Sunn Beta Series - Heatsink the Drivers

just a short blurb on the Sunn Beta Bass and Beta Lead amps,

got a few in with the same problem, TIP29 and TIP30 driver pairs,

they get hot, the glue ages, the undersized clip-on heat sink falls off, the transistors get hotter, collector trace pads start to cook,

rest of the amp is very dependable, so just fix the driver problem and you are set for a life,

why do people play these old relics?

it's that ch1 - ch2 mix thing, plug into the middle input,  set one up for treble crunch, the other for clean, mix em up and it's like instant Lemmy from one amp, you beta freaks know what i'm talkin bout,

anyhoot, here is what we do down at the shop, grab a 2 x 4, cut it down to 2 x 5   

cut up an old reverb pan and no need for insl kits as you are isolated, we even used wood screws for the transistors,

this is overkill as the drivers don't get That hot, but the amps never come back which is a good thing in a small town, where people remember the bad and forget the good,
there is a bar next door and i can hear em thru the wall,  ;D

You can oversize when you don't have a bean counter watching everything.  I remember playing a Beta Lead amp one time and was blown away by how good it felt to play through.  I've always been a tube guy but that amp inspired a creative streak in me.

for a transistor amp is has a cool sound, better than this 2700 dolllar Mesa mark 5 we have on the bench with a front panel that looks like a 747 console,

I have a Beta Lead on my bench now. It sounds distorted but works otherwise. I checked some voltages and have been tracing the signal with o scope. Each time I suspect a component, I’ll flip the board to the solder side and see recent solder work, so someone has been there before. This repair is not going to be just replace some parts and it works, someone already tried that. I need to understand how the circuit works and figure out what’s wrong. I’m not sure what Q3’s job is but it feeds signal from the output back upstream. I’m thinking the output is not the problem as the signal looks fine where Q3 goes but the signal looks funny after Q5 and Q14. I’m having a hard time reading the low res schematic I downloaded, and relating the schematic to the layout but I did figure out that the board has Two Q6 locations, Q16 is screened Q6 on the pcb. Q16 and Q7 are socketed and have been replaced. Q16 was originally 2N4888 and replaced with MPSA94. I have a sack of random transistors and I have been looking up spec sheets to find reasonable replacement parts. The 2N4888 hFE is 500 and the MPSA94 hFE is 40-50. I’m not sure that’s causing the problem, but I do have a BC556, about half the voltage rating but hFE of 125-475. I still need to check if the MPSA94 was installed correctly. I’ll look at the TIP heatsinks as well, thanks for the TIP tip.

CJ

Did the drivers have long wires stock? 
If they did not do you have any stability issues with the wires?

Reads like it might be fun to build the preamp section.

Drivers? tip 29/30 are voltage regulators, no?
http://music-electronics-forum.com/attachments/4095d1231809469-sunnbeta_part2.zip

yeah Gus no problems with the long leads, added C and L probably help, done about 4 this way and scoped them out,

TIP 29 and 30 are NPN-PNP comp pairs,

one way to fix the pwr board Walter is to just change all the transistors on that board, takers about an hour if your digikey order came in, this shotgun approach saves time and prevents returns later,

fixed a Solarus Combo today, kind of rare, 7199 driver, solid state vibrato, EL 34's on this one, green Cornell-Dublier caps were not leaky can you believe it? nice wiring harness on that amp,

> tip 29/30 are voltage regulators, no?

No. But in our world they are often *used* as pass-elements of a voltage regulator. Ample size for the small-stuff of a box, and TI pushed them heavily, so they became old friends. In recent decades, not much used as audio-amp drivers, because there are better (faster) parts introduced since TIP29/30. But in restoring an already designed amplifier it is best to stick with the speed of the original parts. Something much faster will open-up octaves of new oscillations.