JTM Build - Bravo!

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

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JTM Build - Bravo!

Postby SoK66 » Mon Nov 17, 2008 9:11 pm

Built my Metro Amps JTM 45 kit over the weekend and it worked great from the first switch flip. I do have a few questions, but I first want to congratulate the Metro Amps team for assembling the best amp kit I've yet encountered. From the high quality parts to the excellent instructions and support, everything is simply top notch. With some prior experience building other vendors kits I can say without question Metro Amps is the best in the business by a long shot. :D

Ok, here's my questions:

1) Does anyone have a good method to safely tighten the Marshall-style toggle switch nuts? I've used everything from a pair of long needle nose pliers to a pair of snap ring pliers and they're just clumsy as all get-out. Is there a special tool available anywhere?

2) I noted that the JTM 45 has the mains fuse on the neutral line. I've been taught that the fuse & switch should always be on the positive / mains line to prevent current from being present after a fuse blow. I went ahead and built the amp fused on the neutral / commons line per your instructions but wondered if George or someone could explain why you do the circuit this way.

3) My internal voltages seem to be proportionally close to the chart included with the kit, but run 10 - 20% higher. In particular, my phase inverter voltage readings are much higher than you indicate in the chart. For example, pins 1 & 6 are 20%+ higher, and pin 3 voltage runs 85.5v, vs your chart showing 48v. I've triple checked my resistance values and all seem to be ok. Is the increased PI voltage a result of using your PI cap mod (i.e., bridging both cap pole on the PI cap can)? FWIW, my line voltage runs from 122 - 126v.

4) What was the reason for using a 330uf cathode cap on V1, rather than the original's 220uf?

5) I noted that you have the bias circuit grounded on the pot bus, rather than with the PT center taps, etc. I'm mostly a Fender guy and have always been taught to ground the bias circuit at the same PT stud as the CTs and never with the preamp. Is your method preferable to what I've been doing?

6) Finally, do you have a Metro Amps logo available? I hate to just have the amp "logoless".

Thanks again for a great product, I'm sure I'll be enjoying it for quite some time!
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Re: JTM Build - Bravo!

Postby Flames1950 » Mon Nov 17, 2008 9:16 pm

The answer to many of these questions is in George's signature on his posts -- and I quote, "Because that's how they did it on original plexis"...........whether for better or worse.:wink:
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Re: JTM Build - Bravo!

Postby ThunderOne » Mon Nov 17, 2008 9:25 pm

"1) Does anyone have a good method to safely tighten the Marshall-style toggle switch nuts? I've used everything from a pair of long needle nose pliers to a pair of snap ring pliers and they're just clumsy as all get-out. Is there a special tool available anywhere?"

I used 2 quarters! They might not be the ideal tool but they worked great! :D
Last edited by ThunderOne on Tue Nov 18, 2008 9:53 am, edited 1 time in total.
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Re: JTM Build - Bravo!

Postby lcampz » Mon Nov 17, 2008 9:43 pm

SoK66 wrote:1) Does anyone have a good method to safely tighten the Marshall-style toggle switch nuts? I've used everything from a pair of long needle nose pliers to a pair of snap ring pliers and they're just clumsy as all get-out. Is there a special tool available anywhere?


:lol: Great question!! Anyone?? I have never been so careful with a pair of pliers before in my life. You'd think that StewyM or Weber Tech tools would have something for this. They've got just about everything else. Maybe I just missed it.

Hmmmm, there's a machinist out there about to make a whole lot of money. :D
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Re: JTM Build - Bravo!

Postby monsterwalley » Mon Nov 17, 2008 10:38 pm

lcampz wrote:
SoK66 wrote:1) Does anyone have a good method to safely tighten the Marshall-style toggle switch nuts? I've used everything from a pair of long needle nose pliers to a pair of snap ring pliers and they're just clumsy as all get-out. Is there a special tool available anywhere?


:lol: Great question!! Anyone?? I have never been so careful with a pair of pliers before in my life. You'd think that StewyM or Weber Tech tools would have something for this. They've got just about everything else. Maybe I just missed it.

