Bridge rectifier choice for bias supply

[CJVx]

I am updating my Scott 222c, mainly the bias/12AX7 supply. First thing, the selenium rectifier was failing, so updating to a modern bridge and adjusting the resistor is a no brainer. However, I have read that using much higher rated rectifiers with a small load will distort their outputs.

Basically, I have a nice 25A/400V chassis mount bridge that fits perfectly into where the old selenium was, but I don't want to use it if the experienced persons on here would indicate that it is too much to the point that it won't operate correctly due to the minimal load of the filiments and bias requirements <1A.

I also have a 1.5A/300v bridge that I could use, so which would be better for the application?

Thanks

 

[kward]

Silicon junction diodes will conduct with very little current, as little as a few micro amps. I don't see a problem in your application. On the other hand, if you use those large bridge rectifiers for a DC filament supply or some other application where many amps of current are drawn through them, they do tend to exhibit a lot of switching noise. But for a bias supply, it should not be an issue at all.

Between the two choices you presented, I would use the 1.5A bridge.

 

[gadget73]

I'd use the 1.5 amp one. No need for anything more.

I have a 20 amp bridge doing B+ duty in an EL84 amp. That thing might need 300ma under worst case conditions, and it works fine. I had the part laying around so I used it but something rated for considerably less would have done the job fine.

 

[primosounds]

Since the negative bias supply is placed on the input grid of the power tubes any noise from the rectifiers will also be amplified along with the audio signal. For this reason many use low noise or Schottky rectifiers in this instance. The voltage is low so that type of Schottky rectifier is readily available. Just beware of counterfeits from you know where.

 

[primosounds]

I am not sure if they make Schottky bridge rectifiers but they are easy enough to construct with 4 individual diodes on a perf board or using a 5 place solder lug.

 

[kward]

I would agree if there were any sort of current flowing, but at the minuscule currents involved with a bias supply, any normal diode will work just as well.

That being said, schottkys wil work just as well also.

 

[CJVx]

Thanks for the tips. I have some 100v/3A Schottky's that I assembled a bridge out of, will be installing that later.

 

[PakProtector]

In the bias supply, where quiet operation is quite important, Schottky types are called for. I would spring for the SiC by Cree. They are bomb-proof, and couldn't care less about how little current they are conducting. The reason the higher amperage P-N junction types can get noisy is that as the junction gets larger, so does its capacitance and reverse recovery time.

IMO, the additional cost involved for a clearly better part that can be ignored, and perform flawlessly for the life of the unit is well worth it.
cheers,
Douglas

 

[gadget73]

push-pull bias supplies really don't need to be all that clean. Noise goes in common mode so it cancels. SE amps are more fussy, but a 222c isn't that.

 

[PakProtector]

push-pull bias supplies really don't need to be all that clean. Noise goes in common mode so it cancels. SE amps are more fussy, but a 222c isn't that.

While with you in principle, the PSRR benefit depends on better balance than is available usually. Also, I'd rather try to avoid having to cancel stuff if I can avoid it in the first place...
cheers,
Douglas