Help... Fender 6G15 reverb AKA is something wrong with my p. transformer???

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, Feb 18, 2006.

  1. brad347

    brad347 Member

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    I'm posting this here because this thing is basically an amp...

    OK, so the three electrolytics on the board of by fender reverb unit needed replacing, so I replaced them. No sweat...

    Anyhow, I seem to have made a mistake. All are inserted correctly, and I checked after replacing each one (I was working right-to-left looking from the back of the cabinet) and the first two I did fine. Well, after the last one, the unit won't power up. Fuse is fine, not blown.

    I noticed that one of the twisted pair of black wires coming off the 12AT7 toward the 6K6 (heater supply wires?) has come disconnected... the other is connected to pin 2 of the 6K6.

    The wire in question is connected to pin 9 of the 12AT7 on one end... what I need to know is where the other end should connect to. Any help (especially pics!) would be appreciated. I have the schematic and layout, but they don't show how the heater supply wires are wired up.

    I've done a few repairs but I'm sort of a novice so sorry if this is a dumb question. Thanks.
     
  2. brad347

    brad347 Member

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    well, I think I figured out where this heater wire is supposed to go to... I've got it temporarily connected to pin 7 of the 6K6.

    Also, when I switch the power on, nothing seems to happen (pilot light doesn't come on, etc) BUT the electrolytic filter caps charge... I know because I discharged them and they made a spark. Then I repeated the procedure and they still sparked.

    I know it sounds dumb, but I also checked the light bulb in the pilot light, it's good.

    I sure hope I didn't somehow fry my power trans.

    any help?
     
  3. John Phillips

    John Phillips Member

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    It seems unlikely if a wire just came off (provided it didn't short against something - and even then it should have blown the fuse before the PT).

    Pins 2 and 7 are correct for the filament supply on the 6K6, which go to pins 4/5 and 9 of the preamp tubes.

    If the filter caps charge (but be aware that they can 'recover' charge after they've been discharged, although it's small and doesn't usually cause a spark more than once) and the filaments in the tubes light up, and the fuse doesn't blow (also meter that in case it looks good but isn't), the PT is fine. If the bulb then doesn't light, check the bulb (even if it looks OK) and the connections in the holder.

    If the tubes don't light but the caps definitely charge (meter the voltage if you have a multimeter safe for 300+ volts), you have a problem.
     
  4. brad347

    brad347 Member

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    The wire didn't just come off. I probably bumped it when I was removing the old caps... one of them was a bit stubborn and i had to jiggle it a little and I must've knocked it loose(?)

    At any rate, earlier in the day when I was playing with it, before I even opened it at all, I noticed the light wavering... getting dimmer and brighter intermittently... my amp which was on the same circuit was not doing this... I thought this was curious.

    The caps definitely charge. The filaments in the tube DO NOT light up. The light bulb has been tested in another amp. I also tried another fuse.

    Looks like I have a problem...

    Do I have to go with a Mercury Magnetics transformer, or should I go with the Mojo musical supply replacement (which is half the cost). How critical is the tone of the power transformer to the reverb unit?
     
  5. brad347

    brad347 Member

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    In light of this fact I'm wondering if either (a) the wire didn't get knocked loose somehow in shipping or in a previous repair, and was only making intermittent contact, or (b) the power trans. was on its way out anyhow. I only got this thing in the mail the other day. :(

    I'm wondering to just what extent I should blame myself for this one!
     
  6. John Phillips

    John Phillips Member

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    Now I've thought about it, I find it highly unlikely that you could have just 'knocked the wire loose'. I've never seen that sort of thing happen, on something like a vintage Fender. The wiring is sturdy and well-soldered. My honest opinion is that it may have been deliberately disconnected during attempted troubleshooting and then reattached badly - there clearly is a problem with the filament circuit and someone may have been trying to tell if it was a tube socket short or something. I'm guessing the PT may have been on the way out anyhow. I can't say so with enough certainty to definitely blame the seller though.

    I'm not familiar enough with these things (I've only worked on one original, they're extremely rare in this country) to know for sure, but I think it's unlikely that the choice of PT will significantly affect the tone, as long as its output voltages are correct. The whole unit runs in pure Class A (all the stages are single-ended), so the current draw is always constant and sag is not an issue. The dry signal goes through only one preamp tube stage (the PT will have no effect on the tone of this) and the wet via the whole reverb circuit including the tank (which must have a much greater effect on the tone) anyway.

    I would go with the Mojo personally, if it's a lot cheaper.
     
  7. brad347

    brad347 Member

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  8. slider313

    slider313 Silver Supporting Member

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    Before buying a new transformer,go back over the caps you replaced and make sure all wires in the eyelets with the caps are making secure connections. Use a solder vacuum and clean out old solder.The 6g15 layout I have shows the following wire layout for the 12AT7; pin 1 to the 100k resistor on the board showing 105 volts. Pin 2 going under the board to the input.Pin 3 going to the board to the 25/25 cap in parralell with the 1500 ohm resistor showing 1.8 volts.Pins 4+5 tied together.Pin 6 to the board with the other 100k resistor and a .002 cap in parralell showing 120 volts.Pin 7 under the board to the dwell pot.Pin 8 to the board to the 1500 ohm resistor tied to the top of the ground end of the 25/25 in parralell with the 1000 ohm 2 watt resistor. The positive end of the 1000 ohm and the 25/25 cap goes to pin 8 of your 6K6. I hope this helps. Mike
     
  9. brad347

    brad347 Member

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    Crisis averted, problem solved with minimal (zero) expense. Took it to Andy Fuchs today (hell of a guy, check out his amps!) and he noticed that the two terminals at the base of the lamp assembly had gotten twisted around and were touching (shorting).

    A simple twist and all was well!

    Thanks for all of your advice... there's another one for the books when troubleshooting power supply problems in a fender amp.
     
  10. John Phillips

    John Phillips Member

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    Have you got the correct value fuse in? A dead short on the filament circuit should take it out immediately.

    Funny - I did suggest to check the bulb holder in my first post, but I was expecting you to find an open circuit not a short... :)

    It's nice to be wrong, especially when it means not having to replace something expensive.
     
  11. brad347

    brad347 Member

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    sure did have the correct value fuse in! (750 mA) and it wasn't blown!
     

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