Hmmmm, there's a machinist out there about to make a whole lot of money. :D


Check the auto stores. A lot of the early gm cars with external entenae'e had a similar nut (60-70's) I use two a prong wrench
that was from a 57 corvette I restored years ago. IIRC the knobs on the dash had the basically the same fastener. that and on the wonderbar radio.
I'll have to dig up were I got it. I know it was cheap.
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Re: JTM Build - Bravo!

Postby BrianH » Tue Nov 18, 2008 12:08 am

I have some cheapo metal wrenchs that came with old IKEA furniture. They work great for tightening those nuts!
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Re: JTM Build - Bravo!

Postby Mars Hall » Tue Nov 18, 2008 12:59 am

Sok66,

I'm sure you're the same guy that visits the "FDP" forum. You've always been helpful in answering my posts over there. Thank You very much. I would like to take this time to welcome you here at The Metro Amp forum. The people here are just as eager to help! :D
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Re: JTM Build - Bravo!

Postby SoK66 » Tue Nov 18, 2008 1:38 am

Hi, Mars! Glad to see one of my Fender Forum Homeboys hangs out here, too. Sure enjoyed this kit build. I started on it Saturday afternoon and we were paying "Crossroads" by 8PM last night. I spent about a week reading the instructions, so once we got going it was really a smooth process. Played it a lot tonight, too, getting a feel for any changes I'd like to make later. Just want to be sure I've got everything dialed in before making any tweaks. Only concern right now are the high voltages on the PI circuit.
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Re: JTM Build - Bravo!

Postby Roe » Tue Nov 18, 2008 6:20 am

4 - the 330uf might give a tad more gain and bass than the 220uf - if you can hear the difference. I use a 50uf iskra in my jtm45
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Re: JTM Build - Bravo!

Postby Roe » Tue Nov 18, 2008 6:21 am

6 - yes a metro logo would be great! the stickers are good but a more solid logo would be great if people didnt confuse it with the metropolious logos on the amps george build himself
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Re: JTM Build - Bravo!

Postby 908ssp » Tue Nov 18, 2008 11:29 am

I put the fuse on the hot line not the neutral side. Won't make a difference in the sound and it is safer as you said.
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Re: JTM Build - Bravo!

Postby SoK66 » Tue Nov 18, 2008 6:58 pm

Thanks, all. I might as well be a show off (like everyone else!) & post some build pics.

Metro JTM45 Internals.jpg
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Re: JTM Build - Bravo!

Postby SoK66 » Wed Nov 19, 2008 1:18 am

Geez, hate to reply to my own post, but, I made a couple of changes to the kit today. Changed the 500pf cap cap on the #1 vol pot to 100pf, the 500 was peeling paint. Also added 5.6k grid resistors to the power tubes after I'd notice a couple of times that plate voltages were fluctuating wildly. I assumed it was an inaudible oscillation, and I did have a couple of instances where it squealed when switching out of standby. Piece 'o cake to install. As noted in another thread, I had some pretty high plate voltages on the PI & preamp tubes. I experimented with differnt values for the 8.2k 2w I dropping resistor and settled at 18k, which brought nearly all voltages just a bit below or above spec. Also removed the PI cap jumper and went back to 16uf only. Seemed to sweeten & fatten the tone a bit. Ran it quite a bit today through the 16 ohm G12M Greenback in my 18 watter's cabinet and it sounded incredible.
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Re: JTM Build - Bravo!

Postby SoK66 » Fri Nov 21, 2008 7:30 pm

Yeah, well, then Steve pointed out that I'd essentially wired the presence pot like a tweed Bandmaster/Super/Twin, which was why I was getting goofy PI cathode voltages. Kinda made the presence control "over active". With that mistake cured I reverted all the internal values to George's build spec. and we're kickin'. Again, can't compliment Metro Amps more for giving us a such a great product and support.
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Re: JTM Build - Bravo!

Postby SoK66 » Sun Nov 23, 2008 12:15 am

Hot DAMN, is this a great amp! :!: :!: :!: :!: :!:
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