Fender Showman '63 tech questions AA763

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by donnyjaguar, May 6, 2015.

  1. donnyjaguar

    donnyjaguar Member

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    Howdy all, a friend asked me to check over his amplifier, which is in rather astonishing condition, and I have a couple of questions for those more familiar with this particular model. I replaced all the high-voltage electrolytics which measured okay but some were leaking electrolyte so it would appear they were caught in time. I replaced all resistors that were outside of 10% tolerance (screen-grid x4, 68k input) and re-tubed with matched set of Tung-$ol 6L6GC.

    I noticed that the NFB resistor was 100Ω whereas the schematic says for Single Showman (8Ω) it should be 47Ω. I piggybacked another 100Ω across it to bring it to 50Ω. I'm not sure why it would've had the 100Ω fitted, guessing error at factory? Thoughts?

    I also added a 22n across the 20n in the Tremelo circuit, which slows it down to about 1 cps. All the component values in the trem circuit measured fine so guessing all these amps have fast tremelo?

    Lastly, I did a baseline measurement with the old tubes (unmatched set) and got 69Watts. With the new set the best I could muster was 82Watts (1kHz spot tone, 8Ω load, before onset of clipping). Pretty close to the claimed 85 Watts but I mention because generally I've found Fender amps serve up what they claim power wise - albeit only just.

    Any input appreciated. I still have the amplifier here while I *break in* the new tubes. ;)
     
  2. mark norwine

    mark norwine Member

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    Sounds like you did well!

    I would only offer the following:

    1.) that wasn't the NFB resistor, that was the inverter tail resistor.....although it certainly acts as R2 in the NFB voltage divider. You are correct: all 4- and 2- ohm amps used a 100-ohm resistor while 8-ohm amps used a 47. Personally, I'd use the right component; I'm not a fan of making "composite values" with multiple components. But if all you have is 100-ohm resistors, then "50-ohm" is certainly close enough. That said:

    1a.) the 100-ohm tail could have been an error.....or this amp *might* have a 4-ohm OT. Long before the "Dual showman" hit the scene, Fender put 4-ohm OT's in this platform. Check the codes on the OT to be sure.

    2.) you said you replaced the high-voltage electrolytics. Good! But you need to change *ALL* the electrolytics...the bias cap and the bypass caps. Just do it.

    3.) If your ac output calculates to 82W at the onset of clipping (as observed on a scope...not guessed at), then you're more than likely OK. Also, and now that I think about it, it would also stand to reason that you can discard what I said in "1a" because a mis-match would have resulted in a lower calculation.

    4.) The slightly-lower power could also be because of the output tube choice. I don't know what they are (other than they were made by Reflektor / Mike Mathews), but I know for certain what they are NOT: They're most certainly not Tung-Sol 6L6GCs! Others will correct me if I'm wrong, but I don't think TS ever made a GC. These days, that is just a marketing name used by MM / EH.
     
    Last edited: May 6, 2015
  3. pdf64

    pdf64 Member

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    Just in case / for curiosity's sake, maybe check it into a 4 ohm load, or check the OT ratio.
     
  4. gldtp99

    gldtp99 Member

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    What is the p/n of the OT ? Both 4 and 8 ohm output OT's can be found in early Showman amps.
    125A29A is the 4 ohm OT----- 125A30A is the 8 ohm OT................. in most cases.
    Here is a pic of the OT in my '64 AA763 Showman with factory 7355 output tubes----- p/n 125A18A, different OT than either the 4 or 8 ohm 6L6GC amps:

    [​IMG]

    [​IMG]

    [​IMG]
     
    Last edited: May 6, 2015
  5. donnyjaguar

    donnyjaguar Member

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    Good data, thanks. I suppose I should've mentioned I checked the turns ratio of the output transformer and it was definitely an 8Ω unit. The owner noted that there was some jiggery-pokery back in the day and wanted to confirm it was the "right stuff".

    Mark:
    I didn't want to remove a perfectly good 52 year old 100Ω resistor hence the piggyback 100Ω. :)

    Replaced also the bias cap and added an additional 33uF from the wiper to ground to get any residual pulsed DC off the signal, and to de-couple push/pull signals. Also reversible for future purists.

    I left the 25uF low-voltage electrolytics in place for now. They are all working fine with no leakage or internal resistance measured.

    Good on the 82Watts. I'm going to try another phase invertor tube, but if it doesn't improve things I'm not going to waste time/effort chasing down 3 missing Watts.

    Thanks for the quick response on this.
     
  6. Mighty Melvin

    Mighty Melvin Member

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    I never measured anything near 80 watts on a Showman. Usually much less. I know it was a consistent number but I don't want to post something from a memory that is fallible.
     
  7. mark norwine

    mark norwine Member

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    Odd...

    I never got under 80!

    Usually 84 - 88 when dialed in
     

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