1966 JTM45 with EL34s

[M0usey]

Hi. I've had my 1966 JTM 45 since about 1978. It's always had EL34s. A while ago I delved a little deeper into the details and found the following.
The output tranformer has no markings, but using calculation appears to be a Drake 784-103
There are 1k grid resistors on the output valves with a green 150r flying resistor marked "J/H"
As I understand, this transformer was used with KT66s and is rated at 8k.
The OT specified for EL34s is 784-139 rated at 3.5k
The amp is used regularly and sounds great. I'd like to know what effects this "mismatch" is having on the sound?
I once visited the Marshall factory and Jim explained that they used whatever they could get hold of at the time.

 

[neikeel]

The primaries on your OT suggest that it may be a 784-103 OT which is as you say an 8k OT that is best suited for KT66s or 6L6s than EL34s. It has black/brown/red primaries whereas the later -128 and 139 are white/brown/red.
The amp has had had some work done (choke wires have been moved, those screen resistors are replacements, as is the flying resistor and the preamp filter can together with added bias slider.
I can't see from your pics but I expect the chassis to be block end with white rear panel with a serial between 1800 and 1900 range?
If you have can take more pics we can be sure.
I must say that good KT66s biased at hottest safe range (70%) do sound stellar in one of these.
Ideally to get an idea of how your EL34s would be at their best would try it without the mismatch so ideally use your 16ohm amp tap into an 8ohm cab (or 8 into 4 depending on what you have) and see how it sounds to you.

 

[Ken Underwood]

Lets not forget that the early JTM45s had 6L6s, so this may well be an early one.

 

[neikeel]

Lets not forget that the early JTM45s had 6L6s, so this may well be an early one.

I think the ones you built with Ken and Dudley used the RS Deluxe OT with 9k, 8k, and 6k6 options and used the Tung Sol 5881/6L6WGB output valves with OT wired for 6k6 option - all the originals I've seen use that option but of course that could have been changed over the years.
The amp in the pics is probably a few years later using the Drake 784-103 OT which only had the 8k option (judging by the board type and components). The octal sockets are the cheaper black ones (I believe you chaps used ceramic after a few sparks with the cheaper ones) which were used in '66.
I think the first few amps in 63 used the american valves following the Fender Bassman line before divergence to use the UK GEC KT66s which I think sound better - at least in my amps back to back.

 

[Seanxk]

I'm guessing but that choke looks like a 3H version (from the steel work) and would tie in with the last period of the KT66 amps.

 

[Kinkless Tetrode]

My JTM45 has the 103 OPT. But I do like it with EL34s. If it sounds great with EL34s then keep rock-n them.

 

[M0usey]

It is indeed a block end. The original case had a Marshall small script logo, but due to transport limitations I had it built into a 1x12 combo where it still lives. I had concerns about the wax dripping out of the power transformer. On checking the bias, one of the valves was drawing 110 milliamps! Once adjusted, it's now 39 mA on both valves. The original Tungsram GZ34 dated to the 2nd week of August 1965, but is now replaced by a 1962 version due to arcing in the original one. The large silver RS MFD cap dates to Jan. 1966, as does the red Hunts cap. connected to the rectifier.

 

[neikeel]

That’s interesting. The board has some early components seen in 65 (those slim piher type 68k and the pi couplers are 64 date codes (they must have had lots of those as my 67 50w had those). The serial is interesting for a block end chassis more like mid 66 and the block chassis are late 66 and early 67.
Looks good though. Have you tried it at 8ohms yet?

Edit: just noticed the red/green heater wire. Must have been made around the same time as the Hemdrix/Dickinson amp which was mid to late 66

 

[M0usey]

Not yet. The rear panel has "organ" as well as "lead" which adds to the confusion. It can be heard here on a single we made in 1981. No effects were used other than a valve Copicat (which I still have) used without the motor on to get a little more gain out of my Telecaster. I replaced a "fish" cap (not original) with a NOS mustard cap and one resistor which was way out of spec.

 

[neikeel]

Not yet. The rear panel has "organ" as well as "lead" which adds to the confusion. It can be heard here on a single we made in 1981. No effects were used other than a valve Copicat (which I still have) used without the motor on to get a little more gain out of my Telecaster. I replaced a "fish" cap (not original) with a NOS mustard cap and one resistor which was way out of spec.

Sounds a lot like Motorhead witout Lemmy!

 

[Acvox]

You know, this is a truly great guitar tone, it's so fat, full of harmonic overtones I'd never guess the guitar if you hadn't said so.

