Bias Voltage Transformer

[J Rindt]

This bias tranny is 36-0-36. Can i change things, get rid of the Center Tap, to get 72 Volts. I guess i would need a Bridge Rectifier.?
I am kind of confused...perhaps it is simply the result of the cap-job and new tubes.?
The PCB has/had a 22k bias resistor. I replaced that with a 5k to ground and a 47k pot. But my "problem" is that this circuit only generates about 43 Volts.
I have the pot at maximum now and the tubes are all running at about 65%...25-26 Watts.
Could i reasonably  get 72 Volts out of the bias transformer.?
Thank You

http://el34world.com/charts/Schematics/files/orange/orange_mkii1972.pdf

[sluckey]

This bias tranny is 36-0-36. Can i change things, get rid of the Center Tap, to get 72 Volts. I guess i would need a Bridge Rectifier.?

That's correct. A bridge will give you 100V. Then just use a voltage divider to get down to 72v.

[kagliostro]

Remember you need also higher voltage caps for the bias circuit


Franco

[J Rindt]

10-4...Thanks
Looks like some 200 Volt caps would be in order for the bridge.
Thanks Again x2 

I forgot. This was running at well over 500V B+.......so i built one of the "Wall-Dropper" boxes with a 12 Volt tranny.
I never checked the bias voltage before, but i imagine i lost some bias voltage going from 120-122 to 110-112.?
 

[DummyLoad]

with no dropping resistors and 72VCT, the maximum output with full-wave you can achieve is -50V (sqrt 2 * 36). with the parts shown, with simulation i see 45V at 22K and 1K node. close to what you observed. with no resistors in the circuit, then you'd see around -50V.


what bias range of bias voltage are you expecting/wanting?


attached are 3 schematics including as built by orange the other two are suggested PS circuits mods that i ran a simulation on. there are two circuits: FWB & Half Wave. the half wave circuit is obviously the easiest to implement.


simulation shows that FWB and half wave circuits should have a range of ~ -35V to -60V.

--pete

[drgonzonm]

I believe you only need half wave rectification,  look at the bias circuits on popular amps such as the Bassman. 

It is my understanding that the bias circuits don't take a lot of power, so why not tap off the B rail? 

[J Rindt]

I am not exactly sure "Why Not".
I am a mostly a "beginner" .....i just have a basic knowledge of this stuff.
This amp uses a PCB, so i guess was thinking it would be A Lot easier to try and use what is there.......

[drgonzonm]

I am not exactly sure "Why Not".
I am a mostly a "beginner" .....i just have a basic knowledge of this stuff.
This amp uses a PCB, so i guess was thinking it would be A Lot easier to try and use what is there.......

One of the Gurus on this forum indicated how to do this, if my memory is correct it was either HBP or PRR. 

[robrob]

https://robrobinette.com/5F6A_Modifications.htm#High_Voltage_Tap_Adjustable_Bias



I have another circuit for amps with bridge rectifiers.

[Tony Bones]

I am not exactly sure "Why Not".
I am a mostly a "beginner" .....i just have a basic knowledge of this stuff.
This amp uses a PCB, so i guess was thinking it would be A Lot easier to try and use what is there.......

You already have a bias pot wired in, so the absolute simplest thing to do is to convert the FWCT rectifier to HW:

1. Remove on of the two existing diodes.
2. Disconnect ground from the center tap and reconnect to the end of the winding where the diode was removed.

Here's a crude drawing showing what I have in mind. The blue line shows the new connection. Without seeing the PCB, I can't say how difficult it would be.

[J Rindt]

There are "flying leads" from the tranny to the board, so your eloquent solution would be quite easy.
My friend recently bought this...not a real joy to work on compared to e Fender/Marshall of 1972 ilk.
Also, somebody had dome a cap-job on this amp, and they Burned/Lifted EVERY Single trace and pad that was associated with those parts. So i had to do the obvious of bending component leads over and onto all the traces.
It is fine now, but if new power tubes draw more current that these EH Tubes do, i will have to do something as you suggest.
Thank You So Much