JTM 45/100 - Build story

[Xplorer]

Very cool !!
i'll quickly finish a work for my client and then do these steps. I checked the continuity for the heaters, and the continuity for the wires between the preamp board and the preamp tubes, it's all ok but i'll verify better.

The V1 cap 220uf isn't a new one, it's an older phillips mullard cap, measured 250uf last time. so i unsolder one end and keep the 820 ohms in the circuit, ok.

the pots, last time i measured, with the pots installed already, i think it was ok. i'll re verify.

grid wire is the green wire coming from the 2 x 270k and 560pf ?

- if still no better do this: open up your SL and have them side by side remove the grid wire from V2 on each and tape them up temporarily.
- take a wire and solder it across from the green wire that comes from your superlead 470k/470k/470pF and extend it to the correct lug on V2a.

so i'll have both green wires from the 2 x 270k unsoldered from the V2's of each amps , and ... going nowhere, just with tape on it for isolation ? ( is that it ? )
then a wire from the superlead's preamp board's green wire to pin 2 on V2 of the JTM ?
+ aligator clips on a cable between both chassis.
Then play from the SL's input to hear what's hbappening into the jtm's speaker, both amp being switched on, standbye off ( down )

is this process ok to verify some other preamp pins ? ( except for the heaters i guess )

Many thanks ! i'll report later.

[Xplorer]

check the heater on all pins (not a dry joint under on of the bridged red preamp tubes.

- is your V1 cap NOS or new? If NOS just lift one leg of it temporarily and retry.

I did it but it didn't change. Actualy, what's the use of this cap ? it sounded great even without it ! and i'm not sure ( cause i didn't experiment this amp enough ) but i have the feeling that it's maybe a tad better without it.

- check you have the right pots wired up in the right order ie not slipped a 1 meg in the wrong place

checked, they're all ok.

- if still no better do this: open up your SL and have them side by side remove the grid wire from V2 on each and tape them up temporarily.
- take a wire and solder it across from the green wire that comes from your superlead 470k/470k/470pF and extend it to the correct lug on V2a.
- put both on a load with all tubes in with a temporatry earth between the two chassis as well
put speaker on the JTM45/100 and load box on the SL. Switch both on
- plug guitar into the SL input and see what you get out of the 45/100 speakers.

If you get full signal and fully responsive tonee controls etc then the problem is v1/v2A.

No, no break up, no saturation this way. just a dynamic clean like before i'd say.

If it is no good do the reverse, feed the SL V2a from the 45/100 inputs into the SL after swapping load box.

This is to help when you have not got a 'scope and signal generator.

Post back when you have done these things (remember to bleed caps both times on both amps ).

Done ! There's some break up/saturation, but a bit wierd, not very pleasing like the SL as usual.

So the conclusion would be that V1a/V2a isn't the problem ?

Thanks again Neil.

[neikeel]

Yes the problem is tone stack and beyond.

I wonder if excessive NFB could be a problem.

Suggest you replace the grid wires and whatever V1a cap you want.

Then lift one leg of the NFB resistor see how it plays then.

IIRC didn't someone (Shatki) have issues with his Black Flag clone which had incorrect OT secondary colours?

[Xplorer]

Ah.
which are the grid wires again ? i should cut them and replace them ??
V1a cap is the 250uf, right ? at the moment one end is disconnected.
the nfb resistor is the ... 27k ? i disconnect one end ?

[neikeel]

Ah.
which are the grid wires again ? i should cut them and replace them ??
V1a cap is the 250uf, right ? at the moment one end is disconnected.
the nfb resistor is the ... 27k ? i disconnect one end ?

Grid wires are the ones you removed - put them bak how they were

Yes V1 cap is the 250

nfb is the 27k, just desolder one leg

off to bed now

[Xplorer]

MYSTERY !


Brian Wallace made me do an interesting trip into my amp, with a probe. wow !

i had the superlead aside.
one jack cable plugged into a strat, the other side going into the top left input of the jtm.
on the SL, a jack plugged into the top left input, the other side has wires : the ground goes to the chassis of the SL ( or jtm , since both go to the ground ) , and the tip is attached ( all this through aligator clips ) to a 0,022uf 400v mustard. the other lead of the cap being the probe, used into some points of the jtm.

both amps powered on, volume between 4 and 10 on the jtm, and about 2-3 on the SL.

