Original 1960s JTM45 vs. a 1990s re-issue JTM45

SirJackdeFuzz

SirJackdeFuzz

New member
Friends of the Marshall Fan Club.

Is there anyone who has knowledge of both these amps ?
I can get a 1990's RI 2245 JTM45.

Does anyone know how these might differ in circuitry, valves/tubes, and, and, and, from the real deal from the 60's ?

greg jtm45 SD Forum.JPG
The actual amp in question.

Any info will be great.
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

i dont remember details but there are quite a few differences, not bad amps though
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

The main difference is higher value filter caps. There are some minor differences such as the values of the mixer resistors and such. I was surprised to learn that many 60's 45's used 100pf bright caps instead of 500pf. Some of these differences are not bad from a functional perspective, though. As Jeremy wrote not bad amps.

Some of the important bits are close to 66-67 JTM, such as the Drake transformers and the large amount of negative feedback.

The reissue uses 5881s instead of KT66s but this is easily changed. Back in the 90's people were pulling the 5881s and putting in KT66s. The Sovtek "5881" during the 90s is actually more like a 6L6GC than a 50's 5881. Today's current production reissues of real 5881s might be interesting because the earliest 45's used real 5881s before they switched to kT66s.
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

Lake Placid Blues said:
The main difference is higher value filter caps.
There are some minor differences such as the values of the mixer resistors and such.
I was surprised to learn that many 60's 45's used 100pf bright caps instead of 500pf.
Some of these differences are not bad from a functional perspective, though. As Jeremy wrote not bad amps.

Some of the important bits are close to 66-67 JTM, such as the Drake transformers and the large amount of negative feedback.

The reissue uses 5881s instead of KT66s but this is easily changed.
Back in the 90's people were pulling the 5881s and putting in KT66s.
The Sovtek "5881" during the 90s is actually more like a 6L6GC than a 50's 5881.
Today's current production reissues of real 5881s might be interesting because the earliest 45's used real 5881s before they switched to kT66s.
Click to expand...

In laymen's terms, what would higher filter caps do to the tone of those very early JTM45s?

How will a 100pf bright caps differ from a 500pf then ?

According to you, how might a 90's RI differ in tone and feel to the real deal 1960's JTM45 ?
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

There are a few vids about of comparisons. The new one have a few other changes that make them not sound like the originals if you are interested in vintage accuracy.
There was a RI just recently I think where they got things like transformers and chokes made to original spec - this version sounded much closer, especially paired with an accurate speaker as well. Of course the cost was huge too.
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

SirJackdeFuzz said:
In laymen's terms, what would higher filter caps do to the tone of those very early JTM45s?

How will a 100pf bright caps differ from a 500pf then ?

According to you, how might a 90's RI differ in tone and feel to the real deal 1960's JTM45 ?
Click to expand...

Filtering affects the feel of the amp, more than it does the sound. I have my JTM45 with original filter values and it is not spongy or flubby at all. I am using a 5AR4 rectifier.

A 100 pf makes the volume taper and the gain sensitivity more bassman and less plexi. 500 pf is the correct value according to Ken Bran, but apparently the 100 pf cap was also used. The reissue uses 480 pf because it is more common today.

The use of higher filtering in an off the shelf production line amp makes sense because the affect on the sound and feel is minimal but avoids possible problems of hums ghosting and so forth.

One the the unexpected things about my JTM45 experience is the importance of the rectifier. You can completely change the personality of the amp by rolling in different rectifier tubes or even using a solid state plug in rectifier. The rectifier can override all the above subtleties of tone and feel and differences between an original and a reissue. A rectifier that supplies a lower voltage and more dynamic current supply will change the headroom of the amp and also the amount of sag when the amp is run loud. 5881s will have less head room than KT66s and a different bias point for the sound you want. Obviously, different rectifiers affect the bias as well.
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

AlexR said:
There are a few vids about of comparisons. The new one have a few other changes that make them not sound like the originals if you are interested in vintage accuracy.
There was a RI just recently I think where they got things like transformers and chokes made to original spec - this version sounded much closer, especially paired with an accurate speaker as well. Of course the cost was huge too.
Click to expand...

