Some EVH on my 1968 JMP 50W

[mean_street]

Hey guys,

I've just acquired an 1968 Marshall JMP 50W. Original transformers, largely untouched, some coupling caps and the filter caps have been changed. The first time I played it Ive been wondering that it has not the amount of gain I know from my other ´68 speced 100w amps (owned a 1959hw that I modded to 68 specs and currently own a Suhr SL68).

Here is a clip, all controls on 10, Vol II at 3 or 4, Clinch Fx Ep-pre, guitar is a Gibson Les Paul with Bare Knuckle Mule Set.

https://soundcloud.com/benjamin-kreibe/ ... vh-content


Maybe someone can help me with this?

[Tazin]

I don't think it sounds bad. It's sounds like it has enough gain but it's just a bit mushy sounding...It needs a bit of clarity and bite.

[johnnybgoood]

Awesome acquisition! DO NOT MOD THIS AMP! *** See post below regarding bright cap.*** You have an incredible piece of history there!!!

"Want that VH1 tone? You gotta slave it...i would go as far as saying,..Edward did run this setup." -Rockstah RIP ( FYI https://en.wikipedia.org/wiki/Mark_Abrahamian )
viewtopic.php?t=20611&start=45#p207396

Examples of amp - slave amp setup.
TheMarshallmaniac - https://www.youtube.com/watch?v=d0uUXZejA_A

Jim Humble - https://www.youtube.com/watch?v=STj8RDDqbcY
https://www.youtube.com/watch?v=5A7MsY3hMGk

[johnnybgoood]

Ok I've calmed down. Your bright cap on the Volume 1 pot looks like it is soldered from wiper to ground. Please confirm yes or no. If yes, all your highs are going to ground. For '68 specs, the bright cap needs to be soldered from Pin 3 (input form the 0.0022 coupling cap) to Pin 2 (wiper) of Volume 1. This will give you more gain as all the highs from the 5000pf cap will directly go to the treble peaking circuit (470k/470pf) right before the second gain stage of V2a.

[mean_street]

Man, thank you so much! I will fix this immediately when I'm home from work. Yeah, somebody seems to have changed the pots, might reflow all the joints because some of them look suspicious.

[mean_street]

So, I checked the bright cap now. It was soldered correctly, but one of its legs touched the back of the pot slightly. Just bended it a little bit away and the amp came to life.

Many thanks for the tip!

[Tazin]

Good catch regarding the volume pot bypass cap. Now we need another sound clip to hear the amp in proper working order!

[mean_street]

Will make a clip today

[mean_street]

https://soundcloud.com/benjamin-kreibe/ ... clip-fixed

Just a short clip of the sound now. Again: thank you all so much!

[Tazin]

https://soundcloud.com/benjamin-kreibe/ ... clip-fixed

Just a short clip of the sound now. Again: thank you all so much!

Sounds pretty good to me...It has that "typical" '68 lead sound.

[Tazin]

Did the previous owner mention why he changed all the potentiometers?

[jnew]

That was a great observation and catch. Congrats on scoring that amp. It's a killer piece. You can also swap out the NFB resistor for 100K for more gain if you still need.

[efraser68]

Sounds killer!! So you're not attenuating this at all? What speakers are you running? Would love to hear what the amp sounds like without the EP Pre.

[MrBeasty]

Sounds stellar.

I had a 1970 JMP50 which I sold to get a JMP100, I always regretted it. This twists the knife a little.

[MrBeasty]

Just picked-up a '68 JMP50 ... it looks very similar, .68s on the pre-amp, etc.