Asking For Opinions From Little Evidence: JTM Super PA

[TrueFifth]

I may be offered a JTM 100 Super PA with the black flag logo but so far have only seen very little pics, I of course asked for interior pics, and so far I got only from the back.

I know how knowledgeable you guys are here, and if I'm to drop money on this one, I would wish to make decisions from as good background info I can get.

I see some spots, especially on the lay down transformer, and perhaps other places as well, I hope this is not caused something wet.

Compared to other amp pics, the choke looks right, but the writing on it, is that a date?

Can the pics show that the OT is original? Should one be able to see cloth wires coming out of it?

I will post more if I get more info / pics, but hope I get some comments for now, it will be greatly appreciated!

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

 

[ampmadscientist]

I may be offered a JTM 100 Super PA with the black flag logo but so far have only seen very little pics, I of course asked for interior pics, and so far I got only from the back.

I know how knowledgeable you guys are here, and if I'm to drop money on this one, I would wish to make decisions from as good background info I can get.

I see some spots, especially on the lay down transformer, and perhaps other places as well, I hope this is not caused something wet.

Compared to other amp pics, the choke looks right, but the writing on it, is that a date?

Can the pics show that the OT is original? Should one be able to see cloth wires coming out of it?

I will post more if I get more info / pics, but hope I get some comments for now, it will be greatly appreciated!

2

• TrueFifth
• Apr 12, 2019

1

• TrueFifth
• Apr 12, 2019

8

• TrueFifth
• Apr 12, 2019

5

• TrueFifth
• Apr 12, 2019

4

• TrueFifth
• Apr 12, 2019

9

• TrueFifth
• Apr 12, 2019

6

• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

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• TrueFifth
• Apr 12, 2019

It doesn't really show enough, but from what I can see (very little) it so far looks original.
It also looks like somebody spilled a drink into the chassis...

 

[TrueFifth]

It doesn't really show enough, but from what I can see (very little) it so far looks original.
It also looks like somebody spilled a drink into the chassis...

Thanks, and yes, those spots, I’m very affraid that should turn out to be because of some liquids too!

The seller claims no technical knowledge, the amp being far from me, so gotta try make further decisions on very little info...

 

[CoyotesGator]

Have the seller pull the chassis from the cabinet and send you clear/well lit photographs.

 

[neikeel]

The transformers are the later Dagnall type that you see in 100w amps from the end of '67 into '69 (so yes I would expect cloth covered secondaries on the OT). The yellow covered filter caps and with the preamp can up top would suggest late 68 or early 69 (there is a member here who goes by the name Tazin, he has these details documented properly) the rust is a little concern if the boards etc got wet so more pics would be useful.
Normally a Black Flag amp would be purely mid '67 but the PAs mixed up parts being used up so these timelines are blurred, but almost certainly at least late '68.

 

[ampmadscientist]

I may be offered a JTM 100 Super PA with the black flag logo but so far have only seen very little pics, I of course asked for interior pics, and so far I got only from the back.

I know how knowledgeable you guys are here, and if I'm to drop money on this one, I would wish to make decisions from as good background info I can get.

I see some spots, especially on the lay down transformer, and perhaps other places as well, I hope this is not caused something wet.

Compared to other amp pics, the choke looks right, but the writing on it, is that a date?

Can the pics show that the OT is original? Should one be able to see cloth wires coming out of it?

I will post more if I get more info / pics, but hope I get some comments for now, it will be greatly appreciated!

2

• TrueFifth
• Apr 12, 2019

1

• TrueFifth
• Apr 12, 2019

8

• TrueFifth
• Apr 12, 2019

5

• TrueFifth
• Apr 12, 2019

4

• TrueFifth
• Apr 12, 2019

9

• TrueFifth
• Apr 12, 2019

6

• TrueFifth
• Apr 12, 2019

3

• TrueFifth
• Apr 12, 2019

7

• TrueFifth
• Apr 12, 2019

One thing you can do to a late '68 is make sure the output sockets screen resistors are installed.
On some of these amps there is no screen resistors, this will make the new tube draw too much current.
Mysteriously, the fuse will blow when the volume is cranked up (now you know why).

 

[TrueFifth]

One thing you can do to a late '68 is make sure the output sockets screen resistors are installed.
On some of these amps there is no screen resistors, this will make the new tube draw too much current.
Mysteriously, the fuse will blow when the volume is cranked up (now you know why).

Thanks, I already have a mid '69 Super PA, and that one does exactly that! So will definitely check that one, thanks!

