This Marshall is from the transitioning periode and is actually called a JTM50. The chassis is definitely a late 1966 one because its still made out of aluminium and also has the big block ends like the JTM45s had. Marshall switched over to steel chassis in 1967, because the aluminium ones tended to bend. The faceplate however already has the JMP marking on the left side like the late 67 and all the 1968 models had. A 1966 Faceplate would have had MKII on the left and JTM45 on the right printed on it. A mid 1967 faceplate would normally be a "black flag" variant with an inverted JTM printed on next to the mains switch. Circuit wise it already has the 784-128 outputtransformer, so it has the 3.5k primary and was shipped with EL34 output tubes. The resistors mostly seem to be the original Iskra ones, only the 820 on V2A, the 220ks for the EL34s and the bias resistor have been changed. The 1k in the powersupply should be 8.2k. The channel mixing resistors on this amp are still the 270k ones like the early JTM45s had, so you get a little more interaction between the channels and a different midboost with the regular 500pF bypass cap. For whatever reason that bypass cap is missing in this amp so both of the channels will sound the same, similar to the pa heads. For some reason Marshall didn't install screen resistors or gridstoppers on these amps. You should definitly add 1k 5Ws on the screens and 5.6k on the control grid of the EL34. Until 1968 the lead and bass versions were essentially the same the only difference was that the lead had an additional 100pF bright cap on the volume pot. The split cathode and the lead style tonestack were introduced in 1968. The Superbass pretty much stayed the same throughout the years and continued to use the JTM45 style preamp.
@@benburnett8109 it’s a transition amp whereas it is transitioning from JTM 45 (5881) to JTM 50 (el34). Those amps were black flags etc. most closely resembled a 1986 amp circuit
Can't wait for part 2! Super cool old amp. Great stuff as usual, Jason. I bet lowering those voltages and removing the excess filtering will really help that amp breathe and sing.
Nice! I never get any 60s in the shop. Always 70s, usually late, sometimes early. Just picked up a mint 79 JMP SL (front serial number)! Everything original....components, even the PCB studs and screws, never been MODded and no extra holes drilled! For this one, those old Radio Spare caps should all be replaced as worn and dried out or will take a shit soon, so not worth keeping in after 56 years and old technology. Funny, looks like this has 220k/220k mixers instead of 470k/470k along with all of the other changes not in the usual later model JMPs...250pF treble cap instead of 470p. Can't read the pink resistors going to the power tube PIn 5 grids (typically 220k/220k). Which NFBH resistor is in this and which tap going to or to the output jack?? Can't wait to hear it AFTER the restoration! One thing I did not what was white Jim Marshall Products sticker and the frequencies 40/60 cycles (instead of the modern traditional 50EU/60US! Likewise, voltages 110V/250V, now typical modern is 120V in US and 240 EU, and total power consumption wattage of 160 watts!!
Thats some high preamp voltages. Might be good opportunity to a video on how preamp voltages affect sound/feel. Test with high/stock/medium/low voltages would be super interesting.
That sounds really close to my stock 70 Superbass. That said, not surprised it’s lacking some clarity with those settings. Drop the Bass to zero and it’ll be much clearer.
what tubes are in there? probably not the original mullards. ive recently retubed a 1968 jtm 50 with nos mullard tubes and it sounded amazing. if you need nos get them now before they go into the twilight zone of crazy prices. for the preamp you need mullard i63's(1965-1974) these are still available and for the power section xf2(double o getter) el 34's are the best but are extremely pricey and rare so xf 3 or xf 4 will be fine as well. enjoy!
You’d be surprised. Old tubes take a long time to lose transconductance or die. Have a trace Elliot bass head that runs 6 x 6550’s at pretty high plate volts for a 380w clean output… Still on the original trace Elliot branded 1991 tubes.
Diiiiaaaamonds and duuuust! Fine sounding amp and I'm sure you'll tweak that little extra sweetness back into it! I've got a JTM45RI put GEC 5881 power valves, Brimar GZ34 and Mullard ECC83 valves into it and it rings like a bell, remarkably clear and articulate. I do have a collection of old Mullard mustard caps that all measure OK. I'm toying with the idea of recapping it with old stuff to see what it does. What do you reckon? Daft idea?
Strange to convert the tube rectifier to SS and then put in a 15 ohm sag resistor. The GZ34 tube will do this by itself due to the internal resistance of the tube. So wondering why the conversion? Didn't accomplish anything.
The following MODS MUST BE DONE TO THIS AMP. NO QUESTIONS ASKED: 1) SIR #34 MOD 2) SIR #36 MOD 3) JOSE MOD 4) VARIAC ADDED 6) BJB MOD. This mod is most important. This mod alone will deliver an amazing amp from crystal clean to ultra grind and scream I can send you a detailed schematic. But beware; the BJB MOD is all about attitude. 7) Fully adjustable stereo effects loop. 8) leapord skin print tolex with chrome plates. Log chain carry handles. And a chrome mud flap naked girl added to each side. 9) Amps like this must be modded. They've been boring for 55 years, now it is your responsibility to give this amp real character Failure to MOD this amp will create a firestorm on the internet. The experts will hound you. Restoration is the easy way out; and Headfirst Amplification is all about pushing boundaries and rules into the IONOSPHERE!!!
@@HeadfirstAmps As a leader in the field you must make Bold decisions. What would EVH do? What would Angus do? They would say make that thing rip!!! Do the BJB MOD straight away......no questions asked. That is what Eddie would say.
There’s ways to recreate tube rectifier sag with silicon one. Brian Mays been running silicon rectifiers for awhile now… However I don’t actually buy the whole rectifier tubes are unreliable thing. They last years and it’s not hard to carry a spare and put some protection diodes before the tube to avoid a failure trashing anything.
Because the (M) is broken on the logo, M is the 13th letter in the alphabet, therefore you are missing 13 kilohertz in the frequency band. That’s why the amp sounds like farts.
