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5y3 Negative Bias???

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Posted on December 17, 2014 at 14:01:41
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Ok looked all over the net and only a few comments on tube rectified neg bias. Some say it can not be done without multiple transformers (because of shared cathode) others said come out the center tap of a single transformer.

I can get the negative voltage off the center tap but is being rectified half or full wave???

Thanks Tom

 

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full wave, posted on December 17, 2014 at 15:11:32
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002



.
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Some issues:, posted on December 17, 2014 at 15:33:45
gusser
Audiophile

Posts: 3649
Location: So. California
Joined: September 6, 2006
 Depending on the amp design, tube rectified bias could be a liability. We all know what happens if bias voltage is lost while B+ is applied. And along those lines, the bias voltage must be present before the B+ starts to climb.

There has been some mis-information posted on this forum previously about the importance of bias supplies. OK, so it ties directly to the output grid, but:

1) There is typically at a least 100k series resistor as well as an adjustment pot.

2) Half wave rectification was used very successfully back in the golden days. I too would use full wave today only because it's so easy to do with modern diodes. But halfwave is still perfectly acceptable.

3) Due to the very low current demands of the bias supply, it can easily be filtered to virtually pure DC with a simple CRC.

4) While there are mixed opinions on regulated bias supplies and unregulated B+, if regulation is used, some form of fail safe should be built in.

All in all, it's quite easy to get excellent performance using traditional circuits.

 

RE: full wave, posted on December 17, 2014 at 16:13:07
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
 Hey Tre

Your schematic is showing a indirect heated tube. The nice thing about the 5y3 tube is it comes on now and the damper diodes in the B+ come on slow.

Was also reading where they will put a 1 to 10 ohm 1/8 or 1/4 watt resistor in the cathode to check the current and to act like a fuse if you lose the bias and the tube want to pull some REAL current.

Lots of theories out there. guess I will have to do the comparisons if I want to know, in my system, which sound the best.

Thanks Tom

 

This is not practical and there is no upside to this whatsoever., posted on December 17, 2014 at 16:41:44
Michael Samra
Dealer

Posts: 36118
Location: saginaw michigan
Joined: January 30, 2005
 You can go back to the the late 1940s and they always used sand diodes for rectification on the negative bias. They sometimes use the OB2 or OC3 or this type of regulator which works well but the idea of a full wave negative bias supply is brilliant and it will improve sonics if done correctly.
I would use four Schottky diodes as they are very quiet and very linear and if you want,you can put a small choke in series with the bridge.The idea is to eliminate any standing DC on the secondary of the power trafo and to keep any of the audio signal from getting filtered out thru the bias supply being the audio signal does ride on the line that feeds the control grid.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken

 

RE: full wave, posted on December 17, 2014 at 16:55:28
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 The direct heated 5y3 would work the same. If your filament winding is center tapped, use the center tap and make it the ground connection of the bias supply. Otherwise just ground one end of the filament winding.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: 5y3 Negative Bias???, posted on December 18, 2014 at 08:51:40
Eli Duttman
Audiophile

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

Mike Samra gave you good info. A bias supply should rise as quickly as possible. A filamentary rectifier rises quickly, but SS rises more quickly. Schottky diodes are every bit as quiet as vacuum rectifiers and their forward drop is tiny. OTOH, a 5Y3 has a huge forward drop.

JMO, build bias supplies with Schottky diodes and rest very easy.


Eli D.

 

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