JTM 45/100 - Build story

[Xplorer]

Today we tested V3 of each amp.

the processus : each 0,022uf pi cap of the SL feed each 220k of the jtm, so we're using the preamp of the SL into the jtm.
at this level, it sounds like the clips i've posted yesterday : distorted ( almost ) like it should;

But then, to isolate V3 of the SL and use it into the jtm, i unsolder both amps treble pot whiper wires. then i run a wire accross the whiper of the SL's treble pot, and the wire left on the jtm, going to the preamp.

to hear V3 from the jtm into the sl, reversing the whole process.

both results were almost the same : quite a weak sound, clean.
You remember that from my previous clips we can say that the jtm preamp sounds clean once into the jtm itself, and distorted once into the SL ..

i suppose that V3 of the SL should sound distorted normaly ( i should try with the probe test there ) .
It seems, as a conclusion, that something into the jtm either bleeds the signal off or doesn't amplify the signal, after V3. And for either amp's V3 tested.

when we'll have some time, we'll try with a scope and a signal generatot ( never did it before ) BUT , with a mc gyver solution i've found : a soft scope !

http://www.zeitnitz.de/Christian/scope_en" onclick="window.open(this.href);return false;

all the watts will go into the spl transducer ( speaker simulator ) , which has its output going into my soundcard, and then into the virtual scope.
the signal generator will also go through the soundcard, but through its output, and then a jack plugged into the amp.

the input of the spl transducer will be connected with a speaker cable ( as usual ) to the amp this way : the ground to the chassis, and the tip, to the stages tested.

[neikeel]

Something struck me. Are the wires to the OT primary correct.

Not just a correct centre tap with reversed outer coils, I mean the centre tap hooked to a pin 3 and one of the outers as a centre tap etc?

Just a thought........................

[Xplorer]

mmh .... checking ...

the centre tap goes of the impeance selector goes to the hot side of the output jacks, the 4,8,16 ohms wires of the OT go to their respective lugs ( verified with Brian with voltages ) , the black wire goes to pin 3 of V6, the red wire goes to the pin 3 of V5, the orange wire ( 0 ) goes to the ground side of the output jacks, and the brown wire goes to the preamp turret with the choke, linked to the mains filter caps.

So you think that the orange, brown, red and black wires could be mixed up ? i'm not sure how to verify it. But we checked the OT, in my previous posts, at least the 4,8,16 ohm taps + output of the impedance selector.

[Xplorer]

Also, when i use the SL's preamp into the JTM ...it has the power amplifier of the JTM + the OT section of the JTM , and it sounds "as it should", so doesn't it confirm that the JTM's OT ok ?

it would let us tend to think that everything after the 220k is ok. BUT, the JTM's preamp itself seems ok since once heard through the SL, it distorts ( but farty sounding, not so right )

this problem is quite a non sense ! wierd .... difficult to locate its origin.

[Xplorer]

okay .... checked the amp for continuity, no cold solder joint found.
i'll pull and replace V3's components with some others, one by one, test the amp ...
then V2,V1 ...

[neikeel]

mmh .... checking ...

the centre tap goes of the impeance selector goes to the hot side of the output jacks, the 4,8,16 ohms wires of the OT go to their respective lugs ( verified with Brian with voltages ) , the black wire goes to pin 3 of V6, the red wire goes to the pin 3 of V5, the orange wire ( 0 ) goes to the ground side of the output jacks, and the brown wire goes to the preamp turret with the choke, linked to the mains filter caps.

So you think that the orange, brown, red and black wires could be mixed up ? i'm not sure how to verify it. But we checked the OT, in my previous posts, at least the 4,8,16 ohm taps + output of the impedance selector.

No the COMMON of the secondary goes to the jacks (ie the end of the winding), the coloured taps to the selector (the 16ohm being the end of the winding)

The primaries are the black/brown/red. I know that different Drake trannies used different colour codes. As you obviously have a weird problem I am just going through the wierd possibilities. On this trannie the brown should be the centre tap and goes to the choke/HT feed. The other two wires go to the plates, if you get the outers swapped over you get positive feedback and a scream but imagine if the black was incorrectly wound as centre tap and you had HT going to one end of the primary, it might give the symptoms you describe - like a single ended amp very asymetric in the extreme you would still get the roughly correct plate voltages (if as you say everything else that can be checked and double checked is correct).

