Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bakerlite on June 28, 2011, 08:01:17 am

Title: question about bias feed secondaries
Post by: bakerlite on June 28, 2011, 08:01:17 am

Sorry if this is a no brainer folks,

Up till now the amps i have built have had a single bias feed secondary from the PT - for instance my 5e7 build had a 53v tap (with a 0/ground tap aswell)

I notice with this marshall transformer i have it has  21v-0-21v bias feed?

Can anyone help explain this or at least know if it is suitable for a build that calls for no bias feed tap or indeed a single bias feed tap.

thanks

Title: Re: question about bias feed secondaries
Post by: sluckey on June 28, 2011, 08:32:47 am

Most bias circuits use a simple half wave rectifier (1 diode) connected to one of the HT leads or a lower voltage tap on the HT winding.

Your PT has a totally separate low voltage winding (21-0-21, or also called 42vAC with CT) and can be used in a variety of ways...

1. A simple half wave rectifier using one diode. Connect one 21v lead to ground and the other 21v lead to the diode. Don't use the center tap. Probably work fine for EL84, but not enough voltage out for bigger tubes.

2. A full wave conventional rectifier using 2 diodes. Connect one 21v lead to one diode and connect the other 21v lead to another diode. Connect the CT to ground. This circuit will give 29.7v and will have less ripple and be easier to filter than the half wave rectifier circuit. 29.7v is plenty for an EL84 but may not be enough for 6V6, 6L6, EL34, etc.

3. A full wave bridge. Connect one 21v lead to one AC input on the bridge and connect the other 21v lead to the other AC input on the bridge. Don't connect the CT to anything. The bridge will have 42v AC input and will produce 59.4vDC output. 59vDC will cover most tubes up to 6L6 or EL34, but may not be enough for 6550 or KT88.

4. And you could use a voltage doubler circuit to produce even more bias voltage. However, bias is a critical element to fixed bias amps. I prefer to keep the bias circuit as simple as possible (think reliable) just for this reason.

Page 5 of this PDF shows some common bias circuits...  http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf (http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf)

Title: Re: question about bias feed secondaries
Post by: bakerlite on June 28, 2011, 08:58:00 am

thanks for taking the time to write that sluckey.

That helps alot and actually makes sense to me!

I'm hoping to use the PT for 100w 4x el34 jtm45 esque build.

cheers