Modding a silverface amp to
Silverface amps all sound better when
modded back to blackface specs in my opinion. I will outline the mods below.
However, some of the changes that Fender did in the silverface years are
not easily changed back to blackface specs.
go into the amp unless you know how to not get shocked! These amps will
even shock you when unplugged. They store extremely harmful voltages that
will jump out and grab you if you put your paws in the wrong place. I know
this from experience! It hurts!
Suppresser caps: Fender added .002uf
(or sometimes 1200pf) capacitors to the grids of the power tubes (pin 5).
These bleed off frequencies that they thought were too high to be heard
with guitar. The effect of these is to bleed off any frequencies that the
amp will oscillate at. Oscillation is like feedback with a microphone.
It feeds itself and runs away from you if you don't do something about
it. Amps can do this internally, and it's a big problem. For you tech types,
somebody emailed me and said that the effect of these caps is -18 dB at
6 kHz! Ouch! The caps are actually a good idea, but they tend to bleed
of high end "sparkle" from the amp. These caps were added because of problems
with the lead dress that was causing the amps to oscillate. It's unfortunate
that Fender needed these. The best way to control oscillation is to design
the amp not to oscillate in the first place! But for cost cutting's sake,
it was cheaper to put these caps in than spend the extra labor to make
the amp more stable. These caps can usually be removed and the amp will
run fine. I haven't run across an amp yet that started oscillating without
them. But, I have heard from some techs that have seen many amps that can't
live without these caps. Oscillation suppression is nothing new in
Fenders. Tweed bassmans had a cap across the plates of the phase inverter
to help with this (47pf). If your amp plays fine after the mod and then
all of a sudden acts like it looses most of it's power, it is probably
oscillating. Try the 47pf cap like the tweed Bassman and see if that fixes
it (look in schematics to see how it is done).
There is also a much rarer oscillation suppression method Fender had, it
involved a .02 cap and a 220k resistor going from one leg of the power
section then held 100 ohms above ground via the feedback loop. This is
rare and only occurred in the early 70's.
Phase inverter: Fender changed
the phase inverter in a few ways. Starting from one end and going to the
other: The coupling cap increased from .001 to .01. This has the effect
of letting more bass through to the power section. This sounds like a good
idea, but to my ear makes the bass get overbearing. The .001 cap sounds
tighter with more defined bass to me. After that, Fender used 330k resistors
on the grids of the phase inverter instead of 1 meg. The cathode resistor
on the phase inverter was made smaller which ran this tube a little hotter.
The plate load resistors were decreased from 82 and 100k to 47k. This reduces
gain. Overall, the changes to the phase inverter look like they were designed
to tame the amp and change the way the amp sounds to what Fender wanted
at the time.
Reverb circuit: Fender tended to
run the reverb driver tube very hot in the silverface years. Blackface
amps came with 2.2k resistors bypassed with a 25uf-25v electrolytic cap.
Sometime in the 70's, they started to change this value. I've seen 1.5k,
820 and 680 ohm resistors here without a bypass cap. Fender also added
a cap from the plates of the reverb driver to the cathode. They added a
cap to ground after the reverb tank as well. I've found that I sometimes
like the silverface method for reverb better than blackface, but I do take
out the added caps to ground. I will leave the amp otherwise stock if the
reverb sounds good to the owner.
Bias supply: Fender changed the
bias supply from a level to a balance control. The balance control allows
the tech to match the tubes, but not set the bias level which is what can
determine tone. Also, some extra filtering was added to the circuit (two
70uf-100v's? Don't remember exactly). I always change the bias supply
to blackface specs because we use matched tubes these days! When replacing
the bias supply filter caps, I usually just use one 100uf-100v.
There are a few other changes that I don't
mod back to blackface specs because I don't feel they change tone all that
much. The main ones have to do with the power supply.
Changes in the silverface years that
can't easily be changed back to blackface specs: Sometime when Fender
went to the 5U4 rectifier tube, they increased voltages on the high voltage
secondary of the power transformer. The 5U4 has a higher loss, so the increase
makes the voltage after the rectifier tube similar to what it was in the
blackface years with a GZ34. When installing the GZ34 in these silverface
amps, the voltages will be higher than those in a blackface amp sometimes.
The only way this can be changed back to blackface is to change the
power transformer. This is expensive. In the late silverface years,
Fender changed the power supply and power section of the amp quite extensively.
They do not look like the earlier blackface and silverface amps at all.
These changes included new power and output transformers and solid state
rectifiers for all of the larger amps. Voltage was greatly increased on
the plates of the power tubes. These changes are not easily reversed. The
way to tell if you have one of these amps is to look under the speaker
jacks. If it says. 70 or 135 watts, you have the later model that cannot
be easily or cheaply modded back to blackface specs. I'd recommend selling
this amp if you are not happy with the tone before trying to do extensive
and expensive mods to it. They actually do sound pretty good clean
though. And they are extremely loud!
*Disclaimer - My lack of electrical engineering
knowledge means I don't fully understand what Fender was trying to accomplish
with some of the changes in the silverface years. If you have something
to add or a correction to make, please let me know! I'm always happy to
hear from someone that knows more about this than I do.
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Last updated 2/19/2000