Kustom 200 B-1 - shorted output

[Tone Meister]

Dusting off a vintage Kustom 200 B1 bass amp for a buddy and initially it wouldn't power on and was blowing fuses. Already replaced a shorted bridge rectifier and now have stable power on the amp. Located one shorted RCA 36892 output transistor and have it disconnected, so the amp will power on and stay running with the shorted transistor out of the circuit. Feeding it 123 VAC wall voltage from the variac, I measured 30 VAC out of the PT secondary and right at 40 VDC at the rectifier.

Did a forum search and picked up a few nuggets, enough to ask a few more questions.

Q: besides replacing the shorted output Q, is there anything else to check on the PC703 board for possible replacement?
Q: in a previous thread from 2012, Enzo stated that a MJ15015 or even a MJ15003 would be a suitable substitute. is there a better option for an output transistor these days?
Q: are there any preventative maintenance "must do's" while I'm in here?
Q: in another thread it was recommended to swap the transistors in pairs if using a substitute part. How are these paired up in this amp?

[R.G.]

Q: besides replacing the shorted output Q, is there anything else to check on the PC703 board for possible replacement?
...
Q: are there any preventative maintenance "must do's" while I'm in here?

By now, everyone's tired of hearing me say it, but with the amp already open on the bench, you could, and I think should, replace every electrolytic cap in the amp. The caps are about 44 years old now. Eventually they'll start failing one by one. Give it another 40 years of life by replacing them all now. Including power supply caps. It's cheap insurance.

I personally would put a series inductor on the output to the speakers, and a Zobel from the output to ground. In 1970, it was not clear that these ought to just be there on every solid state amp for stability reasons.

Q: in a previous thread from 2012, Enzo stated that a MJ15015 or even a MJ15003 would be a suitable substitute. is there a better option for an output transistor these days?

Better is relative. If you can get real MJ15015s or 15003s, they'll be fine. I would go big. You can get the MJ21194 or MJ21196 for about $4 each. These have massively more safe operating area than the originals.

If you do this, get four new ones of the same type. Also, put in new insulators and thermal grease, don't reuse what's there. I personally would use the Bergquist Q-pads if you can get them instead of mica and grease. Much neater, and more consistent. About $0.50 per transistor, but a premium job.

Q: in another thread it was recommended to swap the transistors in pairs if using a substitute part. How are these paired up in this amp?

The amp is set up as a discrete-darlington quasi-complementary output with paralleled outputs. I personally would match Q1-Q2 and Q3-Q4, which is counter to what lots of people would say. The top and bottom outputs are being driven in fundamentally different ways, so matching between the devices in the two darlington emitter followers is far more important than matching from top to bottom. The quasi-complementary bottom-rail outputs largely don't matter, as they're tied up in a tight little feedback loop. Matching their *drivers* could matter, but that's not the problem you have.

[Tone Meister]

Thanks RG; all your recommendations are duly noted. I don't know if these qualify as "real" or not, but I can source these MJ15015s locally. Also, these 2N3055s are available locally as well. If not, then I will just swap out all four with the MJ21194s from the link you provided.

Thanks again for your thorough answer.

[R.G.]

If you can get real MJ15015s, great. Mojo is generally OK, and they're aware of the counterfeiting problems with audio output transistors. I tend to only buy from the bigger (and sadly, more expensive) suppliers as they've had to face up to parts-tracing from the manufacturer to hold down counterfeiting.

I would not bother with the 2N3055s. A 3055 is right on the hairy edge of voltage death with a +/-30Vdc power supply in an amplifier. It's only rated for 60V. This rating also ignores safe operating area, which is limited in the 3055. The reason the 200V+ devices are better is that the very high BVceo makes ofr much improved safe operating area, which audio amps need.

By the way, Mouser electronics sells MJ15015Gs for $2.85 each in ones.

[Tone Meister]

Have a few parts on order and will check back in to either report a healthy amp or seeking more guidance.

Thanks RG!

[Tone Meister]

After shelving this amp for quite a while I finally got down to it and did a thorough circuit restoration following RG's advice very closely.

I replaced the bridge rectifier and all the output transistors with closely matched MJ15015s. Replaced every drifted (>10%) CC resistor in the amp with 1/2 metal films and every tantalum cap with premium Panasonic electrolytic caps. The entire regulator section was smoked so I replaced both regulators with 2N3055s and all the transistors in the regulator section with 2N4001 (NPN) and 2N4003 (PNP). All the bias diodes were shorted so I replaced CR700, CR701 and replaced the obsolete 1N3754 CR702 with a 1N4148.

I did not install the series inductor or the Zobel and I may live to regret that decision. I'll probably cobble together the parts and put them away to shore up the output in the future if I get a chance to see the amp again.

FAST FORWARD: My buddy picked up his 200 B1 a few days ago and called me yesterday with a report. He plans to use the amp for guitar, not bass, and wasn't specific about the speaker cab and speakers he's using. He said the amp sounds great with a Strat, but he raved about the tone with an SG plugged straight in. Instant John Fogerty - CCR tones and he said the amp was very quiet!

Many thanks to RG for his stellar advice and to Bill and Steve over at the VintageKustom Forum for their patience in helping me sort out the multitude of small problems I encountered while restoring the circuit. Basically, it was a complete circuit restoration and the end result exceeded all our expectations.

-Stan