The SL's output goes into an attenuator set low, and then to a speaker.
The JTM goes into a dummy load ( an spl transducer )

with the probe, i tested the outputs of the jtm's preamp tubes : the pins with the blue wires.
It produces sound ( the guitar with strings played ) on V1 pin 1 and 6. On V2 it sounds different and quieter with pin6.
On V3, there it is ! the big hendrixy distortion !!

then we followed the distortion, using the probe after the 0,1uf caps, where the 220k resistors start before joining together in one point.
the distortion is still there.
And then we went to the hot side , tip, of the output jacks : still there !!!! Just like with a 4x12 cab, only at a lower loudness.

But when used with an attenuator and a speaker, or with a transducer + cubase, etc ....
It does a BIG clean !!!!!! the distortion has disappeared !!! so strange ..... Especially since with the SL, and the attenuator or the transducer, the saturation really is there !!!!
But no ... V3 gets lost for a mystery reason. i have some kind of beautiful "BLUUUUE HOOTEEEEL" .........
ha ha ha ha ! No idea ...

And thanks Neil, i'll put a hold on this 27k nfb resistor for now, since these results are quite strange.

Good night.
Adrien

PS : if we find the trick, i'll call the "Adrien's Clean mod" !!!!!

[Roe]

have you tried the different OT secondaries? have you measured the OT primary impedance?

[Xplorer]

Hi Roe, no i tried only the 16 ohms output.

How should i properly measure the OT please ?
for the primary should i measure together the black and red wires of the primary going to pin 3's of V5 and V6 ?

But it's strange cause after all these stages, and OT settings, by using a probe on the ground and the tip of the output jacks, heard into the SL, it sounds correct .... ( ?? )

[VintageCharlie]

I have no proper idea of electronics, so this is just speculation, but afaik the OT interacts with the load attached to it. The load of an attenuator is surely different than from real speakers. So it wouldn´t be a impossible that the problem really is in the OT wiring.

[VintageCharlie]

btw. did you try to switch out different output tubes?

[Xplorer]

Yes, during the test the jtm has a 16 ohms load, and the 16 ohms tap is used for the OT. So it leaves a safer "low watts signal" to test into the input of the Sl during the probe test, from what i understand : a mirror of what it's supposed to sound.

No, i didn't try to switch the power tubes or the impedance settings.
before i'd like to be sure of how to test the secondary impedance of the OT, and primary ( but why ? )
Brian Wallace checked the wiring, from the pictures, and it seemed fine to him so ... mystery !
would an impedance mismatch with the load create such clean tone, with V3's output missing totaly ?

[shakti]

I would definitely check the primary impedance - there are instructions on the Marstran site on how to. If the colour codes are mixed up, it wouldn't be the first time, as I painfully discovered with my Black Flag build.

If you have, for instance, the 4 and 16 ohm leads mixed up, you would have a primary impedance 4 x as high, on an already high primary Z OT! It would be something like 16k, way high! This would greatly reduce output, but I'm not sure if you would have lacking preamp gain...negative feedback would also be affected, but it would reduce NFB, so that kind of works against my theory as that would make the amp looser and hairier...

[Xplorer]

I'd like to do it ' Especially since it's wierd .... the transformers don't do noise like my SL, they're dead silent !!! or i have to bring my ear very close to it to hear it working ) .
But i don't have a 1 kHZ signal generator ...

http://www.marstran.com/Measuring%20Pri ... edance.htm" onclick="window.open(this.href);return false;

i'll ask to Brian how to do it another way, especially as it's a operation that is a bit complex. i could mix up something during the process...

[neikeel]

Have you tried lifting the 27k NFB resistor yet?

BTW I can help you set up a 1kHZ sig gen if you need to with an on line package.

[Xplorer]

i didn't try the 27k nfb resistor yet ... doesn't it cut the circuit ?

i'd like to test the OT in a more simple way if possible before.
i asked the question to brian, we'll see ..