Q7 Base to emitter open
All other transistors check good with tester thanks

UPDATE TO LAST POST: Q7 on the power board is bad. All other transistors are good. My Question, Is this the cause of my distorted output. The collector of Q7 is tied to the Base of Q1 via R1 and also to CR11 and also to the collector of Q1 via R3-150u/5w. 

Start by unhooking the speaker and testing it with another amplifier, does it work properly? YES
Now the opposite: try your Sunn with another, known good speaker. OUTPUT DISTORTEDThen, with no speaker attached, do you have any DC at the speaker out? NO DC
There seems to be a connector between the board output (TP6/L1/R52/C15) and the actual speaker jacks.
Drop a tiny amount of WD40 or similar on its legs and push/pull it a couple times, it may be rusty/grimey. DEOXIT, not the cure
Wipe any excess.
Do the same on output jacks, work a plug in/out to self-clean it, always aided by a little WD40; wipe any excess afterwards, we don't want dust clinging there.
Do the same on the input side, on the left side of C1. DONE PREVIOUSLY
Lift its left leg, inject clean sound there (a CD/MP3 will do).Injected sound at MASTER ACCESSORY POWER AMP LINE IN. Same result.
By the same token, the preamp out from the same place yields a normal output.Any improvement? No

here's what it sounds like:  http://www.drewswebsite.com/beta_bass_output_distortion.htm

So far these are the best schematics I could find.

Enzo, I rechecked the fx loops, no difference. Diodes CR4 & CR10 read shorted in circuit but good with one leg pulled. They arejust before  L1 going to speaker output.  I powered the amp up with those diodes still disconnected on one leg and the amp sounded the same!

Hello folks,
I have a Beta Bass amp that is distorting at the speaker output. Supply voltage is good. Can't find any bad transistors. Preamp out from the back panel is clean, but power amp in is distorted like the speaker output.
The amp sputters and cuts out. The power amp has plenty of output level but I have to crank the "Drive" control ( first gain stage) At this point it easily goes into self-sustaining mode. At low "drive" level the amp acts like the signal is being compressed and distorted at the same time. Has anyone seen these symptoms before? Thanks in advance DrewV

 It took awhile to find all the damage in this amp. When I got it the power board was floating inside the chassis, held up by the wiring that wasn't disconnected, the circuit breaker had been jumpered across. Every single electrolytic was or went OL before it was over, some (locations) even twice! All four emitter resistors were "going with the flow"! Power resistors were burnt and for a few seconds as I observed them smoking! Three of the power transistors were the wrong type. The two that were the right type were burned out.All three rectifiers were toast and various outher parts were missing or loose...and the jacks and pots were dirty, dirty, dirty! But... the transformers were good and most of the resistors and remaining capacitors. The jacks solid, switches clean. And the most beat up tiny little part, CR306, the only zener  in the house looking like it spent a little time in the garbage disposal, well, that worked. This amp is loud and clean and a tribute to the TONEQUEST of the early 1970's. Did I mention that this amp is LOUD?  Thanks Drew
P.S.The overdriven tone I was getting was coming from the speaker not the amp! I hooked up a Mesa Boogie Black Shadow and got nothing but loud & clean.

  J M Fahey & Phil, Enzo, It's sinking in! I started checking the voltages on and around Q104 tracing the circuit path of the -15vdc supply. When I got to R130 I still had the proper voltage. When I got to R128, I had zero vdc. A continuity check showed that the connection between R130 and R128 was open. After a little prodding the end of R128 closest to R130 lifted right off the board. I resoldered it and checking the voltage. I now have C= 14.3vdc / B= 3.4vdc /  E= 2.9vdc. Vbe = 0.6vdc. The amp now works. Current draw appears to be normal. All of the nasty artifacts are gone. The amp tone is overdriven. That may be normal ( I don't think I have heard another concert bass amp). The " left" distortion works (shorted tip to ground), but not the right. How am I doing? Much Thanks Drew

 The following are the most recent measurements. (Ignore the source and drain voltages on the schematic (in red)) Q101: D= 15.6vdc / S= 0.4vdc.  Q102: D= 15.8vdc/ S= -19mvdc. Q103: D= +15.7vdc /S=106mvdc.  The measured source voltage is off. Enzo I have no scope! thanks

Sorry, I have posted a cleaner schematic and the preamp board layout. I will check the voltages and get back. Thanks

The output level drop and the  current draw was caused by the speaker wires touching on the 1/4" phono plug. Using the poweramp in the amp works and draws what I would call normal current. The sound has a definite buzzy edge to it especially noticable as the sound decays. It sounds as if a nice distortion box was on, mixed way back. The normal preamp input is still breaking up, esp. at higher volume.
I have +35.6vdc on pin 43 on the preamp board and -16vdc on pin 44. C122 and C124 have been replaced. Q104 has 29.6vdc on the collector and Q105 has 12.2vdc on its' collector. Q301 has 11.9vdc on its' collector. I have attached

