• Guys, we've spent considerable money converting the Les Paul Forum to this new XenForo platform, and we have ongoing monthly operating expenses. THE "DONATIONS" TAB IS NOW WORKING, AND WE WOULD APPRECIATE ANY DONATIONS YOU CAN MAKE TO KEEP THE LES PAUL FORUM GOING! Thank you!

Amp Of The Month (1): 1965 JTM45 Matched Set.

F-Hole

Well-known member
Joined
Sep 2, 2015
Messages
2,171
Charlie asked me to start a series of "Amp Of The Month" threads, so here goes.

As ever, luck plays a big part in finding old gear. In this case, I was at home in London last week with Joe B (PBUH), Roy and Clay, gear geeking as you do. My computer pinged to tell me an email had arrived. A forum member, Rob L, had dropped me a message telling to urgently call a small music shop that had just got a walk-in from an old boy, an early Marshall. Needless to say, conversation stopped while I made the call. Pictures arrived, and it looked very promising. So, a deal was done during that five minute chat. I drove the 3 hours on Saturday, arriving in rain of monsoonal proportions. A quick coffee and a bacon sandwich to fortify me while waiting for the shop to open, screwdrivers et al in hand, the anticipation killing me.

The amp and cabs turned out to be better than I had hoped, and the "old boy" turned out the be the original owner. They're very early 1965, and were purchased new as a set directly from Marshall. When ordering them, the owner asked Marshall to install a 8/15 ohm tap on the back panel, which they duly did. I've never seen another like this. The original owner said that he would sometimes use the head as a true PA, so needed the flexibility.

The OT is the early version of the RS Deluxe, and this will be one of the very last amps with that iron. It shares some hardware with the earlier metal badge amps, with particular tells that are unique, although totally correct for that period. The board is untouched, with caps dating to 1964.

The cabs are equally clean. It's very unusual to see gold block logos on white front cabs. These cabs are untouched, and house T0652's with the early silver Hanwell factory stickers (pre gold sticker era). The cones are original. The input sockets are the recessed metal cup type, as used on the metal badge cabs.

It's been a really good month on the vintage Marshall front, and I'm still pinching myself that I managed to get to this before anybody else.

Thanks again Rob.



Note the 8/15 ohm slide switch on the back, factor original per original owner's request.






A photo of the ohm tap from the inside. Original solder joints, and original spec wire all support the original owner's claim, which I had doubted at first.










Original fuse.




This is the earliest inspection tag I've seen, by quite some way.






 
Last edited:

ourmaninthenorth

Well-known member
Joined
Mar 28, 2009
Messages
7,119
Fabulous write up John.

School's in.....:biggrin:

If your stock of male Bulgins is short, I've got one here you'd be more than welcome to.

Thanks again for taking this feature on.

:salude
 

rob livesey

Member
Joined
Feb 28, 2007
Messages
655
Well, that turned out well ! What a find !

I'm glad to be of service good sir.

I did contemplate buying it myself, however having just moved house, recovering from surgery and being currently bereft of gainful employment, I thought better of it and gave you the tip.

It's a lot better than I expected to be honest, in fact, it's as nice as anyone could dare to hope.

Congratulations John.

The impedance selector might actually be unique, wow.

Cheers :salude

Rob
 

springhead

Member
Joined
Feb 12, 2016
Messages
262
Congratulations, that's a fantastic find. Does it sound as good as it looks?

Couple of geek questions if I may.... just out of shot in your pic of the board - as it was used for PA are both channels the same or does one have the volume pot bright cap and hf bypass cap on the mix resistor?

You've taken the output valves out - presumably GEC 66's?
 

Luke Gibson

Active member
Joined
Jul 15, 2001
Messages
1,324
Man, it doesn't get much better! Congratulations John, you are on a roll!
 

CDaughtry

Les Paul Forum Co-Owner and Moderator
Joined
Jul 16, 2001
Messages
12,646
Thanks for undertaking the "Amp of the month" chores. I can't think of anyone more qualified. And what a way to start!!!:yah:wow:dude:
 

Michael Minnis

Active member
Joined
Feb 12, 2004
Messages
1,597
Congratulations on another great amp, John. It found its way to the sanctuary for long lost, wayward Marshalls. Thanks for taking on this new thread and sharing your vast knowledge!
 

shakti

Member
Joined
Jan 17, 2007
Messages
494
That is about the coolest amp and gear story I've read! Congrats on a major find, I know you've been looking for one of these! And from the original owner...drool!

I've built a clone of this exact type of amp, right down to the smooth tolex and Clapton knobs. While I am not going to pretend it can touch an original, it might just be the best sounding Marshall clone I've played. I also built mine with PA specs except a 100pF across bright channel volume, and I am not missing any treble. It sounds glorious.

It is extremely interesting to see the factory original impedance switch! I had been struggling with this quite a bit; I only have a cab with 8 ohm 20-watters (67 restored pinstripe with non-original speakers) but the amp never sounded exactly right at 8 ohms because the negative feedback is dependent on the impedance setting. It has less negative feedback in the phase inverter at 8 ohm than at 16 ohm, and the JTM45 sound *is* a high primary impedance (8k in this case) with lots of negative feedback IMHO. So after all this while I've stuck with the amp set for 16 ohm and playing it through a later cab. I finally found a '66 pinstripe cab I am restoring and will have set of 16 ohm speakers for that one, although possibly a mix of T0652s and T1221s.
But seeing this factory original switch has me thinking...would be cool to implement a similar switch in mine.

