Any Landmines Waiting for JTM45 Builder?

[Structo]

The third video was with full band so it sounded better.

I think that is a very iconic tone which you will hear on countless albums.

But in the end, you are the one that has to be happy with your rig.

[Lynxtrap]

I listened to those Youtubes, and they didn't do a thing for me! I'm just not a Marshall guy, I guess.

We got the board populated, and the sockets are installed. I decided to take a break so I could study up on Merlin Blencowe's heater-wiring tips. We're also planning to stick diodes in the rectifier, along with the tube, as a safety measure.

I can't believe how much prettier turrets are. I'm all done using eyelets.

That sure looks nice! What circuit are you making now? Is it the stock JTM45?

[renshen1957]

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

Hi Steve,

If you can get yours hands on a copy, read the information in TUT3 on capacitors and can caps, especially Chapter 2, Making the Right Connections. There is a section on multi cap cans with a common ground and why you wouldn't want to use them (and radial caps are quite a bit cheaper). TUT3 has a Chapter on the Marshall Plexi much of which is applicable back to the JTM45 as well as entire chapter on the Tweed Bassman, which I recommend reading.

The JTM 45 is in essence Tweed Bassman made with English parts, in the earliest form with radio spares Hi-Fi output transformers (check out the OT Impedance of 6k6 ohms compared to the its American parent).

Built in the Euro fashion of a tubes up head with separate speaker(s) cab. Jim Marshall, a drummer who had a music shop (I believe he distributed Fender products until they parted over these amps) and the talents of Ken Bran and Dudley Craven cloned the 5F6-A to build a bass amp, as the import duty for USA products was high in the UK at the time added to shipping and Fender's High MSRP.

The amp possessed an aluminium chassis in place of Fenders mild steel chassis and Turret board construction instead of eyelet circuit cards.

The first JTM45 were fueled by US 5881 tubes which became too expensive and subsequently changed to (according to some, first to the EL 37 the Brit equivalent to the 6L6GB and then), the KT66 (hi-fi tube) and then to the EL34 (another Hi-Fi tube) all for economic reasons.

I have lent on semi permanent loan my copy of the Tube Amp Handbook to a friend so I can't scan the chassis shots of a vintage JTM45 and the Tweed Bassman, worth the time to check out. I believe I would be covered under fair use, if I had.

The amp was of greater interest to Guitarists in England at the time than bass players, but the guitarist wanted a brighter amp that wouldn't sound farty when played distorted. The amp was tweaked to be brighter to deal with the original circuits flatulent sound when overdriven.

If you want the old school sound of the Original JTM45, then you might want to investigate Edcorusa.com for a heftier output transformer if that is the sound you are attempting without going into hock over MM offerings. Edcor have a variety of 6k6 impedance OTs (even their 30 watt is heftier than the magnetic component offering) at http://www.edcorusa.com/category/67-cxp ... edance=268

Last thought, if you guitarist wants a Bluesbreaker instead of an EL45 version and the PT doesn't have the 6v3 heater current Tung Sol and Sovtek's KT66 are just 6L6 tubes, the TS in a KT88 bottle and pull 900 milliamps current. JJ, Shuguang,, Gold Lions and Saratov's KT66 draw a specified 1.3 amps per tube. Will these get you the Bluesbreaker tone? Your mileage will vary, tubes tone depend on the bias, a studio recording changes with mike placement, most people haven't heard an original MOV/Genelex KT66 (those who have say the new tubes come in a distant 2nd).

Make sure you get the circuit correct, the best schematic I've seen to date (and compare/contrast with the Tweed Bassman schematic).

http://marstran.com/JTM45.gif



Best REgards and good luck on the build.

Steve

[ToneMerc]

We got the board populated, and the sockets are installed. I decided to take a break so I could study up on Merlin Blencowe's heater-wiring tips.

Steve, here's a grounding scheme that you may want to consider. A lot of folks including myself have had good results with it. My JTM45 is quiet as a mouse.

TM

[NickC]

.................. Make sure you get the circuit correct, the best schematic I've seen to date (and compare/contrast with the Tweed Bassman schematic).

http://marstran.com/JTM45.gif



Best REgards and good luck on the build.

Steve

Greetings,

I noticed a notation regarding placement of the choke in circuit in the schematic referenced in the Marstran link above. I believe that schematic is in error (as inferred by the writer of the notation), and that the choke is placed after the center tap of the OT and not before it as that schematic shows.

Contrast the above placement with choke placement in this schematic:

http://raw-sewage.net/images/jtm45-readable.jpg

Since I am currently building a JTM45 also, that detail caught my eye.

Best Wishes

[The New Steve H]

Thanks for the grounding photos. I will look at them after the caffeine takes effect.

Right now I'm doing the heater wiring. Mojotone supplied 18-gauge cloth wire, which is stiff and somewhat annoying to work with. They only provided one color, so it's a pain, trying to make sure the heaters are in phase.

I have some Winged C 6L6 tubes I don't like at all. They have a hard sound to them. I'm thinking we should stick them in the JTM45 before my friend blows a wad of cash on expensive KT66s, and if he likes them, he can have them at a discount.

What's a good amount of component lead to insert in the turrets? I'm thinking just enough so they'll sit there while I solder.

The board is just sitting on the bench with the components inserted loosely. I don't want to start soldering until I have as much stuff connected as possible. I don't want to end up opening up old joints so I can cram new things into them.

[The New Steve H]

That sure looks nice! What circuit are you making now? Is it the stock JTM45?

I'm making whatever Mojotone drew!

My friend is giving me way too much credit. I'm just putting stuff together. I still want to make my own amp with a Bassman front end and different tubes on the output.

[jelle]

You do not need 18 awg..... 20 is fine and easier to do.

[M Fowler]

The last amp I built before my surgery last week was a Mojotone JTM45 which I will call a JTM45/50 since mojo uses EL34 tubes and low primary OT.

Great sounding amp and it's owner loves it so that's best part.

Enjoy whatever you come up with Steve.


Mark

[Reeltarded]

Sorry Steve.. sidetracking again.

Hey Mark! This place feels good today. I hope you do too.

[The New Steve H]

I want to slap whoever invented cloth-covered wire.

If this was going to be my amp, I would have burned this crap already. I'm doing microsurgery on the heaters, trying to get these silly wires soldered, and the fibers insist on hanging out over the joints while I'm soldering. Couple that with the totally unnecessary thickness of the wire, and it's an experience I wouldn't wish on Hitler.

The heater wire is going on okay, but I'm not happy with the pressure it puts on the pins, and doing it the Merlin Blencowe way means going across the sockets, so they're buried in wire before I even start on the other pins.

Argh. Argh. Argh.

One and a half sockets left.

[Reeltarded]

Trick! Don't drip superglue on it, but use a tiny amount on the frey while it's pushed back away from where you'll actually solder AFTER tinning the wire so no glue gets in your joint.

It helps. Little is A LOT. Teeny bit. Use a 10th of a drop.

[Zippy]

I want to slap whoever invented cloth-covered wire.


Argh. Argh. Argh.

Sheesh, such a whiner...

I love the stuff.

[The New Steve H]

It is now BANNED from my life. I will never use it again.

I got the heaters wired, but I'm concerned that the fraying of that stupid cloth will cause something to short.

How do most people jump pins 4 & 5 on 12AX7s? I made a little piece of wire shaped like a staple, and I ran it through both pins and mashed it shut.

[jelle]

That looks nice....(JTM45 envy....)