"Wrecking" a JTM45

[LeftyStrat]

I've been chomping at the bit to build a wreck, but I promised myself I would build a JTM45 first. Though I've built a couple of AX84 designs and have been doing fx builds for many years, I wanted to get my experience level up.

Well, my JTM45 turned out fantastic. It is the quietest amp I've built to date, absolutely no hum. I've tweaked it a bit away from the original circuit, using EL34's (1k on the screens and 5k6 grids, mismatching the 6k6 load to get 3k3), and split the first cathode (820R/220uf and 2k7/.68uf). The clean to "Jimi-clean" (think "Wind Cries Mary") are to die for.

But now I've started wondering about applying some of the wreck ideas to this circuit, without losing some of the magic it currently has. I didn't want to move the tone stack, so I thought about just replacing the second gain stage's 820R cathode resistor with 10k like the third stage in a wreck. Strangely it seems to lower the distortion, but up the sustain.

Anyone else try anything similar? I have a pretty beefy power supply, so now I'm wondering if I could do something like add another 12ax7 and have a wreck style channel (minus the tone stack) and a JTM45 style channel, both feeding the standard Marshall cathode follower/tonestack.

Also, if any of you are in the Seattle area, the "Old Technology Shop", where I score all the NOS tubes at great prices is closing down. The last date is June 28th, and he's started selling all his old transformers by weight at $1 a pound! I just spent $20 on five transformers, ranging from PTs to chokes to OTs.

[erigm]

I would suggest converting the cathode follower (V2b before the tone stack) to a gain stage, and more specifically like a Twreck's 3rd stage (with a 10k cathode resistor). So now you would have V1a and V1b as typical marshall inputs and the next tube as a Twreck stages 2 and 3, followed by a plate driven marshall tone stack. Does that make sense?

[LeftyStrat]

Interesting, I'll give that a try.

[JimiB]

so change V2a from 820 to 2.7K
and V2b from 100K to 10K ?

[LeftyStrat]

so change V2a from 820 to 2.7K
and V2b from 100K to 10K ?

It is slightly more complicated. Change v2a from 820 to 2.7k and add a 22uf bypass cap in parallel with the 2.7k resistor. Then you'll need to modify v1b from being a cathode follower (with the signal coming off the cathode) to a standard gain stage by:

- Add a 100k plate resistor from B+ to pin 6 (it is currently connected directly).
- Take the signal off of pin 6 and route to the tone stack (it currently comes off pin .
- Change the cathode resistor on pin 8 to 10k.

Of course, there is a high pass filter between these two stages composed of a .002uf cap and a 150k resistor. So the cap would go from pin 1 to pin 7 (they are directly connected in the original) and then run the 150k from pin 7 to ground).

I haven't tried this yet. I'm so satisfied with the tone of this amp, I'll probably leave it alone and build a wreck from scratch.

[erigm]

LeftyStrat,

Yeah, you got it right. That was exactly what I meant. Let me know if you try it and your results. I was curious how this small mod would sound in a Marshall ... and I wondered what Ken Fischer did with his marshall mods and thought maybe this was one of them.

[LeftyStrat]

LeftyStrat,

Yeah, you got it right. That was exactly what I meant. Let me know if you try it and your results. I was curious how this small mod would sound in a Marshall ... and I wondered what Ken Fischer did with his marshall mods and thought maybe this was one of them.

Yeah that was what I found interesting. I've read about Ken modding Marshalls and wondered if he made a similar mod to the wreck preamp specs. It should be interesting, I'll probably give it a try tomorrow.

[LeftyStrat]

Well I gave it a try and got complete havoc. I think the standard JTM design is more forgiving of layout.

Also rather than reconfigure the cathode follower, I cascaded to two inputs and used the standard wreck values:

v1a (1k5/22uf) -> v1b(2k7/22uf) -> hpf -> v2a (10k) -> cathode follower

I had some squeal issues and once fixed I got a splatty sound when I turned it up and hit it hard. I don't know if the CF was being overdriven or what. But just as it was going into a sweet sustaining overdrive, I would get this wierd crackle on the attack of each note.

Anyway, I modded it back to a JTM50, because I'm in love with the clean to slight grit tones on this amp. However, I did leave v2a with the 10k resistor, it seems to sweeten things up and increase sustain.

Another issue is with the differences in plate voltage of the jtm vs the wreck. My preamp and PI plates are much lower than the wreck, not sure if this has anything to do with it.

[markd]

But now I've started wondering about applying some of the wreck ideas to this circuit, without losing some of the magic it currently has. I didn't want to move the tone stack, so I thought about just replacing the second gain stage's 820R cathode resistor with 10k like the third stage in a wreck. Strangely it seems to lower the distortion, but up the sustain.

