B25 Bias circuit(s)

[pentode]

Hi everybody!

Just moved over from the old site - looks all nice and shiny around here

I've got a very tired B25 which I'm recapping and rebuilding the power supply to start with.

As well as adding an adjustable bias I was thinking about going one step further and having a separate adjustment for each output valve with sockets for plugging a DMM into to monitor the bias voltage.

Although adding a single adjustment is within my rather limited knowledge, having two individual ones is a little beyond me...

So...... Any thoughts anybody?

Many thanks
Grahame

[Hangman]

Welcome to the new site!

I'm glad it feels new and shiny. I've got a lot of things in the works, but not a lot of time, so the site will slowly continue to expand.

If you want independent bias controls for each tube there are a few ways to do it. here is what I came up with.

There are two slightly different versions of the B25 bias circuit, so I have drawn two different ways you could do the bias. there are many different ways you could do this, and I encourage others to chime in if they have other ideas they think will work better, or use fewer components. this was the first solution that popped into my head.

   

[pentode]

Hi Steve,

That's great, thanks very much for that!

I'm thinking about mounting the components on a small circuit board on their own as there's plenty of space in the B25.

I should have mentioned this earlier but I'll eventually try KT88s in this, will the component values you specced give the necessary -ve voltage?

Cheers
Grahame

[Hangman]

the component values I gave were the original values.

If you use KT-88s I would change R38 and 39 to higher values. maybe 68k. the potentiometer value is flexible, you just need to make sure that you have a wide enough range, but not too wide. I would keep the potentiometer value at 25k or so. the resistor at the other side of the potentometers should probably be at least half the value of the pots if not higher. it will determine the range of swing that the bias has.

[pentode]

Sounds good to me!

Again, many thanks Steve