Every once in a while an amp comes along that breaks a few rules, and is all the better for it. So familiar, but with a subtle character all its own. I love it and wouldn't change a single thing. You instantly know its a Marshall, couldn't be anything else.
If you love it, don't change a thing, I certainly wouldn't.

 

[tomsvintage]

Not yet. The rear panel has "organ" as well as "lead" which adds to the confusion. It can be heard here on a single we made in 1981. No effects were used other than a valve Copicat (which I still have) used without the motor on to get a little more gain out of my Telecaster. I replaced a "fish" cap (not original) with a NOS mustard cap and one resistor which was way out of spec.

Nice tone and riffing there MOusey !!

 

[M0usey]

Thanks for all the info. Here's a few more pics including the "dog bone" bright cap.
Way back in about 1981 the amp started to make "not nice" distortion and lacked power. The guy replaced the tubes. It may be possible that the 1k screen resistors were changed at the same time. I know that the tone was never the same afterwards, which may have been due to cheap Chinese EL34s. He also replaced the ECC83s with the red Chinese military type, which I understand aren't so bad. I purchased a pair of "matched" yellow script Mullards which were far from matched. I ended up using one Yellow script and a Zaerix RFT. This produced a perfect match pair technically, although perhaps not sonically. If anything, the amp is a little too clean now. Maybe I need to make the bias a little "hotter"?

 

[M0usey]

Sounds a lot like Motorhead witout Lemmy!

Yeh Lemmy couldn't make it that day. He said he working on a song called the Queen of Hearts. I told him it would never get anywhere!

 

[M0usey]

Nice tone and riffing there MOusey !!

Thanks Tom. A long time ago now!

 

[Ken Underwood]

I think the ones you built with Ken and Dudley used the RS Deluxe OT with 9k, 8k, and 6k6 options and used the Tung Sol 5881/6L6WGB output valves with OT wired for 6k6 option - all the originals I've seen use that option but of course that could have been changed over the years.
The amp in the pics is probably a few years later using the Drake 784-103 OT which only had the 8k option (judging by the board type and components). The octal sockets are the cheaper black ones (I believe you chaps used ceramic after a few sparks with the cheaper ones) which were used in '66.
I think the first few amps in 63 used the american valves following the Fender Bassman line before divergence to use the UK GEC KT66s which I think sound better - at least in my amps back to back.

Yes you are right about the octal sockets, they flashed over so were replaced with ceramic types, as for the initial valves then no, they were not American but British Mullards, thats one thing that there were plenty of.

 

[Ken Underwood]

Looking closer at the pictures revealed one definite thing and that it is not a very early one and that it because of the heater wiring, Orange and Blue, no way, it should have been Red and Black and twisted much tighter that in that one, and dressed better as well.

It would have been made in the first factory in Silverdale Road in Hayes Middlesex

Little things that you remember from back then as i made up yards and yards of twisted pairs, which took me all week end, just using a hand brace and a bench vice, thats how it was done back then.

 

[junk notes]

Kool info! Thank you, sir ;-)
Yes in most all of Plexi's you only see two or three wraps. I was informed too many twist (drill assisted) defeats the purpose of keeping unwanted RF noise (shielded) out. I would strategically guess that is why you never see tightly twisted heater wires.

(From the tranny, looks closer to a late 67 or early 68?)

 

[M0usey]

Kool info! Thank you, sir ;-)
Yes in most all of Plexi's you only see two or three wraps. I was informed too many twist (drill assisted) defeats the purpose of keeping unwanted RF noise (shielded) out. I would strategically guess that is why you never see tightly twisted heater wires.

(From the tranny, looks closer to a late 67 or early 68?)

Didn't the JMP50 replace the JTM45 in '66?

 

[neikeel]

Didn't the JMP50 replace the JTM45 in '66?

Not so simple.
JTM45 ran just into 67 then the aluminium block chassis were used for a batch of JMP50s (maybe 50 of these using 784-128OT and 1202-118PT with GZ34 rectifier) then the steel chassis JTM50 with reverse logo (aka Black Flag) came in (these first used 784-128OT and 1202-118PT and a GZ34 rectiifer, giving way to the 784-139PT and 1202-133PT for higher voltages).
The more traditional JMP50 arrived late 67 with 784-139OT and 1202-118PT which ran from late 67 to the early 80s with the hotter split cathode amps arriving in mid 68. Lots of blurred lines and oddities, some of which can be explained by parts bins being mixed and the parsimony of using up what was in to get an amp out (hence batches with odd component combinations, random appearance of an early version choke in a 68 amp). The PAs seemed to be a year behind other part introductions (eg BF faceplates a year after the lead/bass amps had gone over to JMP).