my first amp: the PSU

[jerome]

hi,

as part of building my first valve amp, I am trying to understand
how tube work and how PSU and amplifier are designed.

Regarding the PSU. If one hasn't got a tranny with a center tap,
is it possible to use a 'bridge' made of four rectifier tubes to obtain the fullwave rectification? From what I understand so far about tube it should be possible but I just haven't seen it anywhere.
If I use such a design, is their any drawback?

thanks

Jerome

 

[Tom Bavis]

Drawback? The number of tubes, the extra voltage drop, extra filament windings... you could do it with two 6BY5s (two isolated diodes in each) or a 5U4 and two 6AU4s, or four 6AU4s. Watch out for filament cathode voltage ratings, or use separate windings for tubes on opposite sides of the bridge. You could also mix a tube rectifier with two solid-state ones and retain the slow warmup.

 

[Colt45]

biggest drawback IMO is the fact that youre burning off half of the filament winding on the rectifier stage.

but, 2 6AL5's would make for a nice little FWB, maybe for bias or something. i dont know.

 

[45drive45]

well instead of using 4 tubes, why not just build a hybrid with 2 SS diodes and one tube rectifier for example GZ34 etc.

Much simpler!

For details see:

http://www.audiodesignguide.com/

I have used this and its great!

 

[Brett]

<b>You could also mix a tube rectifier with two solid-state ones and retain the slow warmup.</b>

The drawback with this arrangement is you still get the switching spikes from the SS diodes, which are not eliminated by either the filter capacitors or chokes. IIRC Lynn Olson and Matt Kamna did some research into this and measured the results as well as listened.

Full tube bridge, hybrid or SS all have their advantages and drawbacks. If it was me, I'd get a little filament trans for the rectifiers or a power trans with the correct secondaries and a centre tap, and use a pair of good damper/booster diodes like the 6D22S.