Ampeg B25....30uF/600V caps

[acorkos]

will two 68uF/450V electrolytics connected in series work as a replacement for a 30uF/600V cap? will i need to include balancing resistors, and if so, what value? thanks!

[Gtr_tech]

That'll work fine. 220k 1w is the std value bleeder.

[soundguruman]

It actually sounds better when the caps are in series, and bypassed w/ resistors. I would even use 220uf, and have less ripple. I would not be able to explain exactly why it sounds better but I can hear an improvement in sound quality.

[Gtr_tech]

I don't know if there's a difference in sound really, but it will be easier on the caps since they are not running on the ragged edge of working voltage.

As far as changing values, I wouldn't exceed what the recto tube is rated for. SS diodes don't care so much, so you can increase filtering as needed. But recto tubes are rated for a max first filter cap value so they don't get ate up in a hurry. In this guy's case with the GZ34, 60u is top end. I wouldn't exceed that....

[soundguruman]

Please explain WHY 60uf is top end, and why the tube would be eaten up if we exceed that value.
I don't believe you, but I'm listening...

[Gtr_tech]

I'm not going to explain a tube data sheet to you.....

[soundguruman]

The data sheet, created 50 years ago, recommended 60 uf because that was the biggest practical capacitor at the time, and the ripple voltage was deemed "acceptable".
Amplifiers many years ago had a lot of hum, and since then we have raised the standards.
That does not prevent you from updating the amount of ripple, or the value of the capacitor. In other words, 220 uf, 110uf, etc is perfectly useable with no damage to the tube.

[Gtr_tech]

Nope.....a tube recto can only handle so much peak current. You start using huge filter caps, you greatly reduce the life of the tube. Plain an' simple. If you need giant filters, you need a SS recto.

[soundguruman]

If you are using leaky caps, that could draw more current. But with a good quality low leakage cap, I fail to see what is going to make it draw significantly more current. It's the active circuit load that draws current, not theoretically the capacitor. If I use a bigger value, I will get a bit more B+ voltage, and compensate the amp current by adjusting the output bias accordingly. Hence big or small cap, the final current draw is very close to the same.
And the rectifier has limits in current, but as long as it's not exceeded, big or small value cap, it works fine.
I generally don't classify 220 uf as a "huge" value, since many modern amps use 10,000 or 20,000uf in the power supply, or in the filament DC rectifiers. Again, as long as the rectifier rating is not exceeded, no problems.

[Gtr_tech]

10,000u in a *tube amp*? No way in hell. Show me an example.....never seen or heard of such a thing. And values that excessive would not have any benefit anyway. Thats a value you find in SS amps that deal in high currents....certainly not tube stuff.

Tube rectos are used these days for 1 reason....voltage sag. Therefore bigass filters ain't part of the game plan. Put some 220u caps in a tweed deluxe and see what you end up with....

[soundguruman]

10,000 uf is the stock value for a filament rectifier in a McIntosh tube preamp. No way? check again. There are certain individuals that try to do away with 60 cycle noise, including the amps that I have built. I have built several projects with 10,000 uf + filtering.
But I did not say that this was only for tube amps. I said that 220uf was not a "huge" value, it's not.

[Gtr_tech]

Thats DC filaments. Apples and oranges to plate supply with a tube recto.

You can pretty much put as much filtering as you want on a SS rectified supply. The tube recto will not like it...guaranteed.

[Jazz P Bass]

Maybe this will help.

[Gtr_tech]

Can't argue with that.....

[Enzo]

SO does that McIntosh filament supply have a TUBE rectifier? If not, then it has nothing to do with the issue.