Mojotone JTM45 Schematic in Emailable Form?

[The New Steve H]

Steve has been asking me why his "JTM45" had EL34 tubes. I have not been lucky enough to be promiscuous with a lot of amps, so I mainly know about the few that I own. I don't know one Marshall from another. We figured it was sort of a JTM50. It looks like he is finally getting some useful answers.

He was expecting KT66s.

Stick with it, Steve. Now that you are getting some decent input, you should be able to turn this amp into exactly what you want. It will sound great, believe me. You just have to keep working on it until you get where you want to be. If there is anything else about it you don't like, let these guys know, and I'm sure they'll hammer out a solution.

Now that I think about it, looking at the PT and OT drawings, I don't think there is any reason why you couldn't rig it for KT66s if you wanted. I'm sure someone will correct me if I'm wrong, but the PT looks just like a 5f6a PT.

Question for the experienced builders: is there a brand of kit that would be more suitable for guys in Steve's situation? I mean clearer instructions and so on. I can alert my buddy at the VA in case they end up helping other vets get into amp building. It looks like Mojotone is not the optimal choice.

[ToneMerc]

Question for the experienced builders: is there a brand of kit that would be more suitable for guys in Steve's situation? I mean clearer instructions and so on. I can alert my buddy at the VA in case they end up helping other vets get into amp building. It looks like Mojotone is not the optimal choice.

Metro had one of the more comprehensive kits and asembly instructions, however George no longer offers kits. IMHO, if one is going to get introduced to amp assembly via the "paint by numbers" method then a much simplier circuit should be attempted first.
A lot of these troubles originate from folks trying to walk before they can crawl. There is absolutely nothing wrong with building a simple 5F1 Champ circuit as a first build to get aquainted with vacuum tube electronics.

[ToneMerc]

Merc....I'm ALL FOR IT....can we make sumthin' ?

Steven here's your sanity check, your best bet is to get through this project as intended. When it's finished, you can then decide what you like or don't like and if needed gravitate towards a few tweeks or another Marshall circuit type. From what you mentioned above I can think of about 7 different Marshall circuits/topologies in that range of amps that you stated.

Don't get sidetracked.

TM

[steven21042]

Merc, I desperately want to build this thing as "intended". I have an affection for all things 34 including rectifiers and women. I particularly like the Tone the Marshall 1987 puts out. Short of finding and then PAYING someone to build the rest of this kit for me I guess I'll have to ask several thousand questions and put it together to the best of my ability and let the chips fall where they may. In my defense and as a bit of history I've been gasing for a 45 for YEARS - ever since I saw UDO PIPER playing a Vintage 45 equipped with 66s in several YouTube clips. I was told about this VA program for disabled Vets that included Vocational Rehabilitation projects and it was suggested that I should look into it. I did and was accepted. Long story short I had requested a Metro 45 kit with instructions because I have friends online that had built them and they spoke highly of George and his operation. I waited and waited and was finally told many months later that a kit might be doable but by then George had stopped selling them. I suggested a Ceriatone as a replacement but found out that a Mojotone kit had already been ordered. It really wasn't a matter of me picking this kit as a starter. This is the kit I have and I'm grateful to have it..I just don't have any instructions and only my will and you guys to get me there.

[Phil_S]

I built my JTM45 with EL34's. The schematic says KT66 or EL34. It's a great sounding amp. Don't get hung up on the choice of power tubes. Besides, in a reproduction amp very little is 100% true to the original. That and if you don't actually know what a "real" one sounded like, you have no legitimate basis for comparison. Even the memory of an original is likely to be faulty.

I say, move forward. Have fun building it. Enjoy playing it. It will be a great amp.

If you insist on KT66 or want to be able to change, remember they have big glass bottles and make sure the sockets are far enough apart to accommodate the diameter of the tube.

