Any Landmines Waiting for JTM45 Builder?

[The New Steve H]

Finally, the Rock has returned to Amp Garage.

Well. Not the Rock, exactly, but me.

After a year or so of teasing, the musicians at my church have finally produced an individual who is going to go forward with a new amp. He loves the 5f6A clone I built, and he wants a JTM45. We will probably use a Dirty Dawg box instead of a prepunched chassis, because we are cheap, and we are considering getting a Woodcraft drawer instead of spending his money on a cabinet. Also looking to use a Classictone transformer bundle instead of going broke on Mercury Magnetics.

Here's what I want to know. Are there any big pitfalls I have to look out for when building one of these things? I would think it would be like a 5f6A, except a little easier, since we can use can capacitors instead of soldering little caps in a row.

I also have a question about bridging the inputs. When I do this on my 5F6A, I get added hum. I assume this is because I'm creating a big wire loop for EM fields to go in and out of. I'm wondering if I can reduce this by putting wiring inside the amp, between the jacks, with switches to do the bridging.

[Jana]

Look at the schematic for the original Sunn Model T. It had 5 inputs--4 that were just like a Marshall and a 5th jack that, if you plugged into that one, plugged you into both channels. You could leave out one of the "low" inputs in your JTM 45 and use that as the both in jack. You won't have to drill any new holes and it will look just like it is unmodded.

[The New Steve H]

This thing won't look anything like a factory JTM45, so I'm not too concerned about the appearance, but that's a neat idea.

[Lynxtrap]

Can caps aren't necessarily a good thing. They might even be one of the pitfalls with that amp. In a worst case scenario, at least.
Hum, ghost notes etc...

[The New Steve H]

I didn't know that. I liked the idea, in principle.

[Alexo]

Had a hard time finding new production KT66's that could take JTM 45 voltages, despite their spec's saying they can. That and the load can be dangerous to the screens in these beasts sometimes.

[The New Steve H]

I'm totally in love with the Bassman clone, but this guy likes a harder sound.

I told him he really needed to check around and get tube advice, because that's the main area where I will be of little use to him. I can build it and make it work, but I have no experience with the Marshall sound.

[overtone]

Can caps aren't necessarily a good thing. They might even be one of the pitfalls with that amp. In a worst case scenario, at least.
Hum, ghost notes etc...

I have to ask, because I have no experience with the JTM45, but isn't the ghost note issue due to the low filtering chosen for the power supply in that amp? Why would the shape of cap make a difference?
Best, tony

[ToneMerc]

I'm totally in love with the Bassman clone, but this guy likes a harder sound.

I told him he really needed to check around and get tube advice, because that's the main area where I will be of little use to him. I can build it and make it work, but I have no experience with the Marshall sound.

Welcome back, I wondered if you were still on this side of the dirt.

I don't think a JTM45 sounds really that much "harder" at all. More than once I've seen people build them and become disappointed because it's not what they thought it was. What's his point of reference for a JTM45?

TM

[The New Steve H]

Danged if I know. Good question.

I know changing the 5F6A's tubes from 12AY7 and Svetlana 6L6 to 12AX7 and Winged C 6L6 will give a harder, drier sound. Did that the other day. I didn't like it at all.

[The New Steve H]

He's talking about rigging the amp so he can use EL34s if he wants. I don't know much about the JTM45, but looking at the schematic, I see where Mojotone specified different biasing resistors for different tubes. Are the transformer requirements for these tubes so different that using the same transformer for both is a stupid idea?

[Structo]

The only thing I can add since I used to play electric guitar at my church is that a JTM45 is a very loud amp for a venue that is picky about stage volume.

I tried three different tube amps and finally resorted to my Line 6 POD XTL plugged straight into the board.
So then the only thing I got on stage was a wee bit of my guitar in a wedge monitor in front of me with the other things I wanted in the mix.

I was completely at the mercy of the soundman and my wife told me many times she couldn't even hear my guitar.
Now if I was a complete hack on the guitar and my playing disrupted the whole picture I could understand but it was usually due to a soundman that was asleep at the wheel.
I would look at him during a solo and he would be chatting with somebody totally oblivious to what we were doing up front.....

[Cliff Schecht]

Steve are you dead set on a JTM45? Take a look at the JCM800 series stuff, the circuits are close but the JCM series is a bit more robust than the JTM45. Circuits are very similar though. The 2203 Master Volume might be your ticket..

[The New Steve H]

I didn't choose the amp. I would have picked a Bassman. I want EVERYONE to have a 5F6A!

I can pass the info along. I'm encouraging him to join Amp Garage and ask his own questions, but the big danger is that he'll get so much information he won't know what to do.

[M Fowler]

TriodeStore spec the JTM45 with KT66 using 8k OT

Mojotone spec the JTM45 with EL34 using 3k4 OT

Hope this helps.

Mark