ASK AMP MAN
JEFF BOBER
More Twin-Taming Techniques
Thanks for your February 2010 article (“Can
You Turn That Thing Down?”) that discussed
removing one set of output tubes of a Twin
Reverb, or using a switch to disconnect their
cathode connections, to lower the amp’s
output. The article also discusses removing one speaker from the circuit, increasing
the output impedance, and connecting the
screen grids to the plate of the output tube.
to be as leaky as the old paper-foil caps, but
yes, some still can become leaky. The biggest
offenders in my experience are what are known
in the field as the “chocolate drop” caps, used
in late-’60s and early-’70s Fender amps. When
these become leaky, they can cause the amp to
become very anemic and distorted.
If one set of tubes are pulled, the output
impedance doubles, does it not? If that’s
so, then removing one speaker would
double the output load and reflect the new
output impedance value. The output transformer matches the impedance of the output stage to the load, and the ratio of the
windings remains the same. So would that
keep the impedance balanced?
balance between the transformer’s primary
and secondary sides. In order to do this, you
would need to reduce the load on the secondary by half. This can be done in two ways. On
a transformer with multiple taps, you can simply select the next lowest impedance setting.
Say your rig consists of a 100-watt head and
16-ohm cabinet. If you removed two of the
output tubes from the amp, you could simply
select the 8-ohm impedance setting. Now your
8-ohm output has a 16-ohm load, which is half
the impedance load it would normally see.
The quickest way to measure for leaky caps
is with a voltmeter. Most times, signal caps in
a tube amp will have one side connected to
a high-voltage source, such as the plate of a
I’m one of those old, frustrated BSEE
[Bachelor of Science in Electrical Engineering]
people that knows he didn’t get out of college what he went there for. I’m working on
my old Twin and want to decrease weight, as
well as output power, as much as possible.
The newer-style signal caps do not
tend to be as leaky as the old paper-
foil caps, but yes, some still can
become leaky.
This also matches the two-tube load on the
primary side of the transformer, which is half of
the load it would normally see, so the primary
and secondary loads are balanced.
I also know that capacitor leakage can be
an issue, as an amp I built out of spare parts
had to have all the caps replaced. Do the
non-electrolytic capacitors of the last decade
or so still degrade and leak like the old
paper insulators? I don’t have a lot of equipment to troubleshoot with and have a ton of
distortion that sounds to me like bad caps.
I replaced all the electrolytics about six or
seven years ago and the amp has no hum,
but it hasn’t been used much in 25 years.
Gerald
tube. This side, of course, will have substantial
DC voltage applied to it. To check for leakage, you need to measure the DC voltage on
the opposite side of the cap with reference
to ground. Most caps will have some form of
measureable voltage on this side of the cap,
but if you begin seeing voltages that are in
the hundreds of millivolts to the 1-volt or more
range, I would consider the cap to be suspect.
Replace the cap and retake the measurement.
If it is relatively the same, the voltage may not
be leaking through the cap. Its source may be
some other part of the circuit and the original
cap may be just fine. If the voltage is substantially reduced, continue this procedure through
the amp and replace as many caps as necessary. Once you do this, you will probably have
a much better sounding amp.
Hi Gerald,
Thanks for reading the column. You have a
couple of questions here, so let me address
the impedance question first.
Hopefully that will give you a lean, mean Twin
Reverb machine.
You are correct that when you remove one
set of output tubes from a Twin, you are
effectively removing half of the load from the
primary side of the output transformer. In
a perfect world, you would like to maintain
On an amplifier such as a vintage or reissue
Twin Reverb, you do not have the option of
selecting output impedances. In this case, in
order to match the primary and secondary
loads, you would need to reduce the load to
half of the original load. This can be done in
one of two ways. You could replace the two
existing 8-ohm speakers with two 16-ohm
speakers. With the speakers wired in parallel,
as they normally would be in these amps, the
resulting load would be half, or 8 ohms. The
other way would be to disconnect one of the
stock speakers, effectively reducing the load to
8 ohms. (In your letter you referred to this as
“doubling” the load, but it is actually cutting
the load in half.) Keeping in mind that in real-
ity, removing two of the amp’s output tubes
only reduces the output by approximately
40 percent, and assuming that the remaining
speaker could handle the amp’s new full out-
put, you would have the optimal scenario.
Moving on to your question regarding signal
caps: The newer-style signal caps do not tend
Jeff Bober
Jeff Bober, one of the godfathers of the low-wattage amp
revolution, co-founded and was the principal designer
for Budda Amplification. Jeff has just launched EAST
Amplification. He can be reached at pgampman@gmail.com
