2nd Build-Rob's JTM45 Micro

Tom Kamphuys

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I just did the headphone check myself, now on the actual aluminum chassis instead of some wooden template.

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Clearly audible hum. Equal for both orientations.

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Dead quiet.
 

willholt92

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Wow that's intereting. So spacing the OT off the chassis with a few washers to raise it would be beneficial. I'll bare that in mind.
Thanks for the tip
 

BigDaddy23

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Bizarre! Did you try rotating it (whilst listening) from position 1 to position 2 in your photos? How you thinking about achieving the 'tilted' position whilst still making sure it is structurally strong and wont stress the chassis? I'd be concerned about damage to the aluminium chassis around the screw hole locations.
 

Tom Kamphuys

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Listening while rotating around the vertical axis didn't change the hum a lot.

I'm still not sure where to put the OT. A bit further away also helps. Will also help against the yet unknown choke influence. I'm a bit afraid of coupling of the output current to the circuit inside the chassis.
It crossed my mind that I could attach the OT to a small plate which was then attached to the chassis with four bolts, but not resting on the chassis, but on nuts. One could then change the tilt by adjusting the nuts. Even after the amp is complete.
I decided to let it rest for a while, because I feel my overthinking kicking in.
 

willholt92

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Bizarre! Did you try rotating it (whilst listening) from position 1 to position 2 in your photos? How you thinking about achieving the 'tilted' position whilst still making sure it is structurally strong and wont stress the chassis? I'd be concerned about damage to the aluminium chassis around the screw hole locations.

To be honest in a "head" IE transformers sitting on top of the chassis I can't see it being an issue the bolts won't really stress the aluminium due to the transformers weight sitting on the aluminium surface.
If the transformers are hanging from the chassis IE in a combo maybe more so, I'd put a large steel washers under the head of the screw inside the chassis to spread the load.
2mm aluminium will be pretty strong though as long as you are using decent sized bolts I can't see there being much of an issue.
 

willholt92

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I've been busy with a few other things the past month and not spent much time on the JTM micro.

So while my Mrs and the little one are away this week I've made a start on the cabinet after work.

Cut some 12mm (15/32") plywood for the sides and top and bottom.

I made a finger joint jig using pine 18mm x 32mm glued together and will use a flush cut router bit and the opposite sides to make some finger joints. I saw someone make one of these on Youtube and it is a really neat idea so hopefully I can get some good results with it.

On the plus side I got the plywood for free as an off cut from work so free cab up to now.

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Still not got into designing the chassis yet unfortunately but hopefully in the next week or so. Amazing how much time a 10 week old takes up on top of working.
 

Paul-T

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wow, that's impressive (and saves the $80 etc for a template i paid).

You'll use a template flush router bit ? How do you control the depth? Do post pix when you're doing it!
 

willholt92

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wow, that's impressive (and saves the $80 etc for a template i paid).

You'll use a template flush router bit ? How do you control the depth? Do post pix when you're doing it!

I know I couldn't bring myself to spend some much on a proper template and I liked the idea of this DIY version.

Yeah that's the plan I bought a couple flush router bits and I'll just have to try and set the depth of the router, but as the ply is only 12mm it shouldn't be too bad. Hopefully the bits will be long enough, if not I'll probably try and cut the jig down with the band saw worst case scenario.

I'll take some photos when I do it, I'm going to test it with a few off cuts first
 

willholt92

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Slight update.

Happy new year all.

It's been a long time since I've posted any updates on my threads.
I bought my first house last year and completed at the end of November and have worked on it everyday since doing renovations which has ended up being every single room in the house. We are starting to see the light at the end of the tunnel so I'm hoping I might have a little more time to spend on my guitar and amp projects, even more so when I get my shed/worshop set up.

Anyway I've been back lurking on Shock Brothers the last week or so and thought I'd give a bit of an update after seeing @D_Hallett 's JTM micro thread.

I had a change of heart for my speakers and cab, I've always wanted to get an alnico speaker to have in a cab and a mate of mine sent me a facebook listing which I couldn't pass up.

