EICO ST 40 Kegger Mods ??? 6L6 - 6V6- OTHER?

[nerdorama]

DaveG says on this page that the ST-40 OPT is 8250 ohms.

7591A tube comparisons

 

[S Swanson]

well the st 40 Eicos - Both -sets= that is all 6 - are the stock units the PTs are 30047e and s30047-6 --- the outputs transformers are marked 32020-3 138415E st

 

[analogadict]

The conversion to 6V6/6L6 may have been the way to go in the intervening years when vintage 7591a were not easily available and new ones were not around. As new production is available since the last few years, I would suggest sticking with a 7591a circuit or the Fisher SA100 clone that successfully uses trafos meant for 7591a .

Regards,

 

[Andyman]

You HAD his amp -- i am guessing its down the road - when you say that a stand alone amplifier - should be considered- did you have something in mind , any particular design?

Nope, but Keg had no problem gutting stuff for fresh builds. I'm sure others have done so and a quick Google search should find something on any number of audio forums.

 

[ceylon]

for which types of 6L6 does is the kegger schematic a good fit?

 

[gadget73]

DaveG says on this page that the ST-40 OPT is 8250 ohms.

7591A tube comparisons

that would indeed make a good fit for a 6bq5 family output tube, if paired with a more appropriate power transformer. Maybe choke input on the stock could do it, not sure but it looks like it would want to be about 100 volts less than the stock ST-40. As-designed with a cap input though it doesn't make a lot of sense, especially since it would end up as an AB type amp and having big drop resistors in a power supply for an AB stage just doesn't work too well. Driver could honestly remain stock here, the voltage to drive a 6bq5 is basically the same as the stock config 7591, or any number of other 6bq5 front ends could be used.



in pentode mode if you basically drop the 6l6 in and bias it to about 45ma per-tube that would also get it done, needs about double the stock drive voltage which it may or may not be capable of without modification. It wouldn't be running too crazy for dissipation, about 370 across the tube assuming about 400 plate volts, and 45ma at 370v is under 17w dissipation. Any 6L6 family member would survive that.

Could also run it triode mode. I'm not into zero-feedback designs but in triode mode you could reduce or possibly lose the feedback. i would not advise that in pentode mode though.

 

[nerdorama]

DaveG's SA-100 clone uses 6BQ5's at over 400V B+ but with a controlled screen voltage via his EFB circuit. The ST-40 B+ is about 400V so could possibly work for 6BQ5's in the right circumstances.

 

[wlgrant6]

Here is an amp I built using the ST-40 transformers, and original eico tubes. One of the best sounding amps I have.

 

[S Swanson]

Here is an amp I built using the ST-40 transformers, and original eico tubes. One of the best sounding amps I have.

Lets Hear more ! what is the design ? yours ?

 

[wlgrant6]

I built it using the amp portion of the ST-40 schematic. Sounds much better using a separate solid state preamp, than with the original Eico preamp section. Love the 7591 tubes. IMO, it sounds better than the Dynaco ST-40 with EL34's

 

[spantou01]

DaveG's SA-100 clone uses 6BQ5's at over 400V B+ but with a controlled screen voltage via his EFB circuit. The ST-40 B+ is about 400V so could possibly work for 6BQ5's in the right circumstances.

If OP is interested in this direction of SA-100 clonish mods, in addition to DaveG’s, Jason R. Thorpe published the schematic he developed here on AK (I think, some time ago, but I can’t find it now). Jason used the AA-100 transformer set, but the Eico ST40 set is close enough. But the feedback will need to be revisited with different transformers of course.

 

[Lavane]

Nope, but Keg had no problem gutting stuff for fresh builds. I'm sure others have done so and a quick Google search should find something on any number of audio forums.

I've done that. I turned a Matnavox 175 6v6 amp into a 6L6 amp using 6SL7 drivers.
The power supply had just enough oomph to do it along with using both 6.3v heater windings. It didn't put out anymore power, though.

 

[S Swanson]

Something a bit more offbeat might be to convert to 7189a type tubes. You could look at Dave Gillespie’s SA-100 clone thread. ST-40 voltages are in the ballpark (a little low actually) and the 7247 fits perfectly.

Teaser Pics -- Fisher SA-100 clone in progress

With a number of members working on some version of this amp -- either by way of circuit, use of Fisher transformers, or just that it's a 6BQ5/7189 amp, I thought I would mark my progress on my own version of this great amp. I had mentioned the basic features in a thread kpaxfaq started for...

audiokarma.org

Looks like the EL 84 strategies are a viable alternative here --- I like the comfort of a design by Dave _ a respected designer - so this will be on the short list
of options - probably the better choice based on the documentation . so thank you for the link -
Other designs ( Thorpe design) and the simplified st 40 with the (7591 tubes) stand alone are attractive as well , again I have 2 sets of transformers - One set in the Chassis - original st 40 - and another set .-- Decisions decisions ---

 

[S Swanson]

I've done that. I turned a Matnavox 175 6v6 amp into a 6L6 amp using 6SL7 drivers.
The power supply had just enough oomph to do it along with using both 6.3v heater windings. It didn't put out anymore power, though.

So the limiting factor was the Power Supply -- A do over would look like what ?

 

[Dandy]

sounds very interesting - has anyone done this ? - i have both a Eico st 40 and just the transformers from another , - so two options with that iron.
Schematic --- Thread----

Yes, I did it with Kegger's patient guidance, as documented in the back half of his thread at the time. I was a novice builder at the time.

His thread is a kind of documentation of his mod, though written in typical Kegger style. He is still much missed by me, and I only knew him here on AK.

 

[Lavane]

So the limiting factor was the Power Supply -- A do over would look like what ?

Just a little different sound really. I had two 175 amps. One restored original other than what needed to be done to make it a stand alone amp.
It still had 6V6's and 6EU7 driver tubes. Both amps sounded good, the 6L6 amp sounded different is all. Maybe a little better sounding since a lot was changed including the out put transformers. I basically just the chassis and power transformer and built a new amp with them.

 

[spantou01]

@S Swanson
I wondered how your project turned out. How did you rebuild your Eico ST40 and how did you like it?