6V6 Plexi Bias Adjustment

[GS1100]

Hey guys/gals,
New to the forum here, finishing up a Hoffman 6V6 plexi build and I have a question as to how to set the bias. All of my other amps I have built are either single ended or cathode biased to this is a little new to me. The instructions I have seen online talk about measuring the plate voltage and the voltage across 1 ohm resistors wired to the cathode to then calculate current that is adjusted for proper bias. The layout diagram for the amp shows a spot for "bias range" that looks like it measures screen voltage? How is this measurement used when setting the bias on this amp?

Thanks in advance, RJ

[AZJimC]

Welcome to the forum RJ.

The bias range on that drawing refers to the resistor. Changing that resistor changes the range of negative voltage available to adjust with the trim pot. That is only required when the pot will not adjust to an appropriate voltage for the tubes installed. Chances are, you won't have to be concerned with that.

What you are doing when adjusting the grid bias voltage is setting the idle wattage being dissipated by the plate. A common setting is at 70% of the max plate dissipation. Another value of concern is the screen wattage. Often, I will estimate the screen current based on the tube data sheets, rather than taking an actual measurement. That amount should be subtracted from the total calculated when measuring the current being drawn through the cathodes, and thus the 1ohm resistors. 1 ohm is used so that the millivolts measured equal the milliamps current. It is a matter of convenience. Once screen dissipation is subtracted from the total, what is left is the plate current. Hope this is clear, and helpful.

Jim

[GS1100]

OK, that clears things up. Thanks a bunch Jim! Hope to have it up and running in the next day or 2.

[tubeswell]

Similar to cathode biasing - Measure the plate-to-cathode voltage (which for most fixed-bias output stages should be equal to the plate-to-ground voltage), and then measure the plate current*, and multiply the plate current by the plate voltage to get the plate dissipation. For Class AB1 operation you want to bias between about 60% and 85% of maximum recommended plate dissipation. 70% is kind of in the middle.


*Measuring the voltage drop across 1R resistors in series with the cathodes and the ground return will give you the equivalent measurement of tube current (1mV/1R = 1mA). Remembering that plate current = tube current - screen current, if you want to be finickity about working out the plate current, you can either measure the screen current (by measuring voltage drop across Rg2) and subtract this from the tube current, or you can just use the tube current measurement and subtract ~2mA** for the screen current for each 6V6 (under idle conditions).


**Ballpark screen current for a 6V6 under idle.