EICO ST-40 Iron and Tubes

[retox]

I am wanting to build an amp using the Iron from an EICO ST-40. I have new tubes and caps that I was going to use to restore the EICO, but changed my mind. I have searched the web for a DIY amp that I could use this hardware, but no luck. Does anyone have the schematics for an amp that i could build using the EICO Iron and the new 7591s and ECC 832s I have? I tried the amp only portion of the EICO schematic and the outcome was very boring. I have built 5 different amps so far and i am now addicted. All suggestions welcome. BTW, I am a mechanical engineering designer with a basic understanding of electronics. I can read a schematic but can't design an amp on my own.

thanks

[kheper]

Here are some mods for the st-40. Some suggest that the 7591 (and the ECC 832) tubes be scrapped for better tubes. I saw a 6550 triode mode mod for the power amp, but it is not around anymore.

http://db.audioasylum.com/mhtml/m.html? ... t%3Daikido

http://www.audiokarma.org/forums/showth ... p?t=211563

[retox]

Thanks for the quick response kheper...

If new tubes are a good direction, then what about a schematic that would use just the iron, the PT and both OTs are good. A completely new project is an option. I have a Sylvania AV4 GA for a rectifier tube. Building a chassis is the easy part.

[kheper]

On the Audiokarma page in the schematic, the mod is to run 6l6 tubes in triode mode and to use a single 12at7 as volt amp and phase splitter. A simple, down-and-dirty mod. Maybe try the 12at7 mod with the original 7591 tubes in triode mode?

[retox]

Thanks, looks simple...i have a problem with R77 and R78 the 100ohm hum pots. What is a good replacement or a means of elimination?

[kheper]

I would keep the hum pots. In the owners manual, they are listed as 100ohm wire-wound pots, but they do not give the wattage rating. 5W maybe?

http://nekhbet.com/Eico_ST40_Owners.pdf

[retox]

one of the pots fell apart, i am not sure what to replace them with. Will any 100 ohm pot work?

[kheper]

They need to be wire-wound and have a sufficient wattage rating. Don't use cheap, rat shack 1/2 W pots.

You can remove them, but test the filament voltage after turning the amp on. It should be 6.3V.

[msmpe]

Check out Valve Wizard, he discusses hum pots.

I think you ghave a couple options. Change the rating plus/minus 50 as long as they're the same. Change to two matched 50 ohm ww resistors grounded. But then you lose the balancing aspect.

[kheper]

Check out Valve Wizard, he discusses hum pots.

In this page, he has a few more tricks on suppressing hum.

http://www.valvewizard.co.uk/heater.html

[retox]

I have made some progress, though it is moving slow. I have some pictures I could post but not sure if I should use a site like photo bucket or if there is a preferred method on diytube. Any help would be great. Once I post the pictures then I have a couple of questions.

Thanks

[battradio]

I use photobucket here all the time

[retox]

Ok, I added photos to photobucket and this is the link...

http://s1081.beta.photobucket.com/user/retox77/library/EICO%20Mods

[retox]

thanks battradio, this works nice

[retox]

My question is about the hum pots, since i am building three boxes, the power supply, and two separate amps is it ok to put the hum pots in the amp boxes?