuits as diagrammed by Mr. Luckey and others that mimic Marshall bias circuits, running off one secondary tap, but toroidals get weird compared to the EL PTs that I'm more familiar with. In using the toroidal, I will have 200VAC on each secondary at full power, and around 400v B+ at the first node - probably end up with about 440-460v on the plates.
I'm unable to get the bias circuit to work correctly - I've been using my variac to dial in about 70vac going in to the amp, rather than the usual 120 from the wall, and it's clear from plate currents at that lower voltage that the bias circuit is not working as intended - the tubes ain't happy, and plate current wants to run away with the spoon. There is little or no bias current on the grids. I conclude that I'm going about this ass-backwards due to lack of understanding.
This is the first time I've tried building a grid biased amp. In fact, I've bitten off more than I can chew with this one, but a local guitarist asked me to make him one with everything, and like the hot dog vendor, I couldn't resist. Schematic attached. It shows cathode biasing, but I'm hoping for more headroom by using grid biasing - the cathode bias got a bit mushy.
I need help in figuring out a biasing circuit that'll give me around -52v when using a toroidal PT. Thanks for any guidance - always much appreciated.
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Here's the schematic...
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I think you need to do something like this.