JTM45 Build

[pdf64]

I just read this entire damn, thread, took me two hours, hoping to learn something.

Well, hopefully you did? eg that if enough variables and tolerances happen to line up in a certain way, even great circuits that have been well implemented might sound bad.

[Littlewyan]

That account has only posted once and it was the post on this thread.

[JazzGuitarGimp]

That account has only posted once and it was the post on this thread.

Yup, that's why I suspected reel

[Littlewyan]

This is ridiculous.

[Littlewyan]

Well I've been playing the amp a lot and it just doesn't have enough grit for my liking. Its very smooth. Tried different valves which have changed the tone a bit but still theres not much crunch. Weird considering Johan Segeborn on youtube has played through a lot of 67 Marshalls and they had slightly more balls than my amp, but I suspect the resistors in those amps have drifted upwards over time, particularly the feedback resistor.

I'm going to try increasing the NFB resistor to 47K.

[JazzGuitarGimp]

A clean boost pedal between yor guitar and amp might get you where you want to be.

[pdf64]

It took me a while to get used to the way KT66 respond.
As mentioned, high presence settings (which 27k from the 16 ohm secondary acts to increase) and bigger bright caps help.
And recording it helps to re-calibrate your expectations, ie hearing it as a listener, rather than the musician using it.
Now I've made that re-calibration, my Marshall EL34 and BF Fender 6L6 type amps can seem harsh and overly aggressive.

[Littlewyan]

Yeah you're right. I'll give it another go later before modding it. Someone on the Metroamp forum suggested some settings for me to try.

I'm not a pedal guy am afraid JazzGuitarGimp . I plug straight in. I do have a boost on my Les Paul but all it does normally is increase the high end and make it sound nasty so I never use it. Although tbf it can be adjusted, so might play with that at some point, see if I can make it useful.

[martin manning]

IIRC, players who used them back in the day (Clapton, e.g.) wanted to play JTM45's flat-out to get the sound they liked. To the the gimp's point, boost and distortion pedals changed all that.

My 2 cents on chasing the problem that didn't exist: It's only an academic question since the settings that produce it are highly unlikely, but it is odd that even Marshall enthusiasts don't seem to be aware of this quirk.

Remind me what the settings that bring it on are- was it B & M all the way down, and T up?

[Littlewyan]

Not even George Metropoulos knew about it.

Thats right, bass and middle at 0, treble and volume at 10.

Just played through it again and its quite mid scooped, even with the mids at 10. Going to try some different pre amp valves before modifying it.

[martin manning]

I put together a JTM45 simulation following the Metro schematic and found the PI wants to oscillate at ~90kHz with high Treble and low Bass and/or Middle settings. If I drive it hard enough to square-off the output the oscillation will stop, and under some lower drive conditions the oscillation will damp out. I also found I could kill it by increasing the fizz cap (47p across the PI plates) to 220p. The simulation is also better behaved with a 100k slope resistor, where a 100p fizz cap will keep it from oscillating.

[Littlewyan]

Yeah I tried a 250pF Cap across the plates in addition to the 47pF but it just wouldn't play ball.

After playing with it for a bit I just couldn't get the sound I wanted. Changed the feedback resistor to 47K and now its rocking! It now sounds like the Marshalls that Johan Segeborn tried out. They all had 27K feedback resistors, so either the resistors have drifted upwards or they're on the 4ohm tap or speaker socket. Mine is on the 8Ohm Tap.

It would definitely benefit from having a split cathode and the later 33k/500pF Tonestack for playing 70s and up music but for now I'll keep it at 67 spec.

[Colossal]

Have you measured the PI swing/symmetry since you changed the feedback resistor?

[Littlewyan]

I haven't no. Been getting ready for a gig tonight and I just wanted to make sure the amp had enough crunch so it could be used as a backup. The TW Express is my main amp

[martin manning]

The PI output amplitude will go up for sure with less FB, but the symmetry won't be affected.

In the simulation, the presence setting is important too, so it seems the problem is rooted in the FB of high frequencies, which can be reduced by lowering the FB across the board (larger FB resistor, 47k e.g.) or setting the presence pot higher. Perhaps the HF is phase-shifted enough to get near positive FB.

Another way out is to put a small 1n-5n across the presence pot to dump the highest frequencies to ground. Then you can increase the NFB if that suits you.