SUNN Coliseum Bass 300 Pwr Transistors?

[CJ]

working on this classic, people in this town, for some weird reason,like old Sunn amps, Beta Lead, Beta Bass, Concert Lead, Concert bass, now this, the Coliseum Bass, supposed to have a good eq section,

some folks even play the git thru the Sunn Coliseum PA lord have mercy,

 

[CJ]

http://bmamps.com/Tech_sch.html

above is a very impressive collection of schematics,some times the pdf's wont save, so you screen  capture,

question is why this output section uses 3 of the same transistors in the top section,

Q8,9,10  - MJ15022

and two MJ15023  transistors for Q18,19  and  and a different one for the driver, MJ15016

first two p/n's have 200 V-ec    16 amp I-c              and hfe = 60
MJE15016 has V-ec = 120 V, I-c = 15 A and hfe = 70,

thanks for any help!

 

[PRR]

PNPs cost more those days.

Maybe they picked a lower Vce part for the PNP side to save a buck; whereas on the NPN side it was not worth it.

At this power level, 120Vceo seems a bit shy, but I don't know the impedance or if "300" means 300 Watts. I do think few modern transistors can be real reliable at 150W/pair.

 

[CJ]

Thanks PRR!  will order 3 of the same PNP's then as they have about the same gain and current ratings,

 

[JohnRoberts]

Another consideration is the current that beta was measured at.

High current output devices may not provide as much current gain at lower current where the drivers operate.

It seems using the same part in all three sockets would be desirable unless significantly more expensive, since there is an overhead cost to carrying more different line items in inventory.

JR

PS: Mr. Sundholm was more popular on west coast than east, and was behind a few different companies back in the day. As I recall he did some interesting low cost packaging for rack mount gear (80s?).

 

[Brian Roth]

Conrad Sundholm also founded Biamp Systems, which built mixers, amps, etc. for PA applications.  I found this about his current business:

http://www.conradamps.com/www.conradamps.com/About_Conrad.html


Bri

 

[CJ]

fixed the power amp, somebody plugged the preamp cable  in  backwards and tore up half the 4558's so we replaced all of them,
should have designed cable so when you reverse it nothing bad happens,

be careful with this amp, if you get a short on one of the 15 volt supplies, the output rails out to 55 volts at 300 watts and your speaker cab is gonna fry and maybe catch fire, 

so we socketed the preamp board and stuffed the 14 ea 4558's and 3 TL072's    and now the amp works,

however, with a 500 Hz signal applied, turning down the 250 Hz eq knob all the way sets the amp into oscillation  ,

and when you max out the 4 K pot, same thing, any ideas? here is the eq section>

 

[PRR]

Did you replace the elderly and abused rail bypass caps?

 

[CJ]

that was it!  kinda,  there was a 0.1 uf spliced on the back of the board as a factory mod, when we resocketed, we re-installed it backwards so instead of going from pins 4 to 8 (pwr supply) we had it from pin 1 to 5 which is basically, an oscillator circuit, 

let's bolt this thing up after we replace the bypass caps, thanks!