OT: Fixed Bias Supply.

**mah** · July 30th, 2009, 03:42 AM

Tube Guys,

I have a PT which is 0-270V on the secondary( NO CENTRE TAP) and require a 'fixed' bias supply for a pair of el84(6bq5) operating AB push-pull.
Can anyone help with a bias supply circuit? This is all I need to finish a current project and then I can start the Altec speaker cabs.

My goof re the lack of a centre tap.

Thanks, Marshall.

**bowtie427ss** · July 30th, 2009, 08:28 AM

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**armandhoff** · July 30th, 2009, 02:53 PM

I had the same problem and got -er-done

I assume you will use a voltage doubler for the B+
I had about 250 ac and needed 450 dc
and for fixed bias I put a 2.2 mfd cap to one end of the winding and the other to a diode to gnd. [cathode to ground and anode to the cap]

the cap is pumped when ever the part of the ac wave matches the current flow of the diode leaving the connection at the cap/diode -40dc then some decoupling and filtering.

if you need more voltage use more cap.

this was on a dynaco 35 with fried transformers. I used whatever was handy.
old color TV power transformer

if you plan on keeping the B+ under 300 vdc you might try 6CW5 [EL86]

cheap and long lasting with plenty of kick

PS I could not reset my password and in the end opened a new account to post.

**mah** · July 30th, 2009, 05:34 PM

Thanks for your replies.

B+ is 270V into a full-wave diode bridge, not a voltage-doubler. Will the circuit work for this arrangement?

Marshall.

**armandhoff** · July 30th, 2009, 09:32 PM

it should

**armandhoff** · July 31st, 2009, 03:27 AM

this is all from memory
the amp I fixed fried in 1971 or 2
while scratching my head about getting the bias voltage
a friend [tv shop guy] showed me the cap & diode trick
the cap was one of those "orange drop" with a 400 v rating

exact same situation of no winding for a fixed bias.

**armandhoff** · August 1st, 2009, 09:54 AM

and if you think this is strange
at the end of lecture 10
there is a demonstration of kV generation from dripping water
MIT OpenCourseWare | Physics | 8.02 Electricity and Magnetism, Spring 2002 | Video Lectures

**mah** · September 17th, 2009, 11:24 PM

Ended up using a tiny PC mount transformer for the bias supply. One end of the secondary winding is connected to DC ground, the other end to the 'reversed' diode which supplies the negative DC for the rest of the bias circuit. All fitted onto a board no larger than the original. Cost me $2.00 for the transformer.

OG, if you are looking, the Redhead came into being in 1946. She still looks good, eh?

Cheers, Marshall.

**Horn Savant** · September 19th, 2009, 12:50 PM

Why not use cathode resistors --130 ohm--(no separate supply required)(bypass them -100uf)

If using bias supply, then...
Should require a negative DC voltage decoupled & fed to 6BQ5 grids (cathodes grounded)

Bias Voltage: 23 v for 250 v plate/screen, or 28v for 300v plate/screen

Reference: RCA Receiving Tube Manual