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adding a bias tap

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Posted on November 25, 2015 at 10:35:35
tubeflash
Audiophile

Posts: 223
Joined: December 22, 2001
 i have several power transformers intended for use in tube type
power amplifiers, however none of these transformers have a bias tap.
please advise on how to add a proper bias voltage source.
should i just tap off the B+ secondary? should i add a small transformer?
(i have several 48 volt 23 milliamp transformers) please recommend
a method that will provide an adequate and safe bias source? TIA

 

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Use your little transformer for a bias supply, posted on November 25, 2015 at 10:51:33
Chip647
Audiophile

Posts: 2653
Location: The South
Joined: December 24, 2012
 I would not suggest tapping into a power transformer unless you really know what you are doing.

 

RE: Use your little transformer for a bias supply, posted on November 25, 2015 at 11:26:19
tubeflash
Audiophile

Posts: 223
Joined: December 22, 2001
 tapping off the B+ would be a copy of the design in the
RCA receiving tube handbook page 696





















=









 

RE: Use your little transformer for a bias supply, posted on November 25, 2015 at 11:31:44
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Agreed! Be safe and use the little trafo for the C- supply.

Full wave bridge rectify the rail with 4X SB190-T Schottky diodes. CRC filter. A Nichicon KZ series part in the reservoir position works well.


Eli D.

 

What he said!, posted on November 26, 2015 at 09:23:03
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 I recently bought a large quantity of small transformers for exactly this use. Adding a separate bias transformer is the simplest way to deal with the issue.


 

RE: adding a bias tap, posted on November 26, 2015 at 10:03:35
Caucasian Blackplate
Industry Professional

Posts: 8313
Location: Seattle
Joined: June 18, 2004
 How much bias voltage do you need?

 

Use a reversed filament tranny., posted on November 26, 2015 at 11:09:54
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
 Take a 12v 100ma transformer (could be larger but of no benefit) and connect the 12v winding to your 6v filament buss. That will give you 60 volts at the transformer primary which is actually now a secondary. Then proceed with a CRC circuit or whatever you fancy.

If you need more than 60v, just use a 6v tranny and get 120v at the secondary. This is minuscule current so there is no danger of overloading your main filament winding.

 

RE: Use a reversed filament tranny., posted on November 26, 2015 at 11:55:16
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 This cracks me UP.

When I built my First VTVM in the 60s, I wanted to check the HV scale. I think it was 1500vac max.
So, I hooked up 4 or 5 transformers BACKWARDS. I think I had bell transformers.
It even worked for a SECOND or 2 until the last transformer or 2 SMOKED. I suspect HV breakdown of the insulation? But what do or DID I know?
And the awful Aroma of the SMOKE. I had to open up every window I could get to and try to air
the house out. I'm sure it was TOXIC as hell.

Meter DID kick up to maybe 3/4 scale before meltdown.


Too much is never enough

 

RE: adding a bias tap, posted on November 26, 2015 at 15:05:44
DAK
Audiophile

Posts: 2712
Location: PACIFIC
Joined: August 8, 2010
 Guitar tube amps also use this method. But, hardly any audio amps do it. If you run a separate trans for the bias voltage it will help protect the tubes because the bias will be on as soon as the amp is turned on. Tapping the B+ will have a lag period which leaves the tubes unbiased until the B+ starts to develop. cheers, Dak

 

RE: What he said!, posted on November 27, 2015 at 22:12:19
tubeflash
Audiophile

Posts: 223
Joined: December 22, 2001
 what voltage/amperage transformer did you purchase?

 

RE: What he said!, posted on November 28, 2015 at 07:24:34
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000



While you don't use an elephant to squash an ant, just about any low voltage winding of modest current capability can be adapted to bias supply service. Only a few mA. are drawn from a C- rail.

Even windings of 6.3 VAC can be employed. Just use a voltage multiplier, like that uploaded, containing enough stages to accomplish the task. Don't forget a RC reservoir section, after the multiplier. That too is shown in the graphic.


Eli D.

 

RE: What he said!, posted on November 28, 2015 at 10:03:17
Triode_Kingdom
Audiophile

Posts: 10049
Location: Central Texas
Joined: September 24, 2006
 The transformers I bought are 28V @ 175mA. That's more current than is needed for most bias supplies, but these were available in quantity on eBay at a good price when I was looking. They're small enough to fit under most chassis, they're US-made, and the voltage is about right for many tubes if I use a FW bridge. If more voltage is needed, it's simple to use a doubler as Eli notes below.



 

RE: Use a reversed filament tranny., posted on November 28, 2015 at 18:06:39
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Around 1961(?) - I was maybe 14 - I tried to measure the high voltage winding of a surplus power transformer. I had only the wire colors to go by, but I "knew" that black was always the primary so I wired that to a power cord and clipleaded the meter to the red HV leads.

I was right about red, but the black wires turned out to be a 2.5v filament winding. My meter acquired a curved needle and never worked again. Blew the house fuse so I could not disguise my mistake from mom...

 

RE: Use a reversed filament tranny., posted on November 28, 2015 at 18:37:07
pictureguy
Audiophile

Posts: 22597
Location: SoCal
Joined: October 19, 2008
 OOOOOPS!

Thar she Blows!
Too much is never enough

 

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