Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on January 18, 2014, 05:47:17 am

Title: Bias Supply - Can be done this way ???
Post by: kagliostro on January 18, 2014, 05:47:17 am

With a 340v PT that has no CT but has a tap at 60v

which is the right way to do a bias supply ?

is possible to arrange a bias supply in this way ?

K

Title: Re: Bias Supply - Can be done this way ???
Post by: sluckey on January 18, 2014, 09:30:54 am

You can't do that! See pic. There is another diode short across the 60V section. See if you can spot it.

Forget about using that 60v tap and just copy the Marshall JCM900.

Title: Re: Bias Supply - Can be done this way ???
Post by: jojokeo on January 18, 2014, 10:32:04 am

What would be the point of the tranny manufacturer of providing that 60v tap considering it is unusable? Or is there another way it could be used like going to ground instead maybe as a half wave?

Title: Re: Bias Supply - Can be done this way ???
Post by: sluckey on January 18, 2014, 10:55:40 am

Quote

What would be the point of the tranny manufacturer of providing that 60v tap considering it is unusable?

It's not unusable. Just can't be used as K suggested.

But, think of it as a 280 or 340 secondary. You have a choice of one or the other. Kinda like an OT with a 4Ω and 8Ω tap. Just a bit more versatile.

Title: Re: Bias Supply - Can be done this way ???
Post by: kagliostro on January 18, 2014, 10:56:06 am

Thanks Steve

The transformer was self wrapped by a friend

he wasn't thinking that without the CT that tap could be unusable as bias tap

the JCM900 schematic was tested and all is fine

at the begin a try was given also connecting the cap of the JCM900 bias circuit to the 60v winding instead than to one HV branch, but seems that in that way there is no function

Grazie ancora

Franco

Title: Re: Bias Supply - Can be done this way ???
Post by: Willabe on January 18, 2014, 11:34:10 am

Quote from: sluckey on January 18, 2014, 09:30:54 am

There is another diode short across the 60V section. See if you can spot it.

I give up, where?


            Brad    :dontknow:

Title: Re: Bias Supply - Can be done this way ???
Post by: kagliostro on January 18, 2014, 11:57:55 am

This seems the way

K