Phatt I had already substituted the 4304's with new 2N5458's and got no improvement so I put the original JFET's back in. I went back and rechecked today also swapping the 4304's between the different sockets with no improvement. Then I substituted a new BC107 in the three places it is found and still got no improvement. I also put BC141 (Q302-power brd) on a transistor checker and it read good.
Touching Q104 with a pair of  needle nose pliers produced a substantial increase in static like noise. I then broke the circuit by removing Q104.At first the results were: 1) The "avalance" type distortion detailed in my orig post was completely gone. 2) the amp sounded like it was putting out about half power ( and wasn't drawing much current). 3) The sound was nicely overdriven. ( distort control not affecting the signal). I then powered the variac down and off to type the results. Powering back up the I found output level had dropped dramatically and the amp started drawing massive current as the channel volume was turned up. (My setup for testing is: amp into lightbulb limiter into variac.) Putting Q104 back in the circuit the amp output level is still very low and the amp is drawing massive current as the channel volume is turned up. Even the poweramp in (A/E guitar) is producing the same effect. I think i fried something on the power brd! I'm going to recheck everything. I will post when I got some new information Thanks Drewv

  I've been working on this amp since March so please forgive any confusion on my part. Put a signal into the power amp in jack. Plugging an A/E guitar into the poweramp in produces an acceptable loud and fairly clean output at the speaker. Plugging in to the normal input with a regular electric guitar produces the symptoms outlined in my initial post. Taking the preamp out and plugging it into a mixer also produces the same symptoms. The problem appears to be coming from the preamp. ( Patching from the preamp out to the poweramp in has no effect) As with the power amp board, all preamp board components have been tested and replaced as necessary. Preamp transistors read good on a transistor checker. I'm thinking that the problem is connected to the dual distortion circuit, but I can't find any bad components, solder bridges or broken traces.   I removed the dual distort pot from the board, cleaned it with Deoxit and checked the resistance on each pot which reads 25k and 28k. I have the component layout document  and have checked the board against it. Here's a link to the Concert Bass schematic. There are a few differences between the actual amp and the schematic. The amp I'm working on is a 1973 model. There are six wires (three pairs) coming out of the transformer. One pair going to rectifier (CR1) with the two other pair going to CR306 & CR307. The sunnampage.org server is down temporairly. Loudthud has posted a touched up schematic and parts layout there. Until the site is back up the following link shows the schematic. http://www.schematicheaven.com/newamps/sunn_concertbass.pdf thanks DrewV

Hello, I am restoring a Sunn Concert Bass amp. I have tested replaced, as necessary all resistors and capacitors including all five 2N3055 transistors, all electrolytics, all three rectifiers, and all emitter resistors.

Voltage readings on the power transformer (in circuit) are:
Lt Orange pair = 22.5vac & 22.7vac
Green pair = 19.9vac & 20.7vac
Dk Orange pair = 16.1vac & 18.5vac

Speaker terminal: acv = 0/  Dcv = 6.4mv
Collector to ground resistance:
In circuit Q1= reads like a capacitor / Q2= 0.5ohm/ Q3=465ohm/ Q4=465ohm/ Q5= 7ohm.  Q2 & Q5 pass continuity test. The 910ohm resistors associated with Q3 & Q4 also read 465ohms. With the transistors mounted but not electrically connected all collectors read OL.

The + rail voltage is 5 volts too positive (reads 30v when it should read 25v). The – rail voltage is 1volt too negative (reads –16v when it should read–15v).
               
Here are the symptoms:         
With the guitar plugged into either input the signal sounds clean if picked lightly at low amp volume but goes into massive ugly distortion if amp volume is turned up or guitar is played hard, drawing a good amount of current, thou not enough to burn or heat any components..

With the right distortion footswitch shorted tip to ground/ output level is reduced and the distortion control attenuates the signal as it is turned up with near total attenuation at "10".  (something backwards?)

With the left distortion footswitch shorted tip to ground/ idle output level is increased by a substantial margin with a buzz/hum like quality. Rotating the distortion control from zero to "10" increases the overall level and causes the amp to go into massive ugly distortion drawing a good amount of current as described above.

With both the left & right distortion footswitches shorted tip to ground/ amp draws very little current and displays the same massive ugly distortion if amp volume is turned up or guitar is played hard. Rotating the distortion control to 10 produces good distortion at very reduced amp output levels, drawing very little current.
Thanks in advance
DrewV