Changed valves - hiss

hungrymark

Just changed the valves on a blues junior (don't start) and there's a hiss as the volume is turned up. No guitar, just letting them warm through a bit. Any ideas?

ICBM

Do you mean the master volume, or the 'volume' (gain)? That will help identify where the problem is - if it's the gain, it must be coming from the first preamp valve.

If it's the master volume it could be either the first or second preamp valve. Change the old ones back one at a time and see if either of them are the main culprit.

What brand are the new ones?

Has the amp been reset for 240V?

hungrymark

Haven't reset any voltage, the valves are JJ and apparently matched. Seems to be the gain, but it's quietened down a bit now. 'Bedding in' a bit maybe?

ICBM

Possibly. Some valves are noisy for a while until impurities in the cathode coating are burned off.

It's worth resetting the voltage - it makes the amp run cooler, slightly quieter, and reduces the tendency for some of the common problems including arcing and catching fire .

To do it you just need to swap two push-connectors - the one with the white/black-stripe wire on the power switch, and the one with the black wire on P6 on the PCB. You may have to remove the cable tie on the wiring bundle, although Fender don't do it up very tightly and you can often pull the wires through.

hungrymark

I swapped the preamp valves around and it's fine now, thanks for the help!

Winny_Pooh

It's the tone vapors escaping, check you don't have a leak.

Keefy

ICBM said:

..,

It's worth resetting the voltage - it makes the amp run cooler, slightly quieter, and reduces the tendency for some of the common problems including arcing and catching fire .

...

Problem? Nah mate, that’s a feature of these amps.