Adding Bias circuit

[Nathan]

At the risk of sounding like a noob,

Is there more than one way to add a bias pot to an amp which is fixed bias?

I am about to start a scratch build which is a modified Blue Angel, which itself is pretty similar to a 1-channel Deluxe Reverb.

This one is awful close to what I am building: Single Channel ''BlackVibe'' AB763

I want to have the bias circuit incorporated with external bias jacks per David Allen, but when I look at most schematics, they use a 50 VAC tap from the PT and mine does not have one.

Believe it or not, I have done some research but have not been able to tell conclusively how to do what I am trying to do!

Thanks for your help!!

 

[Ten Over]

At the risk of sounding like a noob,

Is there more than one way to add a bias pot to an amp which is fixed bias?

I am about to start a scratch build which is a modified Blue Angel, which itself is pretty similar to a 1-channel Deluxe Reverb.

This one is awful close to what I am building: Single Channel ''BlackVibe'' AB763

I want to have the bias circuit incorporated with external bias jacks per David Allen, but when I look at most schematics, they use a 50 VAC tap from the PT and mine does not have one.

Believe it or not, I have done some research but have not been able to tell conclusively how to do what I am trying to do!

Thanks for your help!!

There are plenty of ways to make an adjustable fixed bias supply.

What PT do you have?

What are the spec's on that PT?

Are you using 6V6's?

 

[King Fan]

Let me mention a couple great sources. In our DIY world, years ago (check out the dot matrix) Doug Hoffman wrote a nice primer on some basic Fender-y options, with and without a bias tap (the alternative, using HT voltage, is shown with his Princeton Reverb adjust pot.) As noted, resistor choices depend on how hot your HT is and how cold your tubes need to be to approach cutoff (be safely cool).

Right about where Doug stops (with his 'better' bias), Merlin steps in with a few elegant and straightforward paragraphs, answering that cutoff question and showing us 'better' and 'better still' near-universal layouts (as he says, resistor values may need some tweaking). If you only read down to the part where he moves back toward the rectifier, you'll know more than most builders about bias.

 

[Nathan]

That's really great guys -thank you.

I will be using the Hammond 270EX PT with a 1760JB OT and my goal is to be able to use 6V6 or 6L6 interchangeably (with a simple re-biasing). Check my sketch for bias circuit and added jacks - I think I have it right?

 

[sds1]

How to derive bias voltage from the HT secondary of your 270EX transformer:

Amp Mods

Generic Guitar Tube Amplifier Modifications

robrobinette.com

 

[Ten Over]

That's really great guys -thank you.

I will be using the Hammond 270EX PT with a 1760JB OT and my goal is to be able to use 6V6 or 6L6 interchangeably (with a simple re-biasing). Check my sketch for bias circuit and added jacks - I think I have it right?

The bias jacks need to be connected to the cathodes.

Your bias circuit will bias your tubes way too cold. Also, if the wiper fails, then the 100V capacitors will charge up to 389V.

Here is a suggested bias circuit. It looks good on paper, but I will build it later on to see what it actually does.


EDIT: I built it and it takes a 68k dropping resistor instead of 100k. I changed the the schematic above accordingly.

 

[Nathan]

The bias jacks need to be connected to the cathodes.

Your bias circuit will bias your tubes way too cold. Also, if the wiper fails, then the 100V capacitors will charge up to 389V.

Here is a suggested bias circuit. It looks good on paper, but I will build it later on to see what it actually does.
View attachment 1186869

EDIT: I built it and it takes a 68k dropping resistor instead of 100k. I changed the the schematic above accordingly.

That's insanely generous of you man! I don't know what to say but thanks a million!

 

[Nathan]

The bias jacks need to be connected to the cathodes.

I have to ask because I saw this done on my Allen Tonesavor a few years back: could the 1 ohm resistors not go from pin 8 to the jacks then to ground? Sorry for the grainy image - it's a printed layout from 1999!

It changes nothing for me, I won't really get into building this until Christmas break...

 

[King Fan]

@Ten Over, I should thank you too. Your scheme shows me a better way to tune the voltage span with a 10KL pot. I recall your note that a sim doesn't sweat all the bias details, but FWIW, the Falstad sim gets pretty close to your actual bias voltage range on this one. (I get -27 to -39V).

https://tinyurl.com/yvwcty3l

@Nathan, that link may give you a working sim -- you can slide the pot's resistance back and forth, and it's kinda fun/curious to watch the current flow (hint -- slow down the current speed). Feel free to tinker -- the linked version won't change.

I'm probably missing the point on the bias measure question, but here's how I've done it on two-output-tube amps (one tip jack each).

To make bias adjust fully external with a 10KL pot, I like a way to measure plate voltage as well. Got a plan for that? Several options out there. Oh, of course a tip jack for the black/COM probe too, somewhere out of the way but accessible; in this case I used V1.

But this can be elegant too:

 

[tele_savales]

@Ten Over, I should thank you too. Your scheme shows me a better way to tune the voltage span with a 10KL pot. I recall your note that a sim doesn't sweat all the bias details, but FWIW, the Falstad sim gets pretty close to your actual bias voltage range on this one. (I get -27 to -39V).

https://tinyurl.com/yvwcty3l

@Nathan, that link may give you a working sim -- you can slide the pot's resistance back and forth, and it's kinda fun/curious to watch the current flow (hint -- slow down the current speed). Feel free to tinker -- the linked version won't change.

I'm probably missing the point on the bias measure question, but here's how I've done it on two-output-tube amps (one tip jack each).

View attachment 1187782

To make bias adjust fully external with a 10KL pot, I like a way to measure plate voltage as well. Got a plan for that? Several options out there. Oh, of course a tip jack for the black/COM probe too, somewhere out of the way but accessible; in this case I used V1.

View attachment 1187784

But this can be elegant too:

View attachment 1187799

I like that last one- de-clutters the tube sockets a bit.