Safe voltage rating for 2nd section of cap

[joeriz]

I'm currently (finally!) in the process of re-building my ST-70. I am using one of the 80/40/30/20 @525 can caps for the power supply. I'd like to add more capacitance to the 80uf section (which will be used as the second section). I've seen folks use 450v rated caps here but didn't know if that was pushing it in terms of safety.

I happen to have some 100uf/450v Panasonic snap-mount electrolytic caps lying around and thought I might use a couple of those. The question is: do I run them in series to get one 50uf/900v cap OR can I run them parallel to get 200uf out of them even though the voltage rating is only 450v. I know the voltage rating of the second section is not as vital as the first but I don't want to push things...

Also, has anyone found that there is a practical limit to how much capacitance to add in the second section? Is there a point beyond which one is just 'gilding the lily'?

Thanks!
Joe

[Ty_Bower]

I wouldn't feel safe with a 450 volt rating. The PA060 transformer is a 360-0-360 unit. If you somehow run the thing without the output tubes installed, you'll end up with over 500 volts (360 * 1.414) across every cap in power supply.

If you believe PSUDII, the stock circuit is going to see startup surges over 500 volts on the second cap every time you turn the amp on. Of course, if you start significantly increasing that second cap the peak voltage will be lower, but you still haven't done anything to protect yourself if the outputs aren't conducting.

Isn't the stock cap rated for 525 volts on all sections? I'd stick with that as the minimum.

[erichayes]

The original run of ST-70s used a Quad rated at 525-475-475-475 or 525-475-450-450 (I've seen them both). Later runs were 525 all around, for one or more reasons I can think of--most likely cost. In any event, the second section was always higher than 450 volts.

[Bob01605]

That CE distribution 80, 40, 30, 20 cap is a very good quad cap. All sections are rated at 525 volts. In talking with the people that distribute this cap they mentioned that it is designed to accept surges higher than 525 volts. I have used over 30 of these 80, 40, 30, 20 quad caps in Dynaco ST-70's the last year or so and to my knowlege none have failed.

Putting this same cap in a Dynaco Mark III, however, is a different story. The Mark III has more voltage than the 70 and this cap may be marginal (as was the original stock quad cap) in a Mark III. The best quad cap other than an underchassis quad cap replacement board is the quad cap sold by www.dynakitparts.com. These caps are rated at 550 volts continuous and 600 volt surge. These are specially made for the Mark III but may also be used in the ST-70 if you want or feel that you want a higher voltage cap.

Bob Latino

[Ty_Bower]

I'll second that. The CE cap doesn't last long in a Mark III. The guts come oozing out.