Review of AB-4 autobias board.

[kodabmx]

Mini review of the AB-4 from Pavel at TES elektronika.

It does exactly what it says. Right now I'm using it to bias 6P45S tubes in triode. It's well built. I like it better than his AB-Q (blew up, repaired, blew up again). The only downside for me, is it needs AC. I tried it on 12VDC and it didn't work. Still, a 6$ transformer solved that issue.

I fully recommend this board. If I could afford to, I'd use one in every amp I build. It even has an error voltage so if the tubes can not be biased, you can use it to kill the power (relay on B+?)

It has worked flawlessly for 6 months now in a chassis that hits 70C...

Automatic bias control module for four push-pull (PP) tube AB-4. Dimensions 74x77 mm. TES

 

[Burnedfingers]

Whats wrong with old school bias that you can adjust yourself? I'm a firm believer in the Kiss principle. Less to go wrong that way.

 

[kodabmx]

Nothing per se, but one control to set all tubes, with no need to rebias when changing tubes is very nice. As is the DC balance You can even set it up using fixed resistors which you could switch, one setting for high idle current/larger class A envelope, and one for tube life/heavier class B.

 

[Lingwendil]

Different strokes for different folks.

I like self bias, and garter bias especially well. Fixed bias for higher power designs.

 

[kodabmx]

Totally, but with this you get the efficiency of fixed bias with the convenience of self or garter. Best of both worlds, really

 

[Lingwendil]

Yeah, but 'spensive

You gotta pay to play!

 

[kodabmx]

Just another good piece of info. I accidentally swapped the -110V and the +340V when I was redoing the power cable. This board lived to tell the tale! So did the rest of the amplifier... Considering the bias caps were reversed biased at 3 times their nominal voltage, this passes some of my failure mode testing.