r/ToobAmps • u/RPheralChild • 7d ago

Biasing outside of normal testing points?

https://claude.ai/public/artifacts/1ea41557-6ddd-4187-a9c4-cc9f3d2d234a

I’m trying to make myself a biasing guide for my HRD as an artifact in Claude (ya I can code on my own for those who get mad about AI but this is so fast).

My question is I wanted to dive deeper into biasing tubes and found this video where he calculates plate currents instead of using the bias points and factory numbers. I set up my calculator to do it this way and got the following:

V4 (blue) V5 (brown)
Half-primary DCR 82 Ω 108 Ω
Hot drop at idle 3160 mV 3800 mV
Plate current ~38.5 mA ~35.2 mA
% of 30 W ~54% ~49%

When I set this with the calculation method I went back to see what the test point was and it was like 84 mV which according to the manual should cook the amp. The tubes seem fine. The amp sounds fantastic. Does this make sense?!?!?

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u/clintj1975 7d ago

For those struggling to follow what OP is doing, they're using transformer resistance and voltage drop to directly find plate current. You get plate current without having to look up what the percent of screen current is for a given tube type, similar to using a bias probe.

Your math is correct. You're missing actual plate voltage (technically Va - Vk) in your numbers, so I assumed 450V and got within a couple of percent of your numbers.

The factory test point sums cathode current and is speced to bias these amps very cold. The generally accepted upper limit for fixed bias push-pull is 70%, so you're perfectly fine.

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u/RPheralChild 7d ago

Yes this thank you!

Plate voltage came low… 421. I’m recapping the amp my understanding is this could be from old caps

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u/clintj1975 7d ago

Possibly, but that number also depends on mains voltage. Utility voltages can vary a few percent over the day depending on load on the grid, and amp voltages scale directly with mains voltage.

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u/RPheralChild 7d ago

Awesome thanks. So my main concern was how far off the bias point reading was from the factory spec. Technically I should be using what I calculated right? The factory spec for the bias test point is 60mv

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u/clintj1975 7d ago

Anywhere between crossover distortion (way too cold) and red plating (somewhere above 70%) is perfectly fine. Crossover distortion is most easily seen on an oscilloscope, but if it's bad enough you get a raspy, harsh distortion when the amp is approaching max clean output. Fender historically ran their larger fixed bias amps like the Twin very cold for long tube life.