jtm45 mods

kolumbus · Post by **kolumbus** » Tue 04/25/06 4:41 pm

Hello everybody!

Got my Ceriatone JTM45 2 weeks ago and after testing it at home and gigs,the low end seems a bit muddy for my taste. I know a lot of people like that sagging tone,but I`ve used to 100W SPL and it`s too different.

There`s 2 choices I plan:
1)another 250 uf cathode bypass cap in parallel with 820ohm resistor for the first preamp tube(pin 8),without the jumper wire
2)SS rectifier

Any ideas? Is worth to try?

Thanks!

Tof · Post by **Tof** » Tue 04/25/06 4:43 pm

the speakers ?

Post by **rjgtr** » Tue 04/25/06 4:46 pm

Some ideas...

What are the Power Filters? If they are vintage 32x32uf and 16x16uf cans, change them to 50x50uf cans for a tighter sound. Even just changing the first one and moving the 32x32 down will help.

Try a plug in Solid State rectifier and rebias.

Change the power tube coupling caps from .1uf to .022uf for more of a classic Plexi bite.

You could Replace the 250 uf with a 1uf or 25uf but that might be too much.

kolumbus · Post by **kolumbus** » Tue 04/25/06 5:23 pm

Thanks for the replies.

I mounted the amp into the 1*12 combo with V30,but have checked it with bigger cabinets also.
I think it has something to do with preamp stage because it gets muddy even at low volumes when I use pedals like TubeScreamer and SD1,which don`t produce much of gain at all.

Groundpath · Post by **Groundpath** » Wed 04/26/06 1:04 am

rjgtr wrote:Change the power tube coupling caps from .1uf to .022uf for more of a classic Plexi bite.

You could Replace the 250 uf with a 1uf or 25uf but that might be too much.

This is were I would start. Simple mods to do. Changing the filtering caps for me would be a last resort.

Post by **zaphod_phil** » Wed 04/26/06 5:18 am

kolumbus · Post by **kolumbus** » Wed 04/26/06 1:29 pm

Thanks for the replies!

Noticed that I was wrong with the 2nd bypass cap and resistor values. They should be .68uf and 2.7K as Plexi preamp.

I will try it as soon as possible.

kolumbus · Post by **kolumbus** » Wed 04/26/06 1:47 pm

Here`s the sound sample of the amp

kolumbus · Post by **kolumbus** » Wed 04/26/06 4:51 pm

So this is a quite common mod?

The switch is a very nice idea. Should the jumper wire between 3 and 8 pins of V1 also swithced off/on with the 2nd cathode bypass cap and resistor switching?

kd · Post by kd » Wed 04/26/06 5:51 pm

Go to the link below and look around on the site, you will find a wiring diagram for a DPDT switch that will allow you to run either a shared cathode on V1 or run split cathodes and at the same time add a .68 cap across the cathode of V2.

JTM45@yahoogroups.com

Ken

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