A scope used for biasing? High fidelity amplifiers want the lowest possible distortion so they can more faithfully RE
-produce the sound amplified through them. Guitar amps are not reproducers of sound, they are part of your instrument. Distortion is not only inevitable, it is usually desired.
Using a scope to set bias implies, I am assuming, that your tech is setting bias for least crossover distortion. In a guitar amp, the general consensus is to set bias by dissipation of the power tubes. If you are burning up power tubes, back off on that bias.
The bias test points built into this amp monitor the bias VOLTAGE, not current. Most of us consider that useless, HOWEVER, when set to factory specification by those test points, the result is a cooler tube that should last a lot longer.
Just for the sake of science, try setting the amp to factory spec, not some latter day "better" method. See how it works out.
Hiss is a product of gain. The more gain there is, the more hiss there will be. if the level of hiss changes, all other things remaining the same, then something in the amp or tubes is also changing.
Hiss is not hum, so ground loops are not causing hiss. They cause hum.
You can localize the source of your hiss. Do ANY controls have ANY effect on the sound of the hiss? If you can turn it up and down or change the tone of it with a panel control, then that means the hiss is coming from a point in the system BEFORE that control. Likewise, if a control has zero effect, then the hiss is coming in after that control. I don;t doubt the preamp is mainly responsible, that is where most of your gain lies.
Does the noise gate control on your amp
have any effect on this hiss? Ask your tech if the noise gate circuit has been removed or disabled. Some guys take them out. They may prefer it without the noise gate, but it is noiser that way. Or perhaps that is the issue, your noise gate circuit has an intermittent connection or part.
I have attached a noise service buttetin from the factory, ask your tech if this has been performed or incorporated into the amp. It seems to be manufacture date sensitive.
Ive read an artical that suggests the main pcb in the JSX is rated at 63vdc but the Filter Capacitors installed are only rated at 60vdc... This means as the tubes start to age and draw a little more current we are going to have a problem... And you know what the Hiss gets louder as time passes...
Well, 63v is a standard cap rating. There may be some 60v rated caps out there, but I have never seen any. In the bias circuit, which is where we would find such caps, the main bias filter uses two of them in series, so that means 126v rating effectively, and the remaining one is across the bias voltage, which is a lot lower than 60v. I am not at all aware of the relationship of tubes aging, drawing more current and somehow that causes stress on the bias caps?????
I am aware that in times past some JJ power tubes did have narrower pins, but was unaware that continues. Easy to check with a micrometer.
You are using the EL34, an not the E34L, is that right?
I like the Winged C tubes, myself. And Marshall ships them in recent amps.
Just what is the nature of the tube failures you report as short life? Are we talking power tubes or preamp tubes? Are the tubes actually failing? Or is this a situation where you hear the hiss, send it in for service, they stick in new tubes, and the whole thing repeats every couple months? In which case what you are calling short tube life is really more a matter of a recurring problem that the techs always try to solve by swapping tubes. MAybe? if that is it, the tubes are probably fine, and it is the act of swapping them out that "fixes" the amp.