Amp Repair Articles
Here are some amp repair articles I have written that I hope will be informative.
Recapping . . . the Saga Continues
Please remember that the information on my site is for everybody with a vintage amp. Some are touring pros, some play at home, and some are collectors or dealers. Some people have tube recording gear, some have silverface Twins, some have tweed Champs, some have amps that are 70 years old, and some have what I consider to be "new," i.e. amps from the eighties or newer. It's a big world of amps out there and I deal with all kinds of amps and all kinds of people every day. It's not always black-and-white and I take each repair as a separate job - what does the owner want? Is the amp going out on the road or just sitting in a collection? Is it a pre-war Gibson EH-100 or a JCM 800? The goal is to optimize that particular amp for the customer. This is a bedrock principal of my amp repair philosophy.
First, let's define "recap." Back in the nineties, when "how-to" articles began appearing in Vintage Guitar magazine, recapping meant just that - replacing every single capacitor in the amp. It did NOT mean just replacing electrolytics. The authors promoted the idea that replacing a good original coupling or tone cap with an "upgraded" cap would "improve" the tone of a classic amp. The end result was an awful lot of old amps that were full of Orange Drops, and a lot of disappointed amp owners! Today, recapping is usually defined as replacing electrolytic caps only, especially the big filter caps used in the power supply. Filter caps certainly can go bad, and I replace lots of them, but I don't consider it a "must," especially on amps from the eighties or newer. Replacing them will not cause a loss of tone, or make the amp sound less "vintage." Coupling caps and caps used in tone circuits are a different story. I usually suggest leaving them in a classic vintage amp as long as they test good. Replacing them can have a BIG effect on tone.
Servicing should always come before parts replacement, and here is a service tip for Marshalls that illustrates this point perfectly. The filter caps will often have their "negative" connection made by soldering to a ring terminal that is bolted to the chassis. On JCM 800 (or older) amps, the little nut is OFTEN loose and, therefore, the filter cap has a poor ground connection. You might think about tightening it before deciding that the cap needs to be replaced!
|Pair of Twin Reverbs
On the bench this time is a pair
of mid-seventies Twin Reverbs. The first amp was
brought in by its original owner, who stated
that he had been gigging with it in country
bands 150-200 times per year since 1976. Much of
the Tolex was gone, the control panel had been
smashed in several times, and the amp looked
beat to death. The owner said that he had
replaced the tubes several times, but had never
needed to have the amp repaired. He finally
brought it to me because it had been “getting
noisy and cutting in and out sometimes.”
I opened the amp up. It had
never seen a soldering iron . . . every part was
original. The 6L6 screen-grid resistors looked
and tested great with no sign of overheating,
and every one of the blue Mallory “drop”
coupling caps tested great. After a good basic
servicing and new filter caps, the amp sounded
great and was ready for another thirty years of
The next silverface Twin came
in after a couple of years of hard touring in an
indie rock band. It had broken down several
times and had been serviced by a variety of
local techs while on the road. Opening this amp
up told a different story than the first Twin.
Although there had been loads of capacitors and
resistors replaced, there was a distinct lack of
On many Fenders, some of the
main ground wires are soldered to ring terminals
that are secured to the chassis by one of the
power transformer mounting bolts. On this Twin,
one of the bolts (all of them, actually) had
become so loose that the ground connection was
intermittent at best, causing all kinds of
problems. In addition, the filter caps had been
replaced very recently, but the soldering was
not done well and one of the capacitor leads had
already broken loose from the part board.
This amp took some time. I had
to carefully look over all the previous repair
work and ended up doing most of it over. After a
thorough servicing, the amp was ready to go back
on the road with good solid tone and
The first Twin illustrates
just how much use and abuse a well-designed tube
amp of that era can withstand and still operate.
The second Twin shows that new components are no
substitute for good servicing, and that when new
components are needed, good soldering technique
is a must.
|Tale of A Bassman
On the bench this time is a
1960 Fender Bassman. Of course, this model
has been the inspiration for countless other
amps, and is certainly a classic circuit.
This particular amp had been used
extensively in gigging situations by its
owner, and had acquired several repairs and
modifications over the years.
The first thing that I
noticed was a mod that involved separating
the cathodes of the first preamp tube
instead of paralleling them as in the stock
circuit. I always wonder a little when I see
a mod on an amp like this. Here you have one
of the most highly regarded guitar amp
circuits of all time…sure, you can change
it, but can you really improve it? I put it
back to stock.
Most of the capacitors had
been replaced, but fortunately there were a
few nice original caps in the tone circuit.
I used Mallory 150 and vintage Pyramid-brand
oil caps where necessary. The filters had
already been changed to the Illinois
Capacitor 22mfd 500-volt cap, a part that
has proven to be extremely reliable over the
Turning on the amp after
servicing presented a couple of twists.
First, it had a very high B+ voltage. With
the power tubes running at 25 ma, the plate
voltage was 490VDC. The Sovtek 5881 tubes
were running well as they had been in the
amp for a few months, and a set of Sylvania
6L6GCs from the seventies ran 32 ma and
480VDC. Both sets of power tubes sounded
The really odd thing about
this amp was that the bass pot had been
incorrectly wired at the factory. The wiring
was reversed, which must have confused a few
people over the years! Factory wiring errors
like this are extremely rare on Fenders,
although I recall a couple of brown-tolex
Princetons with the wrong value volume pots
and an amazing 1968 Super Reverb that had
run it’s entire long life with only one of
it’s high voltage secondary wires connected.
The other secondary wire had been mistakenly
soldered at the factory to a blank terminal
on the rectifier tube socket! The owner of
the Bassman had been gigging with it
professionally for many years and was really
pleased when he turned up the bass knob and
actually got more bass!
It would have been very
easy to overlook the incorrect wiring on the
bass pot and spend a lot of time (and
possibly a lot of parts) trying to get the
tone controls to work properly. Keep your
eyes and ears open for the unexpected. In
addition, style and construction of the
capacitors used for coupling and in the tone
circuits have a big effect on the tone of
the amp. “Upgrading” these components to
more expensive or “higher quality” caps
really changes the sound of an amp, and if
you were lucky enough to own a tonal icon
like this ’60 Bassman, why would you want to
change the tone?