6G6-A bias circuit ?

[worth]

I'm adding one of Dougs' 50K bias pots  to this Bassman bias circuit... is the 47K resistor shown in Dougs 5F4 Bassman bias layout correct here ? Wouldn't a 10K resistor be enough ?

[HotBluePlates]

Yes, his values indicated on the layout are correct.

I think you're looking at the fact the original amp had a 56kΩ resistor in the place where you now have a 50kΩ pot and a 47kΩ resistor. So you're thinking "50k + 10k should be close enough to the original value."

That's correct... but you don't want the total available resistance to equal the original circuit.

Why? Because the original fixed values were intended to land on the desired bias voltage. If you use the 50k pot and a 10k resistor to ground, you can get a total of 60kΩ resistance to ground at one extreme of the pot (more that original, so more negative voltage, so less tube current) but only 10k to ground at the other end of pot rotation. That's much less negative voltage and much more tube current.

Using 47kΩ to ground will give good adjustment in either direction from a likely happy bias voltage.

Build the circuit as Doug drew it. If after building you find you need to alter the range of adjustment available from the cirucit, you can tinker that resistor up/down somewhat.

[PRR]

> 47K ... Wouldn't a 10K resistor be enough ?

So you save 37K?

Doesn't save any money or time.

Here's the way I see it.

I *NEVER* want to be able to dial bias down to zero!! Not even close.

If I've picked the Max (cold) bias reasonably close, then I probably never want to dial-down to half of that. Say target of 52V. I want a COLD bias for initial smoke-test, say -70V. If it lives, I dial-down to a reasonbly hot bias. -52V with Leo's 1962 tubes, but asian clones may need -57V or -45V.

With -70V, 50K pot, 50K fixed, I can dial-down from 70V to 35V which is really ample range for the worst 5881 clone.

With 70V, 50K pot, and 10K fixed stopper, I can dial-down from 70V to 12V. That's way too much range. NO 470V power tube will live with just 12V bias. Yeah, maybe I know better than to turn the pot all the way. And maybe Jim borrows it and diddles it. Or maybe I'm in a dark bar with good toot or bad mood and forget which way is which.

2:1 range is usually the most a bias pot network should run. If your tubes are very consistent, maybe less. Never more.