Bias

[jonrya91]

Guys,

This is my first post, i normally just read.

I'm building a d'lite from a normster layout. I ran into a dilemma that was caused by stupidity on my part.

The transformer that i ordered does not have a bias tap. I never noticed. The bad part about all of this is that i never noticed until i had the thing mounted and wired 90%, and i found that the bias tap was missing.


Here is the question:

Can i save myself from buying another trans and get away with not connecting the bias circuit?

If so, is there anything that i need to do?

If not, do i have any other "out" to get away with using this transformer?

Thanks

[ayan]

Guys,

This is my first post, i normally just read.

I'm building a d'lite from a normster layout. I ran into a dilemma that was caused by stupidity on my part.

The transformer that i ordered does not have a bias tap. I never noticed. The bad part about all of this is that i never noticed until i had the thing mounted and wired 90%, and i found that the bias tap was missing.


Here is the question:

Can i save myself from buying another trans and get away with not connecting the bias circuit?

If so, is there anything that i need to do?

If not, do i have any other "out" to get away with using this transformer?

Thanks

You dont have to have a separate tap for the bias. While some people prefer a separate winding all together, others a seprate tap (with common center tap ground with the high voltage windings), some people are fine without any of that.... many Marhsalls are built that way, in fact.

Look at the attachment for inspiration and work out something similar that will yield the voltages ytou need for your B+ and tube type.

Cheers,

Gil

[jaysg]

...and get away with not connecting the bias circuit?

Just to make sure you're clear, the answer to this is no. There must be a bias circuit in your amp. Gil has given you all the right info.

[jonrya91]

thanks guys

[heisthl]

You guys forgot to point out plan B - although the marshall method works great - Ive used it many times and had no issues, you can also cathode bias the amp. A 25 watt aluminum heatsink style power resistor 330 - 500 OHMS mounted to the chassis works well for 2 6L6 and if you use a 100uf bypass it's hard to tell the difference. You do "lose" around 40 volts of B+ but it actually sounds louder and slightly warmer.

[jonrya91]

I looked over the Marshall schematic this week. It is exactly the bias circuit option that i needed. Thanks.

Do i need to use a voltage divider to get 45v, or can i pull straight from one leg of the transformer as Marshall did?

Thanks

[heisthl]

I actually like the Yamaha method better:

[Luthierwnc]

I've only used the Marshall version but it works fine. A couple points: you are dropping a lot of volts on that 220k so use a stout 2 watter. Also, the 56k tail resistor might not put you in the right range since it is based on the draw of EL34's. If you have to order that resistor, get a couple values on either side so you can dial in the bias about the middle of your pot sweep.

FWIW, I use 150 volt bias caps, at least for the first one. If you are fiddling with the tail resistor above, your first try might get pretty close to 100v. They aren't much larger or more expensive. Not an issue with a 50v bias tap.

Skip

[Luthierwnc]

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