Kustom BassIII distortion

[Netlist]

I’m searching for any info regarding an old Kustom Bass III head unit.
A fellow musician complains that the amp is distorting too fast. Say at less then full output.
I applied a squarewave and indeed the output has nothing to do with a good looking squarewave.
This could however be normal if the designers deliberately colored the signal.
Any info is welcome, schematic most appreciated.
Also a schematic from the same series could probably help me.
http://www.broadwaymusicco.com/kustom4.htm


/Hugo

[Centauri]

And on what basis does he estimate full output ? 135W is quite low power for bass these days - I would imagine this could easily be achieved at practice.

I am pleased the output didn't look like a square wave - it shouldn't! Ideal square wave response would require a very wide band amp such as found in good hifi gear - something which no instrument amplifier is.

To test the amp, you will need to hook up a dummy load (resistor bank) in place of the speaker. The resistance should be the lowest the amp can handle (probably 4ohms - should be stated on the back panel). Apply a SINE wave at around 1kHz, and look at the output waveform as you turn the amp up. When you see the top of the waveform flatten, back off a fraction, and for this amp you should be reading around 23V RMS (65V peak to peak).

Cheers

[sreten]

I would suspect a preamplifier problem.

It appears to have send and return effects loop sockets, this
will allow you to isolate and test the two sections seperately.

Generally I would not describe distortion as a feature of bass
amplifiers of this period, only compression and limiting.

My old Peavey MkIII bass head has very similar specifications
and the only way of making it distort is overdriving the input,
the output stage simply won't distort, it compresses / limits.

sreten.

[Frank Berry]

I suggest that you replace the coupling capacitors throughout the amplifier. You might want to replace the power supply electrolytic caps too.

[sreten]

Originally posted by Centauri
135W is quite low power for bass these days.
Cheers

Your right about that.

My old Peavey mkIII is rated 150W/40hms/23Vrms.

But it has a massive transformer and 8 TO3 output devices on
a huge 15" x 3.5" x 1" heatsink, they don't make them like they
used to, not surprisingly its still going and sounds excellent.

sreten.

[Centauri]

Originally posted by sreten
My old Peavey MkIII bass head has very similar specifications
and the only way of making it distort is overdriving the input,
the output stage simply won't distort, it compresses / limits.

Yes, the Peaveys have their DDT compression built in, meaning you can't clip the power amp. Without this, the Kustom will distort quite painfully when driven to clip.

Cheers

[Netlist]

Centauri
The amp is back to the owner for now, rehearsal time.
I recall measuring something a little above 20VRMS with a sine wave, speakers attached, no dummy load, just before clipping.
As for the square wave, I’m willing to believe its normal to have it that bad though it looks VERY ugly.

Sreten
There are no Send/return busses, but I did compare the same square wave (100-200Hz) at the in- and output of the power amp module.
With the tone controls its possible to make a more or less good looking figure at the input of the power amp.
The colorations seems to come from the power section. But as Centauri says: this is no high bandwidth amp.
Also, I’m not aware of compression circuits in there, as in the Peaveys.
The way it’s build from a mechanical point of view, PCB’s, connectors, components and wires are very similar to old Peavey gear. Only a little bit nicer and better.
If it were a no-name amp I would identify it as “Made by Peavey”

Frank
I measured the ripple on the 4500µF/50V Mallory caps and one of them was too high so I replaced both with 15.000µF/90V cans I had, spares for Peavey CS800’s.
I noticed less hum and a subjective tighter bass.
Other caps in the power amp section are tantalum. I didn’t change them, they are odd values, something like 30µF and 60uF.
Without schematic it’s working like a blind men.

The guy owns the amp for more then twenty years and only heard that distortion the past two years.
That’s what worries him. Of course, he is playing bass for more the 35 years, could be his ears. He also admits that the band plays at a higher volume these days….
For those interested this is the band.

/Hugo