1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

Eico ST40.....

Discussion in 'Eico Place' started by Cogsinister, Oct 27, 2012.

  1. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    This amp was bought off the local internet classifieds by a friend of mine the other day, the owner said it "worked fine"

    My friend passed it to me to clean it up deoxit the thing and generally see what was what.

    On opening the top i noticed a missing tube i take this to be a 5AR4 rectifier tube... see the pics below.

    I cleaned it out a lot and i took the cans off the 12AX7's to photograph them.

    [​IMG]


    [​IMG]



    The rest of the tubes seem original and LOOK ok, the power tubes are made for Eico by Westinghouse, and all the rest are made by Mullard.

    I have little idea about tube amps so what would be the best replacement for the missing tube at a decent price ?

    My friend paid $95 for the amp by the way.
     
    Last edited: Oct 27, 2012

     

    Please register to disable this ad.

  2. Ohighway

    Ohighway Wannabe Minimalist Subscriber

    That big 6L6 doesn't belong in there unless the amp has been modded.... and even if it was ALL should be 6L6. The amp originally came with 7591 output tubes, and it appears 3 of 4 are exactly that.
     
  3. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    Right, i thought it looked out of place.

    So one of those tubes will need to be ordered as well i take it ?
     
  4. hionfi

    hionfi Well-Known Member

    Messages:
    738
    It does use 7591's, the amp looks clean and for $90 with Mullards your friend got a nice deal.
     
  5. Ohighway

    Ohighway Wannabe Minimalist Subscriber

    Correct. Best would be to determine the condition of all the tubes to make sure they're useable. Then place your order.

    More of concern is whether the previous owner actually modded the circuit to run that 6L6 or not. If he did then the mod needs to be undone to run the correct 7591.

    Also, if it was advertised as working then it damn well should have come with that 5ar4, and those aren't exactly cheap tubes. Some compensation is due to your friend for the fact that it's missing.
     
  6. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    It looks clean now after i vacuumed it out and polished the tubes, even the mismatched one !!

    :)
     

     

    Please register to disable this ad.

  7. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    I was just on the phone to my friend telling him to get on the the seller and ask him for some sort of redress.
     
  8. Ohighway

    Ohighway Wannabe Minimalist Subscriber

    Only other thing that comes to mind is if the PO did some mod to it to install a solid state rectifier in place of the 5ar4. If that was done then I guess it could work, though most folks would agree that's not the best way to go.
     
  9. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    I will take the bottom cover off later and have a look, pics will follow.
     
  10. GordonW

    GordonW Speakerfixer Subscriber

    Messages:
    17,867
    Location:
    Marietta/Moultrie GA USA
    One other detail, is that the four small tubes nearest the rectifier, are supposed to be 7247 tubes, NOT 12AX7s. Be sure someone hasn't substituted those! 12AX7s will actually make sound, but it will NOT work CORRECTLY... the 7247 has 1/2 like a 12AX7, and 1/2 like a 12AU7- the half that's supposed to be like the 12AU7 will obviously not bias up correctly...

    Also- speaking from personal experience- unless a significantly big power resistor was also installed to drop voltage, using solid state diodes in these, will cause an increase in B+ voltage. In mine, the B+ was like 455v, with a solid-state replacement rectifier! At that voltage, the output tubes were trying to draw like 62ma each! That's a dissipation of over 27 watts in each output- in a tube (7591) that's only rated for 19 watts dissipation each...

    I wound up putting a 5U4GB rectifier in mine- I found one that was only 4.5" tall from the bottom of the base to the top of the bulb, and that fit fine under the cover ("standard" 5" 5U4s won't fit without touching the metal cover!). I was concerned about the extra heater current the 5U4 draws compared to the 5AR4- but, I measured the heater voltage with the 5U4 in place, and got 5.03v, with the output of the variac set at 117v. At this time, the 6.3v heater (for the rest of the tubes) was 6.34v. Given that, I concluded the PT had enough capacity to do it's job under the increased heater current. With the 5U4, I wound up with right at 390v B+ at 120v in from the wall, and about 50ma of cathode current per 7591, using the stock 125 ohm cathode resistors- subtracting out the 13.7v of cathode bias voltage to get about 376.3v across each tube, that's 18.8 watts per tube. High, but within specs, and with decently rugged 7591s (the EH 7591A is preferable, since they have larger plates than old-stock or JJ), it should be just fine...

