JMT 45 kit & build - Advice wanted - GuitarTalk Community

lapdawg · 2012-05-10T01:07:27+00:00

I want to build a JTM45. I have little knowledge of valve amps & electronics, so I need to learn. From the reading I have done so far, it seems I have t...

JMT 45 kit & build - Advice wanted

lapdawg

I have already been looking at stuff on Antique Electronic Supply(www.tubesandmore.com). I am in dispute about an order with them...
I've sent a mail & am am waiting for feedback. I think sites like this raise interesting questions about online buying - particularly relevant to this topic. In SA we often look to the US as the only source of what we're looking for.
My complaints:
1. the site is not well structured or intuitive i.e. procedures not like common on online buying. Crucially - I expect to know shipping costs when doing the cc transaction..
2. I expect < 3days for a query response. I waited longer.
3. I made a purchase & and was misinformed (I believe) about what I was buying - i.e. the source of the dispute.

That said - they do offer good stuff at seemingly good prices. Bad site does not necessarily = bad dealer. The personal touch still counts... I'll let you know how they they respond.

MikeM

Honestly, for a first build, particularly something like this, stick to an actual kit.

There are loads out there, if AES can't supply what you need, I'm sure there's someone else willing to sort you out!

shaunf

MikeM wrote: Honestly, for a first build, particularly something like this, stick to an actual kit.

There are loads out there, if AES can't supply what you need, I'm sure there's someone else willing to sort you out!

That's why I suggested Valvestorm. Easy ordering and you end up with all the same components as used to be included in the Metro kit (Metro not supplying kits themselves at the moment). The other sources include the other bits not supplied by ValveStorm that would normallybe part of the Metro kit.

All that remains is to download the build instructions from MetroAmp forum,and the guys there are very helpful for support too.

lapdawg

I have been putting together my shopping list and trying to work things out. My call is rather take a bit longer to put it together than rush in with substandard parts.
I see that the:
Marstran OT, PT & Choke: $125 + $145 + $28 = $298 i.e. R2 400
ClassicTone bundle at Triode: $167 i.e. R1 336

How significant will the difference be?
What do these do to the sound quality?

makepeace

I would say that the within the group of popular manufacturers of a similar power transformer (ie a JMT45 PT), the actual brand doesn't make a huge difference. So yeah.. marstran, hammond, classictone, mercury magnetics.. take your pick (whatever's cheapest/looks the coolest ?). The choke is also fairly irrelevant. As long as its designed min of meer for the application, it'll do fine.

The OT though is the place where you will have to place most of your concentrations. They are very different and this is where a lot of the tone in these amps lies. Unfortunately its difficult to get a feel for the quality of something over the internet without actually A-B testing the transformers, and I think with the JMT45, you're probably largely on your own. So you're going to have to take an educated gamble. My money would be on the Classictone though.

Here's something on transformers, particularly OTs: http://www.tone-lizard.com/Electronic_Parts.htm.

That whole website is actually a great read. You should go through all of it.

You could also try this

lapdawg

Further thanks for the advice. I been reading & learnt a bit. I now know what a carbon comp resistor is, what a KT-66 is etc..
For those interested the '45 is now in progress. I sourced & ordered most of the parts. It is possible to save money if you do your research:

Basic electrical parts were ordered from Valvestorm, a few from Modulus Amplification
Chassis, faceplates, knobs, wire etc from Modulus
OT & Choke - 2nd hand Drakes for a song on eBay
Turrets & turret board - sourced locally. I've put it together
Most build info from metro amp. George is THE man
12" Green Beret (Greenback copy) & 12" Reaper (G12H30 copy) from Warehouse Guitar Speakers
PT still needed. I think it'll be a Classictone
Valves still needed

I've also build a 2 X 12 "speaker cab & covered it original Fender tweed + a few coats of lacquer. I like the look but it won't fit with the '45 though.
The WGS speakers sound brilliant. Here are a few pics...

If there's any interest, I'll post the build as it happens...

Reinhard

Great work on the cab, looks fantastic! ?

makepeace

Man! Nice work! :applause:

I've also used the chiseled finger-joint method :-[. Dado racks are mad expensive.. Your tweed turned out perfectly! Cool to see you found an OT on ebay. Is it from an old JMT?

lapdawg

I might try *outsourcing* the cabinet joint cutting in future. The OT & choke are originals but I don't think they're '60s originals. They are definitely more than a few years old.

shaunf

Good luck with your build, and I'd very much like to see your build progress, so please document your process.