I would unhook them and put a meter across each pair of wires so that you can determine which is the centre tap. If I am wrong you can curse me - just trying to think outside normal wierdness!!

[Xplorer]

Isn't it what i explained ? Sorry then. Yes, this is the common of the secondary ( orange - 0 - ) which goes , on my amp, to the ground side of the output jacks.

i meant : the output of the impedance selector ( not the center tap, though it's the center lug ) goes to the hot side of the output jacks.

On this trannie the brown should be the centre tap and goes to the choke/HT feed. The other two wires go to the plates

yes, this is the case on my amp.

what "HT" abreviation means again ?

imagine if the black was incorrectly wound as centre tap and you had HT going to one end of the primary, it might give the symptoms you describe

We can verify it if it's a potential "out of the normal wierdness", though it looks good. Ok, just to be sure.
so what should i measure with the meter, to determine which wire is the center tap ( supposed to be the brown one ) ? accross which wires ? ohms ? voltage ?
You're talking about the black wire from the primary that would have a wrong color, and swapped with the orange one ?

[neikeel]

without the output tubes in put an ohm meter across each wire of the primary. Two readings will be roughly the same the wire that is in the middle will be the centre tap.

ie on a 50 watter 45 ohms between white and red, 47 ohms between red and brown, 92 ohms between white and brown means that the red would be the centre tap.

Your readings will be different but the principles are the same.

[Xplorer]

i'm not following you so well.
i have a spare OT to compare if needed.

Well : i put the meter accross black and brown of the primary, black and red of the primary, red and black of the primary ? is it what you're saying ? ok.

i measured also the secondarys on the spare OT. about 1 ohm between the orange ( common ) and the green. arround 7-8 ohms accross the orange and blue, and about 5,5 ohms accross the orange and yellow.

[Xplorer]

on the spare OT :

88,3 ohms accross the red anc black.
41,2 accross the black and brown
47,2 accross the brown and red

[Xplorer]

on the amp :

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

[Xplorer]

The brown in every case seems to be the center tap as it should, and it is connected to a turret where one choke wire is going too, and both are connected to the mains filter caps.

[neikeel]

Good, although still no further on!

[Xplorer]

ha ha ^^

Yeah but we know things now ...

- The PT and the OT are just fine, the wiring is just fine ( i checked the continuity and cold joints, following the layout and erasing with a pencil what i had checked ).

-We know some voltages ..

-pots were measured ok

-tubes are fine i think

-SL's preamp sounds ok through the power supply of the jtm

-JTM's preamp sounds different and saturated ( all when tested at 10 on the volume ) through the SL than through the jtm itself

-something between the power supply and the preamp doesn't work fine.

-there are not so many components on the jtm, i'll remove and replace the components of the preamp, tube by tube, to see if it does any good.

-testing with a probe into the input of another amp and listening to it gives different results compared to the tests with the mix of tubes from two amps, to isolate a specific tube.

-the 470 ohms and the 47pf could potentialy reduce the strenght of the signal, the saturation. I'll check them, as well as the 16uf feeding v1 and v2, from what i understand.

-the 250uf cap has no effect on the problem

-the 27k nfb resistor has no effect on the problem

[Xplorer]

okaaaay ..... Now it's the turn of my SL !!! i have no idea of what happend, but now it has a very weak output and i have to hit the strings hard to hear something, some kind of saturation, just like when you still play the guitar when switching the standbye on.

could it be some fried transformer ?? tubes ? ( they show their usual light though )
I checked for continuity of these only two solder points and they're ok. i didn't touch anything else !
maybe when my whole room current shut down and i had to turn the AC fuses on again.... would that have caused something ? i don't remember if the amp was on or off by then. and i think it was another day, not yesterday.

i just resoldered the 2x0,022uf to their 220k and the treble wire to the whiper of the treble pot. it played ok. i shut the amp off, hen later played on it, it did this problem.

i'm so unlucky !