I've just had a guitar amp returned from whence it came, supposedly repaired. It's a Vox Cambridge 30 twin reverb, a transistor amp. The opto coupler went on it, because the parts are crap, and I had it replaced with a Mesa part instead. Apparently, this is a common problem with these carelessly constructed amps.

So now i've had it returned and it makes a new and dreadful hissing noise. When the amp is turned on it makes very little self-noise, but once you plug in a lead, it sounds like a Whitehouse record, even with the volume control fully down so no guitar sound comes through. This noise is unaffected by the volume pot.

I've eliminated lead and guitar faults. Obviously i'm sending it back, but i'd be interested to know what the fuck is up with it, as the engineer is reluctant to admit any fault.

gravenhurst wrote:it makes a new and dreadful hissing noise. When the amp is turned on it makes very little self-noise, but once you plug in a lead, it sounds like a Whitehouse record, even with the volume control fully down so no guitar sound comes through. This noise is unaffected by the volume pot.

This sounds like a transistor in the first gain stage (after the pot) has gone bad. This is not an uncommon fault in transistor equipment. It is sometimes called "waterfall" failure, because that's what it sounds like.

It could be a later stage as well, but it is almost certainly unrelated to the opt-coupler, which I'm guessing is for the tremelo circuit.

In other words, you have had a second, unrelated failure. Lucky you.

Steve, do you know anything about Silverburst Les Pauls? I heard Gibson stopped manufacturing them around 1978-79, mainly due to complaints realted to tone. Adam Jones from Tool uses one. I also wanted to know about Diezel VH-4 head amps and if they're worth the money.

Gibson stopped making Silverburst Les Pauls because nobody bought them.


Jay

steve wrote:

gravenhurst wrote:it makes a new and dreadful hissing noise. When the amp is turned on it makes very little self-noise, but once you plug in a lead, it sounds like a Whitehouse record, even with the volume control fully down so no guitar sound comes through. This noise is unaffected by the volume pot.

This sounds like a transistor in the first gain stage (after the pot) has gone bad. This is not an uncommon fault in transistor equipment. It is sometimes called "waterfall" failure, because that's what it sounds like.

It could be a later stage as well, but it is almost certainly unrelated to the opt-coupler, which I'm guessing is for the tremelo circuit.

In other words, you have had a second, unrelated failure. Lucky you.

Thanks a lot Steve. Could the (pretty much token) 12AX7 in the preamp have anything to do with it?

Unbelievable. I take it back to the shop Sound Control (I imagine the US equivalent is Guitar Centre) and explain that it now has another unrelated fault. The engineer plugs it in and it is not making that disgusting noise anymore. They look at me like i'm a total dick; very humiliating. I ask them if it is even conceivable that it could have been roaring at home and now sound fine, and they answer with a resounding 'no'. According to them, any faults with it are unlikely to be intermittent. Now they won't give me money back as they can't find anything wrong with it, whilst I think it is at the very least unreliable. I'm sure if I bother to lug the fucking thing home again the fault will manifest itself with a shit-eating grin.

What the fuck is going on?

Give it a good whack with your fist to see if you can get the noise going again. Then get a video camera and document it.

You said it had one tube gain stage. Well, tube preamp stages are notortious for intermittent problems, but luckily, on older amps at least, are pretty easy to fix.

My advice is to try and document the problem then return with amp and documentation.

ben adrian

Thanks a lot.