So I have ended up with these beauties, bought locally only a couple months old for a bargin £150. So the Greenback has gone in favour of these.

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I had a play around with some more cabinet designs to use the 2 x 10" Celestions, including a bluesbreaker style.

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But it was around the same time last year that Marshall launched the Studio Vintage series amps and as soon as I saw the SV212 I knew it had to be a vertical 2x10 in a similar style.

So I've settled on the below design for the head and cab. I've sourced the cabinet handles and some other parts ready.

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I've also decided I'm going to have a switch to choose between Valve and Diode rectifier to see how this changes the sound, I think it will be quite nice to have a more JTM 45 option and a more JMP hopefully. Probably won;t make much difference but it's there anyway.

3 way NFB may also make an apperance on the back panel.

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That's all for now, I will get more into the build at some point this year I promise haha
 

willholt92

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Some progress.

Finally had some time with my engineer to draw up the chassis last week for an hour or so.
Will be going for 2mm aluminium this time rather than stainless steel, not only is it lighter and I've read that it is beneficial in terms of noise, but it's also a hell of a lot easier to work with.

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Back panel with Fuse, IEC socket, Rectifier switch, speaker and headphone outputs. I was thinking about having a impedence selector like a proper Marshall but I don't think you can use the 125B output transfer to give 4, 8 and 16ohm output? Unless someone knows if you can.

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Still need to work out positions for valve sockets, I am thinking that I will leave the transformer mounting points and wire holes until I have the chassis, I can easily do this with the pillar drill and give me a bit more freedom if things need to move around a little, it's already looking a little tight.

I've decided I'm going to take the V1A grid stopper resistors off the board and solder them directly to the V1 socket, I've bought some shielded wire to go from the input jacks. I remember reading that Metal film resistors are better for this application is that right?

I have also been thinking about my 2 inputs and how I can make them a little bit different? I was wondering if I could have different value cathode bypass caps for each one but from my reading that won't work with out a switch.

Would a different value grid stop resistor have much of an effect? I'm thinking maybe 1 input more suited to single coil and one more for humbuckers?
 

Tom Kamphuys

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Metal film resistors have low noise and the input gridstopper is most important as signal levels are still low.

Grid stoppers form a low pass filter with the Miller capacitance of the tube. Normally input grid stoppers are chosen to have no influence in the audible range.
 

willholt92

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Metal film resistors have low noise and the input gridstopper is most important as signal levels are still low.

Grid stoppers form a low pass filter with the Miller capacitance of the tube. Normally input grid stoppers are chosen to have no influence in the audible range.

Ah right okay, that's great thank you very much.
 

willholt92

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Small update

I've been waiting to hear back from multiple laser cutting suppliers to try and get my chassis cut and folded, it seems everyone here in the UK is crazy busy at the moment, I'm hoping to hear back and get the chassis in the next couple weeks all being well.

In the mean time I finalised my board design, I drew out all the wiring in illustrator to finalise the hole positions for the board.
I then printed the layout, punched and drilled all the holes in the board and installed the turrets.

Looking forward to progressing on this amp when I get the chassis.

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willholt92

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Some progress.

Happy new year everyone, I've been ticking along with the JTM last year between other projects and working on my house.
I finally found a supplier to cut and fold my chassis for me, this arrived in about October or so.

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I need to drill the mounting holes for the transformers next and the holes for cables.
I have been thinking of a way to be able to remove the transformers without pulling the board out, I don't know if anyone has used rivnuts? Or captive studs before for this? Any advice or suggestions would be very welcome.

Any way I mocked up and everything fits well.
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I printed out my faceplates and checked them before ordering them from Tubetown in Germany.
I used up the remaining sheet I had to order for some name plates for a few more amps I am planning to build in the future, I'm hoping to start selling one or two once I've built them.

These arrived just before christmas and I've painted the lettering in last night.

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I'm hoping to get a few hours to myself this weekend and hopefully start soldering up the pots and jacks.

That's all for now. I do apologise for the glacial pace of this project.
 




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