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Power Transformer substitute for Symphonic 6BQ5 amplifier?

Discussion in 'Tube Audio' started by HiFi-Stereo, Nov 9, 2018.

  1. HiFi-Stereo

    HiFi-Stereo Active Member

    Messages:
    217
    Picked up this neat symphonic console and it had a nice little 6BQ5 amp inside!
    I was told the power transformer is shorted though:
    anyone know if the Hammond 270ex would work as a suitable replacement?
    IMG_0055.JPG IMG_0057.JPG IMG_0055.JPG IMG_0057.JPG ?
     

     

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  2. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

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    that must be a typo or a missing number on the specs shown for the PT. Probably 270vac. One thing for sure is a 70 vct thru a 5y3 is not going to output 330vdc.
    I would suggest checking epay, as there are lots of PT for sale and you might get lucky with one that is close to your needs. You could use a trans with lower output an use a different PS scheme like a voltage doubler or a higher output rectifier tube like 6ca4 or 5v4. Also adding a choke or chokes or another RC to adjust the voltage to get the required B+. There are many different options to get to your requirements.
     
  3. Mike Stehr

    Mike Stehr Poverty Audio

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    I think that is a typo...it could be 570 VAC with both secondary taps measured, or around 290-299 from CT to a secondary leg. Keep in mind he mentioned it's a console stereo, and the PS transformer may be providing power for other things than just the amplifier. That may be why it has a 4.8 amp 6.3 volt heater winding, to power all the tubes for the console stereo.
    It appears the heaters are wired in series.

    The Hammond 270EX look's rather close in specs I would guess...a little higher in secondary DCR if that matters...plus or minus 20%
     
  4. gadget73

    gadget73 junk junkie Subscriber

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    6CA4 rectifier, so 570v end to end makes sense. Thats 285-0-285, which is reasonable to get 330 vdc output from the rectifier.
     
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  5. Brice

    Brice AK Subscriber Subscriber

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    Based on another amp I have, I calculated 275-0-275, so Gadget is spot on, but not more than 285.
     
  6. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

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    The document shows ( 5. 70vct @ .145A dc) So, 570 vct would be too much. My guess is 270 vct which is 540 vac end to end.
     

     

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  7. cademan

    cademan Addicted Member

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    Besides the typo in the schematic, it might not have a bad power transformer. What is your level of expertise?

    Just because someone told you it was bad, don't believe it until you can do some basic troubleshooting.

    If the transformer is smoking:
    1) It could be shorted but is rare.
    2) It has no fuse so if a filter capacitor shorts, it will also cause the power transformer to smoke.
    3) Could be a shorted 6CA4 rectifier tube.
     
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  8. Mike Stehr

    Mike Stehr Poverty Audio

    Messages:
    1,620
    Location:
    East Wa.
    My bad. I should have read your post better. I thought 270VAC from the full winding, not from CT to each secondary.

    The document appears to be a Sams Photofact, and they do have occasional errors. The best way to find out is to measure the PS xformer unloaded.
    The input voltage on the primary is for 117 volts. So if used with today's 120 line input voltage, the will be some extra AC voltage on the secondary.
     
  9. HiFi-Stereo

    HiFi-Stereo Active Member

    Messages:
    217
    Thanks for all the info! I thought that sounded low, 5 .70vct.
    So it's probably 270-0-270.
    Of course I can try to test it, never did that before; I'll read up on how to measure it correctly...
     
  10. gadget73

    gadget73 junk junkie Subscriber

    Messages:
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    Southern NJ
    usually you can just pull the tubes and power it with a dim bulb tester if it passes a basic ohm check. You should get under 100 ohms or so across the prongs on the power cord, and infinite resistance to the chassis. When powered, a good transformer should draw nearly zero current, not get hot, and have output on the secondary windings. If it passes those tests and doesn't reek of incinerated dead dinosaur, its probably fine. Transformers that are shorted or have been badly overheated smell absolutely terrible.
     
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  11. primosounds

    primosounds SE KT120 w/ 6J5G drivers. Subscriber

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    No biggie. Those Sam's fotofacts are notoriously full of mistakes and misprints. It is advisable that one uses the schematic from the manufacturer and Sam's as a back up. Although sometimes it is the only schematic that one can find in which case you need to trust the original build components and use the Sam's to verify components that are missing or when the values on the component is no longer legible.
     

     

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  12. Tom Bavis

    Tom Bavis Audiophool Subscriber

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    If the power transformer is bad, you likely have an open winding. Usually they short, but with no fuse, the transformer becomes the fuse and primary burns open - though maybe there was a fuse in the tuner. Check for resistance across the AC plug (around 10 Ohms), across the full secondary (130 Ohms). If those are OK, you can plug it in with a light bulb in series (AKA dim bulb tester) and if the bulb lights brightly, you have a shorted winding in the transformer.
     

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