Oh, BTW, my clone amp is still missing a handle...have got a long brown MG handle as used on Vox amps, but would love to find a similar short brown handle as seen on this amp if anyone has got one.

I'll shut up now and let the talk about this particular amp go on. One of the coolest I've seen!
 

shakti

Member
Joined
Jan 17, 2007
Messages
494
BTW, is that not a red RS indicator light? Or do my eyes deceive me?
 

F-Hole

Well-known member
Joined
Sep 2, 2015
Messages
2,171
BTW, is that not a red RS indicator light? Or do my eyes deceive me?

Certainly looks like it from the photos, but it's actually amber.

I'm at the "self harm" end of the electronics knowledge spectrum, so I can't comment on the construction of the ohm tap. Some educated friends tell me, however, this it shouldn't work on an RS transformer......so I'm taking it to a geek friend who will figure out what's actually going on. I'll report. back. It's quite a mystery.
 

ourmaninthenorth

Well-known member
Joined
Mar 28, 2009
Messages
7,119
The geeks among us will understand the significance of the long retaining staple.




Would I be correct in assuming that the later pinstripes used small staples?

I'll be the silly bugger to ask that question .... how does it sound?
 

F-Hole

Well-known member
Joined
Sep 2, 2015
Messages
2,171
Would I be correct in assuming that the later pinstripes used small staples?

I'll be the silly bugger to ask that question .... how does it sound?

Yes, the early stuff (metal badge and early block logo) heads and cabs used the longer staples, while the mid-period block logo onwards into pinstripe used the short staples. The fakers very rarely get this correct. Below is a photo of the pinstripe era staples, from another block logo head.




I've only played it through a custom cab thus far, and it sounds great. I'm trying to figure out the wiring in the cabs, as they're wired series to put out 30 ohms.....so need to make sure all is correct before rigging it up fully as a set.
 

ourmaninthenorth

Well-known member
Joined
Mar 28, 2009
Messages
7,119
Thank you kindly mate.

I'm sure some necromancer will be along shortly to aid you with the black magic...I mean....wiring in the cabs. :teeth
 

shakti

Member
Joined
Jan 17, 2007
Messages
494
Certainly looks like it from the photos, but it's actually amber.

I'm at the "self harm" end of the electronics knowledge spectrum, so I can't comment on the construction of the ohm tap. Some educated friends tell me, however, this it shouldn't work on an RS transformer......so I'm taking it to a geek friend who will figure out what's actually going on. I'll report. back. It's quite a mystery.

Apparently it *is* possible to set it up for switching, I know others have done it with their clones using RS Deluxe repro transformers. But I am not sure if it is possible to make the negative feedback independent of the impedance setting. I can't really make out how the switch is set up from the photos, I'd have to see the turrets on the OT very clearly to decipher it, and even then I'd be scratching my head, so any info from your friend would be very helpful.
Simlarly, I am curious what primary impedance setting the OT uses. The RS Deluxe had multiple primary impedance taps. Later JTM45 OTs were hardwired for 8k, but the RS can be set for 6.6k as well. I greatly prefer it at 8k with KT66s though. But the turrets are concealed by the preamp board.
 

shakti

Member
Joined
Jan 17, 2007
Messages
494
Yes, the early stuff (metal badge and early block logo) heads and cabs used the longer staples, while the mid-period block logo onwards into pinstripe used the short staples. The fakers very rarely get this correct. Below is a photo of the pinstripe era staples, from another block logo head.




I've only played it through a custom cab thus far, and it sounds great. I'm trying to figure out the wiring in the cabs, as they're wired series to put out 30 ohms.....so need to make sure all is correct before rigging it up fully as a set.

Your speaker jacks are wired in parallell, so if you plug in both cabs with the amp set for 15 ohm you will be good to go.
 

springhead

Member
Joined
Feb 12, 2016
Messages
262
I've only played it through a custom cab thus far, and it sounds great. I'm trying to figure out the wiring in the cabs, as they're wired series to put out 30 ohms.....so need to make sure all is correct before rigging it up fully as a set.

I was given an original RS Deluxe OT and built a JTM 45 around it, but wanted to have it 8/15 ohm switchable - it can be done! Looks like they've got it right in this one.

The RS OT is not, unlike more modern transformers, tapped for 8/15 or equipped with separate windings. What it has is three separate secondaries - one at 6 ohm, two at 4 ohm, brought out to 3 pairs of solder posts - visible in the top left corner of the photo showing the board. They've strapped it as per normal 15 (actually 14) ohm wiring, and presented it on the black and blue wires, but also brought out one extra point - the red wire.

The switch is a DPDT with both halves strapped together - so an SPST (common trick with slide switches as they can be unreliable) and gives you either the 6 ohm and both 4 ohm secondaries in series = 14 ohm, or the two 4 ohm windings in series for 8 ohm.

A second blue wire from the switch takes the wiper off to the output jack. The purple wire is the feedback tap to the Presence pot which is from the 14 ohm combined output.

My busy-day-at-work addled brain is telling me that possibly the phase will be reversed 8 ohm relative to 14 ohm. I used a DPDT switch to achieve the same result but preserve the absolute phase. Not entirely sure it matters!

So Marshall got it right - you've got an RS Deluxe equipped JTM 45 with 8/14 ohm switchable output!

If your cabs are 30 ohm I suspect (and the photo doesn't quite show it) the output jacks on the amp are wired in parallel so the two cabs together gives you 15 Ohms. Why the original owner wanted an 8 ohm switchable option is an interesting question.


Hope the above helps.
 
Top