This 22-watt Fender Deluxe Reverb is one of
the world’s classic 1x12 combos.
I have a 1972 Fender Deluxe Reverb.
It sounds great, so I don’t want to alter
the tone too much. I just want to beef
up the power supply for when the voltages
are high or the amp is really being
worked. I’m curious how much power
filtering is enough to make a tube amp
super-reliable without choking the sound
too dramatically, destroying rectifier
tubes, or causing other ill effects.
The AB868 schematic calls for stock
values of the filter caps to be 16 μF
450V, but I’ve read that the microfarad
and voltage values can be increased to
achieve the aforementioned goal. That
said, some contend that an increase in
cap value is often accompanied with a
mod to a series-wired “totem-pole” stack
configuration of the main filters, if the
amp doesn’t have one. I’ve heard that
this was common on blackface amps, but
the Deluxe schematics (both AB763 and
AB868) appear to show the two main
filters wired in parallel.
So here are my questions: Exactly how
many caps should be replaced in this particular
amp for adequate filtering? Should
I just be concerned with increasing the
values of the main filters, or should the
second-stage filter values be increased as
well? Would a totem-pole stack configuration
in a Deluxe blow the rectifier? Should
the value of the bias supply cap(s) be
increased too? Should I replace all of the
bypass caps on the tubes as well? When
possible, should I remove the boards to
de-solder the leads from the bottom first?
Your question about filtering is both interesting
and subjective. In my opinion, the
amount of filtering in each stage of an
amplifier’s power supply is discretionary.
Increasing or decreasing the amount of
filtering in any given section of the power
supply can change the amp’s performance
and response. The result can be an amp
that sounds warmer and feels softer, or
sounds full-bodied and feels tight, or even
sounds cold and sterile. The latter can happen
when you employ too much filtering.
Yes, the amp will likely be less noisy, but is
that worth the compromise in audio performance?
I think not. So with that in mind,
I’ll try to answer your questions.
Your Deluxe Reverb’s power supply has
four sections—A, B, C, and D. Section A
is where the initial filtering of the DC voltage
occurs. In its stock configuration, there
are two 16 μF 450V capacitors in parallel,
resulting in a total capacitance of 32 μF.
This usually works well enough in a Deluxe
Reverb-style circuit, but if you want to have
a little more reserve in the power supply,
you could start by replacing the two 16 μF
capacitors with two 22 μF caps. This would
yield 44 μF of filtering, which is an increase
of approximately 37 percent.
At this point, the amp may be marginally
quieter and stay a bit tighter when
pushed hard. Theoretically, the maximum
capacitance load on a 5U4 rectifier tube is
40 μF, but I’ve not seen where 44 μF has
caused a problem for any decent rectifier.
But I’d caution against going much higher,
as this might affect the 5U4’s reliability.
You might consider using a 5AR4 rectifier,
as this would increase the high voltage
in the amp a bit. Increasing the voltage
would marginally increase the output power
and help the amp remain cleaner at higher
volume levels. You could also increase the
amount of filtering in the power supply, as
the maximum capacitance load on a 5AR4
is stated as 60 μF. To achieve that, you
could install two 30 μF caps, or a 47 μF
and a 16 μF, for a total of 63 μF. It would
be fine to mix and match here, as these caps
are in parallel and the total value is additive,
as opposed to a “totem-pole” style filter.
In a totem pole, two identical caps are
placed in series and the total capacitance is
half the value of the caps. For example, the
two 70 μF caps in a blackface Twin Reverb:
The total value of this section of the power
supply is only 35 μF. However, this is done
to increase the voltage rating of this section
of filtering. Using two 70 μF 350V DC caps
yields a filter section with a maximum voltage
rating of 700V. This is generally more
cost-effective than using a 600V cap in
some amps where voltages could approach
500 volts. This is unnecessary in your
Deluxe—even equipped with a 5AR4—as
the voltages would not reach 500V. But I
do recommend that you use capacitors with
voltage ratings of at least 475V DC.
Regarding the caps in the B, C, and D
sections of the power supply, I’d keep the
stock 16 μF values. If you do increase them,
I’d advise 20 μF or 22 μF at most, as larger
values would begin to substantially change
the amp’s sound and feel.
Regarding the filter cap in the bias supply:
As long as you’re modding, why not
replace this cap? The stock value is 50 μF
70V DC, but I’d recommend a 100 μF
100V DC cap. Even though the main
power supply filtering affects overall hum
level in the amp’s noise floor, a dirty bias
supply can also contribute to noise, so a
clean bias voltage can help.
As far as the bypass caps across all the
preamp tube cathode resistors, unless one
has become noisy this doesn’t really make
the amp quieter or keep it tighter at high
volumes. Actually, increasing values here
can cause things to “muddy up” more at
high volumes, so I’d leave these alone.
And lastly, there’s no need to remove the
boards prior to replacing the capacitors.
The eyelet boards in Fender amps are super reliable,
and you’re unlikely to damage one
with standard soldering techniques.
Warning: All tube amplifiers contain lethal
voltages. The most dangerous voltages are stored in
electrolytic capacitors, even after the amp has been
unplugged from the wall. Before you touch anything
inside the amp chassis, it’s imperative that these
capacitors are discharged. If you are unsure of this
procedure, consult your local amp tech.
is one of
the godfathers of the
low-wattage amp revolution,
co-founded and was
the principal designer for
Budda Amplification. Jeff recently launched EAST
Amplification, and he can be reached at