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Vespasian 6th January 2007 11:17 PM

Project using Eico ST 70 Iron?
Can anyone recommend an amplifier design utlizing the Eico transformers? For a small room, 89db - 92db speakers, varied music via cd player. I don't think I want to rebuild the entire integrated amplifier to original specs (don't want the preamp), but instead do a ground up with home-made chassis. 12 or more watts per channel would be fine.

TIA, Cheers Ed

Eli Duttman 7th January 2007 06:43 AM


IIRC, Eico's ST70 used 7591s. There's no reason to change O/P tube types. If you triode wire 7591 "finals", the sound will be good, power O/P will be about 15 WPC, and (of course) "iron" and O/P tubes will mate well.

The 7591 is slightly easier to drive than a 6V6. That means the differential splitter/driver used in "El Cheapo" is "ideal" for your purposes. "El Cheapo" schematic here. If you have Sitzfleisch, read the whole thread.

BTW, the DynaClone replacement power trafo for Dyna ST70s can be set up to provide for all of the amp's needs.

Vespasian 7th January 2007 12:15 PM

Hi Eli,

BTW thanks for the recommendations regarding the ASR433, Jim sent along the parts just after the holidays - everything worked out perfectly. I think it is a great little amp - difficult to turn off once I start listening.

Re 7591's, I put a set of JJ's in my Scott and after about 100 hours one of them started to do a super-nova thing, the original Scott labeled tube (45 years old) is sitting in until a replacement comes. I'm wondering if it is just bad luck or perhaps the brand should be avoided? I haven't yet had any trouble with the el84's I am currently using.

Would like to use the Eico power transformer w/ 5ar4 unless it is shot. I haven't got the amp yet, don't know.

Yes, I started reading that thread a while back when you mentioned the current limiters etc, it is a bit long 'eh?

Thanks again, Ed

Eli Duttman 7th January 2007 03:00 PM


A partial schematic that shows the EICO PSU indicates you will need additional "iron" for the B- rail. I suggest you use the low cost Allied trafo "El Cheapo" uses. Use the filament winding on the Allied trafo to energize a 6AL5, which will serve as the vacuum half of the hybrid B- bridge rectifier.

Since you are going to execute a scratch build, allow plenty of room and use EH (Russian) 7591As.

BTW, the "fireworks" you saw in the Scott LK72 may be related to the value of the 7591 grid leak resistors. Scott, Fisher, etc. took liberties with the published grid leak value limit. They got away with it using the tubes of their time. Current production 7591s are not as forgiving. Jim McShane has discussed the grid leak subject on more than 1 occasion over on AA.

Vespasian 7th January 2007 06:30 PM

The 330k's?

Vespasian 7th January 2007 07:58 PM

whoops. dbbl posted

Eli Duttman 7th January 2007 08:21 PM

Yup, the 330 KOhm parts. Look at the data sheet. Notice that 300 KOhms is the design max. value.

3 systems of ratings are used for tubes. Design center is the most conservative and you are safe exceeding such spec's slightly. Absolute max. is the most aggressive and is self explanatory. Design max. is in the middle and represents reasonable limits. The 330 KOhms Scott used is pushing the envelope.

Tom Bavis 7th January 2007 08:51 PM

Ahh... but note the additional 500K in parallel (the bias feed and balance pot). This is about 200K grid resistance (better than the 330K Fisher used..). Still, lower couldn't hurt... but lowering 330K resistor alone would affect bias voltage.

Vespasian 7th January 2007 09:05 PM

I tacked a couple 1m's on top of the 330's (one channel) and the tube still wants to burn a hole through the planet. I think this one is shot. It starts going after about 30 seconds regardless of what socket it is in.

Hey Tom!

Vespasian 7th January 2007 11:38 PM

Just replaced the 330k and 270k resistors (they didn't test that bad), cleaned the pots, now the tube seems ok. :att'n:

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