JTM50 Block End Clone

[neikeel]

I cannot really put this in the 50w kit section. Quick pics for now I will take some better ones when daylight permits.
Blank chassis was from Dom, circuit board from BrianH, transformers from Brian Wallace (789-128 and 1202-118), -114 choke from George, NOS sockets, wire, switches and odds and ends from Valvestorm, NOS fuse holders and original BA hardware. NOS Iskras and Mustards, RS clone filter cap from Steve Marshall, NOS LCR cans. Front panel from Rich Molnar (Sparky). I have not drilled the blocks yet as I will be fitting it into one of Chris Huffs excellent headboxes, hopefully at the weekend.




http://s76.photobucket.com/user/neikeel ... t=2&page=1

Click on here for zoomed pics, bit scary (needs a clean!):
http://s76.photobucket.com/user/neikeel ... 4404015305

Compare it with the original:
http://s76.photobucket.com/user/neikeel ... t=3&page=1

[VelvetGeorge]

Outstanding!

George

[shakti]

A+! I have Iskra 10k if you need.

[neikeel]

A+! I have Iskra 10k if you need.

Me too! I have the 56k for tone stack and bias dropper too, but if you look at the pics of my original you will see why! I also used RS cans for the filter caps and although they seemed to reform ok the screens came back with a capacitance of 120uF so I binned them both and used my last 32uF amd 16/32 LCRs (except for some 50/50s I have.

Although it sounds trite it sounds fantastic from the off, same as the original, surprising amounts of gain for shared cathode and extremely responsive to playing dynamics and the guitar pickup selection and the volume/tone controls, basically wind the amp up andwalk away from it. These are the true 'plexis' that the old grey beard pros are referring to, much more so than a JTM45 and less of a one trick pony than a split cathode superlead.

[bill bokey]

Wow !!!

[shakti]

I haven't seen your original Neil, but I have seen plenty of early/mid '67 amps where the odd resistor is a Piher on an otherwise all Iskra board. Does this one seem to come closer than other clones you've built? It almost sounds that way by your description - OTOH I know that honeymoon feeling with an amp, at first I love them all and think it's the best amp I've ever made, but over time the subtle little differences show up. Although I will say that IME amps like this one, built with the exact right parts, can sound on a par with an original after burn-in. Good quality transformers (I have another #1 choice as you know) are key, but filter caps should not be forgotten. I can only imagine the LCRs adding to the overall feel/sound. I've repeated before that I'm not a big fan of F&Ts, but I have never tried LCRs or found a good alternative.

[shakti]

Wow, saw the photos of your original now...what can I say except: You nailed it!

[JimiJames]

oh daaaaang ! These type builds are my fav... My last one took over 2½ to source.
That's a beauty there . What is the destiny of this amp ? Personal Blues Machine ?
And what's it gonna be married to ? I says the question's, three

[neikeel]

oh daaaaang ! These type builds are my fav... My last one took over 2½ to source.
That's a beauty there . What is the destiny of this amp ? Personal Blues Machine ?
And what's it gonna be married to ? I says the question's, three

I built it to gig with, the original is a rare bird and I had to reglue loose finger joints on the cab. I found that for what our band is doing at the moment the original is perfect and this one will stay at home. So far the best combination with the original has been the G12H25 pinstripe cab as it is a bit brighter than either of the M cabs but my road dog is the basketweave A cab. Once it is in its head cab I will audition it with my other cabs. Normally the shared cathode, big PI coupler amps sound best with the G12M cabs. Funnily enough this is a classic rock amp as well as blues machine, oh and scoop the mids, split the bridge pup it will do some pop/funk disco too

[Brian Wallace]

Great build Neil.

[Haze13]

So clean! Very Beautiful!

[Strat78]

Radiospares Cap clones!?!? Where can I get those! Very nice build, I did not realize some 50 watts had block ends, Dom does some really nice chassis.
What is the V1A cathode cap's make? -looks like the ones you see in 12XXX's. This clone is more NOS than the original, especially with that bias carbon slider pre set. Is the OT rotation orientation original to the amp you cloned or was that changed sometime in it's life?

[neikeel]

Radiospares Cap clones!?!? Where can I get those! Very nice build, I did not realize some 50 watts had block ends, Dom does some really nice chassis.
What is the V1A cathode cap's make? -looks like the ones you see in 12XXX's. This clone is more NOS than the original, especially with that bias carbon slider pre set. Is the OT rotation orientation original to the amp you cloned or was that changed sometime in it's life?

Yes it is rare. Until I got this I did not know that they existed, if you look on Amparchiveshttp://www.amparchives.com/album/Marsha ... index.html and on Marstran Archive http://marstran.com/Historic%20Data%20Base.htm they are in the 2070 range with these specs, I know of less than half a dozen others now having looked around. They all have white back panels and internal HT fuse like a JTM45, perfd boards and Iskras like the 10 and 12 series 100watters.
V1a cathode cap is a NOS WIMA. The slider is one of three that I picked up over the past year, I wanted adjustable bias on this even though the original was fixed (so I dropped in a modern pot on the original).
The OT on the original is in the original orientation. The JTM45s and 50watters always had this oreintation, which begs the question as to why the early 100w amps had theirs rotated 90 degrees with the potential noise/hum issues that this brings.

[rgorke]

Very very nice Neil.

I am also intrigued by the RS Pre cap. In terms of the OT, are you saying that this orientation produces less noise? My 12xxx clne OT is orientd thIs way as well.

Again, very very nice. You have me motivated for a 50 w build. Have a good start on parts so far but is moving at a glacial pace.

I have yet to find the old impedance and voltage selectors....any tips?

[neikeel]

I am also intrigued by the RS Pre cap. In terms of the OT, are you saying that this orientation produces less noise? My 12xxx clne OT is orientd thIs way as well.

I have yet to find the old impedance and voltage selectors....any tips?

Steve Marshall sent me some of those RS filter caps in trade, I have a pair of RS copy bias caps too for my offset project, I then bought some more from him, they are RIFAs (my favourite new production caps) inside. Still don't know if he is alive or dead. He had moved jobs and he said that he was in the Oil Fire Prevention/fighting business, hope there was not some accident?

The selectors are being made by a guy in France - he was selling them on ebay but v.expensive, Tazin borrowed one of my originals, hopefully to copy . Explorer was looking at getting some made but I think the tooling/set up was expensive from his source. The design is simple - just punched paxolin sheet and the selectors are stampings of plated steel (grade) which are then folded and sandwiched between the two paxolin sheets. The ones I got from the US were the last ones he did AFAIK for the Vox clones/repairers and he had to say that they did not pass modern safety standards

What do I do next?:
JTM45 offset
68 Superlead resto
Hendrix/Dickinson 7026
2204 plexi
Metro 12 series with extras
They all have all the parts assembled now and are in 'kit' form