Originally Posted by ModMax
Maybe this has been addressed before, but here goes anyway:
To avoid adjusting bias on the tubes, which seems to vary more than any other bias on my MAX, would it be possible to use a voltage regulator IC (possibly adjustable depending on tube in use) to provide a stable tube bias VDC?
I am curious since I am not aware of commercial tube hybrid amps (Schiit, etc) requiring bias adjustments...
There's a catch: commercial tube hybrid amps (Schiit, etc.) do not use three completely different types of tubes. And by different, I mean entirely different plate loadings. They are designed for one tube, period. Then everyone looks for all the other tubes that are functional equivalents and tries them for different sound characteristics (tube rolling). Or, they go further and choose not-quite equivalent - but perhaps similar - tubes for even more differences in sound. Bottom line, all the mfr has to do is simply choose one bias-setting resistor (assuming a similar circuit) and that's that.
Secondly, think for a minute about the voltage difference in commercial amps and the Millett Hybrid circuit. The Millett uses automobile radio tubes - tubes that were designed to be used where the maximum voltage on the plates was around 12V. Say your bias on a Millett is 1 or 2 volts off of optimum. That's 10 to 15% error in the bias setting - somewhat significant. However, consider a commercial amp that doesn't use these DIY-friendly tubes (low-voltage, no killings or electrocutions involved): it uses 100, 200, maybe even 300V on the tube plates. Granted, the tubes may vary somewhat proportionally in bias variance - but not completely. The bias voltage errors from tube to tube then become scatter within those huge voltages.
The Millett Hybrid and its variants - the revMH Millett Hybrid, the Millett Hybrid MAX, the Millett Hybrid MiniMAX, and the Millett Hybrid MOSFET-MAX, use three different tubes with different gain structures. It's liking saying a commercial amp can take a 12AX7 and run the same with a 12AU7 (it can't). That's why the bias is adjustable with trim pots.
Now, it just so happens that the Millett-style space-charge tubes happen to vary quite a bit in their bias settings, even among the same tube types. That's another reason for having the little trimmers. I suppose you might say there's a price to be paid for not worrying about electrocution.
That said, there is a dirty little secret that I will share, but not recommend: assuming you stick with the same tube type, you can probably set the bias once and get away with simply changing tubes out and never re-adjusting. Yeah, it won't have the optimum voltage swing if the bias is off, but it will still play and sound pretty good. It won't if you take out a 12AE6 and replace it with a 12FK6, however. The bias may be so far off that there isn't enough voltage swing to produce an output. Hence, the adjustable-resistance trimmers.
P.S. The Starving Student Millett Hybrid does not fall under this category. It's a completely different design than the original Millett Hybrid and its variants. It uses high-voltage tubes at a medium voltage - and the circuit is designed for that one tube, only.
Edited by tomb - 1/31/13 at 4:38pm