JTM45 Amp Build

[veatch]

I've had a JTM45 kit sitting in a box for about 3 years now. Decided last night to take a run at it.

Note i'm not posting this here *as* the expert, but because i am certain i *will* need advice from the real experts - especially when i get to the point of firing this turkey up. Fortunately, i do have a variac. And a fire extinguisher.

Anywho...

Whole lotta parts:

Always go with "what you know", so i started with the components:

Interesting that the board was designed with the fork terminals too far apart for some of the components - had to "extend" a couple of parts:

Mounting the trannies - i got a screwdriver, so i guess i can handle it:

That's as far as i could get last night. I'm going to go cut a board up to make a stand to make this easier to work on. Maybe more pics later, more likely tomorrow, though.

[Brewmaster]

Tube sockets and trannies go in first. Tube sockets then the heater wiring followed by the trannies. Any components mounted to the pots go on BEFORE the pots go in, same with the jacks. The board goes in last.

Full instructions are here: http://forum.metroamp.com/viewtopic.php?f=15&t=2859

You may need to live on that forum for a while but I have built a bunch of these and the instructions are excellent. Pay attention to detail, don't rush and make sure you have good mechanical connections before you solder. When soldering heat the part, don't put solder directly to the iron to avoid cold joints. I see some potential problems on the board already.

[velorush]

I was still holding out hope you were going to give up on it and sell the kit to me

[gtone]

Jeez Jef - that's pretty fair progress for one evening! I'd still be looking at everything and trying to figure out how to tackle it all.

[Thundernotes]

Boy, you do work fast! Nice job!

When I build, I trace out the chassis from the inside onto a piece of scrap plywood. Then I drill the holes where the pots go, etc., and build much of the amp "flat". Then I can just drop the pre-wired guts into the chassis and everything lines up nicely. That's how I did this one because of the tight quarters:

[veatch]

Thanks, all. Jeff - if it goes up in a ball of smoke, it's yours.

Thanks Brewmaster for the link! Wish i saw it before i dug in further last night, but i don't *think* i put myself into the ditch (too far, anyway). The kit i got was from Mojo, and it only came with a schematic and a wiring diagram. These instructions will be very useful.

I put the components on the board first for two reasons: 1 - it would knock out a lot of parts quickly, and 2 - if they're on the board, i won't lose any of these little buggars. The solder on there in the pic was just a light touch to keep things in place. With the turret, the solder would wick down the channel, and i wanted to keep the path clear for the wiring later.

Anywho - got pretty far, couple of quick pics:

Brian - i had the same idear , but i used some spare plexi:

I tried to keep the power all to the far side:

I was surprised that the caps did not fit the pre-drilled holes (nice file marks, BTW - should have used a knife, but it was late...):

Started wiring up the board:

Ran out of solder:

And, no, i don't work fast - but when i get on a project, i'm pretty tenacious - i was up until 3 last night...

Hopefully i'll get to the point of testing this weekend. The Metroamps doc looks to have a pretty good section on testing. The voltages on the pins helps a *lot* - no voltages on the schem from mojo...

[Thundernotes]

You've given me the bug again.... might try building something this winter.

[Brewmaster]

Nothing wrong with wiring up the board first, just don't put it in until the end. I have several boards here that are just sitting waiting for a chassis. I have templates built for several amps that I use as well. Good thinking on your part.

About Mojo kits, I do like the Mojo Dijon caps but their schematics and layouts aren't always correct. The link I gave will have the correct schematic and layout. The Metropolis site also has good info within the forums for troubleshooting when you get to that point.

You've given me the bug again.... might try building something this winter.

You should never have stopped.

[Thundernotes]

You should never have stopped.

Divorce put my (home) amp shop out of commission a couple years ago. Life has only recently returned to some semblance of normal, so I can pick up a few of the old hobbies.

[veatch]

Hope you get a project going soon, Brian. That amp looked pretty sweet. How does it sound?

Brewmaster - There are a few differences i could find between the Mojo schem and the Marshall schem from the Metro site. Some i could live with, but others i chose not to follow. I really did not like the way Mojo had the output jacks wired. Whoever drew the schem surely has more experience than my newbie behind, but i opted for the Metro wiring. The Mojo schem also has a 5.1k grid resistor that the Metro schem doesn't have. Haven't researched that difference yet to know if that's a good thing or not. If you are aware of any other issues, i'd love to hear about it (PM is fine if you don't want to post that publicly... )

I did get quite a bit done last night and today. Getting *really* close (i hope...). The plan remains to damage some hearing and annoy some neighbors yet this weekend.

One last check before power up - found one missing connection. Which means there are probably 2 or 3 others that i didn't find...

Initial power on tests are promising. Heater voltages are correct. Voltages at the preamp tubes seem fine. I have an issue to resolve when the power amp tubes are in - i'm drawing a bungload of current somewhere, but i'll have to wait until tomorrow to start tracking that one down...

