JTM 45/100 - Build story

[Xplorer]

and thanks for the help, maybe yes, do you mean shipping a signal generator ? or do you think i can build one quickly, so i can test it very soon ? i'm a bit impatient

[BoogieEngineer]

and thanks for the help, maybe yes, do you mean shipping a signal generator ? or do you think i can build one quickly, so i can test it very soon ? i'm a bit impatient

I think he means he's got a computer package that could get the job done. Sorry to hear the mystery of the amp. I would get very angry not being able to play that beautiful thing right away.

[Xplorer]

I'm testing the OT with Brian. Apparently there's something wrong, we'll get back on it. In any case, i have another marstran OT by chance, as Brian sent this extra to me by accident. This was maybe meant to be ?

[Xplorer]

Test : using the AC output of a 220v - 120v Variac, onto the primary wires red and black of the OT, on pins 3 of V5 and V6.
A jack cable is plugged into the output jack, to open the output circuit. And i measure accross the tip of the other side of the jack cable, and each tap of the impedance selector, with the plug set each time on the setting i'm testing. It gives AC volts. i measured also from the chassis to the taps the same way.

[Xplorer]

So actualy the OT is perfect, and we tested my second OT, just perfect too.

and we checked the continuity of the voltage from the output of the selector to the output jacks, to the tip of the jack plugged on the other side into the output jack : no problem.

again we tested the jtm through the SL : it sounds distorted as it should when on ten through the sl ( set on 1 ) , not through the jtm , whatever audio output we use : spl transducer, or an attenuator with a real speaker ( the three others beong simply replaced by resistors, in a classic 4x12 wiring , including the real speaker , as usual ) .

mystery ....

[neikeel]

[VintageCharlie]

+1

[Xplorer]

Ha ha. Waiting for the next episode ?



Yesterday, with Brian Wallace, we checked some more in the amp ... And i'll post some clips of V1, V2, V3...

V1 has some dirt, distortion, just like "ride on" ACDC intro. V3 at ten is Woodstock ² , and V2 .... seems to be the problem, but V1 could perhaps be more distorted.
Anyway, i'll have to do two more clips tomorrow, i didn't have much time lately, and i repaired a Kurzweil synth i just received.

V1 : checked through the SL again : using V1 of the JTM, V2 and V3 of the SL ... disconnecting both wires from both amps, which go to the whiper of the bright channel volume. Then, a cable with aligator clips from the whiper of the JTM, going to the wire of the SL, which goes to the preamp of the SL.

V2 : on the treble pot, whiper lug, same principle. : there it sounded as usual, just very clean, straight.

V3 : with a probe,

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 !!

so all is tested differently.L i had different results with the probe technic, for V1 and V2 , over the use of some single preamp tube from the JTM, completed with the tubes of the other amp. i'd like to try V3 this way, without the probe technic, but i don't know which point to test. ( V1 was the whiper of the bright channel, V2 was the whiper of the treble pot )

I hope that someone may have some genius and find some ideas

[neikeel]

What happened when you ran the total amp but without the NFB resistor?

[Xplorer]

i'll do it Now.

[Xplorer]

So, i unsoldered one lead of the 27k NFB and ..... it give a tiny more saturation when i play the strings stronger. million miles away from the saturation of V3.

Still got to do clip but my neighbours would agree on that . Ha ! it's 23h20 here. Tomorrow probably.

I have ni idea of which new tests we could do on the preamp stages.

[BoogieEngineer]

Build another one

[neikeel]

So lets be clear:

- if you use the SL preamp to drive the 45/100 power amp it works normally. Where are you inserting the signal?)

- if you use the 45/100 preamp it does not drive the SL normally

or are you using different combinations of each?

- easiest place would be the treble wiper (where a Marshall type premv feed to the pi comes).

This is really frustrating - and it is not even my amp!

[Xplorer]

Hi ! I'll go back to the tests and post the results/clips to illustrate it , probably this afternoon, so it'll be easier.
I don't remember in details already what i did with Brian on the amps.

Yes, we used the treble whiper for V2,
and the bright volume whiper for V1.

V3 was tested another way, with a probe, through the input jack of the SL. The probe touched the blue wires connected to the tube socket. ( and it gave different results compared to the other method : clean tones when i touched the blue wires on V1 and V2 )

Different combinations like :

V1/JTM - V2 & V3/SL
V1/SL - V2/JTM - V3/SL
And more : the whole JTM was also tested with a probe, through the input jack of the SL, thanks to the probe on the hot side of the JTM's output jacks ( the ground of both chassis being connected with aligator clips ) .

[Xplorer]

With Brian we tested the preamps by unsoldering the PI caps ( the two 0,1uf going to the 220k's ) of each amp. Then, connecting each cap to the 220k's of the other amp.
So, i had the SL preamp replacing the preamp of the JTM and vice versa, by connecting with aligator clips the two 0,1 u caps of the SL to the 220k of the JTM .
Volume of the SL at ten,
Then swapping everything as well as the output ( for the speaker, while the JTM was silent, into a dummy load : an spl transducer unused in its output ) to hear the preamp of the jtm through the SL.

Well, then the JTM sounded distorted on 10, not as much as the SL. But why can't it do the same when the preamp of the JTM is heard through the JTM itself ???

Other than adjusting the bias hotter by ears ( maybe at 80% ) , there's something that disturbs the amp, cause you remember how clean it sounded ...

Oh, and these clips are recorded with an iphone, and compressed, and with the attenuator set so low ( midnight here, with neighbours under my floor ) that my voice is as loud, when speaking normaly.
The clip of the jtm sounds probably better than in reality.

i have about ten other clips to do with a method and patience ...
hearing the JTM by itself, but without the nfb resistor didn't change the clean tone a lot, and was very far from this "new" saturation.