JTM50 Black Flag build, finally under way.

ivan H

ivan H

Ambassador of Tubes & Grooves
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I guess most here at TTR are aware of my fondness for the early EL34 Plexi circirts, as Marshall moved away from the JTM45 & refined the circuit into the superlead. Having already done a '68 50 watt & '69 100 watt superlead (both of which have been shown here), I thought I'd best do a '67 build.
The chassis is a Valvestorm 1987 type so not period correct, but I'm not trying to do an "exact" clone, (just get the circuit as close to period correct as possible). It (the chassis) needed a little work as it was punched for a power cord & I wanted to use an IEC connector, so made the cutout a little undersized with a Dremel cut-off wheel & filed to size. I also had to drill some ground point holes & drill the sidewall for the AC voltage selector turret strip & internally mounted filter cap clamp.IMG_20191027_145507.jpg
Power transformer is a Marstran/Heyboer 1202-118 that I had. I got as far as seen in the pic & got held up waiting for NOS McMurdo octal valve sockets to arrive (that's a "used" McMurdo socket in the pic). I used the time to lace up the wiring that will run along the chassis sidewall up to the switchesIMG_20191026_125002.jpg
This is the first time I've attempted this & while the actual lacing is easy, tying the start, & especially the finish knot isn't. I also opened up the control pots & doctored their values to that which I like best (CTS pots are the easiest to do this with).
The McMurdo octal sockets arrived yesterday & I got a little more done last night. Pre-laced up wiring in place WP_20190307_20_04_32_Smart.jpg
All valve heater wiring completed & chassis ground lugs addedWP_20190307_20_02_34_Smart.jpg
Rectifier valve socket wiring done & main filter cap in placeWP_20190308_09_47_01_Smart.jpg
I'll get more done this afternoon & tonight so will post more. Cheers
 
67plexi said:
Nice should be interesting on the 32uf x 8 filter board
Click to expand...
The JTM100's (like yours?) have the 8x32uf board, the JTM50's have the main filter mounted above the chassis between the rectifier tube & power tubes, with the screen/PI filter mounted internally on the sidewall. Filtering on most JTM50's was 32uf (40uf) main, 32uf screens, 16uf PI & a 32uf/32uf board mounted pre amp filter. Cheers
 
Here we can see a JTM100 32uf X 8 filter cap board (caps mounted "upright")15643108082743267217017087367904.png
Or a layout diagram of horizontally oriented caps 15642707910171801330916082261510.png
Here is a JTM50 gutshot where we can see the internal sidewall mounted screen/PI filter. 15717278645284660914586610507452.png
Though a good many JTM50's have been converted to a solid state rectifier, most actually left the factory with a GZ34 tube rectifier.
A couple of things of note about the JTM50's, they could be factory fitted with either a 1202-118 power transformer that delivered about 400~410VDC with a tube rectifier (420~430VDC with SS rec), or a 1202-133 power transformer that delivered about 460~470VDC with a tube rectifier or around 490~500VDC with solid state. Early amps used a 784-128 output transformer, then the 784-139 output transformer was also introduced, with both types being used throughout. So JTM50's can be found with any combination of these power & output transformers. Most used a 3 Henry/110 ohm filter choke but some were fitted with the Radio Spares 20 Henry/690 ohm filter choke (the later would be advantageous with the higher voltage 1202-133 PT). Earlier amps used 270k channel mixer resistors while later amps were fitted with 470k mixer resistors. It was during this period that Marshall changed over from using the dark brown Piher brand resistors to the yellow/tan Iskra brand resistors, so amps can be found with either type or a combination of both.
I prefer the way the Piher's "color" the amps tone so will be using them. Cheers
 
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syscokid said:
Oh Hell yeah... School's in session... :cool:



Never heard of McMurdo but I think I've seen them in some internet pics. They sure look cool though. I'm sure they add some extra mojo... :celebrate::celebrate:
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The McMurdo sockets were used in a lot of old British (& Aussie) amps. If you look closely at the pics of the JTM100 & JTM50 that I posted earlier you'll see the octal sockets are the same as I have used. They are a good quality socket made of phenolic or bakelite material. I used them in my '68 & '69 builds too. They can still be found at reasonable prices, though I also have a handful of reclaimed (used) ones that would be ok to use if cleaned & re-tensioned.
The wires that will be run from the board to the filter caps etc will also be laced. Cheers
 
