JTM 45/100 - Build story

[Xplorer]

How to test if an OT got broken ? or a preamp problem ? ( but i think i remember that the transformers did more noise than now, when it was working )
The power transformer seems to work since there's 470 v on the plates.

[Xplorer]

checking the OT of the SL, all tubes removed. Nothing unsoldered :

primary
white ( CT i think ) - red : 17,16 ohms
white - brown : 15,62 ohms

secondarys :
brown ( Common i think ) - black : 0,21 ohms
brown - yellow : 0,27 ohms
brown - green : 0,35 ohms

on the JTM's OT inside, and almost the same on the spare OT :

primary :
89,7 ohms accross the red and black,
42 ohms accross the black and brown,
48 ohms accross the red and brown

secondary :
orange-green : 1 ohm
orange-blue : 0,55 ohm
orange-yellow : 0,72 ohm

[Xplorer]

Do you guys know how to use a scope, some simple principles from the method, so i could save my amps please ?

i imagine that it starts on a specific point tested, and if it's ok, going further ...
but i have no idea of how it's supposed to be used.

[VintageCharlie]

if the 47pF is a nos mica, then i know that Dom had once a problem due to that bugger - i don't recall the exact symptoms it gave, but had a faulty nos lemco (that was also hard to diagnose).

[Xplorer]

Thank you Karlis.
Yes i've heard about it. I might test some modern components temporarly into the jtm, before replacing it with some vintage parts, once i know it works. ( don't know if i may come accross some other faulty nos parts ... )

But my SL is almost dead now, i also have to debug this one !
i'd need to get lessons about debugging an amp with an oscilloscope, if it's the right way to analyse it.

Just came back from the new James Bond movie
Oh Yeah !.... Jamess' mother was French ...
That's why ......


i'm looking at these sites, but wow .... this is the beginning of the nightmare. And since bad stuffs never come alone, my computer does some serious bugs ... This is a conspiracy !!!

http://www.geofex.com/ampdbug/lowpower.htm" onclick="window.open(this.href);return false;

http://courses.physics.illinois.edu/phy ... esting.pdf" onclick="window.open(this.href);return false;

http://diyparadise.com/web/learn-learn- ... ut-a-scope" onclick="window.open(this.href);return false;

http://www.geofex.com/ampdbug/prepared.htm" onclick="window.open(this.href);return false;

[Xplorer]

i did some tests with the virtual scope, and i could get a sine wave read from the signal generator, it works ! but measured after 68k it cuts the signal.

well, it'll be too complicate this way.

all i know that is sure is that i have a fluke 189, it seems that it can read some ac hz or something, and i know i can have a signal generator, for whatever frequency, from an jack going into the input of the amp.

any ideas on how to debug both amps ?

[neikeel]

Remember to put a 0.1uF cap between your meter and the part you are probing (a spare sozo or short lead mustard will do).

[Xplorer]

thank you. Well, i don't even know what to probe and how to probe the amp ^^ .
all the good tools i have are : a fluke, and a signal generator. That's about all. my head is empty. i'm trying to find clues with "debugging a tube amp" on the web but even at geofex, i can't find clues.
i don't know where to start. Or perhaps ... testing the SL OT another way, to see if it's dead or not.
for the JTM, i thought simply about replacing some preamp components, one by one, starting with the 47pf and the 16uf.
That's all.
i'd like to repair the SL first, cause it makes me sad that it doesn't work anymore. so at least i could continue to debug the jtm from where i was, and learn tricks on how to debug the JTM, thanks to the SL.

[Xplorer]

found this :

http://www.geofex.com/tubeampfaq/taffram.htm" onclick="window.open(this.href);return false;

read it once, about "output transformer, live or dead" . i should read it again cause i can't follow some parts of what he's saying.

[Xplorer]

wow ! these are the results of my SL once measured. not great !

btw, these are some more parts i may drop in the jtm once it'll work properly.

[Xplorer]

Ooh ... That's what's happening when i do my job aside then come back to it and forgetting half of the things i've got to do.
Forgit the power tubes and cab in the output, to measure the dc voltages . Lol

[neikeel]

You have lost your B+ supply.

bet you have blown an HT fuse Measure it with a meter - not just eyeball

After that:

Measure the ac off the PT HT secondaries, then the dc at the apex of the rectifier, follow it to the choke in and choke out etc etc

[Xplorer]

Man ! i have no idea of what happend ! now it's alive again !!!
This morning i loaded the power tubes back in, measured the voltages : they're all fine. Then i played it and .... surprise ! just like before ! Ha ha !
That's less pain in the ass suddenly.

so now, back to the JTM !
Where was i ?

Is there something possible to do with the signal generator + the fluke 189 telling me AC Hz or something ? Before starting to unsolder pieces and pieces ?

- Voltages were quite fine, ( although the bias is at -60vdc, quite cold, even if the tubes show 35mA accross the 1 ohms ) .
- Continuity seemed fine, from my tests
- preamp sounded more ok through the SL than through the JTM itself
- SL's preamp sounded ok through the JTM

[neikeel]

That is good and bad! Good for now, bad that you do not know why it did it if it does it again (am I not a source of cheer!)

Back to the JTM.

Set up your sig generator to have 1000Hz with 0.2 to 0.4v signal strength. Put the 0.1uF cap on the probe lead of your meter make sure it is insulated (the other to the chassis ground). You really need that cap in the power amp as it will block the DC coming form the bias supply.

Leave the output valves out for the moment (so no load required)


Plug the signal into the input of bright channel.

Next will be to go through the preamp as far as the output grids to ensure you are getting appropriate amplification (and response of your tone stack controls at each stage). See if you can get that set up.

I need to find my chart of what you should have at each stage (I had to do this with my 68SL a few years ago so need to track down my notes!).

More later!

[Xplorer]

That would be awsome !!!!
yes, i can set all this.

( about the SL, i suppose that i accidentaly had a tube a bit dosconnected or something, since the amps are both upside down to work on it. so on my table i perhaps had it wrong )

Yes, it seems that the important thing is to know what to expect in the different stages of the amp, when using a scope. i don't yet, but learning this would be great, and in some way, i think it's nice to have a whole story of an amp build, with pedagogic tricks and know how, so it would help others than me too.

i'd love to learn how to calculate what to expect in different points, and how to interpret the reults.

the fluke seems to go to 1000 hz max, so we could probably set the ignal generator at 500 hz max ?

ok, i understand that we'll measure the ac hz, and we need to block the dc voltage from the reading, with the 0,1uf cap.

Thanks