Hi,
I am rebuilding a Sunn Concert Bass head. When I got it most of the wires inside were loose and one of the 22ohm/10w emitter resistors had broken loose. Once reconnected it worked poorly for 5 minutes ( heavy wierd distortion) then blew. It was drawing excessive current right before it blew ( determined by watching my lightbulb limiter). The circuit breaker had been bypassed! After replacing the circuit breaker and various out of spec power resistors on the power amp board , the filter caps all other elctrolytics, the three rectifiers and 2 transistors (2N3055) and resoldering the joints, I got the power amp section working, but the preamp was not working. I used my acoustic electric into the power amp in jack on the back of the amp and got a clear clean loud output at the speaker.
I replaced all the electrolytics on the preamp board and tested the two transistors (BC107) with a transistor checker. They tested good as did all diodes. Upon powering up (variac) I got motorboating directly proportional to the amount of voltage applied. I could not apply full voltage (120V) as the noise got too loud to bear. I then replaced the three FET's and resoldered a bunch of joints. Powering up I got motorboating, only this time I could apply full voltage as the volume control was controlling the level of the motorboating.
I pulled the preamp and ended up finding some more questionable solder joints which I resoldered. I checked a bunch of resitors and ceramic and mylar caps(all good). Powering up again, the motorboating was gone. Plugging in an electric guitar turning the volume all the way up on the amp, I am now getting a clean but very low level output through the speaker The amp appears to be drawing little or no current. ( the poweramp in w/ acoustic electric is still loud and is drawing current).
The tone controls and brite switch are functioning fine, but not the dual distortion pot.
I measure +34VDC at the red wire (on the preamp board /in from the power am board) and –16VDC on the white wire and 0vdc on the blue wire (connected to preamp circuit ground. Q201 reads G= 5.93vdc / D= 5.93vdc / S= 5.56vdc. Q202 reads G= 0.0 vdc / D= 11.9vdc (hums through speaker when touching D leg) / S= 1.1vdc. Q203 reads G= 2.6 mvdc (hums through speaker when touching G leg) / D= 12.3vdc / S= 1.33vdc. The original jfets were 2N4304. I replaced them with 2N5458”s.
The voltages at the transformer are: Primaries (dark orange pair to rectifier CR1) = 18.9vac and 16.25vac. Secondaries ( light orange pair to power amp board) 22.7vac and 22.5vac and a third pair (green to power amp board) = 19.8vac and 20.6vac.
So, the power amp section is working fine. The preamp is barely putting out signal. The “power amp in” works fine with an acoustic/electric guitar. The power amp section is very quiet. (I replaced the 3400uf caps with 3900uf.) There are no pops, clicks or components getting hot. I get hum and static-like noise when I touch the volume shaft with a finger that gets louder as the control is turned up. Tapping on the controls with a chopstick does not reproduce the static. No hum on the rest of the controls or when touching the guitar input jack shield. thanks