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by **burnedfingers** » Wed Jan 09, 2008 5:35 am

I got a used SDS cap board that is already populated. Is there anyone that would share the directions and schematic since I have nothing to go on? It would be much appreciated.

Thanks in advance

by **Shannon Parks** » Wed Jan 09, 2008 6:19 am

From Sheldon's site:
http://www.quadesl.com/pdf/st70_capboard.pdf

by **burnedfingers** » Thu Jan 10, 2008 6:12 am

I have a question if I may.

I noticed there is a resistor across both caps that are in series. Now if my memory is correct 2) 100mfd caps for example would give you 50mfd.
The two of them in series would give you a higher voltage rating.

So, if there were 2) 390 mfd /250 volt in series the capacity and rating would be 195mfd/500 volts? The resistor across each cap helps in charging equally and the series resistance discharges the cap bank when powered off?

by **Shannon Parks** » Thu Jan 10, 2008 6:39 am

burnedfingers wrote:So, if there were 2) 390 mfd /250 volt in series the capacity and rating would be 195mfd/500 volts? The resistor across each cap helps in charging equally and the series resistance discharges the cap bank when powered off?

Correct and correct.

by **burnedfingers** » Thu Jan 10, 2008 6:43 am

Thanks,

Its been a few years. Just trying to remember correctly.

by **wiredbecker** » Thu Jan 10, 2008 8:46 pm

No, this is very helpful because soon enough I'll be stuffing my SDS board and begging you guys for information. Every little bit of information helps add up to a greater understanding of the big tube amp picture.

by **Ty_Bower** » Fri Jan 11, 2008 8:41 am

Be gentle while assembling your SDS cap board. The one I put together recently had very thin traces. I found it was all too easy to overheat them and risk lifting one. If you have one of the cheap, junky Radio Shack 25 watt pencil irons (like I do), try to work quickly and don't apply heat any longer than it takes to make a good solder connection.

If you have a good temperature controlled soldering station, you shouldn't have any problems. Just watch the electrolytic caps and make sure you put them in with the correct polarity. The rest is nearly idiot proof (or at least idiot resistant).

by **SteveH** » Sat Jan 12, 2008 5:09 am

I just wrapped mine up a couple of days back. I can vouch for the superglue requirement Yellow_Light_Colorz_PDT_09

by **Shannon Parks** » Sat Jan 12, 2008 6:15 am

separks wrote:
burnedfingers wrote:So, if there were 2) 390 mfd /250 volt in series the capacity and rating would be 195mfd/500 volts? The resistor across each cap helps in charging equally and the series resistance discharges the cap bank when powered off?

Correct and correct.

The leakage current in electrolytics end up making the caps look like two large value resistors in series, and end up being a voltage divider. Modern electrolytics usually have leakage in the microamp range, whereas an old electrolytic could be in the milliamp range (hence dissipating heat which can make one blow eventually).

Let's say you have two electrolytics who have 100uA leakage with 250V across them: 250V / .0001A = 2.5 Megaohms. So two of these in series with 500V across them would be similar to two 2.5 Megaohm resistors in series, dividing the voltage. But if the leakage current was not the same (when tested separately) due to different manufacturers or build variations, you will end up with a mismatched voltage divider. One cap might have 300V across it and the other would have 200V. So these parallel resistors are sized to have much more current go through them to even things out. I think 1mA is more than enough on modern caps, if they are needed at all. I wouldn't have a problem matching new caps with no resistors.

But the secondary effect of being bleeders is a huge plus, and ends up helping the above decision. For example, in my Eiclone board the strapping resistors drain the large capacitance power supply very quickly, preventing a hobbyist fromming zapping himself after powering it down. With no bleeders, the high voltage could remain a long time.

by **burnedfingers** » Sat Jan 12, 2008 6:53 am

Looking at the board again I see a spot for a what looks to me like the filament CT to ground cap. Didn't the Dynaco have a 3 lead ceramic cap to go from Ct of the filament winding to ground for each side? What was the rating of this cap and what would work? I assume this might help with noise?

Thanks

by **burnedfingers** » Sat Jan 12, 2008 6:58 am

Another question... looking at the board again. Could a person put in a three pole double throw switch to allow the usage of either the SS diodes or the tube (5AR4) rectifier? This would allow for comparison between the two. I of course would NOT suggest flipping the switch when it was running.

by **burnedfingers** » Sat Jan 12, 2008 7:10 am

The board I have doesn't have C7 and C8 on it. It shows L1 and Fcap.

by **burnedfingers** » Sun Jan 13, 2008 7:01 am

no answers to my questions? Yellow_Light_Colorz_PDT_14

by **Ty_Bower** » Sun Jan 13, 2008 7:45 am

burnedfingers wrote:Could a person put in a three pole double throw switch to allow the usage of either the SS diodes or the tube (5AR4) rectifier?

I see no reason why not, although if it were up to me I'd just solder a pair of diodes into a tube base. Then you can plug the diode-loaded tube base into the socket where the 5AR4 goes. There would be no chance of accidentally flipping the switch while it's running, either.

by **burnedfingers** » Sun Jan 13, 2008 7:50 am

I see no reason why not, although if it were up to me I'd just solder a pair of diodes into a tube base.

I'm not quite sure what you mean, not enough coffee yet. What is the best way to go about doing this?

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