Amp Repair Articles Here are a couple of amp repair articles that I have written that I hope will be informative. 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 honky tonkin’. 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 experienced servicing. 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 reliability. 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 years. 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 great. 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?