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

[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]

Originally Posted by Gregg

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]

Originally Posted by 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?

I think i'd do it this way!

[Tom Phillips]

Originally Posted by Satamax

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]

Originally Posted by Tom Phillips

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.

Originally Posted by Gregg

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]

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.

[Bruce / Mission Amps]

Here, this does work:

[Bruce / Mission Amps]

If he used a FWB block then he did not connect the anodes of the FWB to ground and only used half the diodes.
You can just use the two or three of the axial diodes.

[Tom Phillips]

Originally Posted by fiesta55

…Can I just follow the Hoffman layout as Bob says with this PT?

Yes, with the P-TM-P50 power transformer, you should use the stock Hoffman circuit. The P-TM-P50 has a center tapped high voltage secondary designed to be used with a full wave (two diode) rectifier set up. The lack of a bias tap does not require you to switch to a full wave bridge rectifier. Marshall amps typically did not have a dedicated bias tap on the power transformer secondary winding. The bias supply was tapped off one side of the high voltage secondary. That’s exactly what the Hoffman diagram shows. This makes it easy for you. I can only speculate that the power transformer in the amp your boyfriend built was originally designed for a FWB and that’s why he didn’t use the stock Hoffman arrangement.

Max – I believe that the capacitor coupled circuit that Bruce just posted solves the “shorted secondary” problem I was referring to. Basically, try as you might, if you connect two diode bridge rectifiers to a shared transformer winding without using those caps, then ground the positive output of one AND ground the negative output of the other, you will end up with the equivalent of a short across the secondary winding.

Regards,
Tom

[KevinOConnor]

Hi Guys!

I can't say I am a fan of capacitively coupled bias supplies even though conceptually they make sense. The reason I don't like them is that never seem to produce enough voltage to really control the tubes.

I prefer the old method of getting a 6V or 12V transformer and reverse-wiring it to the heaters - there is always a heater winding! The auxiliary transformer can be very small and will put out about 80-160V. It will also be much lower impedance than the capacitive supply, so you can have individual bias pots if you wish.

Bruce, how much voltage do you get with your circuit?

Have fun
Kevin O'Connor

[bob p]

Originally Posted by KevinOConnor

I prefer the old method of getting a 6V or 12V transformer and reverse-wiring it to the heaters - there is always a heater winding! The auxiliary transformer can be very small and will put out about 80-160V. It will also be much lower impedance than the capacitive supply, so you can have individual bias pots if you wish.

Interesting. I've never heard of that trick. I guess that if you stay in the "1" operating subclasses, then additional current drain on the heaters will never be an issue.