Mysterious Marshall JTM45 amp

Hosted by Marshall Founding Father G3SDW. Discuss JTM45, JTM50, JTM100, Bluesbreaker, Super Lead, Super Bass, 200W, 100W, 50W, 20W, Plexi Panels, Metal Panels, 4x12 straight, 4x12 slant, Pinstripe, Basketweave, Checkerboard, Park, Kitchen-Marshall and Narb

Mysterious Marshall JTM45 amp

Postby Kalle_in_Sweden on Sat Sep 19, 2015 3:21 pm

Hi Marshall Vintage experts
I am helping a friend with a mysterious JTM45 amp that we believe is from around 1966 as it has alu chassi with diecast alu ends and Heathfield manufactured circuit board.
It has major modifications that seems to be very old (maybe from factory).
The tone control part is very different to normal and uses V2b (normally cathode follower) connected as triode gain stage as an active part in the tone control design (I am still trying to do a reverse engineering on the control design)
I need your help to identify this mysterious amp .
/Kalle
PS This amp might have up in this forum during 2011
viewtopic.php?f=1&t=97471&p=956474&hilit=tone+stack+jtm45#p956474
Here are a schematic showing some of the changes in italic letters and a picture of the amp .
Image
Image
Kalle_in_Sweden
Peasant
 
Posts: 7
Joined: Sat Sep 19, 2015 2:34 pm

Re: Mysterious Marshall JTM45 amp

Postby cerrem on Sun Sep 20, 2015 9:43 pm

Back in the mid 80's a guy from Connecticut brought me one of these JTM45 amps with a weird alternate tone stage....
He wanted it re-wired to "normal" JTM45 schematic...
I did not document the original circuit....
For years it was in the back of my mind as a strange alternate circuit I never came across again...till now..
Looking at your photo brings it all back ...that was the same wiring ....
From what I remember, the amp sounded very flat and clean with not much grind....
I may be mistaken, but I thought it was a 2 channel PA head...

CERREM
1969 SVT Bass Amps x2 + 4 8x10 cabs...
1964 JTM-45 (GEC KT66)
1965 JTM-45/100 Super-Amp100W(GEC KT66)
1965 JTM-45/100 100W SuperPA (GEC KT66)
1967 Super Lead 100W (Mullard EL34)
1968 Super Bass 100W (Mullard EL34)
Various cabinets
100% EVH free
*****SEMPER FI*****
cerrem
Knight
 
Posts: 1900
Joined: Thu Aug 22, 2002 7:01 am
Location: San Diego, California

Re: Mysterious Marshall JTM45 amp

Postby cerrem on Sun Sep 20, 2015 9:48 pm

Back in the mid 80's a guy from Connecticut brought me one of these JTM45 amps with a weird alternate tone stage....
He wanted it re-wired to "normal" JTM45 schematic...
I did not document the original circuit....
For years it was in the back of my mind as a strange alternate circuit I never came across again...till now..
Looking at your photo brings it all back ...that was the same wiring ....
From what I remember, the amp sounded very flat and clean with not much grind....

CERREM
1969 SVT Bass Amps x2 + 4 8x10 cabs...
1964 JTM-45 (GEC KT66)
1965 JTM-45/100 Super-Amp100W(GEC KT66)
1965 JTM-45/100 100W SuperPA (GEC KT66)
1967 Super Lead 100W (Mullard EL34)
1968 Super Bass 100W (Mullard EL34)
Various cabinets
100% EVH free
*****SEMPER FI*****
cerrem
Knight
 
Posts: 1900
Joined: Thu Aug 22, 2002 7:01 am
Location: San Diego, California

Re: Mysterious Marshall JTM45 amp

Postby Kalle_in_Sweden on Mon Sep 21, 2015 7:17 am

Hi Cerrem

Check more pictures of this amp (including my analyze of schematic of the active Baxandall tone stack) in this link:
http://www.ljudbojen.com/viewtopic.p...715334#p715334

What can you say about the mods made to this amp:
- Screen grid resistors changed to only 1Kohm/5W white ceramic per tube.

- Increased pre-amp filtering with one added 10 Kohm resistor and the grey 32/32 uF dual can on the board.
Is this a factory mod ?

