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tubo 14th October 2009 03:12 PM

eico st70 irons!what to do?
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hi fellas,

i have a pair of eico st70 opts, and its pt along with a quad of 7591. i was thinking of building an integrated amplifier out of them ala "el cheapo" by eli. or the pp music machine by poinz. i have built the new version of poinz's 6gk5 6v6 machine and it sounds very good so was contemplating on going that proven route.

ive read that the 7591 is just as easy to drive as the 6v6 so im assuming that poinz's circuit will be able to the the job.

ill just be using the eico st70 power supply schematic for the 7591.

1. how do i make the changes to accomodate the 7591?

pls help. thanks


Eli Duttman 14th October 2009 03:55 PM


Yes, the 7591 is as easy to drive as "12" W. multi-grid types. Therefore, either of the EC or MM splitter/driver circuits will work.

I suggest you use EC style circuitry, given those O/P trafos, which are designed to be used inside a NFB loop.

Both MM and EC require a B- supply. If you SS rectify the B+ rail, the 5 V. winding on the power trafo can be voltage multiplied to yield both the B- for the LTP and the C- for the 7591s. The extra Volts yielded by SS rectification of the B+ can be put to good use by combination biasing the "finals". Combination bias allows you to safely keep the 7591 grid leak resistance in the 300 KOhm area. Current production 7591s, unlike NOS, are intolerant of out of spec. grid leak resistors.

kevinkr 14th October 2009 05:27 PM

Hi Eli,
Might want to tell us what the abbreviation EC and MM stand for. I can't think of a single phase splitter or amp topology with those abbreviations.. Guess I am clueless.

Off the top of my head I can think of split load/cathodyne, differential/LTP, transformer or choke coupled, but these two have left me hanging, and I expect others are too..

How about electron coupled, European Community, moving magnet or motor mount? :boggled: :D

SY 14th October 2009 05:58 PM

I'm mystifried, myself. Assuming EL84 or 7591, the classic voltage amp direct coupled to split load would seem to be a pretty safe approach.

HollowState 14th October 2009 06:25 PM

From reading what's been discussed, I would assume it's:
EC is El-Cheapo
MM is Music Machine

Eli Duttman 14th October 2009 10:37 PM


Originally Posted by HollowState (
From reading what's been discussed, I would assume it's:
EC is El-Cheapo
MM is Music Machine

HollowState is correct. A high gain/high gm differential splitter/driver kicks serious butt in combination with these EASY to drive types. Whether or not GNFB is used is a matter of personal preference and the nature of the O/P "iron" available. Please notice that a separate preamp is needed only for a phono source, as a 2 VRMS I/P signal is more than enough to drive these designs into clipping.

I know the 12AT7 is very good in PP amps. You get an ear pleasing "waterfall" for the net distortion spectrum. Perhaps Poinz will see this thread and give us the "skinny" on the 6GK5 setup's distortion behavior.

tubo 16th October 2009 02:26 AM

hi eli, folks

ill measure the secondary voltages of the power transformer when i get guessing there will be more than 400volts on the secondary. i also do have a 10h 200mA CHOKE for the ps. i want to use the tube rectifier as i do have a couple laying around gathering dust.

ive been wanting to build the el cheapo for a long time now but just can't stop tweaking my 6v6 music machine as it sounds very good to my ears.

i don't have the parts to build the el cheapo but i have all the parts needed to build poinz's 6gk5 driver circuit so i was thinking of taking that route. im still a beginner and know nothing about designing an amplifier so everybody's input are greatly appreciated.

ive seen the new thread about the fisher 500 irons being used but will shy away from that as ive read that it will be complicated.

ill download the psud designer when i get the chance.

thanks everybody


alexg 16th October 2009 08:20 AM

Hi Tubo,

Whereabouts in the Philippines are you?

Would love to see your builds and listen to them. ;)

tubo 16th October 2009 09:24 AM

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?



TonyTecson 16th October 2009 09:40 AM

hello tubo,

you were born at a time when tubes almost became obsolete if not for musicians using them in their gigs...

if i may ask, how did you get interested in tubes?

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