Eico ST-40, Redo/Modo
- Thread starter Kegger
- Start date
tube-a-lou
Addicted Member
Hi,
Wow nice job on that, I was wondering how the phono section sounds on those
the ST-40??
Tube
Wow nice job on that, I was wondering how the phono section sounds on those
the ST-40??
Tube
Little more current through the phase splitter, little lower output impedance driving the 6L6 grids... probably would lead to a little more available clean voltage swing to drive the 6L6s, and maybe a little lower distortion from the driver tube itself...
Regards,
Gordon.
Switched the resistors to 56K and the amplifier still sounds as great as ever-- my ear can't really tell the difference, though I think there's more gain.
Building this has been a good learning experience for me, on top of everything else. St-40's are relatively inexpensive, easy to come by, and have plenty of space to work with. I've now got two ST40's that I've converted to straight power amps (not needing the preamp functions at all). One runs the original amp circuit with 7591's, the other runs Kegger's 6L6 mods, using NOS Russian 6P3S-E tubes.
Interesting to compare. The original circuit, completely rebuilt, with 7591's is more powerful, plenty detailed, a little boomy in the bass. Kegger's circuit is sweeter sounding, especially in the midrange, very clean, easy to listen to, and tighter in the bass. Both sound terrific.
I've also rebuilt an Eico ST-70 with the advice of Dave Gillespie on the Eico forum. I kept the preamp functions but bypassed the tone controls, took away the crazy center speaker stuff, and adjusted the feedback. That sounds pretty great, too, more powerful than both ST40's, but maybe a little 'old fashioned' in flavor.
All three Eicos are brutally ugly, and all three are a great match for my Quad ESL 57 speakers. Of the three, I prefer listening to Kegger's 6L6 version; it's my first experience with triode push-pull, no feedback, and it's very pleasing.
Time for me to stop tinkering and just enjoy the music. Thanks again, Kegger.
Building this has been a good learning experience for me, on top of everything else. St-40's are relatively inexpensive, easy to come by, and have plenty of space to work with. I've now got two ST40's that I've converted to straight power amps (not needing the preamp functions at all). One runs the original amp circuit with 7591's, the other runs Kegger's 6L6 mods, using NOS Russian 6P3S-E tubes.
Interesting to compare. The original circuit, completely rebuilt, with 7591's is more powerful, plenty detailed, a little boomy in the bass. Kegger's circuit is sweeter sounding, especially in the midrange, very clean, easy to listen to, and tighter in the bass. Both sound terrific.
I've also rebuilt an Eico ST-70 with the advice of Dave Gillespie on the Eico forum. I kept the preamp functions but bypassed the tone controls, took away the crazy center speaker stuff, and adjusted the feedback. That sounds pretty great, too, more powerful than both ST40's, but maybe a little 'old fashioned' in flavor.
All three Eicos are brutally ugly, and all three are a great match for my Quad ESL 57 speakers. Of the three, I prefer listening to Kegger's 6L6 version; it's my first experience with triode push-pull, no feedback, and it's very pleasing.
Time for me to stop tinkering and just enjoy the music. Thanks again, Kegger.
Kegger
R.I.P. 1/12/1966 - 6/1/2017
I've built both a KT88 one Jay and a 6L6, worked on a couple others.
It does workout pretty darn well, I do prefer Triode or UL, then either
no feedback or "limited" feedback, but pentode with regulated screens
and decent feedback is quite nice, and yu gotta like glow of VR tubes..
It does workout pretty darn well, I do prefer Triode or UL, then either
no feedback or "limited" feedback, but pentode with regulated screens
and decent feedback is quite nice, and yu gotta like glow of VR tubes..
jaymanaa
RIP 1961-2018
Yeh, I have one that I need to try out for awhile. 6AN8s, 7591s, and Acro outputs. Of course the Acros have taps, they just aren't being used. I rebuilt this amp a couple years ago, but never did bring it upstairs (too busy I guess). This thread and your pair of 5AR4s made me think of it because it uses a pair of 5AR4s. I think I even have a pair of "metal base" Amperex put back for it.:scratch2:
Dandy
Addicted Member
I have a couple of queries relating to this old but excellent thread.
I want to keep running the 7591s but upgrade the power supply, which still has the original cans (all other caps have been replaced). Can I use the power supply mod as it is with the 7591s?
Can I replace the 2 12DW7s with 12AT7s as Kegger describes? If so, which R and C changes do I not make?
I want to keep running the 7591s but upgrade the power supply, which still has the original cans (all other caps have been replaced). Can I use the power supply mod as it is with the 7591s?