Wiring (shouldn't have tie-wrapped the stuff until i finished testing... <doh!> ):

Wiring is clean up front:

Wiring at the tubes needs some practice / experience:

I have a list of things that i would do differently given another chance. (i'll wait till i see what else i did wrong before i list them...)

Which, is really code for "I already want to build another one..."

[Brewmaster]

Stay with the Metro wiring and values and you should have no issues.

A tip for the heater wiring for next time, Take two 3ft lengths of 20 gauge mil spec stranded wire, one black one red or other colors you wish and put one end in a vise and the other end in a drill. Hit the trigger on the drill nice and slow and get a good twist on the wires. Don't over twist. Get some vice grip pliers or something similar and attach to both end of the wires to retain the twist. Let the wires sit for a while, the longer they sit the more "Memory" they will retain. I let mine sit for a least a day.

For grins and giggles when you are done, compare the JTM45 to the 59 Bassman 5F6A and be stunned and amazed at how close they are. We have heard about the similarities for years but after you build one or the other, and in my case both, you REALLY see the similarities. So what I am trying to say is this, if you have been bitten by the bug to build another amp, build a 59 Bassman.

[veatch]

It's *ALIVE*...

4 things to fix. Not bad, i guess, for a first build:

1. Missed connection on board (fortunately found it before powering up)
   
2. Connected bias to the wrong side of the diode at the rectifier (to be fair to mojo - this was a simple mod that was not on their wiring diagram)
   
3. Had the input jacks wired wrong (backwards - doh!)
   
4. Bias resistor needed to be adjusted
   

When i got it up and running, the bias bottomed out at 41mA, which was a bit hot, but i figured it would be fine "for now". Amp sounded *awesome* dimed, then i blew an output tube. :/

Fortunately, the EL34's i was using were the el cheapo's, but still...

I raised the bias resistor, and viola - biased it a little cooler (around 65%), and it ran without issues for about 90 minutes. Hearing protection is king right now. I'd rather use a 50kA bias pot, so i might change that out in the future.

If it stays alive for the week, i'll put some older Seimens tubes in i still have, and see how that goes.

Overall - it behaves like an old JTM45 / 50. Keeping the bass dialed down low, i get a great sound on all three pup positions, and a wide range of dirt from the guitar's volume control. Fairly dynamic amp. And, uh, LOUD.

I haven't put it in the case yet, but here's a look at it au natural:

I don't like the output tube retainer spring thingies. I'll put some regular push-clip ones in when i get a chance.

Brewmaster - thanks for the guidance. That Metro guide saved a lot of grief! I love the drill idea. I didn't twist the heater wires nearly tight enough - i'll have to try the drill trick.

I definitely learned a lot - mostly what not to do. I wouldn't use all the same color wire on the front and back end. On the up side, it forced me to tone everything out to ensure i had the right wire, but color coding is the way to go.

I'm not sure if the ground "bar" was a good idea or not. I have minimal hum - less than most of the other 50 and 100 watters i have here, but i wonder if having the grounding all that close to the PT is a good idea. Logic is telling me it is not.

As it is, the amp is pretty serviceable. I put the cap on the standby switch to a bad place for ground, and i need to re-route one other wire to go above the other wires, but after that, it is pull the pots, and the board will flip right up. I would route the wiring a little different if i did it again to make it look a little neater.

Overall - I definitely won't put any pro builders out of work, but this was a *lot* of fun, and a great project. I have a bucket list of amps that i want to build now, but those will have to wait until i get some money saved up.

I'll post one more pic after i get 'er in the case. Maybe some sound clips, too.

[velorush]

That's great, Jef!

Congratulations.

That was really quick, too.

Can't wait to hear some clips.

Brewmaster - awesome tip on the drill trick!

[Brewmaster]

It's *ALIVE*...

4 things to fix. Not bad, i guess, for a first build:

1. Missed connection on board (fortunately found it before powering up)
   
2. Connected bias to the wrong side of the diode at the rectifier (to be fair to mojo - this was a simple mod that was not on their wiring diagram)
   
3. Had the input jacks wired wrong (backwards - doh!)
   
4. Bias resistor needed to be adjusted
   

When i got it up and running, the bias bottomed out at 41mA, which was a bit hot, but i figured it would be fine "for now". Amp sounded *awesome* dimed, then i blew an output tube. :/

You did really well. The 4 mistakes you made are the most common. I can't tell you how many folks put diodes in backwards and wire the jacks backwards. The bias resistor almost always needs to be adjusted.

As a general rule I like to bias El34's in most Marshall amps at around 38ma depending on the plate voltage. Check this out, it is a pretty reliable site that gives the recommended bias point based on tube type and plate voltages:

http://www.dreamtone.org/Calculate_Bias_Form.htm

[backinit]

Sweet! Congrats!

[teleman]

I have been so tempted to try an amp build. In fact the one I'd want to do would be a JTM 45! Got any sound clips?