NOS-style JTM50 Black Flag build

[emmjaydubya]

Although it sounds trite it sounds fantastic from the off, same as the original, surprising amounts of gain for shared cathode and extremely responsive to playing dynamics and the guitar pickup selection and the volume/tone controls, basically wind the amp up and walk away from it. These are the true 'plexis' that the old grey beard pros are referring to, much more so than a JTM45 and less of a one trick pony than a split cathode superlead.

This was the quote from Neil's JTM 50 block end thread that made me want to build one of these. After finishing my 45/100 (favorite amp!), I knew it was time, since I sold my first build which was a Metro JTM45 kit. I needed something to fill the void but more NOS like my 45/100. It will be a later style JTM 50, like in the Ser.# 10400 range,with a steel chassis, but tube rectified and shared cathode of course, with slightly higher filtering than the JTM45. The only thing I'm not settled on is 270k or 470k mixers?

Been collecting parts for quite some time, Iskras, Mustards, RS silver micas, RS 16/16uf pre filter cap (it formed perfectly, thanks Neil!), M****n iron, old-style black paxoline selectors, Sparky Black Flag front panel, NOS white ceramic 9-pin sockets, Belling Lee fuseholders and FINALLY got ahold of some correct looking red/brown paxoline blank board material which I will fully perf and stake. Found a new LCR 32/32 but couldn't find any NOS 16/32 or 16/16 of any kind so I'm using a F&T on the inside.

Got the package from Valvestorm last week (thanks Robert!) and got started drilling the chassis and fitting the back panel on Sunday. Found the back panel on ebay, it has the voltage selector hole which I wanted and it turned out to be a pretty nice piece:





Unfortunately the speaker jack holes didn't line up, the Metro chassis is spaced out further and spread apart compared to the panel, but the tighter jack spacing of the panel looks more correct to me from the pics on Amp Archives that I looked at, so it's a pretty darned accurate panel I guess. There will just be extra holes in the chassis on the inside that I will have to live with. I cut the power hole for a Bulgin plug but put in one of the handy IEC jacks that fit in said hole:



Next fitted the selectors and fuse holders along with the IEC. Looking like a Marshall already!





More updates coming soon...

[shakti]

Very cool! Love these NOS style builds.

I really prefer 470k mixers with EL34s, at least in my own 50-watter similar to this.

[emmjaydubya]

Ahh so you've tried both? I still have yet to try the 270k there, I used 470k on the 45/100 which was the only off-spec thing I did on it. What is it you like about the difference on the EL-34s?

[neikeel]

I have tried both in 50 and 100w.

My 67 block end JTM50 has mainly Iskras and has 270k mixers (bright channel bypasses with a RS 556pF cap, it has a RS 100pF tubular ceramic bright cap and is running a 784-128OT). It has as much gain and grind as my '68 Lead JMP50.
Funnily enough when I have built JTM50 clones I have found that a 470k mixer with 500pF or 560pF cap seems to work best (this one I posted not long ago seen here before I changed them.
This did not sound the same, it was a little cleaner and less grind at volume pot settings between 3 and 7 so you got nice cleans and it sounded good at higher volumes. I corrected this by going to 470k mixers. I suspect the original is a slight (good) aberration. Previous all Iskra JTM50s with 784-139 type OTs (presume your ****** is one of those?) I had used 470k with good results.
Try both and let us know what you think.
The 67 is my main gigging amp due to its versatility.

[Tazin]

Not sure if any of this info will help ya with your build......
The 50w Metro chassis differs slightly from the '67 - '68 50w Marshall chassis.
The JTM50 amps in the serial number 10400 range seem to be either solid state rectified and tube rectified. Both versions appear to run the same filtering scheme; 32uF mains, 32uF + 16uF for the screens and PI, and 32uF + 32uF for the pre-amp. The mixer resistors on these amps are either 270K or 470K, but the majority seem to run with the 470K.

EDIT: mains filter is 32uF not 16uF....Sorry about that.

[neikeel]

Not sure if any of this info will help ya with your build......
The 50w Metro chassis differs slightly from the '67 - '68 50w Marshall chassis.
The JTM50 amps in the serial number 10400 range seem to be either solid state rectified and tube rectified. Both versions appear to run the same filtering scheme; 16uF mains, 32uF + 16uF for the screens and PI, and 32uF + 32uF for the pre-amp. The mixer resistors on these amps are either 270K or 470K, but the majority seem to run with the 470K.