Can I replace the 2 12DW7s with 12AT7s as Kegger describes? If so, which R and C changes do I not make?
Dandy
Addicted Member
I connected the Tape 1 and Tape 2 sockets direct to the grid of the 12DW7 driver and agree that this alone is a nice way of making it usable as an amp.
Am curious hear the sound of this amp with 6L6s, and like the sound of having two 12DW7s and four 7591s to spare, so will do the full mod.
Am curious hear the sound of this amp with 6L6s, and like the sound of having two 12DW7s and four 7591s to spare, so will do the full mod.
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Sorry Dandy, hadn't noticed any action on this thread.
As to your previous questions, the supply should be just fine for the 7591
but I'd probably use an 5965 in place of the 12AT7 with 7591 output tubes
and either tweak the supply dropper or plate to set the voltage to the input.
I think you'll like the 6L6/12AT7 setup, an like you say it frees up some tubes.
As to your previous questions, the supply should be just fine for the 7591
but I'd probably use an 5965 in place of the 12AT7 with 7591 output tubes
and either tweak the supply dropper or plate to set the voltage to the input.
I think you'll like the 6L6/12AT7 setup, an like you say it frees up some tubes.
Dandy
Addicted Member
Thanks for your advice Keg. Your revision seems really nice and my ST40 is due for a proper clean up.
Another dumb question: are Russian 6n3cEs and similar Mesa 5881s suitable for use here? I see you used as 6L6GCs. I have a random collection of nice old US made 6L6GCs, but all different makes.
Another dumb question: are Russian 6n3cEs and similar Mesa 5881s suitable for use here? I see you used as 6L6GCs. I have a random collection of nice old US made 6L6GCs, but all different makes.
Dandy
Addicted Member
Thanks. I'll report back when I've done it.
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Dandy
Addicted Member
Just finished the work on mine, to be tested tomorrow. I returned to this thread to print out the schematics of your mods, so I can check each change, and see that you did post pics of your work. I had to be creative to find space to add two more big cans with new electrolytics on the top.
Dandy
Addicted Member
I've checked my work and powered it up and it's sounding very good. With sensitive speakers you have to go right up to the speakers to detect a soft hiss and hum. (With no ground bus, there is still grounding to points all over the chassis.)
I made a few value changes to some parts because of availablity:
For R70, 72 etc I used 1.3k resistors in place of 1.2k to the 6L6 grids
In the power supply, I used a first resistor of 82ohms in place of 75 in the first place in the power supply and a 33uf cap for C36A.
I would appreciate any comment on my voltages. They are a bit high, because the 'mains' voltage from my drop down transformer is 125V. Are they too high?
Power supply: 400V at the GZ34, 373V at U to output centre taps (should be 355) and 347V at test point (should be 320).
Plates of the 12AT7: 111V/115V (pin 6, should be 96) but 184/181V (pin 1, should 195). I kept the 82k resistors there. Cathodes of 12AT7: 1.05V at pin 8 (should be 1V) and 110V (should be 97.5V).
Plates of 6L6s: around 362V (should be 352).
I'm pleased with this mod, despite the ugliness of the amp, made worse by raindrop corrosion on the top of the chassis. That corrosion meant that I didn't feel too bad about cutting holes for the extra caps.
I attach a couple of pics showing the mounting of two extra caps to the top of the chassis. The feedback wires were taped off (in case I needed them quickly). Screens are still off the small tubes so I could see if they were okay.
I made a few value changes to some parts because of availablity:
For R70, 72 etc I used 1.3k resistors in place of 1.2k to the 6L6 grids
In the power supply, I used a first resistor of 82ohms in place of 75 in the first place in the power supply and a 33uf cap for C36A.
I would appreciate any comment on my voltages. They are a bit high, because the 'mains' voltage from my drop down transformer is 125V. Are they too high?
Power supply: 400V at the GZ34, 373V at U to output centre taps (should be 355) and 347V at test point (should be 320).
Plates of the 12AT7: 111V/115V (pin 6, should be 96) but 184/181V (pin 1, should 195). I kept the 82k resistors there. Cathodes of 12AT7: 1.05V at pin 8 (should be 1V) and 110V (should be 97.5V).
Plates of 6L6s: around 362V (should be 352).
I'm pleased with this mod, despite the ugliness of the amp, made worse by raindrop corrosion on the top of the chassis. That corrosion meant that I didn't feel too bad about cutting holes for the extra caps.
I attach a couple of pics showing the mounting of two extra caps to the top of the chassis. The feedback wires were taped off (in case I needed them quickly). Screens are still off the small tubes so I could see if they were okay.
Attachments
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