build thread: JTM45-based kit

A local guitarist has commissioned me to build his TubeDepot JTM45+ kit. This kit looks pretty good, so I am eager to get this done before life gets busy in the Fall.

TubeDepot supplies the schematic and assembly manual on their site, so interested individuals can check out the specifics in detail (no affiliation to company; I can remove link if need be https://www.tubedepot.com/products/c...r-amp-kit-head

Some initial notes on the kit:
- this JTM45 "plus" looks to be a mix between a JTM45, JTM50, and some of Rob Hull's own tweaks. It came stock with EL34s and an appropriate OT (3k6 primary), split cathodes, 470k mixer resistors, and 50uF filter caps.
- Classictone iron. Me like.
- One of the 'plusses' is a back-panel switch to select either a GZ34 rectifier or a solid-state rectifier (with added filtering). The solid-state rectifier consists of two 1N4007 diodes (one per AC secondary), which may not be adequate for the 345-0-345 PT.
- The standby switch is located before the rectifier. This is OK for a solid-state rectifier, but apparently tube rectifiers should not be "hot switched" (that is, pre-heated by a standby switch).
- Given the last two points, I will re-design the front end of the power supply. I'll drop the solid-state rectifier and move the stand-by switch to a point after the tube rectifier.
- Despite the move from KT66s to EL34s, the kit still calls for the JTM45 screen resistor setup, which consists of a shared 1k resistor and a pair of 470R resistors. I plan to use a pair of 1k 5W metal oxide resistors instead.

Here's where it's at:

Jet City Amplification now offers a JCA45--their take on a JTM45. Unbelievably touch-sensitive and growls like a beast with KT66 glass on-board. Stock amp has no master volume but they'll install one for you. In true JTM fashion after about 3 on the volume knob it just gets more saturated, but even dimed you can still roll down your volume knob and clean the sound to crystal clear. Definitely worth a listen.

This amp will have better iron than the Jet City. The cost (after my labour) will be about $100 more than the import.

Some photos of the chassis with more components installed. Note that part of the TubeDepot kit build experience is to make your own turret board.

Looks good! I know the panel labeling would be wrong but...ime the PPIMV is used a LOT whereas the Presence is essentially useless at low MV settings (cuz there's basically no NFB) so it basically doesn't get used. So I always put the PPIMV in the Presence hole and put the Presence on the back. And since the Presence is now right next to the Impedance selector the layout works really nicely.

Anyway, I have the itch to build something again but I'm too busy so this thread will help.

SpiderWars":6payy8n5 said:

Looks good! I know the panel labeling would be wrong but...ime the PPIMV is used a LOT whereas the Presence is essentially useless at low MV settings (cuz there's basically no NFB) so it basically doesn't get used. So I always put the PPIMV in the Presence hole and put the Presence on the back. And since the Presence is now right next to the Impedance selector the layout works really nicely.

Anyway, I have the itch to build something again but I'm too busy so this thread will help.

Click to expand...

Yep, this is what I often do as well. I'll leave it up to the owner of the amp as to the final placement of the PPIMV. for now I'll mount it on the back panel in place of one of the speaker outputs, but I kinda hope the owner will want it up front.

Most components mounted on board. Flying leads to be added next. Note the gratuitous use of metal film resistors. Oh the horror! I might suggest to the owner to swap in some carbon comp Rs for the PI plates (the place it matters most, if you are into that sort of thing). Outer foil of the mojo dijon caps marked and caps oriented with foil pointing toward low impedance sides of circuits.

flying leads added, some leads wired to preamp tube sockets...

Looking good, man - nice work!

Steve

racerevlon":2ynj30ud said:

Jet City Amplification now offers a JCA45--their take on a JTM45. Unbelievably touch-sensitive and growls like a beast with KT66 glass on-board. Stock amp has no master volume but they'll install one for you. In true JTM fashion after about 3 on the volume knob it just gets more saturated, but even dimed you can still roll down your volume knob and clean the sound to crystal clear. Definitely worth a listen.

Click to expand...

Badass. Hadn't heard about it until you post. Think I'll snag one of I see one come around.

Just curious but how are you testing the outer foil? Scope?

SpiderWars":3w4810a3 said:

Just curious but how are you testing the outer foil? Scope?

Click to expand...

yep. full details/instructions are on the Aiken tech pages. Basically, you "pinch" the capacitor between your fingers and measure the amplitude of the induced 60Hz noise with the leads connected in each orientation. The noise will differ (even just slightly; depends on capacitance), and the ground lead of the scope is connected to the outside foil when the noise is lowest.

sah5150":1l5z3kl2 said:

Looking good, man - nice work!

Steve

Click to expand...

Thanks Steve.

V2a":2mydrfib said:

SpiderWars":2mydrfib said:

Just curious but how are you testing the outer foil? Scope?

Click to expand...

yep. full details/instructions are on the Aiken tech pages. Basically, you "pinch" the capacitor between your fingers and measure the amplitude of the induced 60Hz noise with the leads connected in each orientation. The noise will differ (even just slightly; depends on capacitance), and the ground lead of the scope is connected to the outside foil when the noise is lowest.

Click to expand...

Cool. I tried it when I had some polyester ODs that had a line at one end and I wanted to see if it correlated with the outer foil (it didn't seem to). My tin ears couldn't really tell a difference so after that Dumble-ish build I haven't done it (I rarely build anything).

Ready for the next phase of testing (power supply was tested earlier).

A few more photos before I finalize the build. The front-panel master volume will be removed since there is now a post-PI master on the back panel.

The amp sounds fantastic!

Below are some photos of the finished amp (completed many moons ago; but then shit happened). Note the back-panel LarMar and its immaculate shielded wiring.

Really nice & clean work!! congrats!

Please correct me if I'm wrong, but your screen grid resistors seem to be rated 1 watt. Thats a bit on the low side if you ask me.

duesentrieb":1z5rceqx said:

Please correct me if I'm wrong, but your screen grid resistors seem to be rated 1 watt. Thats a bit on the low side if you ask me.

Click to expand...

just saw your initial post about the 5 watters.