Source: Musicians Hotline
Edition: December/January 2006
Article: Letís Ask Budda with Jeff Bober of Budda Amps
Q: Iíve got an SR Blackface í66 amp, all original. The sound is very thin, weak and trebly. Thereís nothing wrong with any of the tubes, and it has been biased correctly. Iíve tried the amp through different speakers but found the same result. The amp hasnít been played very much over the years. Can it be old, dried-out capacitors that make the amp sound like this? Can a complete replacement of the capacitors really improve the sound Ė meaning to give it more twang and bottom end, or ďlifeĒ? Ė Tobbe Sweden
A: Beautiful amp. One of my favoritesÖ except, of course, for a Budda!
Iím assuming both channels of the amp are thin and weak. If it were only one channel that sounded bad, I would tell you that the 100K resistor in the tone stack of that channel was probably open. This would also disable the Middle and Bass control, but Iím going to assume that all the amplifier controls function properly. Since you also mentioned that the amp has been properly biased, I have to assume that the primary sections of the power supply have the correct voltages.
Considering the ampís age and the possibility that it might have been sitting idle for some time, a power-supply cap job is probably a good idea. This might not be the source of the problem, but once any other repairs have been made it will make the amp sound stronger and tighter. Make sure the values of the power-supply resistors are checked after the filter caps have been removed. A power-supply resistor thatís substantially out of tolerance can cause degradation in tone, but usually it wonít be as drastic as the symptoms youíre describing. And donít forget to have the bias supply cap replaced. A weak cap in the bias supply will give the amp some additional unnecessary hum. You might also want your tech to do some normal maintenance on the amp, such as cleaning all of the tube sockets and controls while heís in there. A very dirty tube socket, especially in the phase inverter, can really suck the life out of the amp. If the amp still isnít right after the maintenance and power-supply build, itís time to start looking elsewhere.
Okay, letís look at some other possible causes for an anemic Fender amp. The first thing Iíd look for would be the ground connections from the circuit board. These are the buss wires that come off the circuit board behind the controls and solder to the brass grounding plate that runs under the control panel. Iíd check all of these connections, but thereís a particular one that seems to be the most frequent offender. It comes off the board in the area of the tremolo and phase inverter circuits behind the tone controls for Channel 2. For some reason this solder connection tends to break, causing the phase inverter to lose its grounding. That makes the amp lose substantial power and sound thin. If any of these ground connections is broken, it takes a good deal of heat to re-solder them. So, youíll need to use a high-wattage soldering iron or gun. If all these ground are intact, the next thing to check would be the 100-ohm (brown/black/brown) resistor in this same area. Itís the only resistor in the area that is positioned horizontally on the circuit board. It also typically suffers from a broken solder connection and will yield the same low-power situation. Re-solder the resistor, and you should be good to go. If not, itís time to start looking at the other major cause: a bad output transformer.
The best way to determine if the output transformer is bad is to simply substitute it with one you know is okay. A good way to check it is to simply unsolder the leads coming from the original transformer and attach a replacement using clip leads. If you canít come up with a Super Reverb transformer to try, a Twin Reverb transformer will work well for the test. If you need a replacement transformer, I have to recommend (as I have in the past) Mercury Magnetics. Their Axiom line of transformers should have just what you need. They can be found at www.MercuryMagnetics.com.
I hope you can make your Super super again.
Until Next Time, Jeff Bober