YUP, i deff want it so sound as vintage correct as humanly possible.
Can a GOOD amp tech do the necessary changes (exlc. the transformers off course) if asked to do so ?
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

Lake Placid Blues said:
Filtering affects the feel of the amp, more than it does the sound. I have my JTM45 with original filter values and it is not spongy or flubby at all. I am using a 5AR4 rectifier.

A 100 pf makes the volume taper and the gain sensitivity more bassman and less plexi. 500 pf is the correct value according to Ken Bran, but apparently the 100 pf cap was also used. The reissue uses 480 pf because it is more common today.

The use of higher filtering in an off the shelf production line amp makes sense because the affect on the sound and feel is minimal but avoids possible problems of hums ghosting and so forth.

One the the unexpected things about my JTM45 experience is the importance of the rectifier. You can completely change the personality of the amp by rolling in different rectifier tubes or even using a solid state plug in rectifier. The rectifier can override all the above subtleties of tone and feel and differences between an original and a reissue. A rectifier that supplies a lower voltage and more dynamic current supply will change the headroom of the amp and also the amount of sag when the amp is run loud. 5881s will have less head room than KT66s and a different bias point for the sound you want. Obviously, different rectifiers affect the bias as well.
Click to expand...

GREAT info LPB . . . thank you very much !
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

Yes and yes and... yesss.

I have one of those 90's JTM45 reissues (1996).

In 2003, I followed these instructions to get mine to original spec, but I also purchased a PTP kit.

These are the no-BS, real-deal, amp modding notes to make your amp a true mid-60s circuit JTM45...dig it:

http://raw-sewage.net/jtm45ri_mods.html

(note: my amp is no longer a JTM45, but a '68 plexi circuit)
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

LLL said:
Yes and yes and... yesss.

I have one of those 90's JTM45 reissues (1996).

In 2003, I followed these instructions to get mine to original spec, but I also purchased a PTP kit.

These are the no-BS, real-deal, amp modding notes to make your amp a true mid-60s circuit JTM45...dig it:

http://raw-sewage.net/jtm45ri_mods.html

(note: my amp is no longer a JTM45, but a '68 plexi circuit)
Click to expand...

Thank you very much for the info LLL.
Gonna look into it right now.
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

SirJackdeFuzz said:
Any reason in particular for this option LLL ?
Click to expand...

I originally put Mallorys in during my JTM45 mod phase... when I modded again for '68 plexi (not a whole lot of changes) I replaced all Mallorys (except presence pot cap) with Sozo (new make).

I think they're clearer-sounding... but not like Sprague Orange Drops.
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

LLL said:
I originally put Mallorys in during my JTM45 mod phase... when I modded again for '68 plexi (not a whole lot of changes) I replaced all Mallorys (except presence pot cap) with Sozo (new make).

I think they're clearer-sounding... but not like Sprague Orange Drops.
Click to expand...

Understood . . . thank you very much !
 
Re: Original 1960s JTM45 vs. a 1990s re-issue JTM45

Get these power tubes if you're going the KT-66 route:

41yna6EE6EL._SX466_.jpg

https://www.amazon.com/Valve-Art-KT66-Vacuum-Single/dp/B003HEC0QS

They're supposed to be the closest new-manufacture KT-66 to the holy grail NOS GEC (Genelex) KT-66s.

Also note these amps (the originals) have a ton of negative feedback (off the 16ohm tap IIRC); thereby making them pretty clean.

If you wanted more dirt you could move the negative feedback wire to 8 or 4ohm tap... but remember too much and it may get flubby in a hurry... that preamp
820ohm + 330/250uF combo on Channel 1 is really phat to begin with.
 
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