How To get Fixed Bias when you Don't Have a Bias Tap

[Bruce / Mission Amps]

Well as you can see... it was a quick scratch drawing using my mouse!
I have not hooked anything up to see it works but a .1uF to .22uF 630v cap in between the AC legs is real good idea.
I think the old VOX AC50s and AC100s amps have a cap coupled bias supply off a non center taped PT too.

[fiesta55]

Well guys I really do appreciate the help but now your getting me a little paranoid.Will this work?I am a newbie doing my first build and really do need" paint by the numbers" as Bruce put it but I also need a basic circuits spelled out for me that will work.I don't have much trouble shooting skills and this is all I need to finish this amp.It's quite a learning experience.

[bob p]

why make it hard on yourself? just follow the hoffman schematic that you are already using.

[Tom Phillips]

Well as you can see... it was a quick scratch drawing using my mouse! I have not hooked anything up to see it works but ….

Understood Bruce. This is one of those times when it looks good at first but you soon find that it doesn’t work. Been there, done that.

Marie (Fiesta) Don’t try to use that circuit. Bob has your simple solution and the following will explain why.

Could someone help a newbie out? I am building a Plexi from the Hoffman layout and using a full wave bridge rectifier in place of the 2-1N508 diodes. I have a P-TM-P50 power transformer with no bias tap….

I’m wondering why you want to use a full wave bridge rectifier. What led you to that? The P-TM-P50 power transformer has a center tapped HV secondary and will work with the Hoffman schematic that you are already using as Bob stated. Yes, it does not have a bias tap but that has nothing to do with switching to a FWB rectifier. The Hoffman setup will take care of the HV rectification AND the bias supply that you need. The bias is derived from one side of the HV secondary as Marshall commonly did. There you go! Make sense? Attached is a diagram of the P-TM-P50 for everyone’s reference.

Regards,
Tom

[Bruce / Mission Amps]

Intersting... P-TM-P50... where did the need for a FWB come from then?
Bruce

[Satamax]

I've done this once,

Used a bridge recto on a center tapped transformer, still connected the center tap to ground, and used the - leg of the fwb to get the bias

[Gregg]

What if you have a 0-50V-320V non-center tapped PT and using FWBR?

[Tom Phillips]

What if you have a 0-50V-320V non-center tapped PT and using FWBR?

Never seen that done. I suspect that you won't be able to get the ground reference to be correct for the HV and the bias supply at the same time. I think one half cycle would be using the 0-50V portion of the winding and the other half cycle would be using the 50V-320V= 270V portion of the winding. Probably a good reason why we don't see your suggested configuration implemented.

What's happening Fiesta? Any thoughts. Still wondering why you were wanting to use a full wave bridge. Did someone tell you that it was better than a full wave center tapped supply? Or???

Cheers,
Tom

[Gregg]

I'm asking because I have an old PT with such secondary winding. Can I use another FWBR as on the schematic Bruce posted but connect it to the 50V tap?

[Satamax]

I'm asking because I have an old PT with such secondary winding. Can I use another FWBR as on the schematic Bruce posted but connect it to the 50V tap?

I think i'd do it this way!

[Tom Phillips]

I think i'd do it this way!

Interesting.
To determine if that will work my approach would be to draw in the individual diodes, then determine what would happen during each half cycle of the AC wavefrom from the transformer. My first hunch is that there will be a condition where, due to the combination of forward biased diodes and the two different ground points (One on each bridge), the transformer secondary will be shorted.
Comments?
Tom

[Gregg]

I was thinking to do it this way:

[Satamax]

Interesting.
To determine if that will work my approach would be to draw in the individual diodes, then determine what would happen during each half cycle of the AC wavefrom from the transformer. My first hunch is that there will be a condition where, due to the combination of forward biased diodes and the two different ground points (One on each bridge), the transformer secondary will be shorted.
Comments?
Tom

Tom, i'm not verry good with phases and transformers. Would you care to take my aproach at fault yourself. I'm always willing to learn.

I was thinking to do it this way:

Greg, looks a lot like fender's way with a FWB instead of the half wave rectifier and centertap. I guess it's the only way. I missunderstood your description of the trany thinking there was 320 volts between the bias tap and HV tap.

[Gregg]

I'm not very much into theory and I guess the only way to see whether it's going to work is to try it.
I've seen fixed bias designs where zener diode is used for regulation. Considering the voltage sag when playing loud do you think it's a good idea?

[fiesta55]

Bob and Tom

I guess it's time to fess up.I'm building this amp over a bet with my boyfriend.
He says I can't do it without his help.
I watched him put one of the Hoffman Plexi boards in a Traynor amp a while back and he used a FWB in place of the 2- 1N5408 diodes.I assumed that's what I needed to do with this transformer and no bias tap.
Can I just follow the Hoffman layout as Bob says with this PT?
Thanks for everyones help.