This is my JTM 45/100 that was recently finished after a parts-search that took me almost 2 years. Built by a friend here in Latvia. Some of the wiring needs to be tidied up at some point, but other than that it is finished. Sounds glorious - just like what i hear on records from 100w JTM's and very early el34 iterations.
Some of the specs:
- stock 45/100 circuit, except it is running on the 500v instead of the 560v tap and it utilizes a star-grounding scheme.
- Marstran 1202-84 OT and Marstran 1204-43 PT, Marstran C1999 3h Choke
- Alpha pots (almost impossible to find decent RS pots anymore)
- mid 60's 1/2w and 1w Piher resistors
- 66 mustards (the outer foil orientation of all mustards was taken into account, to gain a theoretical increase in shielding - no idea if it was worth the extra effort, but the amp is quieter when dimed and idle than my 18w clone)
- mid 60's RS micas
- nos Erie duoelectric snubber cap
- nos erie preamp filter cap
- RIFA electrolytics on the filter board
- mid 60's RS 1k5w wire wounds on the screen grids
- nos Mullard by100 bias diode
- nos by127 diodes
- original RS hole board material from 60's (toughest find!)
- original RS split top turrets from 60's
- original RS paxolin mains and impedance selectors
- original McMurdo octal sockets - exact ones used in the original Marshalls
- Bulgin mains plug
- Belling Lee fuse holder
- Mullard reissue 12ax7's
- TAD KT66's (selected and relabled Valve Arts)
As you see from the build, i am/was quite a bit into the NOS part cork sniffing craze (hope i'm not the only one of this kind around here
). To be honest, i don't believe there is any real tonal "benefit" to this - i just wanted to make a clone of the parts used back then, just to be sure i don't have any nagging thoughts about how close i MIGHT have come with NOS parts. Might be that all the old parts add up to some slight nuances, but i'd have to swap in boards with modern parts with same exact values to make an assassment on that.There is one issue though - it ghosts quite noticeably. I never had ghosting in any of the amps that i have owned, so i immediately noticed this and it doesn't leave me alone ever since - that is the only thing i wish to change about this amp.
ALL of the parts were tested for their specs and i actually selected them within a tolerance of about 2-3% throughout the amp (at least according to my chinese meter
). I avoided the dodgiest NOS part category - electrolytics - almost completely, except one cap in the preamp, which was formed along with all the new production electrolytics in the amp before it was fired up the first time. All of the caps, including the NOS Erie preamp formed nicely and dropped below 4v fast and easily. From what i have read so far, ghosting is not a rarity with these models, as there is supposedly some sort of a mistake in the design of the pi, which introduces this. It can sometimes be reduced by raising the filtering of the screens. But that's not a real solution (would not want to do it, as that would stiffen up the amp too). I have also read that it might be caused by bad grounding. The only deviation from an original layout is that a star-groudning scheme was used, which is superior to the one implemented by Marshall - as a result, the amp is very quiet and i have to stick my head into the cab to know, if it is on. Another cause might be bad filter caps (not very likely in this case, as they were tested (even for ESR) and formed properly, etc., but, of course, something might have gone wrong).
Is there any sort of procedure i could follow to try to narrow down possible causes of ghosting? What tests could i run to narrow down the cause?
Due to the "vintage correct" cork-sniffing nature of the build, i'm not too fond of blindly swapping out filter caps or do other modifications to the amp before there is an idea what might be the actual cause of the ghosting. Of course, if there is no such "text-book" procedure, that would be all i would be left with - to dig around til i find the hornets nest.
Ghosting gets noticeable on vol 6 (hi input) on the neck pickup, slightly earlier with a les paul. It maxes out at about 8 and doesn't increase noticeably when dimed. I've tried running the amp with 2 tubes pulled, but it still ghosted.
I'd be very grateful for some advice. Please let me know what information should i provide in order to give you a better picture of the problem?
Thank you very much i nadvance!
Regards,
Karlis