- The change of tone stack to an active 3 stage Baxandall

Best Regards
Kalle Sandberg
Kalle_in_Sweden
Peasant
 
Posts: 7
Joined: Sat Sep 19, 2015 2:34 pm

Re: Mysterious Marshall JTM45 amp

Postby Dai on Tue Sep 22, 2015 10:25 am

hi,

I think this may be the correct link (below)?

http://www.ljudbojen.com/viewtopic.php?f=21&t=104919
Dai
Baron
 
Posts: 3377
Joined: Fri Jun 15, 2001 7:01 am
Location: lying in the harmonic marshall filth

Re: Mysterious Marshall JTM45 amp

Postby pdf64 on Tue Sep 22, 2015 11:47 am

Really interesting, thanks!
I think that the coupling cap from V2b plate may be 47nF (47000pF), rather than the 470pF noted on the schematic in the linked thread?

This may be one of those amps that should be preserved for historical record as a museum piece, rather than maintained as a working amp?
I wonder if even Marshall have a documented record of it?
It would have saved them re-inventing the wheel when the idea of active Banxandell type tone controls is raised (ie been there, done that, didn't sound like a Marshall, wasn't well received).

Regarding the screen grid resistors, my impression is that the more normal JTM45 arrangement of '1k shared then 470 ohm individual' provides a slightly smoother tone / more rounded waveform, at high signal levels. Though the higher overall effective resistance may have been more suited to KT66, which, being beam tetrodes, draw less screen grid current than EL34.
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1243
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Mysterious Marshall JTM45 amp

Postby LD50 on Tue Sep 22, 2015 12:09 pm

pdf64 wrote:Really interesting, thanks!
This may be one of those amps that should be preserved for historical record as a museum piece, rather than maintained as a working amp?
I wonder if even Marshall have a documented record of it?
It would have saved them re-inventing the wheel when the idea of active Banxandell type tone controls is raised (ie been there, done that, didn't sound like a Marshall, wasn't well received).

Regarding the screen grid resistors, my impression is that the more normal JTM45 arrangement of '1k shared then 470 ohm individual' provides a slightly smoother tone / more rounded waveform, at high signal levels. Though the higher overall effective resistance may have been more suited to KT66, which, being beam tetrodes, draw less screen grid current than EL34.


I agree, looking closely at the components and solder joints they do look original.
The screens I am sure were changed to accomodate EL34s at some stage, the bias feed resistor has been fiddled with and pot added so I suspect different valves have been tried. I would imagine late 66 that KT66s were used with that OT (presume a Drake 784-103 with 8k primary)
All bleeding stops.......eventually.
LD50
Prince
 
Posts: 4196
Joined: Tue Jun 12, 2007 12:27 pm

Re: Mysterious Marshall JTM45 amp

Postby Kalle_in_Sweden on Tue Sep 22, 2015 9:31 pm

Hi Guys
I have AC measured the OT and it has a primary impedance around 7.2 KOhm (anode-anode) which is typical for KT66 tubes.
/Kalle
Kalle_in_Sweden
Peasant
 
Posts: 7
Joined: Sat Sep 19, 2015 2:34 pm

Re: Mysterious Marshall JTM45 amp

Postby cerrem on Wed Sep 23, 2015 6:42 pm

Kalle_in_Sweden wrote:Hi Guys
I have AC measured the OT and it has a primary impedance around 7.2 KOhm (anode-anode) which is typical for KT66 tubes.
/Kalle


Depends on how you measured it.... Frequency, Voltage level...ect....
You may need to account for the magnetizing current..if not, then you will see what appears to be a lower Z then what is actually there...

CERREM
1969 SVT Bass Amps x2 + 4 8x10 cabs...
1964 JTM-45 (GEC KT66)
1965 JTM-45/100 Super-Amp100W(GEC KT66)
1965 JTM-45/100 100W SuperPA (GEC KT66)
1967 Super Lead 100W (Mullard EL34)
1968 Super Bass 100W (Mullard EL34)
Various cabinets
100% EVH free
*****SEMPER FI*****
cerrem
Knight
 
Posts: 1900
Joined: Thu Aug 22, 2002 7:01 am
Location: San Diego, California

Re: Mysterious Marshall JTM45 amp

Postby pdf64 on Thu Sep 24, 2015 8:23 am

What procedure do you recommend?
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1243
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Mysterious Marshall JTM45 amp

Postby Kalle_in_Sweden on Thu Sep 24, 2015 2:23 pm

Hi Cerrem

I injected 2.0 VAC/880Hz on the 8 Ohm tap and measured 60 VAC between the anode-anode taps
=> voltage "gain" = 30 >> 30 x30 = impedance "gain" = 900 >> 900 x 8 ohm = 7200 ohm

Are my calculations correct ?
Should I have used another frequency or higher test voltage ?