    If you wanted to correct the voltage (down to about 390-400v or so), with a solid-state rectifier, you'd need about a 300 ohm, 50 watt resistor. 330 ohms would work too. That'd be a big sucker to stash somewhere- it's dissipating like 17 watts of heat, continuously... so it needs breathing room!

    Regards,
    Gordon.
     
    Last edited: Oct 27, 2012
  11. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    Bottom cover off......

    Pics with the bottom cover off.....

    [​IMG]

    [​IMG]

    Looks like there was some sort of mod under the rectifier base.

    Back later, the wife just got home from her business trip.....
     

     

    Please register to disable this ad.

  12. derekva

    derekva This ain't no picnic... Subscriber

    Messages:
    4,842
    Location:
    Woodinville, WA
    Yup - someone made it into a solid-state rectified ST-40 (and should have taped off the two 5V taps...including the one that's being shared with the downstream end of the diodes as well as the power resistor and the HV B+ lead). If you have decent voltage on the 5V taps it would be good to revert it to the original tube rectification. However, before you do anything else you need to (1) get a matched set of 7591 tubes and (2) replace all those old capacitors, including the two multi-section can caps.

    -D
     
  13. jaymanaa

    jaymanaa RIP 1961-2018 Subscriber

    Messages:
    54,327
    Location:
    OZ
    Looks like a nice clean amp, and if it survived everything some knucklehead could throw at it, I'd say it deserves a complete resto. :thmbsp:
     
  14. Ken Boyd

    Ken Boyd AK Subscriber Subscriber

    Messages:
    15,988
    Location:
    Florida
    The tubes alone are worth much more than your friend paid for the amp, and I know the seller told your friend it worked but redress seems a bit out of line. If I was the seller I would say sorry refund your friend the money, pull the tubes sell them and then sell the amp without any tubes on ebay for a few hundred dollars. Now I probably wouldn't really do that, but I was just making a point, at the price your friend paid for it, he should just be smiling for what he got for the price he paid.

    As far as purchasing just one 7951 tube to replace the one missing, I am not sure it wouldn't work, but you might check around all the ones I always purchased were quads at a time, I think they need to fairly close to each other on the tube tester to insure they biais up right or balance correctly. Ask someone here that knows about that.
     
  15. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    Point well taken about the great deal my friend got, i had no idea what these amps were worth.

    Right, so we need to undo the mod with the diodes, get a nice new rectifier tube, and a matched pair of 7951's.

    Can anyone guide me in what kinds of caps to use, as i want to replace them all.

    Also, how do i wire up the speakers, the markings on the back are rather baffling ?

    Thanks in advance everyone for your help, Audiokarma rules !!!!!!
     
  16. Andyman

    Andyman Scroungus Stereophilus Subscriber

    Messages:
    28,534
    Location:
    15 miles North of Canada
    Why? Because he only got $175 worth of tubes, plus the iron and chassis for his $95 instead of $250 worth of tubes???


    He got a sweet deal that could of been sweeter, but he's still way ahead and darn lucky.
     

     

    Please register to disable this ad.

  17. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    Like i said, both me and my friend have little idea of the value of tube gear these days, the last tubes i came into contact were on the radiogramme in the front room of our house in England in about 1966 !!!!
     
  18. guitbanger

    guitbanger Active Member

    Messages:
    382
    The socket containing the 6L6 has been rewired for it. Rewire it the same as the other sockets containing the 7591's. Plug in a 7591 and a 5AR4 and give'er a listen. If you hear any hum turn it off and begin restoration.
     
  19. Shadowdog

    Shadowdog Super Member

    Messages:
    2,788
    Location:
    Flin Flon, Manitoba
    Great deal!

    As you mentioned replacing all the caps is a good idea as you risk the transformers. Good to measure the resistors also as they can drift off, clean pots and tube pins and sockets.

    Also never run it without a load (speakers or 16 ohms 25W resistors) or you risk the output trans.

    Randy
     
  20. Cogsinister

    Cogsinister Super Member

    Messages:
    3,586
    Location:
    Atlantic Canada.
    Where do i put the speaker wires on those terminals at the back ?
     

Share This Page