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Very nice Terry ! I love the before and after contrast.

Those are the nickle plated covers that Ned sells, very nice! I might try the brass plated covers next.

My B+ @ turn on is 490 - 500v, and settles to 455 - 460v. The inrush limiter really helps the turn on "slam".

that is one of the better st70s i have seen so far. Excellent work.

Great job on the restoration, really nice looking. Just curious, where did you get the quad capacitor from?

EDIT: Disregard, I did a search and found it. I wish I knew about that cap, I would have put that in my ST-70 instead of the standard value cap.... Oh well, I do notice on heavy bass music (the crap my daughter listens to) the ST-70 struggles a little... On an unrelated note, if you want to see my daughters DJ debut on radio, go to http://youtube.com/watch?v=sN7UDX-7ypw . Sorry, I could not help it, she's daddy's little girl... Yellow_Light_Colorz_PDT_12

Regards,
Sal Brisindi

I think the new can cap has FAILED! It now has that distinct 120hz hum, not loud but noticeable. Maybe SS diodes is too much for it? Since I finished the getsetgo, it took a back seat and doesn't get used very much.

How does the SDS cap board survive with only a 500v rating and SS diodes? I measured 500v at startup and settles down to appx 460v. There is a CL-80 in there also. Tube rectifier in a '70 marginal at best, and I use mine rather aggressively and a 5AR4 will fail in short order.

The SS diodes will deliver a significantly higher B+, and the B+ voltage will surge to well over 500VDC until the tubes warm up and start drawing current. This is one case when the tube restifier is better / safer than SS rectification, ulness some mods or changes are worked in...

A somewhat different approach, is to use the SDS SS rectifiers, then wire in the 5AR4 / GZ34 with both plates connected, as a "series pass" element. this will return the B+ closer to normal voltage levels and the tube will allow the slow warm up of the B+ line, so that the rest of the circuitry doesn't get by the "high voltage hammer" while the EL34s and driver tubes fully warm up, and draw normal plate currents..

Combine that with the older trannies having a 115VAC primary in today's 120VAC + world, and this is definitely a case in which more "or bigger" is not better...

A normal tube rectified B+ at the OPT CT leads shoule be around 420-440VDC, (I get 446VDC with my 123VAC power lines and a Triode PA-060 power trannie, and JJ KT-77s biased at 50 mA each, SS diodes will deliver 10 to 20 V more, even under lfull current oad. Peak unloaded (SS rect) DC voltage can soar up to 550VDC, which is WAAAAAYYYY too much for a long and happy component lifetime.... :o

Does the can cap feel too warm (beyond what the nearby radiant heat from the EL34s would normally generate) ? Overvoltage will be evident with overly hot cap temperatures, and look for any swelling or deformation of teh cap body and check the rubber "burst seal" on the bottom, a little bit of "protrusion" is OK, but if it is poking way out or dribbling electrolyte then the cap is "toast"...

The SDS (and Cyrus Chong's) "bulk" caps are wired as pairs in series with equalizing resistors, and probably are good for well over 500VDC, with plenty of safety margin.


HTH

/ed B in NH

I think the new can cap has FAILED! It now
has that distinct 120hz hum, not loud but
noticeable.

The 80-40-30-20 multi-cap seems to be
more prone to failure than the 30-20-20-20.
It is also said to fail much earlier as well.
Perhaps, there has been a bad batch
produced, recently.

The symptoms are as you mentioned, slight
hum and also severe loading on the power
supply.

I do not think any 525V rated cap is suitable
for an st70 with SS rectification. My B+ is
466V with a 5AR4 in it.

BTW. A really nice looking amp!

I have replaced the can cap with a Chong cap board. Since it's voltage rating is 500v on the second stage, the SS diodes would be too much voltage at turn on.

So I changed the rect socket to a novar socket, hard wired 4 UF4007 (2 each in series) then to a 6DW4B damper tube used as a slow turn on device, then to the cap board. Seems to work very well with no side effects so far.