57 twin web page
This is one in Fender's line of
hand-wired “boutique like” amps. A reproduction of
the 1957 Twin amp.
We had heard
from our friends and customers of mixed views on this amp. Some said it
was fabulous, but others noted some disconcerting problems with it.
Gustavo P. got
one recently and after putting in a Torres Premium Select '57 Twin Tube
set (to replace the bad tubes the amp came with) he still had the
Gustavo sent me a very
high resolution MP3 of him playing the amp (thanks by the way) and the
fuzzy crossover distortion others had noted was right there.
Sounded like a
blown speaker, a fuzz on the edge of the notes.
After trying some
troubleshooting via email we decided to get the amp into the shop so I
could take a look at it.
Right away it was
evident what was wrong. Even testing with regular 6L6 tubes the amp
could not be biased – with the beautiful KT66 tubes we were
using, the problem was even worse.
The bias supply would
not provide enough negative voltage to bias the tubes into the correct
After applying the
normal fixes, and doing some repairs (a bad bias pot too) a check of
the AC showed that the bias tap on the power transformer did not
provide enough raw AC to get the correct negative DC Bias to make this
amp work right.
Rather then build a
whole new bias supply into the amp (always a possibility) I applied a
new design voltage doubler to the negative voltage supply.
A fabulous new Torres
Now I had DOUBLE the negative voltage, plenty to push the KT66 tubes
into the correct AB range – Problem solved.
It sounded immensely
better, the fuzzy tone was virtually gone – very good, but it
wasn't ALL gone.
There was a little hint
of crossover left, audible when the amp was set up very clean with very
It was probably inherent
to the amp's design.
Where was it coming from? A study of the schematic found it –
an unbalanced phase inverter.
It has exactly the same
Concertina phase inverter as a Princeton Reverb, and has exactly the
same unbalance error, so my long-ago modification design, the "Perfect
Prince" is included to correct this pre-existing error (and existing in
ALL the amps I have ever seen with the split load, concertina
phase inverter of this design) - it corrects the problem, well,
The Download includes
all the instructions for both these mods, plus has a parts list for all
the parts. the Highly accurate resistors are available from Mouser.com
- AND the mod includes a needed capacitor upgrade for the signal caps.
Overall a great project
for less then two bucks.
Your download will appear as a DOWNLOAD button on the order
confirmation page. You can also get to it, even from another computer
from the ORDER STATUS PAGE. if you have trouble, please contact us with
your order # and we will send you a copy
|Parts Kit for th '57 Twin fix/mod/upgrade
for the '57 Twin Modifications and bias repair.|
You need to download
the instructions they
are not included with the parts kit.
The parts kit includes
the new diodes,
filter caps for the power supply,
all the parts for the Driver/Phase inverter fix including matched Caps
for the driver.
AND all the signal caps to upgrade the circuit board.