JTM45 Offset Clone - new build

[neikeel]

I have been quite busy recently so had little time to build amps. Finished my sons Superbass (well half 2203, half Superbass) in time for Christmas and had this on the back boiler for years.
Chassis and front panel were a gift from Steve Marshall, so it is his fault!!
OT started off with original RS Deluxe and the PT an original RS Heavy Duty but I needed those for something else more deserving. The board is a 7 hole one from BrianH that I added my own turrets to. Nearly all the resistors are original RS 10% carbon comps, except for a couple of carbon film Hytabs, the caps are Dubilier made RS. The filter caps reformed NOS. PT made for my by Majestic transformers in Dorset to original RS specs. Pots are all old CTS. I had to wire the input jacks opposite to normal to clear the internal anchor screws and you will notice I tried to use the original wrap around the turrets wiring. I ran out of skinny wire for the output heater between V4 and 5 and for the OT secondary wires.
The OT is a Marstran RS clone as is the 20H choke. I made a bottom board up similar to the one Alex (908ssp) made for his offset years ago.
I elected for Phillips 6L6WGB which I understand are pretty much ruggedised 5881s that can handle higher voltages. The preamp tubes are some early Telefunkens I have been saving for this project. Rectifier is a Mullard labelled as a Miniwatt.
A couple of very quick snaps:




I have tried it tonight with a P90 equipped Martyn Booth through a G12M 2x12 (well my 18w combo speakers) and I have to say I am blown away with how loud and crisp it is with a sweet crunch wham cranked. I little hiss when dimed (less than my AC30s) but no hum at all.
I confess that I have not checked the bias yet but plate voltage is 460v (-48v on the grids) which is a smudge higher than I was shooting for (440-450) but heck it sounds good. I will check and adjust the bias properly when I get time.
Now to sort out a head cab for it. Chris Uff in Norfolk usually comes up with the goods and I have some smooth cloth and RS Vynide saved for it. Looks like frets were bent and used instead of gold trim around the top and bottom of the cab?

[danman]

Great looking build Neikeel! I bet it sounds just as great as it looks!

[bill bokey]

[Haze13]

Nice looking amp! Will you post some sounds?

[shakti]

Extremely well done as always, Neil! Nice to see such a plethora of old parts being used for something as nice as this.

BTW, no "flying resistor" on these offset amps? I have never studied them closely. It should be very close to my own block logo clone in circuit except for the lack of the common 1k screen resistor, and KT66 vs 6L6WGB. What primary impedance are you using (8k or 6.6k)? I've found that I really like my block logo best at 8k and running at 16 ohm for maximum NFB - it has this sweet chime and very clear, sweet, quasi-clean almost quacky attack even with volume at 10. I've built a later JTM45 with Drake 784-103 clone OT, and really preferred the RS/20H version.

PS: my 18w build is getting closer now...just scored an original 18w chassis(!) to go with the RS OT. I might be in touch or post over at the Facebook group to try to find the last original parts I need, particularly a PT.

[Roe]

Impressive build! I guess you don't really need the flying resistor with the 690DCR choke.

[neikeel]

Impressive build! I guess you don't really need the flying resistor with the 690DCR choke.

That was my view but I am happy to experiment, I used WW rather than CC for the screens, they don't look too far off.

Regarding Block Logo amps I have all the parts for a '64 clone, having subsequently found another pair of original RS trannies but another Marstran choke. This one has Mustards and Hystab (bumblebee resistors) all white sockets, white panels. Even have a full set of RS pots (64 to 67 dates). Cannot find exact Clapton knobs but found some identical apart from fine ribs rather than smooth outer ring.
If people are concerned re resto parts I do restos and have spares of most bits and if an original is in need I can use correct parts for that (that's why the first set of original RS transformers for this were used).

[DG45]

Very, very nice!

I have a couple dumb questions of a general nature. Were those boards with the pre-drilled holes the standard in the vintage amps? I mean, as opposed to a solid board drilled only where needed for turrets. Also, what is that board specifically called? It would be much easier to work out a layout especially for someone not as experienced with building amps. If you make a mistake or change your mind, you just move the turret. Finally, and this may be dumb, but is there any further potential for noise compared to using a solid board and only drilling where you need the turrets?

Thanks

[coldengray]

Neil, how did you cut the chassis for the OT tag board?

[neikeel]

The first JTM45s had 7-hole width perforated boards.

They later became more solid boards with 8-hole width spacing.

In '66 they once more had perf'd boards with 8-holes but with small hollow turrets rather than the split turrets from 63 to 66.

The chassis came from Steve pre cut, it was cut for a ****** OT spacing (hence the extra holes.

On the next one I will punch holes and file oval. If you look on Marstran you will see how Alex prepped his OT. I did mine in similar style.

[coldengray]

When you say punch and file, are you using a circular punch then filing oval? I need to cut a Metro chassis for an original RS OT and I'm trying to decide if I punch & file or cut square using a Dremmel. Thoughts?

[neikeel]

The early amps were two oval holes Between round greenlee pinches.

Later 64/early 65 had very rough cut square holes (not even filed edges!)

[Tazin]

Damn Neil, that came out really nice. I take it that those gray signal caps are the paper dielectric types? Kind of ashame that Steve up and disappeared since he was good at offering small runs of products for the DIY'er.

[neikeel]

Damn Neil, that came out really nice. I take it that those gray signal caps are the paper dielectric types? Kind of ashame that Steve up and disappeared since he was good at offering small runs of products for the DIY'er.

No Actually they are the moulded Dubilier ones with RS stamps.

I do have a load of the paper ones (of correect values) but they seem to test higher than their value +20%. I did use some in a really early 18w combo resto (despite them testing correctly) and they were no good. I suspect that they will only be good for gutting and putting new PIO or similar inside.

[newking70]

do you have a head cab yet?