/Kalle
Kalle_in_Sweden
Peasant
 
Posts: 7
Joined: Sat Sep 19, 2015 2:34 pm

Re: Mysterious Marshall JTM45 amp

Postby pdf64 on Thu Sep 24, 2015 3:49 pm

Was that 2V RMS, peak, or peak-to-peak?
Did you scope the wave to check that your signal generator was able to maintain good sine wave accuracy into such a heavy load?
Did you measure the voltages with a true RMS meter?
My band:-http://www.youtube.com/user/RedwingBand
pdf64
Knight
 
Posts: 1243
Joined: Thu Dec 14, 2006 9:55 am
Location: Staffordshire UK

Re: Mysterious Marshall JTM45 amp

Postby Kalle_in_Sweden on Thu Sep 24, 2015 4:25 pm

Hi
I used a 10W power amp (after the signal generator) to drive the transformer and a true RMS voltmeter to check all voltage levels. I did not use a scope to check signal quality.

Kalle
Kalle_in_Sweden
Peasant
 
Posts: 7
Joined: Sat Sep 19, 2015 2:34 pm

Re: Mysterious Marshall JTM45 amp

Postby cerrem on Fri Sep 25, 2015 8:50 am

For this type of transformer measurement...
You need to get up and out of the mud..... 2V is way to low on the BH curve for that low quality steel..
Keep in mind that the real operating voltages are up around 400V AC range......

The best place to measure the transformer is in the amp.....
Push a 1KHz or so signal into the amp and turn it up a bit to get at least 100V to 200V across the primary... I use Tektronix P5205 High Voltage Differential Probe across primary @ 50X...
Then another 1X scope probe across the output jack ...resistive loaded...
There is a way to do it with no load, without damaging the transformer but won't mention it here...
Then in the scope you use the MATH function to DIVIDE CH1/CH2 , then scale it for 50X..... the flat line on the scope is the plate load, if not flat, reverse the phase of the diff probe...

If you don't have the scope and probes use your TRUE RMS meter...make sure to use clip and not hold the probes in a live amp...

DISCLAIMER...If you don't have the experience to do this live voltage test ..then don't do it !!!

Personally don't waste your time to measure it... "It is what it is " measuring it won't make any difference to the amp.....
1969 SVT Bass Amps x2 + 4 8x10 cabs...
1964 JTM-45 (GEC KT66)
1965 JTM-45/100 Super-Amp100W(GEC KT66)
1965 JTM-45/100 100W SuperPA (GEC KT66)
1967 Super Lead 100W (Mullard EL34)
1968 Super Bass 100W (Mullard EL34)
Various cabinets
100% EVH free
*****SEMPER FI*****
cerrem
Knight
 
Posts: 1900
Joined: Thu Aug 22, 2002 7:01 am
Location: San Diego, California

Re: Mysterious Marshall JTM45 amp

Postby LD50 on Fri Sep 25, 2015 3:29 pm

cerrem wrote:For this type of transformer measurement...
You need to get up and out of the mud..... 2V is way to low on the BH curve for that low quality steel..
Keep in mind that the real operating voltages are up around 400V AC range......

DISCLAIMER...If you don't have the experience to do this live voltage test ..then don't do it !!!

Personally don't waste your time to measure it... "It is what it is " measuring it won't make any difference to the amp.....


Looks like an original -103 to me so anything measured around 8k (7-9k) is about right?
All bleeding stops.......eventually.
LD50
Prince
 
Posts: 4196
Joined: Tue Jun 12, 2007 12:27 pm

Next

Return to Marshall

Who is online

Users browsing this forum: No registered users and 5 guests