Hi everyone, my Marshall JTM30 sounds extremely low, with the master knob all the way up i can barely hear it, all distorted. I re-tubed the amp, changed all electrolythic capacitors in the poweramp section and powersupply, but i still get low plate voltage (around 140v) when i plug in the 12ax7. If i do not plug in the 12ax7 tubes, and i only plug the 6L6 tubes, plate voltage goes up to 400v. Any ideas?
Thanks in advance
Thanks in advance
Resistor values are fine (10k each), i get 150v on one end and 120v on the other... shoud i replace them?
This is schhematic for poweramp section.
http://drtube.com/schematics/marshall/jtm3-61-02.pdf
What i mean is when i put in 12ax7 i get 140v on all tubes, even 6L6. I'm guessing that's why i get low output volume. Does that sound correct?
http://drtube.com/schematics/marshall/jtm3-61-02.pdf
What i mean is when i put in 12ax7 i get 140v on all tubes, even 6L6. I'm guessing that's why i get low output volume. Does that sound correct?
OK so now i get 380v on B+ with all tubes ON. But i still have very low output volume. And i get a blue glow on 6L6 tubes when i turn on stand by switch.
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There are three ECC83/12AX7. Any one in particular?
The screen grids of 6L6s do glow blue, no problem unless the anodes are red.
The screen grids of 6L6s do glow blue, no problem unless the anodes are red.
My mistake, B+ is now OK, i get 380v for 6L6. But i still have low output volume, it increased a little bit since i changed all capacitors and resoldered lots of joints, but i cant get a decent volume. 6L6s glow blue whenever i strum harder. I assume that is fine. Could it be a biasing problem? can wrong bias get me an output volume this low? thanks
Ok, next time please be careful when reading, we rely on what you measure and post.
Oh my ..... don´t replace parts at random, even less on a PCB based amplifier, it´s possible to do more damage than what was originally there.
Hardly, way overbias might run your tubes red, way too cold might send them into cutoff and sound would lose sustain and act like a poorly adjusted noise gate; in any case JTM30 uses non adjustable Zener stabilized bias .
You must feed 100mV 1kHz or 440 Hz audio signal at the input and follow signal voltage along the path until you find the point where it gets lost or greatly attenuated.
Might also be a connector or switch problem, usually the Loop return one, but please post the full schematic so we might suggest "expected" voltages.
A scope would be a great tool, but any multimeter can read up to 400Hz or so, and won´t be far away at 1kHz, we don´t need *exact* values but "reasonable" ones to check whether an amplification stage is working as expected or not.
So post that full schematic 😉
You can get your test tones here:
Download Audio Tone Files
get the 30 second MP3 and set player for "loop" or "repeat one" so you have continuous sound.
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OK i will.
I did not replace parts at random, power supply caps where too old and swollen up, they seemed damaged, that's why i replaced them.
I already posted full schematic, its here:
http://drtube.com/schematics/marshall/jtm3-61-02.pdf
Thank you!
I did not replace parts at random, power supply caps where too old and swollen up, they seemed damaged, that's why i replaced them.
I already posted full schematic, its here:
http://drtube.com/schematics/marshall/jtm3-61-02.pdf
Thank you!
Explore the speaker side. ANy possible output jack problems? Impedance switch making poor contact?
Try disconnecting the output transformer secondary wires from all circuits and clip wire them directly to a speaker. Now what happens?
Try disconnecting the output transformer secondary wires from all circuits and clip wire them directly to a speaker. Now what happens?
JTM-30
Great amp with proper tubes and speaker.
I have found,
Once warmed up....
Volume loss occurs in some of these amps caused by (microscopic) cold joints at the thick legs of the filament Bridge Rectifier.
Drove me crazy at gigs for a month, intermittent as it was.
I had to remove, clean and re-solder BR62 with a hot Iron.
Worth a try.
Tom
Great amp with proper tubes and speaker.
I have found,
Once warmed up....
Volume loss occurs in some of these amps caused by (microscopic) cold joints at the thick legs of the filament Bridge Rectifier.
Drove me crazy at gigs for a month, intermittent as it was.
I had to remove, clean and re-solder BR62 with a hot Iron.
Worth a try.
Tom
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Thanks a lot for that tip!
So, resoldered every suspicious joint on the tubes board, every wire and every component. Still low volume. I decided to look at the OT. Discovered this:
OT Primary: 390VDC on every pin, B+ and plate connections from 6L6.
This can't be right i guess...
Disconnected every wire from OT, measure DC resistance from B+ pin to plate pins, i get 7.8ohms on one side and 83ohm on the other. I assume that is wrong.
I would expect a slight difference due to the turns/length ratio but not that much.
New transformer required.
New transformer required.
??????????????????????????
OT center tap is connected to +V
Do you have +390V +V? ... then that´s what you´ll find there.
Tube plates are connected to those windings, which have low DC resistance, so you will *also* have +390V DC there, or *a little* less.
lease confirm, did you write 7.8 and 83 or 78 and 83?
Please remeasure just in case.
10% to 20% difference is normal, but IF you have such a large 10:1 difference, your OT is dead.
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