Rebuilding my ST-70...a few questions

[joeriz]

As I posted last week, my Welborne modded ST-70 (done about 10 years ago) developed a problem in 1 channel. Rather than try to mess with it as is, I have decided to re-build it again. What the heck - it will be fun! Also, I'll be glad to get rid of the Welborne mod. I would never do this mod again today as it makes maintenance on the amp a real chore. There are many more driver boards available now than there were 10 years ago when I did this. I'm going with Shannon's board this time around.

I stripped everything from the chassis and am having it replated. I'm going with chrome with a brushed stainless type finish. You might wonder why I would do this given the repro chassis that are now available. Well, this was my dad's amp which he built back in 1959 and it's the amp I grew up listening to so it's a sentimental thing.

A couple of questions...

Does anyone know of a good source for decent metal binding posts that don't cost a fortune? Also, I need to be able to fit three of them on each channel (C, 4, and 8). Most of the metal ones I see are like $30 a pair. As an alternative, I do have a set of barrier strips that I could use (or I suppose I could even buy new terminals like the originals) and use those banana-to-spade converter type things. Any thoughts? I suppose it doesn't matter all that much...

I'm also trying to decide whether or not to use an under-chassis mount cap board in place of the can cap. Or, I'm also thinking about going with the 80/40/30/20 can cap (I've used this on my stock ST-70) and supplementing with some additional under-chassis capacitance. It seems like the latter method would allow the use of more capacitance overall. Any preferences here?

And, one last thing. Does anyone know of a good quality toggle switch that I could put in place of the old slider power switch (i.e. able to be mounted in the same opening without cutting the chassis)?

Thanks,
Joe

[Sal Brisindi]

Joe,
Why don't you rebuild the ST-70 to its former glory just like the way your dad built it in 1959?

Regards,
Sal

[joeriz]

Well Sal - The last time I rebuilt the amp I had it gold (yes, gold...) plated since the chassis was in slightly rough shape. Also, the Welborne mod requires drilling some holes in the top of the chassis so now I'm having them filled before replating it. So, as you can tell, it's not going to be exactly like it was back in '59 no matter what I do.

This time around though I'm not doing anything that requires surgery on the chassis so if I ever decide to go with a straight stock build I can. It's my daily driver though so I might as well beef it up a little bit... Yellow_Light_Colorz_PDT_02

Joe

[Sal Brisindi]

Joe,
That sounds good to me. I am about to rebuild my 2nd ST-70 and the only mod I am doing besides replacing all the caps is to install a heftier quad cap. I noticed on my other ST-70 on heavy bass music the amp struggles a little bit.

Regards,
Sal

[joeriz]

Hi Sal,

My other ST-70 is completely stock except for the slightly beefier 80/40/30/20 can cap, the 'lytics in the bias supply, and a diode in place of the selenium rectifier. It sounds really nice. (Some days I'll get around to installing the new coupling caps I have on hand.) I'm always amazed at how nice they sound stock with refreshed parts.

I think I've decided to forego the under chassis cap board on the one I'm rebuilding now. What I'd like to do is use the same can cap mentioned above and supplement with some under-chassis caps. I'm thinking about something like this JJ cap: CAPACITOR, ELECTROLYTIC, 100/100 µF @ 500 VDC.

I'm thinking I could use 2 of these in series for double the voltage rating and an extra 200uF total. Or, I think I could parallel 2 of them for 400uF extra which, if I remember correctly, would keep me at the 500V rating which I think should be fine since the extra capacitance would come after the rectifier tube and initial section of the can cap which takes most of the turn-on voltage surge I believe. I could use some advice here, however.

Thanks,
Joe

[erichayes]

I think I've mentioned this before, but it bears repeating.

Take the voltage rating on the clamp type can electrolytics (JJ, F&T, LCR) literally. Even 15 volts over will cause them to start boiling. If you think you might exceed the rating (like when things are warming up and there's no load on the power supply) use two caps in series, or go with a different style cap.

[soundmasterg]

There are several problems with can caps, and the biggest is the cost. If you use discrete caps the price is way less, and that will continue to be the case as time goes on. Can caps usually share the ground between the cap sections, and because of this, you cannot connect the sections in a can cap up in series. You can connect them up in parallel however, or you can connect two can caps up in series. If you're going to go to all that trouble though, why not just get discrete caps and connect them in series. You'll save money doing it.

In the Stereo 70, the best choice is probably to use one of the cap boards that use radial caps. They will fit right in and the cap prices are very cheap and the selection is very good. The boards don't cost very much either, and there is a good choice between various boards from different manufacturers. I'll be using the SDS boards in my Stereo 70 when I get around to rebuilding it.

Greg

[EWBrown]

I've seen in some '60s vintage heathkit test equipment, where they hooked up can caps in series for a voltage doubler to generate 300VDC from a 125VAC secondary. The "trick" they used was to mount some of the can caps on phenolic insulatong wafers. It works OK, but also keep in mind that those caps now have "hot" cans in relation to the chassis. Finger biters, indeed :o Yellow_Light_Colorz_PDT_07 Yellow_Light_Colorz_PDT_04 and no protective cardboard or plastic sleeves. But then, Heathkit, in those days, also didn't believe in primary AC fuses, either :o Yellow_Light_Colorz_PDT_11


In a couple of ST70 restorations, I used the cap boards, to do the PSU duties, and left the inactive and disconnected can cap in place just fir looks.

/ed B in NH

[Shannon Parks]

The Eico HF-87 and HF-89 are like that, Ed. Scary! Definitely changing the PS on mine.

[erichayes]

You can spot a voltage doubler with bare naked eyeballs if one of the cans has a cardboard shell on it. That's the "high" cap, with half the PS potential on the can.

[parabellum]

Just today I read in AudioXpress magazine an article about voltage quadruplers where that autor suggests can be used to get the recycled solid state PT suitable for the tube projects.

In theory sounds ok, not sure what the heck it would look like in practice.

[EWBrown]

(Moved to new topic)

HTH

/ed B in NH

[joeriz]

OK...back to my question if you don't mind.... Yellow_Light_Colorz_PDT_02

I'm now thinking about paralleling a cap off of the can section(s) where I want to increase capacitance. I'm a little confused as to how to do this, however, due to the common ground of the can. Would I simply attach the new cap's positive end to the can section where I want to increase capacitance and the negative end to ground? In looking at the power supply schematic, that would seem to be the case. I'm thinking about using something like a 600v Sprague electrolytic here.

[EDIT: Or perhaps even a 500v Sprague (higher values available) would work after the first section as I see that is what the Chong board uses for the second section. It also uses 450v for the third and fourth sections!]

Joe

[mesherm]

Would I simply attach the new cap's positive end to the can section where I want to increase capacitance and the negative end to ground?

Yes, that is the way its done.

[joeriz]

Thanks, Mesherm.

So, per my edit above, would a 500v cap be sufficient (again, it would not be on the first section)?

I'm thinking it would also be a good idea to put an inrush limiter (CL-80?) in. This would go in series with the power transformer primary, correct? (Going by memory here.) The primary is the wire coming off of the fuse holder I think(?). So I would wire one end of the CL-80 to the fuse holder and the power trans. primary off of the other end of the CL-80? Sound correct?

Thanks again,
Joe