halving output of a jtm45. please help....

i'm looking at amps and have been thinking about a jtm 45 head. i'd like something with the top end volume of the 45, but would love to be able to halve the output for smaller situations (a la tonemaster, but who can afford one of them?). is there a mod that can be done to switch between full and half power? how much do you think the mod would cost and has anyone tried it? would it affect the tone at all, or just make it break-up at a lower volume?

cheers

A half power (pentode/triode) switch will alter the tone...mids mids and more mids with the top and bottom chopped off, great if you want to play Boston...not great if you're looking for a good Marshall sound.

One option is to put 6V6s in the amp, a bias adjustment is all that is necessary and if you want even lower power you can swap the 5AR4 out for a 5V4 or 5Y3. The cool thing is that savvy tube dealers these days can match you up a set of EL34s (6L6/5881/KT66 too) and 6V6s that will both be biased correctly in the same amp...Not quite as convenient as a switch, but still very easy to go from one to the other. The 6V6 trick also works great in most '70s 50w amps.

FWIW, I owned one of these until recently when someone offered a trade I could not refuse (a '77 fawn 2x12 combo!).

thanks for the info, tim. did you find there was much of an output drop with the different valves?

tim spells it out pretty plainly....

an attenuator is always an option too.....

d

Use an attenuator and adjust EQ according to the final volume. Which attenuator is the best?? It depends on who you ask. :lol: Starting with about 2~3dB insertion loss all the way down to no volume is pretty important, though.

thanks for the info, tim. did you find there was much of an output drop with the different valves?

Enough to where you can crank it up in places you couldn't before. A more natural sounding solution than an attenuator...and a lot less dough. Also, the JJs are proving to be excellent 6V6s.

What you might want to check out is a Winfield Thomas "Winfield" amp. I have a jtm 45 built into a 2x12 with Weber blue dogs in it and a Tonemaster with the original cab and they both sit around my house lookin pretty while the Winfield gets all the work. It has 4 el84's with a half power switch turning 2 tubes off, is handmade and relatively inexpensive. I can't reccomend it enough.http://www.rfd.cc/winamp/

I owned a JTM45 for quite a while, please don't do it its not necessary. Once you add in a loud drummer and a bass player with a 200 watt or 400 watt rig, you'll need that power.

The JTM45 reissues (I assume that's what you want) is 30 watts before distortion. 45 -50 watts at full crank. I used mine inside at a local blues club that seats 120 max, and another small bar and grill (60-90 people) and the headroom was just perfect for these environments.

Here's what I'd do:

Throw in some KT66's, then change the filter caps to vintage correct values of 32x32 and 16x16. These two small and very inexpensive mods will make the amp open up and give up the goods a little nicer. Now get yourself a nice 2x12 cab with Greenbacks. This is the tone of legends. Use a guitar cable to connect guitar straight to amp, using normal input #1 and use a short jumper cable to jump bright and normal channels.

Dial in the tone stack like this:

Presence: 6
Bass: 2
Middle: 7
Treble: 5
Bright vol: 4
Normal Vol: 6

Play aggressively and you'll thank me later. A Fulltone Fat Boost works very well in this setup if you have to use a pedal, also Vox 847 wahs, if you like wahs.

Frankly I like the straight in approach, and using the volume knob to go from clean to mean, to crunch, and everything in between.

I've also got a Winfield. I seldom use all four EL84s keeping it at 1/2 volume most of the time. The amp is loud enough for most gigs/jams/etc. on 1/2 power. It's nice to have the switch, if I need more juice. The amp has a lot of Marshall vibe.

thanks for the input, lads. first off, the winfields sound interesting, but being in either australia or japan i've never seen nor heard one (importing cops a huge duty, and i'm just not interested buying an amp i haven't heard).

tim, super-valuable information. thanks a bunch. it's good to know the amp can be toyed with and retain it's sound. it's probably topping my list. cheers.

jtm45, i DO play aggressively... i guess that's my problem ;) in some of the settings i've been playing in (using a marshall tsl602 60w 2x12... not mine so don't laugh) people piss and moan about my volume. i've just been turning up to get some tone from the clean channel. my main concern was if the 45 could be jiggered with to make it fit into the quieter situations that i sometimes find myself. turns out it can, so i'm happy. don't get me wrong, i'll push the bastard as much as i can to get tone. i also like the straight-in approach, but do have an old 60's vox wah (cherish it) and like to push the front end with an analogman ts-9 (...and the pulsar trem gets a lot of use these days)...

cabinets are another question. i was thinking 2x12, but would you suggest open or closed for the 45?

I prefer 2x12's to be open back.

One cab type that works exceptionally well with a JTM45 head is the old 60's Vox Thomas Organ cabs, especially the early ones, because they're baltic birch, and have the wonderful Gold Bulldogs speakers. Also these cabs have the unique football shaped cutout in the back so it is mostly closed but has just a small football shaped opening. Much like the new Tone Tubby cabs.

The new Greenback RI's also sound great in a cab like this. I know people always say that Greenies sound best in a closed back, but they also do well in a "mostly closed" back scenario.