Can you create a fixed bias tap?

[Dr d]

Hi all. I've been working on a 4 x 6v6 bluesmaster project for a while now and started the whole thing with a pair of transformers that I was originally going to use for 4 x 6v6 5e3 project. Here's the thing, the power tranny was designed for cathode biasing and therefore doesnt have a 50/60v tap for fixed biasing. I had kind of resigned myself to the cathode biasing but then began to wonder if there was an easy way of creating a tap without having to use an auxiliary transformer. Has anyone got any clever ideas on this one......as always, any input would be much appreciated. Cheers

[paulster]

If the transformer has a centre-tapped secondary (i.e. 250-0-250 or similar) then take a look at the Trainwreck Express or a 50W Marshall schematic for how you generate a bias tap.

If it isn't centre-tapped then you can still do a capacitor-coupled bias circuit but it's just a tad more complicated.

[David Root]

Look at old Ampegs on shematicheaven.com for the capacitor coupled bias design.

[odourboy]

There are a couple of standard circuits for this:
Check out some Marshall schematics - JCM 800 for example.
There's also a 'diode pump' circuit (IIRC) but I can't lay my hands on one at the moment.

[talbany]

Here's one I use..Seem to be less spongy than the Marshall T-wreck type..Good Luck!!
Tony

[glasman]

If the transformer has a center tap you can derive a bias voltage from the center tap. Randall Aiken has a whole group of schematics showing this technique is his technical section.

You should also look at a Marshall 50W schematic, you can derive the bias off of one half of the HV section (similar to what Tony posted). Normally uses an 82K series resistor to a 1N4007 with cap to ground and works pretty well.

Gary

[Dr d]

Wow, guys, thanks for the quick response. Due to the time difference between UK and the States, i've only this minute revisited the site...didn't anticipate such a quick reply to the post!!!!! This is all great stuff and at the very least gives me a choice between fixed and cathode baising. Thanks again all of you.

[Structo]

What Gary is referring to is generally called Back Biasing.
Another method is Zener Back Biasing which Randall recommends.

Randall Aiken explains it pretty well here.

http://www.aikenamps.com/BackBiasing.html

[M Fowler]

This one was floating around.

[markr14850]

In that pdf, the top circuit couples to the HV thru a cap. I'm curious what the advantage is. Any ideas?

[gilgalad101]

Any clue what the ovals are in this schematic that Mark posted?

[Dr d]

Thanks again guys for further input on this one.....its all making sense. The top schemo looks very similar to the scheme Tony offered up and the others very much along the lines of the Marshall design. (The "ovals" are capacitors). I am tempted to go with Tonys scheme. Tony, if you are there, how do you decide which value "entrance" capacitor to use for a given secondary voltage. The secondary on mine is supposed to be 340-0-340v....will the 0.047uf work OK at these voltages? Thanks again for all of your input.

[paulster]

If your tranny is 340-0-340 then the Marshall 50W type (which was copied on the Trainwreck Express, so the schematic is available here) is probably the easiest to use.

Tony's example is capacitor-coupled as it's expecting you to have used a full-wave bridge rectifier on the entire secondary, and that's not the case in your build.

[FYL]

This one was floating around.

"(Bias taps are not necessary for Class A or AB amplfiers)"

The usual Torres crap.

[Dr d]

Thanks Paulster for your input.....I made the assumption(dangerously!) that I could just tap off one of the legs of the secondaries. It makes sense to follow the marshall method. Thanks again.