testing an Gallien Krueger bass amp...

[3nity]

I have a friend who bought a 400RB-III bass head for cheap...
He dont knows ifts working as the seller said was selling it as it is.

Now we dont have a bass cab to test it right away....

What can i do to test it if needed? without a bass cab?

Still on the way but just wanna get ready..before it arrives..

 

[tv]

For sure you will need to solder up some sort of a dummy load (whether it's a 4ohm or 8ohm..) and then figure out how to make a resistive divider to drive headphones.

.. the easiest would be to figure out where to borrow a bass cab for a quick test-drive. Maybe look out for some rehearsal places with gear in it and go there for a check?

 

[clintrubber]

Hi,

If the state of that amp can be 'anything' then I'd pay attention & check before connecting
any cab that you don't want to hurt.

Don't send any signal into the amp, then connect a lead to the speaker output, power-up
and measure the DC-voltage on the output. Should be not too high, say a few hundred mV.
A few Volts is suspicious. A fen tenths of Volts is a fried output-transistor, in which case the amp might have fried that borrowed bass-cab 

If this first 'test' is passed then you could connect any speaker to get sound, be it from a gtr-combo (disconnect the internal amp) or even a hi-fi speakerbox - as long as you don't play too hard. Start with volume down etc (the usual precautions).

The more scientific approach might be to try to get the factory-schematic and check the DC-Voltages before connecting signal-source (bass) & speakers.

Bye,

  Peter

 

[mrphotodude]

definatly have to do some measuring before you hook it up to any good speakers.
I toasted a 12" on my 400rb when the power section was messed up.
You should be able to test the preamp direct out easily enough though.

 

[NewYorkDave]

Here's a tip vaguely remembered from my "guitar amp tech" days: tighten up all the screws on that G-K. They used an odd grounding scheme that relied on good continuity through the chassis. When the screws loosened up, failures would result.

Be sure to check the output terminals for DC before connecting a speaker. I seem to remember that output transistor failures were fairly common. It's been years since I've worked on a G-K--so alas, that's all I can really remember besides the fact that I didn't particularly enjoy working on G-K amps

 

[tv]

BLAH - BLAH - BLAH ...

if you are unsure if the unit --works-- you can test it like that:

-get 500watts worth of lightbulbs and wire them in series with mains (220V...) power supply (more precisely: parallel the bulbs themselves and wire _that_ in series to mains). This should act as an emergency 'guard' if there's some short in the amp. Take care about grounding and safety measures for "deadly" circuitry and voltages.

-get 200watts worth of lighbulbs and wire them in parallel, and use that as a dummy load for the speaker. If the amp works, bulbs should simply:
a) produce no light when there's no signal present in the amp
b) they should glow a little when there is some signal in the amp
c) if the bulbs glow when there's no signal in the amp, it means that there's either some DC at the output or there's some oscillation going on

hope that helps.