JTM45 squealing with presence controll wide open.

[Bluefinger]

Hi,

There is a JTM 45 that starts to squeal when the presence control is wide open. Tubes have been replaced with known good ones without any success. What can cause this? Probably a lead dress issue?

Thx!

[MWJB]

Yes probably, try running the grid wires to the power tubes as straight as possible (no big looping runs, definitely do not twist them together), avoiding the heater wires, shield them too. Won't hurt to see if you can shorten & or shield the preamp grid wires where possible too.

[pdf64]

As per Fender 5F6A Bassman, the JTM45 didn't have grid stoppers on the output tube control grids; after ensuring that there were no glaring problems with the lead dress, that would be my first line of attack (fit 5k6 grid stoppers at the tube socket).
Mark, why not twist the grid wires - I had always thought that to be good practice and is employed on many amps (eg BF Fenders), a twisted pair of antiphase signals in theory having an improved level of common mode noise immunity (compared to non-twisted)?
Also capacitence wise, it's just adding a few pF to the 47pF that's already across the phase splitter plates.
What's your reason for the 'non-twist' method?
Pete.

[MWJB]

Hi Pete,

I'm not aware of any stock Fender that twists together the grid wires feeding the power tubes (usually just the heater wires & PT secondaries & the wire carrying the preamp outputs from the coupling caps to the PI twisted with each other & B+ wires), it's something I have seen on hobbyist built amps though. The main problem being that doing so inevitably increases grid wire length which can make an amp more prone to oscillation...the JTM45 design is pretty borderline in this respect already...it's pretty common for JTM amps to display unwanted artefacts with the treble & presence wide open, or even when just using the bright channel.

[jrfrond]

It's not unusual for older Marshall's to oscillate at higher Presence levels. The items mentioned here i.e. the addition of grid-stoppers if none exist or the 47pF across the plates of the PI go a long way toward squelching this problem. A good percentage of amp designs have this snubber cap installed. In an amp with high-gain with the frequency response sloped as most Marshall's are should DEFINITELY have this cap installed. Don't forget, many favorite amp designs were coined in an era when guitars weren't turned up to 11.

As far as lead dress goes, outside of the heater string (which I twist up firmly on a cordless drill to maximize the CMR/humbucking effect), I will twist grid wires and OT wires for cosmetic appearance. Yes, it increases lead length, but another trick is to make sure that your signal wires are dressed down to the chassis as close as possible. An input to a tube is high-impedance, and any lead left floating up in the air is a potential antenna, even though it loaded with grid resistance.