1987x vs. JTM45

[Telesexual]

from what I understand the 1987x is just a JTM45 with solid state rectifier. What are the tonal differnces here. Possibly alittle bit louder, less spongy?

[jeremy]

Re: 1987x vs. JTM45

a little louder, a little less spongy, a little brighter

im not sure they are exactly the same but similar in many ways. having said that, all the 1987x models ive played have had el34 tubes where the jtm45's ive played almost all had a kt66/6l6 type tube. that changes things quite a bit, but i think that with the same tubes they sound sorta similar but with the differences i mentioned above

[Lake Placid Blues]

Re: 1987x vs. JTM45

+1

The pre-amp has a few things different in terms of caps and resistors. The output transformers and filter caps are also different. The Solid State rectifier runs the power supply at higher voltages. This not only increases clean head room and but also wattage output. The JTM45 will put out about 35 watts at clipping, but the 50 will be about 55 watts at clipping, and 90 watts flat out.

[Rich_S]

Re: 1987x vs. JTM45

Let's see some more discussion here! I'm looking at converting an old PA to a JTM45, but want to be able to option it toward 1987, also.

One thing I found interesting during my recent search for JTM45 schematics - I couldn't find a 2-channel normal/bright version of the JTM, like we expect from the Plexis and other NMV Marshalls. The only JTM45's I found were normal/trem models, or PA's. The tweed Bassman has normal and bright channels, but I can't find a JTM like that. So, I'm thinking of grafting a Plexi preamp onto JTM everything else.

Or did I miss something?

[Gr8Scott]

Re: 1987x vs. JTM45

I'll get you a link to a buck naked JTM-45 schem with no trem. Hang tight.

[Lake Placid Blues]

Re: 1987x vs. JTM45

The main diffrence between bass, lead, and PA, channals on JTM45 preamps were the bright cap values. The lead version had a 500pf cap across the volume control, and an additional 500pf across the 270k resistor on the lead channal. The Bass channal only has the 500pf across the 270K. Otherwise it's like the Tweed bassman. The PA has no bright caps.

The plexi preamp had the 270k resistor upped to 470K on the lead channal. The reissues JTM45's are not correct. Later plexi's have different resistor and cap values between cathode and ground on the preamp tubes. The early plexi 100's retained the JTM45 cap and resistor values in the preamp, but later plexis have similar values to early metal panal amps and have tighter bass.

On edit: The main difference between plexi and JTM45 in terms of caps and resistors between cathodes and ground is the shared cathode for V1A and V1B on JTM45 and early plexi, and independant caps and resistors lines to ground, for the V1A ana V1B on the later amps. The cap value changed from 250uf to 330-or 320uf on V1A, with 68uf past 27k resistor on V1B. Plexis started using a 68uf cap on the V2A cathode.

The EVH amp pre-amp (an early 67 model) is supposed to have used a 250Uf on V2A, and retained the shared older JTM45 cathode design for V1, but with a 320uf cap instead of the 250Uf there. The exact arangement and values were in a constant state of change during those years, so a 66 amp may be different from a 67, and a 68, and so forth....Another difference is the use of .002 coupling caps on the later plexi lead amps (lead CH signal path) in place of .022 used on the earlier amps, and the bass versions.

[Rich_S]

Re: 1987x vs. JTM45

I'll get you a link to a buck naked JTM-45 schem with no trem. Hang tight.

Looking forward to it.

Thanks for your input, too, LPB.

When we're done, I'm printing this thread to store away in my project notebook.

[Scott_F]

Re: 1987x vs. JTM45

I've already told you what I do, so I'll leave it at that. I'd ask Armotron if he can hear teh difference between the V1A V1B thing versus the traditional JTM45 set up as I wired his both ways and ptu a switch on it. My old tired ears could hear virtually no difference.

[SoCalSteve]

Re: 1987x vs. JTM45

Hey Scott, can your Edana be ordered with a master volume or do you suggest an attenuator for less than rehearsal-gigging volume?

[Scott_F]

Re: 1987x vs. JTM45

don't need no stinkin' attenuator! I could probably kill off the presence pot and put a MV in there is someone DEMANDED it.

[SoCalSteve]

Re: 1987x vs. JTM45

Hmmm. Interesting. Thank you.

[Scott_F]

Re: 1987x vs. JTM45

tequila and amp building are dangerous bed-fellows.

[Telesexual]

Re: 1987x vs. JTM45

tequila and amp building are dangerous bed-fellows.

Could be dangerous but both are fun. I think tequila is a bit more fun though.

Although I decided on holding off on the JTM45 and go with a smaller tube combo for the time being. I am interested on building my own JTM45. I know metroamps or something has their own kit. I have 2 years electrical eng. experience but do not know much about amps in general. Although I am no longer in electrical eng. field anymore it is still something that interests me but couldnt imagine wanting to do if for work. Scott you have the right idea on building your own amps for other people and yourself as a hobby. I cant believe you are selling those edenas at those prices. I sure hope you are making some money. I am sure it can be quite tedious unless those clones are second nature to you now.

[Telesexual]

Re: 1987x vs. JTM45

don't need no stinkin' attenuator! I could probably kill off the presence pot and put a MV in there is someone DEMANDED it.

I like the presence pot. but I could see loseing when jumping channels or if you only use the bright channel. The bass side really needs it though IMO.


keep the comments coming you guys. I wanted this thread to be really open in the discussion of NMV amps and possibly mods to them. I love these marshall amps and the Fender bassman LTD.

[Lake Placid Blues]

Re: 1987x vs. JTM45

This talk about the presense control reminds me of the diffrences on the feed back loop among these amps.

The original JTM45 had the feed back loop connected to the 16 ohm lug. The Bassman had it connected to the IIRC, the 2 ohm lug. Later Marshalls have it connected to the 8 ohm lug with EL34's, and the 4 ohm lug with 6550/KT88, if I remember correctly. Or is it 8ohm with 6500 and 4 ohm with EL34?

Additionally, on the power amp side, the coupling caps are .1 on the Bassman, JTM45, and bass model Plexis, but lead plexis run .022 there.