Calling all jtm45 kit experts..cuz I ain’t one...

Get support and show off your MetroAmp JTM 45 kit builds.

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Tcaradonna
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Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Tue Jan 22, 2019 2:57 am

I’m handy with a soldering iron, and finally getting ready to make my first amp.

I picked the jtm45 more or less based on its legendary status, not because I have a particular sound in mind. I’ve done lots of wiring before so I’m sure I can follow the directions and do this nice.
I’m looking at the parts I want to go in it and I basically only want the best stuff in it. (Yes I’m getting all the nice cap and resistor packages available on the site) but I still need some help deciding. (This is where I need you!)

First up...Mercury Magnetic or Mercury Magnetics?
It seems there are several different ones. The radio spares or the tone clones. Patrick at MM tells me RS is the way to go, but it seems I’d lose out on selectable impedence. Bummer. How’s the tone clone? I like the flexibility, but wondering if I’d be missing something by not getting the RS. Also either way I go, do I need to worry about how to wire them up based on the instructions? I want something to drop in without modifications to the instructions.

Second!
Kt66 or el34 or 6l6 tubes. Which would you pick? I’m thinking kt66, but it’s kinda an oddball tube for me (I have lots of 6l6 amps) and more expensive to replace. Is it hard to switch between them? I read somewhere that when using kt66, it changes the output Impedence of the output transformer. Is that true? How do folks deal with that?

Last...the whole package...
I’m curious about people’s thoughts here: I don’t need an amp that looks exactly like a the original, which in my was too huge. I see monotone has something called the tall 20 head cab that is much much smaller. I think it would be awesome to have a more lunchbox sized amp for carrying around. Anyone else doing this?

Thanks in advance for you highly valued replies.
Cheers!

Tcaradonna
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Thu Feb 14, 2019 1:43 am

Ok, a few choices made...
I went with the Mercury toneclone rather than the radiospare. I decided the ohm switch selector was too important to give up.

Kt66 it is!

I picked up a mojotone tall 20. Damn sweet little package, I like it’s compactness, and still has a Marshall vibe.

Ok, so I have a question:

What is the correct way to mount the power transformer?
Do I take the nuts off and mount it flat on the chassis? Or do I keep the nuts on and have them act as standoffs on the chassis? The internet seems to show examples of both. George’s GPM45 seems to show it mounting flush (taking the nuts off) and with the bracket support on the underside. I’m leaning toward the George method, but just want to be sure before I mess up a 200 dollar tranny...help!

Tcaradonna
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Thu Feb 14, 2019 1:49 am

https://ibb.co/hWVKtp8

Pics, case u interested.

harleytech
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by harleytech » Thu Feb 14, 2019 6:55 am

I have always mounted them with the nuts on as standoffs as you say...
And I have used the bracket supports but just with the large stand up transformers like on my 100 watt...
If you are not sure what to do, Contact the Manufacturer.. I have used Classic Tone in all my builds
and they say to leave them on...
:thumbsup:








My Builds:
JTM45 Bluesbreaker , 50 watt Superbass , 100 watt Superbass , Tweed Deluxe w/6L6 Tubes-GZ34 Rec.Tube

Tcaradonna
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Thu Feb 14, 2019 9:25 pm

Thanks Harleytech

I contacted Mercury, here is their response:
It needs to be mounted flush tight to the chassis with the laminations touching the chassis.
You will use his brackets and our screws and nuts from underneath the chassis.
There is no torque spec just tighten accordingly.

https://mercurymagnetics.com/pages/_misc/TranMounts.htm

There is a diagram showing the installation as well.
So I guess I’m going with the flush mount (george’s builds appear to agree with this). But give there seem to be a lot of successful builds done with nuts as standoffs, I’m thinking the difference is small.

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neikeel
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Fri Feb 15, 2019 2:28 am

Code: Select all

https://forum.metropoulos.net/viewtopic.php?f=15&t=46510
:what:
Neil

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by harleytech » Fri Feb 15, 2019 8:43 am

Well at least you know now...
:thumbsup:

Tcaradonna
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Sun Feb 17, 2019 5:31 pm

I think I’ll post some chronicles of my build.

I just spent about a couple of hours sorting thru some of my components.
I doubt this time is worth it in the end, but I’m kinda fanatic for getting stuff “right”, or whatever my subjective concept of what that is. I bought a bunch of Allen Bradley NOS carbon comp resistors. It’s old stuff, may have been rejected in the past, who knows. They aren’t exactly cheap, but not bank breakers either, so what the hell...

I turned the top of an Amazon box into a component holder with an awl (punches every 1/4 inch), ruler, box knife, tape and a cup of coffee. That made a handy testing bench for checking specs of the resistors. As you can see from the pictures I found a number of components that were not passing the printed specs (for gold stripe tolerances). That’s not unexpected for carbon comp components that tend to increase in resistance with age, but fortunately I found enough in spec to make my build.

Question:
According to the metro amp instructions, they suggest using carbon comp resistors on the input jacks. I’ve read in other places that it’s not a good idea to use them there. That actually makes a lot of sense, that at that stage they probably would just add noise to the signal (but is it musical noise???). I obviously have both, probably wouldn’t be that hard to change out if I wanted. Any thoughts or theories here? How did you build yours? Any regrets?

Pics...I can’t figure out how to embed...

https://ibb.co/9ry0gyy

harleytech
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by harleytech » Sun Feb 17, 2019 7:04 pm

I have and am using them at the input jacks and everywhere else George say's to use them and it's fine... It's not noisey at all... :scratch: I have used Carbon Comp's on every build and they all sound great... :thumbsup:
:shred:

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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by RockinRocket » Mon Feb 18, 2019 9:25 pm

Don't use CC for the 68k input grid stoppers. Thas the noisiest spot in the amp. Use CF there instead.

Tcaradonna
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by Tcaradonna » Tue Feb 19, 2019 2:02 am

Are those the 4 68k resistors on the turret board?

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neikeel
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Re: Calling all jtm45 kit experts..cuz I ain’t one...

Post by neikeel » Tue Feb 19, 2019 3:35 pm

Tcaradonna wrote:
Tue Feb 19, 2019 2:02 am
Are those the 4 68k resistors on the turret board?
Yes.
carbon comps give you that slightly hissy sound, particularly on the inputs. My understanding is that they sound best where you have big voltage swings.
I would recommend good quality ccs on V1 and V2 plates (100k ) and the 100k on the cathode follower input, and possibly the 27k negative feedback resistor. Elsewhere just use the carbon films from the kit.
Neil

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