[ToneMerc]

I built my JTM45 with EL34's. The schematic says KT66 or EL34. It's a great sounding amp. Don't get hung up on the choice of power tubes. Besides, in a reproduction amp very little is 100% true to the original. That and if you don't actually know what a "real" one sounded like, you have no legitimate basis for comparison. Even the memory of an original is likely to be faulty.

I say, move forward. Have fun building it. Enjoy playing it. It will be a great amp.

True dat Phil

[ToneMerc]

ever since I saw UDO PIPER playing a Vintage 45 equipped with 66s in several YouTube clips.

Great amp, supposely built with scrap parts from several amps.

TM

[jelle]

I respectfully disagree on the powertube comment....it depends on the turns ratio of the transformer (the reflected impedance that the powertubes will see). This will determine which tubes will work well in this amp.

I have recently built at JTM45 that just kills. All current production parts, nothing NOS. We compared it to a JTM100, a real one with the old caps, and the owner had these EL34's in that amp. The amp calls for KT66.

The JTM45 just sounded much better than the JTM100. No comparison. Bigger, juicier...

Please find someone who looks at these kind of details when you look for a tech to complete your build. It can make or break the project, IMHO.

[jelle]

This is the kit I have and I'm grateful to have it..I just don't have any instructions and only my will and you guys to get me there.

This forum and the people here are great. This is all you need if you have confidence that you can do it.

[smokin' tone]

Jelle is correct. You have most of what you need. You can get a nice transformer for a JTM 45 here http://store.triodestore.com/jtm45outr.html. You could also order a more accurate turret board, you may have to drill new holes for it but that's not a big deal. You could then possibly follow the metro amp instructions for the most part http://metroamp.com/downloads/JTM_45_KIT_V2.1.pdf. Get some KT66's and rock on. You can sale your left over parts and get some of your money back.

[steven21042]

...Let the questions begin then...

My kit obviously has 8 pin power tube sockets. Do KT66 tubes have 8 pins and will they fit the sockets then? Do I need a special tranny for the 34s VS the 66s? Are 66s more suited to a lower wattage amp like say 50 watts or less whereas 34s are usually used for 50 Watts (with 2 EL34s) and up to 100 Watts (with 4 EL34s)? What purpose do the resistors serve when attached to the tone pots? If I use my board as it stands now and go ahead and try to finish the rest as per the supplied layout what are the possibilities that I blow the thing up?



Numb

[jelle]

Did the kit come with transformers? If so, it has the Mojotone OT, see attached PDF.

That OT is decent and is the one I used as well. In order to run KT66 powertubes you will need to ignore the color code of the PDF. (I really do not know why this transformer is sold with this documentation as it is highly confusing for most people and will result in overheating and destroyed KT66 tubes).

What you do is this:

for KT66: Brown 4 ohms
Yellow 8 ohms
Green 16 ohms

For EL34's:
Use the color code of the PDF.

Now, if you use KT66's, I recommend the Chinese ones. They sound good, very much like a real GEC KT66. Get them from a seller that burns them in and matches for both bias current and gm. NBS electronics does this for instance. Then, bias for max 60% and use 1.5K screen resistors to prevent the powertubes to turn into firecrackers when the amp is dimed.

Many people have isues with the JJGZ34, I use chinese ones instead.

Preamptubes, the JJ ECC803 works well for the cathode follower (the middle preamptube) as well as the phase inverter (the one closest to the powertubes).

If you need help, just ask.

Jelle

[jelle]

I just sent you a PM.

Jelle

[steven21042]

Merc,
Just thought I'd share a Champ(ish) Circuit with you....

Skip to 3:28


http://www.youtube.com/watch?v=yKYp-4j4RJY



Now THAT'S INSPIRATION

[Lonely Raven]

Merc,
Just thought I'd share a Champ(ish) Circuit with you....

Skip to 3:28


http://www.youtube.com/watch?v=yKYp-4j4RJY



Now THAT'S INSPIRATION

Wow that little thing sounded great!