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eico st70 irons!what to do?

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alexg

alexg

Member
Joined 2005
tubo said:
hi alex,

i was from bacolod city. visayas. but im in US now for almost ten years. migrated here when i was 15. where are you located in the phils?

thanks,

ivan
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Hi Ivan,

I am in Sta Rosa, Laguna.

I still maintain a house in LA, CA, I immigrated to the US in the early 80's and lived in CA. Now I work in Laguna and visit the US almost yearly. :D

Anyway, good luck with your project. This hobby is so addicting, I have built more than a dozen amps already! (I started building tubed equipment 4 years ago) :D
 
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tubo

Member
Joined 2007
crispycircuit said:
I have a similiar project. I have the pwr and output trannys from a Scott receiver that used 7591s and have them mounted on a new chassis with switches and sockets. I'm gonna use 6L6s but have't decided which circuit to hard wire in ?? Or if I should use diodes or tube rectification? Let me know what interests you? Cheers!
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i think ill just build what eli is suggesting. already have most of the parts needed. good luck on your build.:)
 
C

Captn Dave

Member
Joined 2008
Reviving this old thread

I was given an EICO ST70 without the tube compliment. He's not a very attractive guy and he needs new can caps and coupling caps. I already have a Fisher X-101C and a Scott LK72 (and thy are among my favorites) but I'm not too excited about another 7591A. So I'm looking for help vetting some ideas. Eli and others have already provided some thoughtful content so I thought I'd tack onto this thread.

I'm thinking of using 6L6G or GC in triode. The 6L6 heater voltage is just one tenth higher than the 7591 and I'll be eliminating some small signal tubes and the rectifier so I should be covered in that respect. The OTs are probably okay as load resistance appears to be similar. Bias voltage is similar.

What do you think of the 6L6 as applied here?

Would the El Cheapo or Musical Machine topology make sense here?

Note: See post #18 for the PT specs
 
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Eli Duttman

Member
Joined 2004
Captn Dave said:
I was given an EICO ST70 without the tube compliment. He's not a very attractive guy and he needs new can caps and coupling caps. I already have a Fisher X-101C and a Scott LK72 (and thy are among my favorites) but I'm not too excited about another 7591A. So I'm looking for help vetting some ideas. Eli and others have already provided some thoughtful content so I thought I'd tack onto this thread.

I'm thinking of using 6L6G or GC in triode. The 6L6 heater voltage is just one tenth higher than the 7591 and I'll be eliminating some small signal tubes and the rectifier so I should be covered in that respect. The OTs are probably okay as load resistance appears to be similar. Bias voltage is similar.

What do you think of the 6L6 as applied here?

Would the El Cheapo or Musical Machine topology make sense here?

Note: See post #18 for the PT specs
Click to expand...


The 6L6 is anything but easy to drive, which is unlike the 7591. However, both types take 6.6 KOhm primary O/P "iron". If you are going to use 6L6s of some kind, a Mullard style circuit of some kind seems best. A superior implementation of Mullard style topology that uses 6L6 family "finals" is the H/K Cit. 5. You can't go full pentode, as the Eico O/P "iron" lacks the requisite power handling capability. However, Class "AB2" triode wired "finals" have promise. The best deal out there right now in 6L6 family tubes is the Russian 6П3С-Е (6p3s-e), from a reliable seller.

Oh yeah, you don't have to use the 12BY7, which is getting scarce, as the voltage amplifier. A 6922 cascode will give you similar gm and the gain that goes with it. Of course, use a CCS in the LTP's tail.
 

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Captn Dave

Member
Joined 2008
Eli,

Yes, the Citation V circuit is a very good suggestion. I like the use of the CCS on the 6CG7 LTP.

I'm aware of the Russian tubes, but I have plenty of good vintage 6L6GCs on hand together with an embarrassment of 6CG7s. I'm going to switch to solid state rectification to gain a few volts and use a Maida regulator on one of SpreadSpectrum boards.

When you recommend the 6922, are you doing so in preference to the similar 6DJ8? I have not that much experience of the 6922 and I have a boat load of 6DJ8s.

Also, what do you think of THIS 6158 medium mu dual triode in place of the 6922?
 
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Eli Duttman

Member
Joined 2004
6DJ8s in cascode will be fine, as the voltage amplifier. Regulating cascode B+ is necessary, as PSRR is very poor. Regulating O/P tube g2 B+ is a good idea, as an open loop linearity improvement is realized. FWIW, McShane puts a small choke in Cit. 5s to tighten "final" g2 B+ up. Space is limited in the 5's chassis.
 
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Captn Dave

Member
Joined 2008
I just checked the PS with diodes and the Maida. The Citations' 440 volt B+ will b doable with the regulated supply. I got 475 into the regulator, albeit with a reduced load of about 25ma.

I'm thinking the regulator should serve well as a choke with it's low Z. I wonder if I'm missing your point?

Eli Duttman said:
Regulating O/P tube g2 B+ is a good idea, as an open loop linearity improvement is realized.
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Now I'm sure this went over my head, Eli. In Triode, g2 will be strapped to the plate which will of course get it's current from the B+ regulated supply. You're not suggesting separate regulators for plate and g2 are you?
 
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Captn Dave

Member
Joined 2008
Eli Duttman said:
A "stock" Cit. 5 sets the O/P tubes up in full pentode mode.
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It was my premise that the 6L6 would run in triode as the OTs are not going to handle the power from the 6L6 in Pentode. It would seem (to me anyway) the most minor modification to strap g2 to the plate and use the circuit as a Triode version of the citation. I can't see the need to change anything else, other than perhaps the CCS and 6DJ8 substitution.

Eli Duttman said:
6L6 family tubes are tough. The control grids can dissipate some power. Is "PowerDrive" for Class "AB2" triode finals in your scheming?
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Hummm, are you taking about using a Mosfet as the cathode follower in the first stage? Frankly, I haven't really followed the PowerDrive thing too closely. (I think I read it was in MJ's ed.4) I'm interested, I just haven't made time to read up on it. Is it TubeLabs baby?

EDIT: No, I think your talking about placing the Mosfet between the coupling cap and the power tube grid. I see the advantage that poses. I'll chew on that a bit.
 
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Eli Duttman

Member
Joined 2004
Is it TubeLabs baby?
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Yes! :D Look here. Keen's MOSFET Follies is also a useful guide in the blending of tubes and FETs.

The Eico "iron" can easily handle 30 W. Your 6L6GCs, when triode wired and running Class "AB2" rate to get you there. Original, metal envelope, 6L6s would work in full pentode mode and Class "AB1", as they, like the 7591, are rated for 19 W. plate dissipation.
 
V

Victoria

Member
Joined 2006
Transformer Numbers

Captn Dave,
I just picked up an old custom amp with big iron on it. I have an idea they might be Eico
parts.

The power transformer has this stamped on the bell: 30048

138252E


The OPT that is viewable has on the bell: 32021-3

138309E

Would you please be kind enough to look and see if these numbers happen to match those on your parts?
Thanks, Victoria
 
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