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A customer has imported an old Kustom 200 bass amp from the USA. He wants it converted to 240v and done up generally (he likes period amps). It has a number of 'issues' as I like to call them, and I'd appreciate some guidance on any of these you know anything about.
Power amp - when I feed it a signal direct this puts out 50 watts into 8 ohms then clips. It has four RCA 36892 NPN power transistors; these are stamped '9A'; I understand the NTE130 is an equivalent, so I guess they are likely to be 9 amp as labelled, at least. One pair have +40v on the collectors; the other pair have -40v on the emitters. This voltage holds up under load and signal. Shouldn't I be seeing more output watts? Anyone know for sure what the design output wattage is? Unlikely to be 200 I guess, but you never know, maybe they named it honestly... It has a hefty txfr and huge smoothing caps.
The preamp has another issue... I put in a guitar-level signal and up until about half way up the volume control, it amplifies clean and normal. Then above that it starts to clip... then by the time you're 3/4 of the way up it's clipping so hard there's hardly any signal left. I thought, power supply, but I have a hefty bench supply and rigged up the preamp to this, but it clipped just the same way - down to almost a flat line with the volume wide open. It has two preamps - both preamps do this, but one in a more marked way than the other. I note the unusual volume control arrangement. Any ideas?
I completely separated preamps and power amp to do the tests above. The preamp was completely isolated from the power supply also, and fed +8/-8v from the bench supply.
Because of the power output uncertainty I'm not sure what power transformer rating would be correct.
Power amp - when I feed it a signal direct this puts out 50 watts into 8 ohms then clips. It has four RCA 36892 NPN power transistors; these are stamped '9A'; I understand the NTE130 is an equivalent, so I guess they are likely to be 9 amp as labelled, at least. One pair have +40v on the collectors; the other pair have -40v on the emitters. This voltage holds up under load and signal. Shouldn't I be seeing more output watts? Anyone know for sure what the design output wattage is? Unlikely to be 200 I guess, but you never know, maybe they named it honestly... It has a hefty txfr and huge smoothing caps.
The preamp has another issue... I put in a guitar-level signal and up until about half way up the volume control, it amplifies clean and normal. Then above that it starts to clip... then by the time you're 3/4 of the way up it's clipping so hard there's hardly any signal left. I thought, power supply, but I have a hefty bench supply and rigged up the preamp to this, but it clipped just the same way - down to almost a flat line with the volume wide open. It has two preamps - both preamps do this, but one in a more marked way than the other. I note the unusual volume control arrangement. Any ideas?
I completely separated preamps and power amp to do the tests above. The preamp was completely isolated from the power supply also, and fed +8/-8v from the bench supply.
Because of the power output uncertainty I'm not sure what power transformer rating would be correct.
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