Transformer for bias taps(?)

[jussi]

Hey,

If you take a look at this schematics. What is that transformer the bias taps are connected to? I've only seen amps with fixed bias before and I don't get this (I'm also very beginner).

click for bigger picture

[loudthud]

There are some serious errors on that schematic. DO NOT BUILD IT! Find something else from a reputable source.

[Enzo]

First, adjustable bias is still "fixed" bias. The alternative to fixed bias is cathode bias.

It doesn't matter if it is adjustable or not, a bias supply is a negative voltage source. Either way would be made the same other than the adjustability.

Whoever drew this noted voltages assumed to be like Orange/Matamp. COnsidering he has ??? all over the bias supply, I assume he had no specific transformer in mind. The ??? would be filled in after considering whatever transformer he used. A transformer might have a 40v tap or a 60v tap or whatever. We would alter the parts values to suit what we have.

And I don't think the high voltage supply is right.

[Tom Phillips]

Also note in the lower right that the schematic was drawn by someone named "NUTJOB."
Not very encouraging

[jussi]

I found the schematics from this thread: http://music-electronics-forum.com/t26551/

Here you can listen to this amp built by nutjob: http://www.youtube.com/watch?v=07gSXN9yOfU

But yea, maybe I should look into easier projects at first. I was planning on making a 50W version of this with two KT66 instead of four KT88.

[J M Fahey]

It isn't that it's "difficult" but that it is **wrong**.
That PSU will explode on turn on, etc.
To make it worse, it looks like "somebody" took a ¿legitimate? old schematic, and edited and corrected it big way.
What's right, was already there.

[imaradiostar]

There are a few examples of "bad design practice" but I wouldn't say it's unbuildable, given the correct transformers.

Still, not a beginner's project. Start simpler, with proven parts.

jamie

[imaradiostar]

Wait, I was wrong, there are some WRONG things on that. I should have looked more closely. Don't build it!

jamie

[Steve Conner]

The amplifier section of the schematic looks OK, but the power supply is all wrong. The voltage doubler thing is wired wrong (the centre tap of the winding shouldn't be grounded for a start) and the bias supply is drawn positive instead of negative. It will explode on power-up. If nutjob's amp works, then the insides of it must be different to the schematic he drew.

Now on to the original question, what is the bias transformer. If you use a power transformer designed for use with a valve amp, and a bridge rectifier, then it should have a separate winding for the bias voltage. In this case, there's no need for another transformer, simply use this bias winding.

If you're using some salvaged transformer that lacks bias taps, then there are a number of hacks you can use. You can run a small transformer backwards off the heater winding.

[bob p]

People commonly think of using the 6.3V heaters as a place to hook up a small transformer to provide a bias source.
Another option, especially if your plan is to use SS rectification, is to use the unused 5V winding. At least it will be good for something.

[R.G.]

Good point. If there is an entirely unused 5V (or other) winding, you can make a voltage multiplier out of it and multiply yourself up to a bias supply, given how small the current demands are. Gotta be otherwise unused, though.