My weird one came with 32uF mains, 32uF+16uF for screens and PI and 32uF single for preamp, half way house between a JTM45. I wired it 32+16 (=48) for mains, 16 screens, 32 pi and 32 pre amp, for me is perfect.
The other steel chassis builds I did the PI and screens the other way round and 32/32 preamp, still pretty good, IMO

[Tazin]

Not sure if any of this info will help ya with your build......
The 50w Metro chassis differs slightly from the '67 - '68 50w Marshall chassis.
The JTM50 amps in the serial number 10400 range seem to be either solid state rectified and tube rectified. Both versions appear to run the same filtering scheme; 16uF mains, 32uF + 16uF for the screens and PI, and 32uF + 32uF for the pre-amp. The mixer resistors on these amps are either 270K or 470K, but the majority seem to run with the 470K.

My weird one came with 32uF mains, 32uF+16uF for screens and PI and 32uF single for preamp, half way house between a JTM45. I wired it 32+16 (=48) for mains, 16 screens, 32 pi and 32 pre amp, for me is perfect.
The other steel chassis builds I did the PI and screens the other way round and 32/32 preamp, still pretty good, IMO

Neil you always seem to find the interesting "odd ball" amps produced by Marshall.

[emmjaydubya]

Wow, all three of you responded, that's great! I might just go with the 470k in the mixers since it does have a 139. That's interesting Neil, how the older one has more grind with the 270ks, I wouldn't have thought that. Your '67 sure is a beautiful amp, I have all the pics archived.

Not sure if any of this info will help ya with your build......
The 50w Metro chassis differs slightly from the '67 - '68 50w Marshall chassis.
The JTM50 amps in the serial number 10400 range seem to be either solid state rectified and tube rectified. Both versions appear to run the same filtering scheme; 32uF mains, 32uF + 16uF for the screens and PI, and 32uF + 32uF for the pre-amp. The mixer resistors on these amps are either 270K or 470K, but the majority seem to run with the 470K.

EDIT: mains filter is 32uF not 16uF....Sorry about that.

Yes it does help, that was going to be my other query, the filtering. Seems odd that the screens would be 32uf and the PI would be 16uf, I would think the opposite. What would the tonal difference be?

The filtering scheme you mention Neil is what I'm leaning towards, 48uf mains, 16uf screens and 32uf PI. The thing is, I have to use one section of the 32/32 and one section of the 16/16 to get 48uf mains. Since I'm using Larry grounding, I would normally separate the ground points on the two caps but if I share sections wouldn't that cause a potential ground loop? Does that mean they both have the same ground point? Is it okay to share sections of different caps?

[shakti]

You can get ARS 16+32 caps, so you can use one cap for mains, and the other one for screens/PI.

[neikeel]

You can get ARS 16+32 caps, so you can use one cap for mains, and the other one for screens/PI.

I tink F&T do too.

Tazin - I was very fortunate to find both block end amp (the 50 was £1k needing work and the 100 much more but also need some sympathy after a hifi tech had worked on it but glad to have both, I tend to pass on the less interesting stuff or move it on if I have duplication

[Roe]

yes, the early 100 watters had 16uf on the screens and 32uf on the PI, but the jtm45s and jtm50s typically had 32uf on the screens and 16uf on the PI. There are exceptions though. I've seen a 66 jtm45 with a dual 20uf cap on screens and PI and 32uf (RS) on the preamp. and I've seen a pic of a 67 50w with a 40uf mains cap

[emmjaydubya]

Been really busy but finally got the chassis wiring done, except for the internal filter cap. Working on the board in the next few days...

[shakti]

Very nice work! I never had the patience to attempt the lacing, but I suppose it's possible to do after the amp is finished if I ever muster up the courage to try. You seem to have it down to an art already!

[emmjaydubya]

Thanks! Lacing the twisted wire is not so easy, that's why it doesn't look as good as the straight wire runs. Also, the power wires around the outside edge were done outside the amp first. I left extra wire on each end, laced the straight wire and bent the ends to fit the width of the chassis. Much easier than doing it inside the amp and you can control how it looks and how straight it is. Of course I had to lace the other runs inside, pretty frustrating but worth it.

I've read that twisting the AC wires reduces hum but I wanted this to look as 'original' as possible so we'll see if lacing them straight adds noise or not...

[emmjaydubya]

Been working on this one, finally almost have it finished. All that's left is the bias resistor, but I'm not sure what to put there. I was thinking a 56k or 47k would get me in range, opinions? The paxoline board is awesome and I feel lucky I got ahold of a few of these blanks. 50uf Radiospares on the V1 cathode to shave off bass early in the circuit, RS micas, Iskras and mustards always look cool on Paxoline! The only thing odd on the board is the giant 2 watt Piher 8.2k, kind of out of place. It definitely won't fail though, ha!

Can't test voltages yet as I'm still waiting on a Blackburn Mullard rectifier tube and and some 12AX7s, got some NOS Winged C EL34s though. Here's where we are: