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.

**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