Eico ST40 GW Find - Where do I start?

That sounds unrealistic to me. 20 wpc in cathode bias might be a little optimisitc from a 7591, but 35 from the same thing but with fixed bias sounds like a pie in the sky number. I think my Sherwood is rated at 48 watts, or 24 WPC with it's fixed bias configuration and 7591 tubes.

The transformers in the Eico ST40 are as big as the Sherwood 5500 series amps, easy. And many of those are rated at "64 watts" (32w/ch) or "80 watts" (40w/ch).

The transformers in the Eico ST70 are actually BIGGER than Dynaco ST70 output transformers.

With that sort of iron, they will make spec, from 30Hz and up. Many people have measured them. No, they're not going to put out full power at 20 Hz without mods, but MANY amps of that era wouldn't do that, either...

Regards,
Gordon.
 
hm, so maybe my S-7000 is just a bit on the gutless side then. Its the only thing I've owned with 7591's, so I figured ~25 wpc with fixed bias was about typical.
 
The amp has an f31-series Mullard 5AR4 fat base in it. Those are worth keeping if they're still good. The later was f32 ( look on the glass near the base there are two lines of alphabet soup); lower one starts with a B for Blackburn plant and some digits to indicate the date of manufacture. f31 is the type code.

On those amps, figure on replacing every capacitor and some of the resistors. Not a trivial job, but once done the amp will be good for the next few decades...:)
cheers,
Douglas
 
So a great big thanks to nerdorama for coming over last night and helping me navigate my terrifying entry into the world of tubes. We got everything checked out and it's not to bad.

I'm putting together a parts list for the recap now. I'd love if you guys could have a look and tell me if I'm headed in the right direction.

I've got a shopping cart going at Antique Electronic. The replacement cans suggested earlier in the thread, and a matched quad of 7591s...

Capacitor - Electrolytic, 50/50 µF @ 500 VDC, JJ Electronic (C-EC50-50-500)
Clamp for capacitor, 1-3/8" diameter (S-H122)
Capacitor - Electrolytic, 40/20/20/20 µF @ 500 VDC, JJ Electronic (C-EC40-20X3-500)
Clamp for capacitor, 1-1/2" Diameter (S-H128)
7591A, Electro-Harmonix (T-7591A-EH-MQ)

And another cart going at Mouser with the other caps...

75-TVA1206
Aluminum Electrolytic Capacitors - Leaded 25V 50 uF (x2)
527-CL80
Inrush Current Limiters 3 Amps 47 Ohm (a suggestion from nerdorama)
539-150104J400EC
Film Capacitors 400V .1UF METAL POLY (x8)

That's just about all the caps, but I'm having trouble with a couple of them. The big red cap underneath is a 30uf 400v. These seem really expensive, like $15. Is that about right or am I looking at the wrong thing? Even 2 15uf will be over $10. Also, the "death cap" is a .03uf 600v. I can't find anything to match that. Though I may go to a standard power cord and won't need it if I do that.

So how does that look to you guys, does everything seem about right? I'm sure there's lots more I can do, but this will get it in operating order and I can go from there.
 

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the 40/20/20/20uf cap isn't what you want for C36 imho. Problem is that it is too big (diameter) to fit thru the existing hole and will require chassis modification - which I'd avoid.

One of the twist mount caps although more expensive is a better choice. Watch the height. I've also found that this will usually reform.

You'll also need C37 (30uf/400v) which can be replaced with 47uf/450v

Lastly, you'll need 2 each .022uf caps and 22k resistors to rebuild the loudness circuit.

Also perhaps some terminal strips to mount the current limiter.

- Gary
 
Thanks for the advice, Gary. Regarding that multi cap can, I'm also considering re-stuffing the thing with these pencil caps from Mouser (another suggestion from nerdorama). I've just got to figure out if they're available in the right value. I was going to be lazy and order drop-in replacements, but I'm starting to add up the parts and I think I'm going to need to save where I can.

And thanks for the alternate on that 30uf/400v, that's going to save me a few bucks right there.
 
I might suggest Jim McShane for the EH tubes. He screens and matches them a little more carefully than most and I don't believe his prices are much different than anyone else's.
 
That's just about all the caps, but I'm having trouble with a couple of them. The big red cap underneath is a 30uf 400v. These seem really expensive, like $15. Is that about right or am I looking at the wrong thing? Even 2 15uf will be over $10. Also, the "death cap" is a .03uf 600v. I can't find anything to match that. Though I may go to a standard power cord and won't need it if I do that.

I found this less-expensive alternative at Antique Electronic Supply, IIRC for my Eico ST70 http://www.tubesandmore.com/products/C-ET30-500-FT

C-ET30-500-FT
Capacitor - F&T, Made in Germany, 30 µF @ 500 VDC, Axial
.99" diameter x 1.48" length Axial lead electrolytic.

