JSX problems. Am I screwed?

[Nebulous]

Hi guys

I always hoped that I wouldn't have to make one of these threads, but just then when I was about to do some pre production recording I turned my JSX on, let it warm up as I usually would, then as I took it off standby I was greated by an almighty boom of a ground hum. It's about as loud as the amp is when turned up about half way. It's ringing at around a G (if that matters). The volume knob does not make a difference, switching to a different gain or clean channel does nothing to help or reduce it. I checked all knobs and switches (volume, FX loop, impedance) tried the low and high gain inputs. I turned the amp off for 5 minutes, then let it warm up again.
Still the same.

I have noticed recently that while warming up it would be making some interesting sounds which would come through the speakers, even when standby is engaged.
I should have checked the tubes then for any that may have been microphonic.

I still have my old set of tubes lying around, but I'm a little hesitant to touch the amp at this point. The volume of noise it makes and the amount of times I've tried to turn it on today means that I will not be trying it any more (maybe once in the evening just to be sure, if anything).

WTF is going on with my JSX? Have any of you had a similar problem?

I will probably try swapping the tubes again later in the week, unless the consensus around here is that it's not tube related.

Please help!

 

[Joematthews]

G means grounding problems as far as I know, take it to a tech, I wouldn't do anything so as not to damage it further.

Joe

 

[GuitarHack]

Symptoms sound VERY similar to what I had with my 6505 a couple of weeks ago. Don't try it too many more times as mine got worse whilst I was diagnosing it and ended up doing some minor component damage. Long story short, I suspect you have a dud Powertube. The hum is caused because of the out of balance on your output transformer from 1 half of a pair of tubes being dead. Put your old powertubes in (flick the bias switch back back from 6l6) and give it a quick test. Dont adjust your bias pot since you have it set to where you want it with the 6l6. It will not matter for the purpose of the test if it isn't set exactly right. If problem persists, see a tech. Good luck finding a good one in Melbourne. Come to think of it, I have spoken to the authorized Peavey tech some time ago. Call Davis Music centre and ask for his contact details. They gave them to me, I just don't know what I did with them, it was a while ago.

Mine ended up shorting some flyback diodes and opened some screen resistors, which was an easy repair to do, I would hate to pay for it though.
On the other hand it could be as simple as a dud preamp tube. Do you have any spares to swap in/out? My money is on the Powertube though.

All the best.

 

[Nebulous]

Thanks for the insight guys.

I actually tried it again just before, and it was dead quiet.... I almost with it was still there so I could diagonose it or take it to someone. Last thing I want is for it to go away then haunt me on the night of the performance.

I'll see how it goes over the next few days, I may just throw the EL's back in untill this gig is over.

 

[AudioPhile777]

YEah, i had a similar issue with mine... changed the tubes, all better now.

 

[Ermz]

If you can't work it out yourself, take it to Tim Evans.

 

[Lasse Lammert]

If you can't work it out yourself, take it to Tim Evans.

damn, how did you change your name?
I sent metalages like 3 PM and he never replied...I want my old postcount back, lol

 

[Line666]

damn, how did you change your name?
I sent metalages like 3 PM and he never replied...I want my old postcount back, lol

http://www.ultimatemetal.com/forum/name-changes-285/

 

[Ermz]

It took the better part of a month, but they finally got around to changing it!

 

[bibz]

Def sounds like a tube problem to me, give swapping them a go. If pain persists, 5 year warranty

 

[Nebulous]

Def sounds like a tube problem to me, give swapping them a go. If pain persists, 5 year warranty

Good point!

 

[GuitarHack]

Tubes are covered for 1 year AFAIK.

 

[Nebulous]

Heh, so powertube swapping hasn't helped. As soon as the amp cools again I'll give the pre's a go.

Maybe I see where PODs can be prefered....

 

[Wolfeman]

Screen grid resistor might have gone out... Did you actually try different power tubes or did you just swap their places?

 

[Nebulous]

Took my JJ 6L6's out and put the original EL34's in. As I type this, the amp is warming up again with the original pre tubes in it.

The problem only seems to occur when the amp well and truly warms up. When I had the old power tubes back in, I got about 5 mins of playing and bais time before the hum started.

 

[Nebulous]

Screen grid resistor might have gone out..

Is this a common thing? Is it expensive to fix?

 

[GuitarHack]

Is this a common thing? Is it expensive to fix?

Can be common. Sometimes a powertube shorting can draw excess current and open circuit them. I have had 2 go on my 6505. It cost me 35 cents each for the resistors from Dick Smith and about 2 mins soldering and 20 min dis/re-assembly.

 

[GuitarHack]

If it was this though, it should happen straignt away, not after a warmup period though I would expect. Could be as simple as a preamp tube.

 

[Nebulous]

The old pre tubes went in and 30 mins of playing later, all seems ok, for now.
I'll give it afew hours tomorrow just to be sure before the performance on Sunday. If it's still there, may just borrow a friends POD.

 

[bibz]

Yep, tubes are weird things. They have certain uncontrollable characteristics and lifespans, as you're experiencing. But fuck me do they sound good!