Fender 135W Twin question - - OUTPUT TRANNY Pt. II

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mullytron, Dec 17, 2009.

  1. mullytron

    mullytron Member

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    Hey guys, you were all helpful when I asked before, I had a wonky Twin, and I determined that half the OT primary was gone. So I got a replacement (Heyboer), and dropped it in. I played it for a half hour, it was FINE, loud, Fender-y, a little hum. Sounded like a silverface Twin to me.

    So the guy comes to get it today, and plays thru it for like 5 minutes before the smoke gets out. F*CK! I felt stupid.

    One of the original 1W 470R screen grids was blown, on the same leg as one that had been previously replaced. I am going to get back into it tonight, but my guess is that the OT primary is gone again. RATS. Clearly something upstream that I didn't fix caused the same failure as before. Any idears? Thanks in advance.
     
  2. Fuchsaudio

    Fuchsaudio Member

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    If you didn't replace the power tubes before, do so to be safe. It's tough to swallow that you toasted the new output. Heyboers' are solid as they come, and my gut tells me it's just not likely IMHO. Also check or replace the coupling caps from PI to power tubes, for safety.

    Don't feel bad, I plugged a guys newly repaired amp in (one of mine thankfully) to show him how it worked, but didn't realize my bench AC cord (isolation and step up transformer with variac) was set to 240, to bias up a few Euro amps....by time I noticed the pilot light being as bright as the sun, boom smoke.....oops. Honest mistake, and he was cool with it....but boy, shows that even with burn-in, things happen.....good luck with it !
     
  3. Mike9

    Mike9 Supporting Member

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    Isn't the 135 watt Twin Ultra Linear?
     
  4. 5er Driver

    5er Driver Member

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    Yep. :munch
     
  5. Mike9

    Mike9 Supporting Member

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    Are you thinking what I'm thinking?

    So mullytron - when you ordered the OT from Heyboer did you spec it as an Ultra Linear OT?
     
  6. 5er Driver

    5er Driver Member

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    Yep. These Twins are clinical strerile clean when they are stock. I've got one because I want that clean sometimes. But here's a twist, I pulled the 4 6L6GCs out of it and put in 2 TAD ToneBones with EL84s. Nice touch and they do saturate nicely at a tolerable volume. The 2 pentode ToneBones produce about 22 watts output. A big dif from the stock 135 watts.
     
  7. Mutronics

    Mutronics Member

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    So mullytron, was this modded to run NON-ultralinear?
    I assume it is stock, and you got the correct transformer???!

    How's the bias across the 4 6l6s? Could you wire some 1R resistors in the cathode legs of the 6l6s so you can easily monitor the quiescent current?

    As Lord Andy of Fuchs noted, the Heyboers are built solidly and I'd put my money on the transformer outlasting a(n original?) 470R 1W if things hotted up.

    Sub some known good 6l6s (maybe just a single pair to begin??) and see what happens.

    scx
     
  8. Fuchsaudio

    Fuchsaudio Member

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    Lord Andy ?? lol, I'm hardly worthy sir...

    We should confirm that A) he installed the proper UL-style transformer and B) if he did not, then the amp was properly modified to run a non-UL transformer. The UL iron and switching to non UL requires adding a choke (or at least a choke-resistor), and modifying the power supply accordingly. Not major, and worth doing, but if you switch the output without doing these changes, you could get hum and less than ideal performance.

    I have spoken to him in a few PM's and suggested the 1-ohms (minimally one resistor on the output transformer CT) or ideally one per tube. I suggested he check the
    coupling caps feeding the power tubes, and the bias feed resistors as well.

    It's tough to speculate without knowing more details.
     
  9. Mutronics

    Mutronics Member

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    Yep, sorry I meant to type 'That dumb knave, Fuchs...' ;)

    Amen

    An lo, fwom the West came Saint Fuchs beawing pearls of wisdom and dispensing DIY joy hiverto unknown in veese parts...

    Speaking for myself you unnerstan, I find it pretty easy to speculate...


    scx
     
  10. donnyjaguar

    donnyjaguar Member

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    This is another reason I don't miss the opportunity to extol the virtues of AC line MOVs in tube equipment. :) They've saved my life, or more correctly circuits, on a few very similar occasions!
     
  11. mullytron

    mullytron Member

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    When I looked around, I found a few different conflicting part #s for the UL transformer, but I gave Heyboer the number off the OEM end bell (013691, since we're talkin'), so I assume it's correct. They were very upfront and cool, said it was not a common build for them, and would take a while to get the spec since they hadn't built one in a while, and got it to me in a few weeks. Heyboer is a stand-up operation, and they're great to deal with.

    The amp was dead original SF/70s/135W, other than someone having replaced a single screen grid resistor already. When I asked the owner if he knew when/why it was replaced, he had no idea. I poked around, and couldn't find any other damage or voltages that were off, and the amp actually sounded great for a half hour with the new OT, I think it was just another screen grid resistor failing (I hope...). Post-smoke, the Heyboer metered out fine, as far as I can tell, but I think it got hot again. I got some big 5W 470s, and will R&R all the screen grids. Any reason not to R&R the OEM 1500ohm also? I have some orange drops for the .01 coupling caps, and plan to check the old ones once I get them out.

    I think I will also try the 1ohms in series with the primary halves, and check the currents. Has anyone else had good luck finding problems with this method? I figure all that current hsa to be coming from somewhere...
     
    Last edited: Dec 22, 2009
  12. mullytron

    mullytron Member

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    That's a hip mod. Thanks for the info.
     
  13. mullytron

    mullytron Member

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    Just closing the loop here.

    Found some 5W 470Rs and some metal film 1500Rs in NYC on Xmas eve (PM if you want the super-secret local Manhattan parts source...), so I redid all the grid connections on the power tubes, and she's purring again. If by "purring" I can be understood to mean generating super loud, hard clean tones, bordering on brittle if you set the knobs wrong...

    Thanks for the input fellas.
     
  14. Mutronics

    Mutronics Member

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    Rich, at a shop named after a certain city in Yorkshire, perchance?
    He is very cool indeed!

    Well done, and you're welcome!

    Rock and roll!

    scx
     
  15. donnyjaguar

    donnyjaguar Member

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    I generally replace Sgrid resistors as a matter of course when major repairs of the final stage are called for. I've seen power resistors that go open-circuit when they get hotted up then re-connect as they cool thus cycling forever. Even in amplifiers that are otherwise okay I often replace them. I learned this trick fixing solid state equipment with multiple output devices that use power resistors to balance output currents.
     

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