It's still sort of expensive at $6.25, but I think that higher voltage ratings will cost a little more.

However, the 47 uF capacitor, which I now see has been recommended, may be less expensive still.
 
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I might suggest Jim McShane for the EH tubes. He screens and matches them a little more carefully than most and I don't believe his prices are much different than anyone else's.

I would give Jim a big YES vote based on my experiences. He does a good burn-in on all his tubes and matches them well.

Have fun Corbin.

John
 
FWIW, I've rebuilt 5 ST40's and never had to replace C36. You might see if it will reform nicely. On the other hand C35 (the 500v dual cap) has been bad in every amp I've rebuilt - I'd bet it sees more ripple and is under more stress. The JJ part you've suggested works nicely there.

If you do need C36 - Restuffing a 4 section cap can be a challenge, especially as you are dealing with a short can. I'd break down and get the C-EC40-20x3-525 cap from AES. Expensive at $37 but will fit nicely. You can use 2 of the 20uf sections in parallel for the other 40uf section.


The .03uf/600v cap is very non-critical value wise, but the voltage rating is critical. I'd use C-SD02-1600 which is an orange drop .02uf/1600v part for safety @ $1.76

- Gary
 
Gary knows EICO ST-40 and ST-70 rehabilitation! :)

I suspect he's planning to start an EICO restoration business as a cottage industry when he retires... ;-)
 
Thanks Mark, but it's not by choice. The Eico ST40's and ST70s seemed to have been widely sold in this area. Every time I clear one out, another seems to show up.

I have an Eico ST70 sitting on my floor now that I rebuilt recently. I suppose I should get my act together and post it on BarterTown.

I had good notes from the last two ST40's I rebuilt (yours was one of them) so it was easy to pass capacitor suggestions along to Corbin for his project.

BTW, if Corbin needs a copy of the ST40 manual, or notes on the suggested (and very necessary) tone control modification I can pass along .pdf's.
 
THe ST-40 Gary rebuilt is in the living room hifi in MA and sounds great... currently hooked to a pair of Dynaco... umm... I think they're A-40XLs :)
The ST-70, arguably, is even better - it's up in NH, now (Gary kindly rehabbed that one, too!) and I am really tempted to try it with the Quads.
 
Good luck with the rebuild!

Lots of talk about firing up/reforming caps with a variac, early on. With tube rectification, you don't want to do that. The tube rectifier doesn't work until it gets near normal voltage, and then... Boom. Bye bye caps.

Use a large resistor before the first cap. This limits current, and lets even the most stubborn cap reform over MANY hours.. Without guess work. Won't save 'em all.. but you can watch them reform on your meter(go across the resistor). You'll be pleasantly surprised how many you can bring back from the grave, that otherwise would have died using a variac with a tube rectifier.

Have fun in there.
 
Good luck with the rebuild!
The tube rectifier doesn't work until it gets near normal voltage, and then... Boom. Bye bye caps.

Use a large resistor before the first cap. This limits current, and lets even the most stubborn cap reform over MANY hours.. Without guess work. Won't save 'em all.. but you can watch them reform on your meter(go across the resistor). You'll be pleasantly surprised how many you can bring back from the grave, that otherwise would have died using a variac with a tube rectifier.

Have fun in there.

Any figures here? I find tube rectifiers do work quite a lot below normal voltage, but not of course at very low voltages.

However, the resistor idea is nice. What value and wattage do you mean by 'large'?
 
I've variac'd diode rectification, but never tube, as I was advised against it long ago. IIRC, 50-90 volts AC., so your blasting at a minumum of half power almost completely defeating your goal.

100k ish. Couple watts. If you're a cheap bastard, or desire a time capsule(like me), It's worth a shot. The cap decides when it's reformed, as well, eliminating variac guess work.

Variac might work for diodes, but if you want to save the other stuff, the variac is not as advertised.

Now back to your original programming...
 
Thanks for the details. I'm sure this cap reforming info will be helpful, Corbin, if not for this this ST40, then for your next tube amp. ;) It's addictive when you get one working nicely.
 
The better way to reform caps is to clip them onto a cap checker that will put out working voltages like the Heathkit IT-11/IT-28 or Eico 950b. If the circuit uses a tube rectifier you can usually perform this without disconnecting the cap from the circuit.

If you're using a variac, take a junker octal base and solder in some 1N4007's. I use it for reforming and a slow startup until to about 90vac. Don't go higher as the voltage drop will be less with solid state diodes. The resulting higher B+ may exceed the voltage ratings of the filter caps if you raise the input AC higher. Once you get to 90vac power down and switch back to a tube rectifier.
 
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