Ideas to tweak my Ceriatone JTM45?

[791979]

Hi all,

I'm really pretty happy with my JTM45, however I'd like to tweak a few things and would value some opinions on what to do

So here is my wishlist

1. A little more clean headroom
2. Slightly more Fender tone (maybe more like a Bassman)
3. Reducing Bass to make the bass control usable (mine stays below 2 all the time)

So, what would you do?

[MikeMurray]

Cut some of the bypass caps, and swap some coupling caps. If my right hand wasn't in a cast, I'd explain better

[laird]

Well, a JTM45 is practically identical to a Bassman circuit-wise. Have you tried swapping a 12AY7 into the first preamp slot? That'll lower the gain (more perceived headroom) and make it sound a little more Fendery.

The first component change I'd make to reduce the bass is to change out the first preamp stage's bypass cap (it's something ridiculous like 220uf) for a 1uf cap. I prefer controlling the amount of bass as early in the circuit as possible so it doesn't swamp the middle and higher frequencies in the middle of the circuit.

-Laird

[RomanS]

On my JTM45-clone I used a 0.47uF bypass cap on the first preamp tube (still have to keep the bass pit at around 10-11 o'clock).

+1 for a lower gain V1 tube - I use a 5751, a 12AY7 has even less gain.

I personally love the full-bodied tones KT66 power tubes yield in this amp - but a while ago one of my KT66 burned out, so I used 6L6 tubes (after re-biasing, of course!) until I could get a replacement - those give the amp a slightly more Fender-like tone (less mids, tighter bass).

Most important for a Fender-ish tone: DON'T use a closed-back 4x12" cab, but some type of open-backed cab, preferably with 10" speakers (2x10" or 4x10") for a Bassman vibe.

[JohnnyCrash]

A solid state rectifier might be a nod toward more headroom and a more controlled bass.

You might also consider downloading Duncan's Tonestack Calculator and plugging in some pot, cap, resistor values. Tweaking your tonestack for your taste in Bass response might help.

For reducing gain, you can play with a lower gain tube as mentioned. You may also consider playing with the V1 cathode resistor and bypass cap. Marshall used a variety of values for the "Normal" or Bass channel over the years.

I'm with Roman on this one... an extra step towards a Fender type sound is an open backed cab. 10"s are a great idea.

[791979]

Thanks for all the ideas, I will start experimentation shortly, It will make a nice change from all the garden work I'm buried under at the moment

Being a Brit, I'm more familiar with Celestion 12's and am frankly pretty confused by Fender type speakers, and 10's

It's unlikely that I will be buying vintage speakers, so what modern speakers fit the Fender voiced thing (in 12's & 10's)