New build parts gathered

[beaulieu]

Well since I have built a 12 series and 50 watt NMV and since I don't have any 470k resistors I guess its time for a JTM 45 clone. Here are the parts I have gathered up so far! Still needing a 1 watt 10k Iskra to have a 100% Iskra board. Also guess Im not going to be able to use the RS 1k screen resistors. Guess I need to decide if I really want to do a 45 or track down 2 470k Iskras and build another 50 watter! Decisions...I have a bunch of Iskras if anyone wants to trade for and 100k 470k 220k or 68ks you might have. I have a quanity of each of the other values. Thanks for looking and I will post pics as the build proceeds. Mark

[beaulieu]

So I have decided to build my first JTM 45! I have built several amps but never a 45. One of the reasons is because of the NOS GEC KT-66's. I buddy of mine built one a few years ago and I heard it with RI tubes and NOS GEC's and there was a pretty big difference. Also everyone needs a assorted mix of amps, right? Anyway I will post pics once I start.Now I guess I need to decide what version I want to do! Off set..etc any suggestions? Thanks Mark

[neikeel]

Just to be awkward - why not build a JTM50 with a 789-128 OT and 1202-118PT and 3H choke (more correct with Iskras,although the latest block end 45s did have them). Of course if you already have the GEC KT66s go for it. If I had to keep only one amp it would be my JTM50 and the G12H pinstripe cab

[beaulieu]

Hey Neil, Well since I Have enough parts I would do both. I don't have any KT 66's yet. I do have some phiers and I know the AB are easier to find. What would you suggest I use in the 45? I do see them with Iskras on Amp Archives. Prob get trans from Chris.

[axeman]

Were can I get the same on and off, standby switch ?

[axeman]

Just to be awkward - why not build a JTM50 with a 789-128 OT and 1202-118PT and 3H choke (more correct with Iskras,although the latest block end 45s did have them). Of course if you already have the GEC KT66s go for it. If I had to keep only one amp it would be my JTM50 and the G12H pinstripe cab

Neikeel, Can you post the voltage reading on your JTM50? I would like to have a referance when I build it. I have a pair of nos Xf2 Brown Base and a nos BB Gz34 and lastly 3 nos 1970 Made in Holland ecc83 for this build.

[neikeel]

Hi Art

Between 420 and 430v depending on wall voltage, the Marstran one I used on my clone was about the same (I use Mullard GZ34s). When I made the clone I first used 270k mixers but it did not sound quite like my original, as is typical, a bit louder and a bit cleaner until cranked more, so I subbed in a pair of Piher 470k and they put it back to where I wanted it. Also to note my original has a 25uF 6v WIMA electrolytic on V1 cathode. None of the values have drifted particularly beyond their specs. I do not have screen resistors or 1ohm bias resistors (bias with transformer shunt method) on the original. I used both on the clone.

[beaulieu]

Just to be awkward - why not build a JTM50 with a 789-128 OT and 1202-118PT and 3H choke (more correct with Iskras,although the latest block end 45s did have them). Of course if you already have the GEC KT66s go for it. If I had to keep only one amp it would be my JTM50 and the G12H pinstripe cab

Neil. Can u explain details what makes this spec different then a 45 or 1987? I have two boards started and one will be a 45 and the 2nd is still up in the air.the amps i have now are 69 plexi 73 1987 73 1959. And builds are 12 series (with the larger pt) and the 68 50 watt (spec still undecided) I wanna do the 45 and a 18.the JTM50 would be a great addition to the stable.is the only difference the trannies? Thanks Mark

[Tazin]

The basic difference between the later JTM45 and the JTM50 is the filtering and the trannys. Early JTM50's seem to run the 1202-133 PT and 784-128 OT while the later JTM50's use the 1202-118 PT and 784-139 OT. As for filtering for the JTM50, they tend to run 32uF for the mains, 32uF for the screens, 16uF for the PI, and a 32uF + 32uF dual cap on the tag board for the pre-amp. This filtering scheme holds true for both versions (tube and solid state rectified) of the JTM50.

[neikeel]

The basic difference between the later JTM45 and the JTM50 is the filtering and the trannys. Early JTM50's seem to run the 1202-133 PT and 784-128 OT while the later JTM50's use the 1202-118 PT and 784-139 OT. As for filtering for the JTM50, they tend to run 32uF for the mains, 32uF for the screens, 16uF for the PI, and a 32uF + 32uF dual cap on the tag board for the pre-amp. This filtering scheme holds true for both versions (tube and solid state rectified) of the JTM50.

Yes, but most JTM50s have the -128OT and -118PT. The -133PT seems to be a bit random (also has higher voltages) and only the last ones have -139OT.
Most are tube rectified.
Where my alloy blockend fits in I do not know. There are several out there (they have JMP front plates and white back panels which mine also has)
The JTM I built with the dual 32 preamp. Metro -139OT, Marstran -118PT 3H choke can was a nice all rounder.

[Tazin]

After some poking around I thought it would be interesting to post what I came up with for the tannys used in some of the JTM50 amps. These particular amps had the reverse logo JTM on the faceplate along with the window style serial number on the rear panel. I omitted other amps that seem to fit this particular era because they had the JMP faceplates even though they sported all the other indicators.

S/10019 PT:can't tell OT:784-128
S/10038 can't tell on either
S/10049 PT:1202-133 OT:784-139
S/10054 PT:can't tell OT:784-128
S/10076 PT:can't tell OT:784-128
S/10083 PT:1202-118 OT:784-128
S/10103 PT:replaced OT:784-128
S/10107 PT:1202-118 OT:784-128
S/10117 PT:1202-118 OT:784-128
S/10122 PT:1202-118 OT:784-128
S/10163 PT:1202-133 OT:784-128
S/10184 PT:1202-118 OT:784-128
S/10208 PT:can't tell OT:784-139
S/10344 PT:1202-118 OT:784-128
S/10403 PT:can't tell OT:replaced
S/10408 PT:1202-118 OT:784-128
S/10421 PT:1202-133 OT:784-128
S/10432 can't tell on either
S/10436 PT:1202-133 OT:784-139
S/10458 PT:replaced OT:can't tell
S/10469 PT:1202-133 OT:784-139
S/10476 PT:1202-133 OT:784-139
S/10488 PT:replaced OT:can't tell
S/10508 PT:1202-133 OT:784-139
S/10547 PT:can't tell OT:784-139
S/10554 PT:can't tell OT:784-128
S/10562 PT:replaced OT:784-139
S/10585 PT:can't tell OT:784-139
S/10612 PT:1202-133 OT:784-139

EDIT: S/10208 has a JMP faceplate