Spark & pop in Custom Sound Cube 30

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Spark & pop in Custom Sound Cube 30

Postby Billy Bullshot on Fri Feb 11, 2011 5:04 pm

I'm pretty inexperienced when it comes to valve amps, so please bear with me on this.

I own a Custom Sound Cube 30, an all-valve amp from around 1974, built in Pudsey, West Yorks. with three 12AX7s / ECC83s in the preamp and four 6BQ5s / EL84s in the poweramp.

The other day I experienced a noticeable drop in volume. I tried changing the valves for old spares, in order to isolate a potentially bad valve, with no luck - the volume stayed the same.

Now, moving onto another problem, the amp has four springs used to hold the EL84s in place. Well while I was trying to isolate the bad valve I saw fit to leave these springs dangling into the chassis while testing, and during this testing I experienced a loud pop accompanied by a spark (visible because I was testing with the back taken off). This seems to happen every time I use the amp, seemingly after a few minutes of playing when the valves have become hot, occurring at regular intervals until I turn the amp off. It seems to occur more frequently after a few minutes of playing at high volume, when the valves have become hot. The volume of the pop is affected by the poweramp volume knob, but not the preamp knob. Again I tried changing all the valves but to no avail - every time I use the amp, just when it starts getting good, after playing with the volume turned up for a few minutes I get the same very loud pop (accompanied by a spark in the amp).

I tried turning the amp on without any valves in, left it on for a few minutes, and nothing happened, so I turned the power off and as I was about to re-connect the valves, I got the spark/pop again, which I think was due to the third spring from the left coming into contact with part of the electrical circuitry on the top of the poweramp. So it still happens when there are no valves in.

The spark occurs on the topside of the poweramp board, so I don't think it is being caused by one of the pins on the underside shorting to ground. But I could be totally wrong, as from my Google searching it definitely seems like this could be the cause.

I don't want to get electrocuted with 400 volts or whatever this thing runs at, so I haven't really done anything about it aside from blowing at it a bit because the short to ground could be caused by some random piece of metal that shouldn't be there.

How should I go about resolving this? Is this an issue I can fix myself or should I get a tech to look at it?
Billy Bullshot
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Re: Spark & pop in Custom Sound Cube 30

Postby capt pugwash on Sun Apr 10, 2011 4:30 pm

if your not sure about amps............take it to a "good" tech
capt pugwash
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