Tapping for Bias supply - FWBR

[Decko]

Hi All,

If I use a full bridge rectifier with the transformer listed below, how would I configure the bias supply for the two EL34's?

Here are my power transformer specs:

Blue = 6.3 VAC
Blue = 6.3 VAC

Yellow=5 VAC
Yellow=5 VAC

Red=330VAC
Red w/Blue Stripe=0
Red=330VAC

Thanks
Decko

[pdf64]

See http://en.wikipedia.org/wiki/Rectifier# ... tification
I think you intend a full wave rectifier, rather than a full wave bridge.

See http://www.webphix.com/schematic%20heav ... II_50w.pdf
for an example of how to derive a bias supply from such a transformer.
Pete

[Phil_S]

This is a good example of an adjustable fixed bias circuit, and a nice clean easy to read drawing, too.
http://www.webphix.com/schematic%20heav ... w_1987.pdf
You may have to fiddle with the resistor values to get the bias voltage in the relevant range. The 15K/47K+25K divider is pretty standard. If the range isn't right, I'd swap the 220K for anything from about 150K to 270K depending on what you see. The bias supply draws very little current, so small changes often give big results.

[Decko]

Hi Gents,

Thanks for your response!

I wanted to try a full wave bridge rectifier with a choke.

Is this transformer not capable of providing both full wave bridge rectification and a fixed bias circuit from the 350 VAC secondaries?

Blue = 6.3 VAC
Blue = 6.3 VAC

Yellow=5 VAC
Yellow=5 VAC

Red=330VAC
Red w/Blue Stripe=0
Red=330VAC

In looking at the schematic, FWBR requires grounding one point of the diode daisy chain where B+ voltage comes off the opposite side.

Full wave diode rectification (FWDR) B+ off the diodes and ground off the center tap.

Problem with my current transformer is that I do not have additional (Bias Winding)48 VAC taps to power the fixed bias circuit.

That said, I wanted to understand how I might create the fixed bias circuit using the 350 VAC secondaries while still running a Full Wave Bridge Rectifier.

Is this requirement beyond the capability of this transformer?

Please advise.

Thanks
Decko

[Firestorm]

A bias supply doesn't draw much current, so if the transformer is up to your intended tube complement it won't make a difference.

[JazzGuitarGimp]

If you're using a four-diode full wave bridge, there is no way to get a ground reference for the minus supply without shorting (and destroying) the PT.

[Decko]

Hi Lou,

Well, after some digging, I found this link:

http://ampgarage.com/forum/viewtopic.ph ... highlight=

Might be more trouble than it is worth.

Perhaps I stay with full wave rectifier with choke.

Decko

[JazzGuitarGimp]

Hi Lou,

Well, after some digging, I found this link:

http://ampgarage.com/forum/viewtopic.ph ... highlight=

Might be more trouble than it is worth.

Perhaps I stay with full wave rectifier with choke.

Decko

Okay, there IS a way to do it... Just a bit involved.

[pdf64]

Why do you want to use a full wave bridge rectifier?

With the whole B+ winding that would result in nearly 1000Vdc.
With half the winding you'll get nearly 500V but may require a higher current rating on the transformer, as the Iac = 1.414 x Idc.

If may be helpful to others and beneficial to yourself (due to better responses) for you to provide more detail, eg what are you hoping to achieve?
Pete

[ToneMerc]

Hi Lou,

Well, after some digging, I found this link:

http://ampgarage.com/forum/viewtopic.ph ... highlight=

Might be more trouble than it is worth.

Perhaps I stay with full wave rectifier with choke.

Decko

Full wave rectifier it and just hang a bias supply off of one of the HT 330V legs.

HT winding>180k> diode>10uf>15k>10uf>47k>25k pot to ground

TM