1967 JTM 50 "Black Flag" OT recommendation

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1967 JTM 50 "Black Flag" OT recommendation

Postby gigant0r » Tue Dec 15, 2009 8:00 pm

Hello, my 1st post :D

I see some familiar people around here so I'm sure this is a very nice board!

Anyway, I come here for some advice..

This is my friends 1967 JTM 50 "Black Flag" it's his main gigging amp.
It is working fine but he wanted to know some details about it.
Some very knowledgeable people already told me that this amp have replaced both transformers, V1 cathode cap, mains, screens and PI capacitors, on/off and standby switches and
a few other minor things to keep this amp up & running for over 40 years.. so regular maintenance.

Anyway, my friend is playing in blues-rock band and he would like to have as much headroom as possible with later breakup into some nice overdrive. Basically, he wanted it to sounds pretty much as original. Warm cleans and nice focused overdrive when cranked.
OT is rewound and he would like to replace it with best possible solution in todays market.
I've heard nothing but the good things about Marstran transformers and I'll recommend him to go that route, but I would like to know which OT ( 128 or 139) will suit better in
this amp? I know that 128 supposed to be in this amp but as you can see, few things are changed in it ( no tube rectifier) so I'm not really sure.

Thanks :D

Here are some pictures:



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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby dai h. » Wed Dec 16, 2009 5:44 am

hi gigant0r, welcome! (Sorry!) I can't tell you which one is the correct OT for this period 50W but I'm sure Brian(or someone that knows will chime in--my guess is the 128 since the amp looks relatively older). Just to add to what neil/LD50 said on the PP board, you might want to consider reverting the two blue filter caps to more traditional values (unless you like it that way, of course). It appears they are both 100-100uF instead of the usual 16, 32 to 50uF that usually appear there. Big values may be good for bass or something, but I think most guitar players would prefer values on the lesser side. Might also want to change the 32-32uF in the preamp since (by now) it's quite old.
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby gigant0r » Wed Dec 16, 2009 7:07 am

Hey Dai, thanks for welcome :D

I've noticed a bit too much bass for my taste, it's a bit boomy on '68 4x12 greenback box. I'll recommend him to try different bright cap values and to change those caps.
I also hope Brian will chime in and help me choose the right OT :D
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby David B » Wed Dec 16, 2009 10:50 am

Hi gigant0r,

For an idea what OT your 10562 could have originally had check out the database. fwiw 10415-10652 had 139’s then 10771 had a 128!

http://marstran.com/Historic%20Data%20Base.htm


Possible the amp had a 139 then someone used the covers that are now on the OT but who knows for sure
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby gigant0r » Wed Dec 16, 2009 11:04 am

Hey David,

that amp came with Vox AC50's ( i think) OT so my friend borrowed rewounded, mid 70's JMP OT to try it out.
He decided to buy the best possible OT for it and now I'm here ;)

Since the amp is probably converted from tube rectifier to SS I'm thinking that 139 Marstran type is probably better solution then 128.
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby Brian Wallace » Wed Dec 16, 2009 6:39 pm

Hi gigant0r,

As you stated in the first post, the amp had a 128 in it. This is evident by the ohms selector.

What OT to get is a hard one. I would like to see it have the 128 back in it but if you like the 139, go with that option.

What kind of plate voltage is this amp running at?
Remember....You can fix it with a little extra energy and a positive attitude. ;)
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby David B » Wed Dec 16, 2009 6:47 pm

Oh yeah good eye, 100V on the selector.
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby gigant0r » Wed Dec 16, 2009 7:46 pm

Brian Wallace wrote:Hi gigant0r,

As you stated in the first post, the amp had a 128 in it. This is evident by the ohms selector.

What OT to get is a hard one. I would like to see it have the 128 back in it but if you like the 139, go with that option.

What kind of plate voltage is this amp running at?



Hi Brian,

I'll recommend him 128 then, thanks!

I don't know what kind of plate voltages are running in that amp, I'll have to check that with my local amp tech.

BTW, I have 1971 JMP 50 with 128 tranformer and ohms selector with 100V. That OT looks original to that amp, solder joints are untouched. I find it pretty strange, but it sounds great :D
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby Brian Wallace » Wed Dec 16, 2009 7:55 pm

Post pictures of the amp please. :mrgreen:
Remember....You can fix it with a little extra energy and a positive attitude. ;)
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby gigant0r » Wed Dec 16, 2009 8:18 pm

Here are the pictures I took when I got the amp.. some things are changed since then ( bright cap ( now it's silver mica 100pF),tubes ('65 RFT Telefunken EL34's and 60's Tungsram ECC83's are now in it), pilot lamp is back to stock, original bat switch is back in place.. I like things stock :D I'll take the picture of that ohms selector with 100V on it later.


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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby David B » Wed Dec 16, 2009 10:07 pm

Great pictures, nice half stack ya got :D
what's on the pedal board?
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby dai h. » Thu Dec 17, 2009 5:30 am

wow, nice condition! :shock: (pics definitely saved)
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby gigant0r » Thu Dec 17, 2009 5:59 am

Yeah, I was amazed by it's condition too. I bought it from 1st owner who buyed that half stack in 1971. Sounds as good as it looks ;)

David, my pedalboard is constantly changing because I love pedals but few of them are always on my board; 60's Vox wah ( or RMC),Fulltone OCD, AnalogMan NKT275 Sunface, Retroman Beano, custom made Uni Vibe, AnalogMan CompRossor, Retrosonic analog delay and Korg tuner :D
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby David B » Sat Dec 19, 2009 1:09 pm

Great rig all the way around :D

custom made Uni Vibe ?
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Re: 1967 JTM 50 "Black Flag" OT recommendation

Postby gigant0r » Sat Dec 19, 2009 3:38 pm

thanks :D

That vibe is actually rehoused/true bypassed Danelectro Chicken Salad. I've designed the layout (based on original Shin Ei obviously), a friend of mine did all the rest. I'm using it on every gig for over two years and I'm still very happy with it.
Here's a picture with some of the battle scars:

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