That speaker cab came out very nice.

Also, forget Tubesandmore.com, PM me if you want details on how you can buy all that they sell from their source, at substantially cheaper prices.

JeremyE

Man that cab is just fantastic! Watching this thread with great anticipation!

lapdawg

I'm still waiting for my parts & components to arrive. Some orders have been in SA for over a month. Apparently the domestic postal strike was an issue.
Elbows -> desk
Face -> palms

...

shaunf

Sorry you're having such trouble with the postal system. It's strange because I've placed 2 orders in the last couple of weeks and both have arrived very quickly.

lapdawg

Finally, I got my packages & put it all together. I opted for:

Classictone OT & PT
LarMar PPMV (put an additional hole in the back)
Valve Art KT66
JJ Preamp: ECC83 on V1 & V3 / ECC803 on v2 (apparently improves gain)
I added a voltage selector. Its a bit crammed at the back next to the impedance selector & the mod is not the neatest, but I wanted to have the option of switching between 220/230. I've kept the 240v input wire & will also be able to add that on. I have read that people are measuring mains voltage at different levels & it might good to be able switch it. This might not be a wise decision. The wiring in also a whole lot more complicated & unfortunately its not simple to "read" by looking.

I've biased the valves & played it a bit today. I'm really happy with sound & understand that it'll settle in with some playing time especially with regards the valves & transformers. The Classictone PT puts out 690v. This seems to be higher than others e.g. Mercury Magnetics delivers 646v.

My parts came from:
- Valvestorm
- CE Distribution /Tubes and more
- Tubedepot ( I am more and more impressed with these guys & there pricing)

I am almost finished the case - it was almost cheaper to cover it in leather that have tolex posted, so I've done that. I'll post the amp in cab pics in a few days. I'm happy for a first build & have learnt a bit. I can sense a '68 Plexi in the near future...

Thanks again for help and advice.
Here are the pics so far:

Kalcium

That looks so neat and professional its scary! First build!? WELL DONE!!! :applause:

shaunf

Looks good!

How is she behaving? No hiss or hum to speak of? Any osciliations or funny behaviour with the volume on max?

Psean

Wow! Looks great ? Well done :goodtimes:

lapdawg

shaunf wrote: Looks good!

How is she behaving? No hiss or hum to speak of? Any osciliations or funny behaviour with the volume on max?

No hiss, no hum that I have noticed of yet. Volume on max was done for a few seconds. It all seems OK so far, but because this is a first, I almost expect problems. Fingers crossed, but not when playing.

shaunf

Quickest way to pick up any potential issues is to dime all the controls. A lot of the old Marshall circuits oscillate if you do this. If I take my chassis out of the cabinet and put it rightway up on top of my 4x12 and dime the volume, it oscillates. Flip the chassis over so its standing on the tops of the transformers and it doesn't do it. Go figure!

I see you're not currently running any output grid resistors, which might cause issues, especially when using a PPIMV. Also, are the orange/green wires from pin 5 of the output tubes connected to anything under the board, or are the ends under the board sealed off ? Those wires should actually be removed when running a PPIMV, but like you, I have also left mine in place, under the board. They have the potential to act as an antenna, but I haven't had any problems.

I can't see where your NFB resistor connects to on your impedance selector or output jack, where have you connected it to? I moved mine from the output jack to the 8ohm tap and was very pleased with the results. It's worth experimenting with placing it on different taps as the difference in tone is quite pronounced, depending on your tastes.

Oh, and since you're running a very similar circuit to my amp, a nice mod I have done was to install a 0.68uf capacitor to bypass the V2 cathode, the result of which is a nice boost to the midrange and a little bit of extra gain.

lapdawg

Thanks shaunf for tips. There's a bit there for me to digest & work out. Also, I have given it a bit more of a run through its paces. I am not sure that the sound is 100%. There's definitely no hum. The bass is very pronounced, but I can tweek it out. I am also picking up a little crackle when I crank it. The most disturbing is that when crank the master volume, it fades away from about level 9 - 10. Its perfect up until then, but then the volume dips completely.

I did run the impedance at 16 and not 8 - that's what my cab is.

The KT66's are too hot too touch after a bit of of run time. Is this normal?

Theres still a bit of binding etc. to sort out on the cab, but I do like the way the leather covering has worked out. I took some plain leather, dyed it deep green. It still needs a few more coats of leather dressing. I think the leather will age nicely and get good character. Here's a pic: