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Old 23rd January 2007, 01:19 AM   #1
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Question Eico ST70 OPT / Schematic Question

Hello,

I plan on using the iron from this amp to build a similar amp (less preamp and eq circuits) on a new chassis. Anyway, I've got the original amp running on the bench (after replacing some power supply caps) so I can take measurements, study the design and take a few notes prior to disassembly...

Looking at the schematics I found on the web, I notice the 4ohm opt connection to chassis:

Click the image to open in full size.

Hoping someone can explain this to me (novice), I would have expected the common lead to connect to chassis. AND to make things worse, the actual amp has the BLACK lead as common and the YELLOW lead as 4ohm and the YELLOW lead is wired to chassis.

Lastly, the layout of the terminal strip on the back of the amp is:

16 8 <blank> 4

So if on the schematic you just switch the location of the word COMMON with 4ohm, that would make sense to me but then my amp is wired wrong with the 4ohm lead attached to ground?

There IS music coming from the speakers.

I hope this doesn't win me stupid post of the year...
Ahhhh... I've probably already been nominated anyway.

Cheers, Ed
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Old 23rd January 2007, 01:39 AM   #2
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Yup. I have 3 of these and enjoy them. When you build the new amp, wire the outputs normally. That funky wiring is there for the strange phase adjustment switch and the "center" channel (L-R) Outputs.
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Old 23rd January 2007, 02:05 AM   #3
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Yup?

Which Yup?

The schematic is right, my amp is wrong; the schematic is wrong, my amp is right; both wrong; both right - or just forget it and use the black lead as common and attach it to the chassis for the new build?

Say, have you done any mods to the original tube circuits?
If so, Please share!

Thanks, Ed
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Old 23rd January 2007, 02:09 AM   #4
anatech is offline anatech  Canada
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Hi Ed,
Every Eico I've restored sounds great. I went back to factory original in every case.

4 ohm to ground is normal. Beware that some of these were kits and not factory wired. My current fav is an HF-87 and I have another ST-70 waiting for me to get into.

-Chris
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Old 23rd January 2007, 02:38 AM   #5
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ahhh, yeah, funny thing is with this one, just replacing one of the 4 way multisection caps and a 30/400, a pot cleaning, tubes and some adjustement and I'm rather surprised how good it sounds with all the original pyramids!

If it wasn't for the rust spots on the top chassis surface, I might have gone with a restoration - front isn't bad, she's got all her knobs and the cage is pretty good...

The iron is nice 'eh?
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Old 23rd January 2007, 03:06 AM   #6
pchw is offline pchw  United States
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I have 3 ST-70 and one ST-40. I did the following small mods to all of them:
- Removed the funcky center channel
- Removed the stupid balance check
- Wire the common to ground instead of the 4 ohm
- Fixed the loudness control.
- Replaced all caps and resistors.
It sounds very clean and more than it's rated power.

Recently I rewired one of the ST-70's as power amp only, but it was not quite the Eico HF-87. So, I am looking into some projects that I can use the parts. There is a Eico user group a Yahoo!Groups. There is a lot of info there.
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Old 23rd January 2007, 03:16 AM   #7
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Thanks Fred, what choice of coupling cap did you make for the power amp? And you just used one (half per channel) or two 12ax7's in front of the 6sn7's?
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Old 23rd January 2007, 03:38 AM   #8
pchw is offline pchw  United States
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Hi Ed,

Only one 12AX7 precedes the 6SN7. Each half of the 12AX7 is the voltage amplifer for one channel. The pair of the 12DW7's are for the preamp, and the other pair of the 12AX7 are for the phono. The phono section is suprisingly good, it has both 47k and 100K MM inputs. The ST-70 is not hard on the tubes, one set of tubes can last a long time!!!

I forgot to mention, I did try wiring the OT as the factory circuitry and the way I mentioned in previous post. I didn't hear much of a different in sound quality. All the simple mods can be easily reverted. You can try each one of them and decide which sounds the best to you. The concenses from Eico user groups - the center channel, the balance check, and loudness mods are a must.

edit: if you want the original factory manual, email/pm me your email address. I will send you the pdf copy.
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Old 23rd January 2007, 11:59 AM   #9
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Yes, I've done the loudness mod already because the balance didn't seem quite right and the switch acted more like a mute than loudness...

Was also wondering about the strange center channel setup and the balance switch that connects the feeback circuits - thanks.

For the new power amp build, I was thinking about using separate 12ax7's up front, initially using only one triode section from each, this would give me the option of using the other halves later if the need arises. Not going to bother with the pre-amp / phono stuff, just volume pots...

I'd be interested to hear what you decide on for the new project and will send you a note for the pdf.

Thanks alot, Ed
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Old 23rd January 2007, 04:14 PM   #10
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Fred - cannot access your email - I'm at Vespasian$@$cox.net less the dollar signs. - thanks
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