Ok, it's quite overcast here this morning & bushfires in the area are still raging so the ambient light is low, which doesn't make for good pics, but anyway...
I fitted the OT (output transformer) & filter chokeWP_20190309_09_50_01_Smart.jpg
The OT is again from Merren Audio. I'm really impressed with the OT that Chris wound for my '69 superlead build & have heard equally good things about his 784-139 OT. The filter choke is a Classictone item, 3Henry/110 ohms. Its actually wound on a paper former (like was fitted to these), not a plastic bobbin like most supply. Not that it makes a difference.
I chose to fit screen grid resistors, even though these amps did not have them. The xf2 Mullard EL34's that I have for this can deal with no screen resistors, but they're getting expensive so I don't want to risk shortening their life. With no screen resistors fitted the plate potential falls below the screen potential during half cycle conduction, thus leaving the screen grid as the most positively charged electrode in the tube, resulting in excessive screen current. Fitting resistors results in a slightly more compressed tone. Resistors are Welwyn W22 series 1k/7 watt, though Ohmite's are also a good choice.
One good thing about being in Australia is that NOS British components can be found, like the Belling/Lee fuse holders used here.WP_20190309_09_49_05_Smart.jpg
I also wired up the board that I had previously turreted. This is my last piece of the really nice phenolic board blank that I scored off Brian Haberman when he stopped producing turreted boards about a decade back.WP_20190309_09_47_03_Smart.jpg
Please excuse my fingerprints on the board, it will be cleaned before fitting. The boards in '67 amps (& previous) were wired on the board underside, with the change to wiring above being a '68 thing. I chose to wire it above as it makes any circuit troubleshooting easier. Cheers
 
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Thanks ChasFred.
Sysco, I measured the (center to center) hole spacing WP_20190310_00_23_45_Smart.jpg & it is 3/8" (0.375"). This is the same spacing as the perforated boards used in the old Marshalls, when they had them.
So tonight I wired the input jack sockets & all control grounds. The grounding is as per "Larry's Grounding" scheme, from the Metroamp forum, except for the power tube cathodes which I grounded normally. I also laced up the wiring that will run from the board to the main, screen & PI filter caps, as well as the screen & PI filter ground & AC bias feedWP_20190310_00_11_11_Smart.jpg
Getting these all cut to length & properly positioned before lacing it up is quite time consuming. Tomorrow I'll attach all flying leads to the board & then fit the board, ready for population. Cheers
 
ivan H said:
Thanks ChasFred.
Sysco, I measured the (center to center) hole spacing View attachment 33890 & it is 3/8" (0.375"). This is the same spacing as the perforated boards used in the old Marshalls, when they had them.
So tonight I wired the input jack sockets & all control grounds. The grounding is as per "Larry's Grounding" scheme, from the Metroamp forum, except for the power tube cathodes which I grounded normally. I also laced up the wiring that will run from the board to the main, screen & PI filter caps, as well as the screen & PI filter ground & AC bias feedView attachment 33891
Getting these all cut to length & properly positioned before lacing it up is quite time consuming. Tomorrow I'll attach all flying leads to the board & then fit the board, ready for population. Cheers
Click to expand...

Time consuming but very, very nice - and it lets you know the person making it cared about what they were/are making. When ever I open up gear with wiring wrapped up like that I smile - to me it just says made with luv (not to mention it just plain looks cool)...
 
ivan H said:
The grounding is as per "Larry's Grounding" scheme, from the Metroamp forum, except for the power tube cathodes which I grounded normally.
Click to expand...
Where are you grounding the PT's heater center tap? On my two Marshally amps, they are grounded at the input jack and V1b cathode ground. The boyz at Metroamp tried to explain why this is better, but the explanation was over my head, so I just do it anyways.

Do you like to use any of the PT's mounting bolts as a grounding point? I don't cuz Merlin Blencowe says not to! Once again, I really don't understand why... :unsure:
 
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