Help with JTM-45 kit build

[johan]

Hi

I have some thing coming and a friend if mine will put the amp together.
It's a Ceriatone chassis with populated board but no iron.
I have a set of Marstran RS OT/PT choke.
I also have a set of Sozo Mustard Caps for it.

Tubes are a mix of new production and NOS - Golden Dragon KT66, TAD GZ34, 12AX7 a whole box to choose from, Brimar, Mullard, Telefunken, Tungsram, Philips.

I fell in love with the JTM45 sound on the Udo Pipper youtube clips plus some other similar clips. Low gain, woody, smoky blues tone. I think I'm on the right path with the parts above. What I've read hear, although I don't unerstand all of it is the following circuit / changes


PA Specs : No cap across the mixer resistors, which were 270K's

1.5K screen grid resistors and a common 1K screen pre resistor (1.5k at "2w" CC)
"If you don't have GEC's kt66 use some 10k stoppers, they must be CC also. will make your modern tubes sound a little more like the real thing..."

Removed the bright cap from vol 1, and removed the cap from the channel 1 mixer resistor
No cap across the mixer resistors, no cap on the Volumes!

CORRECT?

What about speakers? I plan to get a Bluesbreaker combo cab made for JTM45 panels.
Golds or Greenbacks? 20, 25 30W? Celestion or Scumbacks?

Anything else you want to add? I'll come back with pics of the Ceriatone chassis.

[youtube]http://www.youtube.com/watch?v=jKlczOVcG-E[/youtube]
[youtube]http://www.youtube.com/watch?v=aJSAudDWxO8[/youtube]
[youtube]http://www.youtube.com/watch?v=g-FrMcXyCfM[/youtube]

PS! I was soooo close to order the Metro kit a couple of weeks before George stopped selling them.

[neikeel]

The idea about leaving off the cap from vol 1 pot and the bypass cap off the mixer resistors is to give the EQ a flatter response. It will depend on your style and sound whether it sounds flat and toneful or flat and lifeless. I tried it but it did not sound particularly good for me.
Regarding the screen resistors this is to lower the voltage/limit the current and preserve tube life some say it gives more grind/compression. I run GECKT66s in my present 45 with only 470R on the each of the screens and no common supply dropping resistor, although my B+ is not particularly high (<420v IIRC) a 60s Marshall would be 450v or so and would probably benefit from these droppers.

[Roe]

Udo was using 1k5 screen resistors but allegedly went back to the traditional 470r, claiming it sounded better. so basically he is running a stock 45

[johan]

Thanks Roe and Neikeel

OK, so with the RS OT and above info you think I should start out with the JTM45 stock?

What about speakers? Green or Gold

Is it possible to put this in a Bluesbreaker cab? (tubes and transformer placing)

[neikeel]

Yes I do, if you can swing it I would go with mustard caps from the outset (the sozos apparently sound good when broken in but when I had them in one of my amps I was too impatient and found some mustards which were very cheap at the time).

Ah but I see you went for a Ceraitone populated board - well see how it sounds to you. For me making the board is quite theraputic as opposed to heater twisting that is still a PITA to get spot no matter how often you have done it before

Happy building

[johan]

Although I'm sure I could make it work a friend will do it for me. He's way more experienced and is building a "budget" JTM45 (Weber) at the same time. We'll see how close they will be.

I'll start with the Sozo's. Hopefully they are ok for me. If anyone has CC Resistors and Musstard caps at a good price I'm interested but I'm tired of hunting for old parts. Hit or miss.

So, stock JTM45 w RS OT for the Udo tone then? http://marstran.com/JTM45.gif

[neikeel]

I am not best qualified to answer this question but have a little experience. My JTM45 journey started with a 1989RI which with hindsight was pretty good with stock trannies, low filtering and KT66s. Using the Toneclone OT was not a great step forwards. I then used that OT and choke in a Metro type kit bit too stiff, changed the OT to a Metro (heyboer) Drake -103 type much better Marshall blues tones IMO, quite like the original mid 60's JTM45s I have played.

I am in the (very slow) process of building an offset replica but with an original RS OT, Marstran 20H RS type choke and M****n RS type PT. I have KT66s for this but may try 5881s too. I have both Golds (in my BB combo), Blues (in my AC30T combo), G12M20 (in my 18w combo), G12M25 (as yet unhoused) to try out once finished as well as the usual 4x12s. Listen to Georges clips of his 45BB with Golds to see what you think yourself:
http://plexireplicas.com/metropoulos-am ... -45-combo/

The Drake is said to give more grit and grind, the RS if set up using the 6.6k primary tends to run a bit cleaner with flatter broader frequency response with more bass and said to be more hifi. You can get the Bluesbreaker/Udo tone with an RS type OT with maximum output by using the mismatched primary trick (use the 8k primary tap on the PT but select the tap on the 16ohm secondary of the OT into an 8ohm load (two 16ohm speakers in parallel) Golds give you more mid range push and can give clean chimes and have a bit more low end than the original silver alnicos - same as original 'blues' except for bell cover and colour, G12M ceramic greenbacks give earlier speaker break up and ideally you want original 20watters or second best 20w Heritages). Marstran trannies in my experience have always been very good and I doubt that you will go wrong with whichever one you use.

That may have given you more food for thought than I intended

[Coot Boy]

For me making the board is quite theraputic as opposed to heater twisting that is still a PITA to get spot no matter how often you have done it before

+1, I enjoy everything that goes into making the board, measuring and marking out for turrets is great because you can customize the board if you want, drilling and staking I like as well. Heaters, I'll never enjoy doing them