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Hey Brian thanks for the input. I've heard the "normal" flutter in my
late 70s Twin but this one is ridiculous!I've already tried
the "caveman" tube swap...unsuccessful. I thought maybe it was the
footswitch, but it wasn't. So I'm driving back up to Austin in about
a week to replace the "photocell lamp oscillator thingamabob..."--
Phillip
--- In , "Brian Neal" <bneal@i...> wrote:
> Hi Phillip,
>
> A certain amount of fluttering or thumping is normal for the
> blackface amps, I think...from what I have read on the Fender Forum.
>
> My Showman thumped like crazy though, and I took it to the tech
> twice. He replaced the photocell (bug) but to no avail. Finally I
> replaced the tremolo driver tube with a NOS tube and WOW! The noise
> simply vanished, and I suddenly had a very rich and thick tremolo.
> Boy was I happy! It was quite a dramatic difference.
>
> I'd have to go home and look at the back of the amp to tell you
> which one it is. But the AB763 circuit used in the blackface
> Showman, Twin Reverb, etc usually has a tube chart like this (I may
> not have the order right):
>
> 4 6L6GC (power tubes)
> 2 7025 tubes (preamp for normal and vibrato channels)
> 1 12AX7 (tremolo driver)
> 1 12AT7 (rectifier?)
>
> The 12AX7 is the tremolo driver that interacts with the photo cell.
> Its just a noiser version of the 7025's. If you don't have another
> 12AX7 handy just swap it with one of your existing preamp tubes
that
> go in the 7025 slot (these are most likely 12AX7's anyway, since
> 7025's are not made anymore and are very scarce).
>
> Apparently the NOS 12AX7 has a lower voltage gain than the brand
new
> JJ tube I had in there and doesn't drive it as hard. At least that
> was the theoretical answer I got from smart guys on the Fender
Forum.
>
> I just know that "me replace tube with another one, it now sound
> better, ugh," (caveman amp repair).
>
> Good luck!
> BN
>
>
>
