Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: shaun on May 21, 2021, 10:18:56 pm
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Heyo. I'm recapping a '68 Dual Showman that has some problems. The PT has been seriously baked but still works, although putting out only around 315vac either side to the rec diodes, instead of the 345ct listed on the AB763 schematic, and supplying only 400vdc B+ at Node A instead of the listed 460. (These are unloaded voltage readings.)
Under the doghouse, the original filter caps had a 500v rating, instead of the schematic's 525v. I wasn't sure what to make of that; I haven't been able to find a Dual Showman schematic that uses 500v filter caps. Perhaps Fender cut corners?
Someone had been in and modded things in what seems like odd ways. The amp had died. I put this down to broken leads I found on the 70uF filter caps, as well as the deadest set of 6L6s I've ever put on my Eico 667 tester. Plus the small bottles were a mismatched bunch of junk.
So I start replacing the caps and resistors that were out of spec, and things seems to be going okay until I hit the bias circuit, which clearly doesn't match the schematic. Replacing caps and resistors in a circuit that I don't fully understand - especially when the PT has been baked to death - leaves me anxious. Alternately, that modded bias circuit may be there precisely because the PT had been baked, and it may actually be an improvement, but if it is, I'm not sure how. I see it reduces voltage on the PI plates (the original schem says 82k and 100k to the plates, not 470k), and instead of lug 2 of the bias pot meeting a pair of 220k's, it now runs into a 68k, with another 68k coming off lug 3 after running through a 10k resistor. Because there were quite a few mismatched resistors in the amp before I got it - carbon comp and otherwise - I'm no longer sure what was original.
I'm attaching pics and a rudimentary schematic of the modded bias circuit. My drawing sucks, but perhaps it may clarify the pics.
My main question regards the bias circuit and whether I should restore the Fender design or keep what's there now. As always, thanks for any help.
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I'm pretty sure that is the stock bias circuit for that amp.
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I'm pretty sure that is the stock bias circuit for that amp.
Hi 2deaf. Thanks for the response. As it does not match the AB763 schematics I've looked at (see attached), could you point me in the right direction or perhaps have one handy?
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Try https://el34world.com/charts/Schematics/files/Fender/Fender_dualshowmanreverb_aa769_schem.pdf
or
https://el34world.com/charts/Schematics/files/Fender/Fender_dualshowmanreverb_aa270_schem.pdf
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Is this a showman or a showman reverb? Whichever, here are the schematics that exactly match the bias circuit in your amp.
https://el34world.com/charts/Schematics/files/Fender/Fender_showman_ac568_schem.pdf
https://el34world.com/charts/Schematics/files/Fender/Fender_showmanreverb_aa768_schem.pdf
Your hand drawn schematic has some errors. First, the PI plate resistors are 47K, not 470K. Second, the ground resistor for the bias pot is 15K, not 10K. Otherwise, your hand drawn bias circuit exactly matches the two schematics I posted.
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Hi 2deaf. Thanks for the response. As it does not match the AB763 schematics I've looked at (see attached), could you point me in the right direction or perhaps have one handy?
sluckey linked the two schematics for your amp.
Here is what it looked like originally:
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Wow. Thank you! A plethora of schematics :). I kinda figured the silverface would be different from the AB763, but I couldn't find it in my searches.
Best forum ever!