Separate bias for each power tube

[jazzyjoepass]

Anybody got a design for separate bias for the power tubes.

I don't have a diagram here but off my head ....

A typical dumble 1.2kohm/diode/100uF-100V with a set of 10kohm Bias POT and 30k to 40k ohm resistor to GND for each power tube (6L6 here).

Anyone comments?

[ElectronAvalanche]

Hi Joe,

you just need to 25k pots, or trimmers, preferably pots with screwdriver shaft that can be chassis mounted.

You take the Bias voltage from after the (reversed) bias diode. There might be a dropping resistor before the diode or after it which is 1k for a seperate bias winding and maybe 220k to 150k if you tap of the B+ winding. Then there comes the bias filter cap (reverse orientation). On the - side of the cap you will have your raw bias voltage.

You have the two 25k lin bias pots in parallel. Bias voltage comes in at either lug 1 or 3 of the bias pot (the other is paralleld as said!). The other lug (either 1 or 3 depending on the input lug of the bias voltage goes to a series resistor to ground for EACH pot. Think 22k resistor (value will depend on the range you want on the pot).

Install a 220k or 100k resistor across the middle lug and the bias input lug of each pot, this will always keep a bias voltage on the wiper lug even if the wiper looses contact.

From each of the wiper lug (2) run a wire to the 220k or 200k bias dropping resistor after the PI 0.1uF caps.

Voila!

Here is a drawing...

Cheers,

Dominik

[Icetech]

Bob talked me into double bias pots on mine, and it was worth the 30 seconds it took to do:) I had a 6mv difference in the tubes and it might just be in my head but i thought it sounded a bit clearer with them evenen out..

[Bob-I]

Here's my schematic and layout. I used 15k pots because my local electronics supply had 15k linear with screwdriver slots. It seems to give plenty of range and I've had 3 different sets of 6L6's in this amp.

[jazzyjoepass]

My trannies got a bias tap at -50V - it's a twin reverb tranny.

Also I realised my calculated values are actually for two tubes only (6L6).

If you look at the dumble circuit, it's a 10k Pot with a 15K ground tail but that's a single bias point. If I want two bias points, I'll have to double the resistance from the diode - since two resistances in parallel is a division of two.

So 10k + 15k = 25k which should get me two 50k. In this way, I'm working backwards by multiply two to get 50k.

I've got only 10k Pots, so it should 10k POT with a 40k ground tail.


Any gurus here can comment on my method? Glaswerks seem to have something similar.


What's the bias voltage range I should be getting with 6L6?

[Bob-I]

My trannies got a bias tap at -50V - it's a twin reverb tranny.

Also I realised my calculated values are actually for two tubes only (6L6).

That shouldn't matter. Bias is a voltage not a current so all you need to do is supply a potential for the grid. One tube or 20 it's pretty much the same.

What's the bias voltage range I should be getting with 6L6?

I think mine is at about -38V.

My ckt is working great so I'd copy it, and measure the bias current. Yes 10k should be fine, you'll just have to adjust the tail resistor to make sure you have the range.

[BobW]

As a alternate method, why not use a combo of the black and silverface designs. Use the blackface adjustable bias ckt into a balance ckt. This way you can fine tune the matching using the balance pot, then coarse adjust both tubes proportionally. just a thought, and am out of town so I can't implement it. May prove to be at least interesting, or not

[Bob-I]

As a alternate method, why not use a combo of the black and silverface designs. Use the blackface adjustable bias ckt into a balance ckt. This way you can fine tune the matching using the balance pot, then coarse adjust both tubes proportionally. just a thought, and am out of town so I can't implement it. May prove to be at least interesting, or not

I thought about that too. It just seemed a bit more complex to bias.

Also, the bias pots are through the chassis so I can adjust without removing the chassis. I have bias test points by the tubes. It's great, I'm gonna do this to all my amps.