Lefty,
A preamp gain stage with an unbypassed cathode resistor(like V2a) loses gain as the cathode resistor value is increased. Going from an 820 to a 10k cathode resistor at V2a will noticeably reduce the gain of the whole preamp. Adding a cathode cap across that cathode resistor(make it switchable if you can) will restore the gain of that stage without losing the smoother tone and sustain you get with the 10k cathode resistor. I have built a few JTM45's and have a test amp with switching for the cathode resistor, and cathode cap, at V2a.

If you can make the 820 to 10k cathode resistor switchable it is interesting to go from one to the other. The 820 will cause that stage to clip asymeterically at the grid first. The 10k will cause that stage to clip asymeterically at the other end of it's swing first(at the plate). Both will cause asymeterical clipping but the difference in tone is clearly noticeable. The 820 has the classic "crunch" of the JTM45. The 10k is much smoother sounding. A Marshall fan will almost always prefer the 820, IME.
I have tried several ways to switch between the two cathode resistors(with them in series and in parallel) and it causes an unacceptable pop. A 10k pot in series with an 820 resistor works better and gives you an opportunity to try other bias options.
hth, markd

[LeftyStrat]

Hi Mark,
Thanks for the info. I just gave it a try and I have to agree with your assessment. Smoother distortion and nice sustain (I had notes feeding back at a low volume), but you loose the Marshall crunch. I also loose my favorite semi-clean tones (Hendrixy clean with a little hair).

Interesting experiment and sort of confirms my suspicion that the 10k stage in the wreck has a bit to do with the magic and sustain. But I'll probably switch this back for the Marshall sounds I love. Plus, it gives me a reason to build a real wreck.

[Analog Assassin]

I'm incredibly curious about this. I built a Ceriatone plexi 50 kit with a PPIMV and while it's got a great crunch, i've been trying to squeeze some extra sustain out of it. I've reduced the NFB, I've reduced filtering to increase touch sensitivity, but it seems relegated to being a "rhythm" crunch amp, where I'd like to smooth it out just a bit and make it easier for leads.

Maybe 10k is a bit extreme, have you tried any values in between?

[Richie]

Although a bit different, look at the 2203 schem, it used a 10k on the cathode. Also some of the different model marshall used a .0022 coupling cap for the lead or bright channel.
The express uses a plate driven tone stack more like a fender, and the values are standard fender or marshall.
People have modded the marshalls fenders etc, useing clipping diodes and a number of different things. Or trim pots in place of the 10k resistor to fine tune it. Switchable etc..

On the 18 watt site, i posted a T-Rex,which was a blending of the 18 channel, with a sort of TMB wreck channel. Had a few people build it and posted some sound files of it.
And over on metro amps, Rockstar [forum name] built a wrectified sort of plexi amp,and posted some sound files of it. The link should be listed there.

Their are so many things that have been tried with the JTM 45 or bassman circut. but you never know when you might try something no one else has.

[PCollen]

[quote="LeftyStrat"]I've been chomping at the bit to build a wreck, but I promised myself I would build a JTM45 first. Though I've built a couple of AX84 designs and have been doing fx builds for many years, I wanted to get my experience level up.

Well, my JTM45 turned out fantastic. It is the quietest amp I've built to date, absolutely no hum. I've tweaked it a bit away from the original circuit, using EL34's (1k on the screens and 5k6 grids, mismatching the 6k6 load to get 3k3), and split the first cathode (820R/220uf and 2k7/.68uf). The clean to "Jimi-clean" (think "Wind Cries Mary") are to die for.
quote]

I'd suggest replacing the 820/220u with 1.5K/22u for a more Fendery clean.

For 'Wrecking' the JTM45, I'd suggest buying another circuit board to wire up, rather than re-working your original. With the JTM45 board topology, you could easily build a TW Rocket pre-amp/PI and couple it to your JTM power supply and output tubes. I'm in the early process of building up a KOMET board to install in my JTM-clone

[Teleguy61]

For 'Wrecking' the JTM45, I'd suggest buying another circuit board to wire up, rather than re-working your original. With the JTM45 board topology, you could easily build a TW Rocket pre-amp/PI and couple it to your JTM power supply and output tubes. I'm in the early process of building up a KOMET board to install in my JTM-clone[/quote]

I've been thinking right along these same lines. don't want to rebuild the 45 clone, it works great. New board seems like an option.
Let us know how it goes.
Good luck!
George

[PCollen]

For 'Wrecking' the JTM45, I'd suggest buying another circuit board to wire up, rather than re-working your original. With the JTM45 board topology, you could easily build a TW Rocket pre-amp/PI and couple it to your JTM power supply and output tubes. I'm in the early process of building up a KOMET board to install in my JTM-clone

I've been thinking right along these same lines. don't want to rebuild the 45 clone, it works great. New board seems like an option.
Let us know how it goes.
Good luck!
George[/quote]

I'm building up the Komet turret board at this time.. a little each evening, taking my time. It's an economy project; I'm not trying to closely duplicate the look or esthetics of the Komet, only the raw circuit. I'll post a pic of my board in a few days..time will tell if it 'works' well.