 

[ampmadscientist]

Thanks, I already have a mid '69 Super PA, and that one does exactly that! So will definitely check that one, thanks!

The fuse blows when the volume is cranked up?
You know where the screen resistors go?
Did you measure the speaker w/ a meter to make sure it matches the amp setting?

Either one of those, the speaker or the resistors will make the fuse blow.

 

[TrueFifth]

The speaker should be good and checked, I have had the fuse blow once when I cranked it, wondered why since, but since it gets inconceivable loud, I haven’t needed to do that again... But that means, when I get a proper attenuator for it, it could happen again...

Locating screen resistors would take a little research, I’m also not super tech minded, but I try to pick up what I can!

 

[neikeel]

The 100w Marshalls all came with 1k screen resistors (except first 45/100 with KT66 that had 470R).
It is the 50w that ran Mullard EL34s that omitted them as standard until about 71, then all 50w had them.

 

[TAZIN]

Marshall 100w P.A. amps are the oddballs when it come to changes/transitions. This amp looks like something built around April 1969 based on what I see. I've seen other Super P.A. amps from 1969 with "black flag" logo front panels and the rest of the amps features being in-line with 1969.

 

[TrueFifth]

Marshall 100w P.A. amps are the oddballs when it come to changes/transitions. This amp looks like something built around April 1969 based on what I see. I've seen other Super P.A. amps from 1969 with "black flag" logo front panels and the rest of the amps features being in-line with 1969.

The serial number stamped on the back plate is also a little peculiar to me, it is written like this "SERIAL NO OP A/020xx"

The owner has delivered the amp to a tech, probably for service and/or appreciation, I have asked for more pics of the interior, but so far have not received any.

 

[RevJamie]

Can't offer any opinion on the amp other than I have a 69 and a 70 Super PA, and I love those damn things!

 

[Jethro Rocker]

Is that a date of 1978 on the choke? Just wondered. Plus there is a similar number on one of the yellow caps. Could be nothing...
The one on choke is 17-8-78 and the can is 29-8-78. Seems kinda weird if it isn't a date of some sort.

 

[TrueFifth]

I have been thinking the same ;-) Still no news from the owner.

 

[neikeel]

Both are service stamps Gert H

 

[Purplexi]

Barter it down as low as you can. She doesn't look too bad and you'll have fun getting her in proper shape. Some of the best tones I ever had was on my modded/de-modded 67 PA head. Wish I'd never let it go.

 

[Scott Johnston]

Not sure how much abuse this unit too but *I* would do two things. First pull out the chassis and look at the components. Make sure this unit wasn't tampered with and second, make it a conditional offer. If the unit does not work at full power with NEW TUBES then pass.

However if you are buying it as a project, it would be worth it. The question is; do you know enough about this basic amplifier tech to fix anything that's wrong?

 

[TrueFifth]

It seems the seller found another buyer, I was too far away to be able to move on it in time I presume, and I couldn't get myself to buy it without inspection and trial. I was promised info from an ordered service, but someone probably got hands on it before that info was to be provided to me... A pity, would have like to get a black flag Super!

But I do have a June 1969 Super PA, and from what I can see on photos and the info gathered in this thread, the transformers are identical.

The Super PA I have is an absolute favorite, with the sweetest top end and big low end, and an amazing compression going on...

But a feeling is nagging me that this amazing clean compression might indicate a near emergency state for capacitors?

It would just be such a pity to change those and lose that incredible compression, it makes clean chords and solos incredible beautiful. But if it is a bad sign, it could perhaps lead to even bigger losses not to change them I guess...
There is also quite a lot of hiss/noise coming on when turning the treble and/or presence up full, increasingly from the last half to max. I have been told noise here could be normal on plexi amps with lay down transformers (directions of magnetic field?), but I think I have read that old capacitors also can give noise/hiss...?

It has led me to wonder if it could be possible to change capacitors to new in smaller size than usually installed, and in that way retain the same kind of wonderful compression?

Or is it a far out thought? (No proper tech knowledge on my part)

 

[neikeel]

Mains hum may represent failing filter capacitors
Hiss does not, that is typically noisy resistors early in preamp (pots are resistors too) 1M ground resistors, 68k grid stoppers.
Earth (ground loops) also cause hum, poor heater wire routing and transformer field coupling cause hum, but not hiss.
Hiss is normal as you turn the volume up on these I am afraid
Stop the paranoia and play it so you cannot hear the hiss!