Two pot bias circuit compared to only one pot?

[Tdale]

The hybrid-a bias circuit says 1.2k and 15k resistors

But what if I want to use two pots, one for each tube (which I do)? Should the same values work, or would I have to do anything with one or both of the resistors?

I ended up haveing to use a 56k resistor to ground to get a bias range from about 20mA to well over 40mA, and I'm wondering if it's supposed to be this way, or if I did something wrong.

Both pots share the same 1.2k, diode and cap.

Tommy

[CaseyJones]

I'm promoting the K.I.S.S. concept. Read my other posts for more details.

My philosophy is anything that can be adjusted can be mis-adjusted. That's why most consumer electronics have a stern warning on the back panel that "there are no user servicable parts inside". Yeah the planet is crawlin' with technician wannabees but only a handful of 'em have done their homework.

Build in adjustability only if the amp will never leave your hands. If you make it adjustable and then sell it it's your duty to provide documentation like a schematic and a service manual with the amp. Unless of course you want to be the amp's "mother", make it tweakable and it will come home to you again and again. Kinda like a pregnant teenager. With a drug habit.

If your tubes are nominally matched individual bias pots are unnecessary.

Every time the finest engineers known to Man make sumthin' idiot proof... God makes a superior idiot.

I'm not callin' you an idiot, you had the skill to build this thing in the first place which makes you Above Average. Just be aware that things that seem sensible and obvious to you may be F.M. (the second word is "magic") to the general public.

[FUCHSAUDIO]

Dual bias (IMHO) is as important as the (oft questioned) AC Balance control in D-style amps. It would allow a precise adjustment of each power tube, and even with "matched" tubes, this sometimes is nice to have. Put a 1-ohm resistor on each power tube cathode, which will provide a nice low-voltage test point for each tube. .036 Volts (36 millivolts) is 36 ma. IF you want a higher voltage to measure (for ease) use 10-Ohms and measure for 360 MV .360 V for 36 MA and so on.

The attached schematic is a simple means to do this in a two tube amp, and works well . The lower "tail" resistor can be adjusted to suit getting the bias pots to work at their middle range for proper voltage. I usually try for 1/3 of the raw voltage at the top of the pot, 1/3 across the pot, and 1/3 at the bottom. So, an 80 volt source has a bias pot range of about 27 to 54. If you need a little higher voltage, raise the lower resistor to 100-K or 120-K. These values are good for most power tubes.

Have fun.

[Structo]

I installed a dual bias setup on my D'Lite.
Works great!
I have read as long as you retain the total resistance of the pot and tail that was originally there you will be fine.
So if you have a 10K pot and a 10K tail resistor you need to use a 25K pot and a 15K tail.
Because in parallel this comes out to the 20K that you originally had.
I'm sure I am over simplifying it but it worked well for me.
This way if I happen upon a couple NOS power tubes that aren't matched to each other (but not too mismatched) I can get them really close.
Also, it seems with the matched pairs that I have bought they haven't been that close of a match as I desired.

[Andy Le Blanc]

it is nice to have control..... built a 200 w bass with a quad of 6550...
but it demanded a good deal of time to balance... distribute the bias voltage
every change for one had to be compensated for by the rest....
the bias become the limiting factor the amp
made me start to consider single bias and separate balance controls instead
might lose less ma. thru distribution..... and less time spent to re-balance as the tubes drift\age... anyhow.... use the suggestion to put 1 or 10 ohm
resistors on the cathodes if you havent...... the control is useless if you
dont have good way to exircise it