JTM50 Build

[Marcomel79]

Woah! So much progress!

The chassis looks great! Next time you might consider electroplating it (with zinc or nickel). You can do it at home with minimal equipment (although nasty chemicals are involved) and will give your metal work a "factory look" and rust protection over the years to come!

Tank you Pietro, i did consider it but i went with two coats of clear sealant. Lets see if there will be a next time!

 

[Marcomel79]

Thank you so much for the insight @_Steve, this is my first big bottle build so all new to me. First lets hope it doesnt blow up when i fire it up, then ill have to test it and play it and then see if i like it or not. Nice to know ive got options!

 

[Whatwhatringrang]

Wow nice work ! I am sure you will be fine with it firing up . I enjoyed reading all this . Very nice job and cool pretty much all from scratch . Inspiring stuff.

Have fun when it’s done . Trying with all the different stuff you can do to these amps. It is amazing what a few little changes in these can do sonically and with how they play/feel even the small stuff . These amps just amaze and entertain me . Always amazing to me what some can squeeze outta amps new or old when chasing it.

 

[Marcomel79]

@Whatwhatringrang thanks man, i had to do this project on a very low budget, so anything i could reuse/make myself, has been done. But ultimately it could not have happened without the help and generosity of the amazing people of this forum. I consider it OUR project.

 

[AndyD]

Really enjoying following this thread! Your build is coming along super well!

Just something you might be interested in... A while back I did a ton of testing on screen resistor values for JMP circuits. One of the tests I did involved having switchable 470k/1K values (as well as just 47K + zeners). In my testing the difference between the values was HUGE, even though I had read several online 'experts' saying the difference is imperceptible (i can only guess these people have never actually A/B tested their theories because the differences are obvious to anyone!). IMO, depending on your risk appetite, the lower the better. 1K adds a significant, noticeable amount of 'squish' that you may or may not like. Lower values have a more immediate, responsive attack. As you know, the originals had none. Just food for thought

Hi Steve, this is absolutely true. The trouble is that modern production valves need the resistors to knock back some current. The old valves could handle current better.

 

[AndyD]

Hi Marco, making great progress! Forgive me for noticing that you have bias resistors installed. I started out using these and then ”progressed“ onto using the OT resistance/voltage drop method after which I realised that I was biasing my amps colder than I thought! The 1ohm/10ohm bias resistor is very convenient…..but it’s not very accurate IMO.
Happy building!

 

[Marcomel79]

Hi Marco, making great progress! Forgive me for noticing that you have bias resistors installed. I started out using these and then ”progressed“ onto using the OT resistance/voltage drop method after which I realised that I was biasing my amps colder than I thought! The 1ohm/10ohm bias resistor is very convenient…..but it’s not very accurate IMO.
Happy building!

Thank you Andy, i installed them so i can get in the ballpark and then i fine tune using the same method as you. Its a little thing that i think makes biasing a little easier. Always nice to have constructing criticism!

 

[AndyD]

Thank you Andy, i installed them so i can get in the ballpark and then i fine tune using the same method as you. Its a little thing that i think makes biasing a little easier. Always nice to have constucting criticism!

I stand corrected! Good idea!

 

[Marcomel79]

Well, i got as far as i could. I only need the fuse holders and a 820R resistor to call it finished. I used the grounding scheme from Modulus, from the top of page four of this thread with a little twist. Hopefully the Royal Mail sorted their stuff and will send my parcel soon. Here is the final progress for now. I would like to thank all the people that stopped by and helped me with this project. You know who you are.

 

[AndyD]

Looks beautiful!

 

[Marcomel79]

Looks beautiful!

Thank you Andy, i wish the PT side had been different, but beggars cant be choosers...

 

[Marcomel79]

Fun shot of how tight the switches are!

 

[neikeel]

Excellent job.
I've got lots of 820R resistors from NOS cc to NOS CF Piher or Iskra and Modern CF
Let me know

 

[Marcomel79]

Excellent job.
I've got lots of 820R resistors from NOS cc to NOS CF Piher or Iskra and Modern CF
Let me know

Thank you Neil, very kind of you. I talked to Michael at Modulus and he said my parcel was sent but most likely stuck in customs. Hopefully it will show up next week if the Royal Mail fixed that bug. If not ill def take you up on the offer.

 

[Nicolas S]

Super clean build

 

[Marcomel79]

Super clean build

Thank you!

 

[paul-e-mann]

Well, i got as far as i could. I only need the fuse holders and a 820R resistor to call it finished. I used the grounding scheme from Modulus, from the top of page four of this thread with a little twist. Hopefully the Royal Mail sorted their stuff and will send my parcel soon. Here is the final progress for now. I would like to thank all the people that stopped by and helped me with this project. You know who you are.
View attachment 124316
View attachment 124323View attachment 124324View attachment 124325View attachment 124326View attachment 124327View attachment 124328

Wow!

 

[guitarzan2525]

A bit of a heads up: i used the press fitting method to fix the turrets on the board, which is basically drilling the holes for the turrets slightly smaller, and then hammer the turrets down for a tight fit. I saw it in a Tube Depot video...
However, after soldering all the connections, i noticed that some of the turrets can slightly move. Nothing much, but they shouldnt. This is because they havent been pressed from the back, and because of the heat from soldering.

What i will do is, after all the components are mounted and theres no more soldering to do, i will put a dab of superglue on the backside of each turret just to have a little piece of mind. Not ideal, but starting to swage fit now with all these wires seems hell. Plus i only have limited time to use my friends workshop (long driving, night time..) and id rather use that time finish the chassis.ld use a dab of 5200lbs JB weld

Lesson learned for next time, and if somebody is thinking about drilling their own turret board, will hopefully avoid the press fitting method.

You could use a dab of JB Weld - stronger than SG

 

[obx351]

Beautiful build!

 

[Marcomel79]

You could use a dab of JB Weld - stronger than SG

I ended up flaring the back of the turrets