Jason McG
Tele-Meister
I know, it does sound like that, but it's not. Each of the 4 speakers sounds the same independently, and also when I plug into my 5E3 combo 12" speaker, known to be clean, same buzz
JTM45 with EL34's, OT, choke questions
- Thread starter Jason McG
- Start date
This site may earn a commission from merchant affiliate links like Ebay, Amazon, and others.
I know, it does sound like that, but it's not. Each of the 4 speakers sounds the same independently, and also when I plug into my 5E3 combo 12" speaker, known to be clean, same buzz
Jason McG
Tele-Meister
well, the new V3 socket and all new leads coming to it, didn't make any difference
Jason McG
Tele-Meister
I was using the troubleshooting guide in RobRob's webpage, for output transformers.
amp unplugged, caps drained, rectifier and power tubes removed
Output Transformer
For push-pull transformers you should see approximately the same resistance between each secondary wire at each power tube and the center tap--typically somewhere between 10 to 200 ohms.
was good here, 39.4R and 44.2R
A shorted primary winding will have much lower resistance compared to the other winding and typically measure at less than 10 ohms.
The secondary windings will often measure less than 1 ohm between all the secondary leads so it is difficult to detect a shorted secondary.
ranged from .2 to .3
If a transformer winding is open (break) it will show a very high resistance between the primary leads or between the secondary (speaker) leads (typically 500k or higher).
83R between primary leads
A short between the primary and secondary windings will typically show less than 10 ohms resistance between the primary and secondary leads.
no continuity between primary and secondaries
Measure the resistance between all the leads and chassis ground--low resistance of less than 10 ohms indicates a short to the transformer's iron core.
chassis ground to:
CT: no continuity
either primary: no continuity
4ohm secondary: .5R
8ohm secondary: .6R
16ohm secondary: .9R
does this indicate a short to iron core? or am I interpreting the instructions incorrectly?
RobRob's notes also state that I should measure an open circuit (no connection) between the primary and secondary wires and between all wires to the transformer outer metal shell, but the transformers metal shell is grounded, so I get the same continuity as described above.
amp unplugged, caps drained, rectifier and power tubes removed
Output Transformer
For push-pull transformers you should see approximately the same resistance between each secondary wire at each power tube and the center tap--typically somewhere between 10 to 200 ohms.
was good here, 39.4R and 44.2R
A shorted primary winding will have much lower resistance compared to the other winding and typically measure at less than 10 ohms.
The secondary windings will often measure less than 1 ohm between all the secondary leads so it is difficult to detect a shorted secondary.
ranged from .2 to .3
If a transformer winding is open (break) it will show a very high resistance between the primary leads or between the secondary (speaker) leads (typically 500k or higher).
83R between primary leads
A short between the primary and secondary windings will typically show less than 10 ohms resistance between the primary and secondary leads.
no continuity between primary and secondaries
Measure the resistance between all the leads and chassis ground--low resistance of less than 10 ohms indicates a short to the transformer's iron core.
chassis ground to:
CT: no continuity
either primary: no continuity
4ohm secondary: .5R
8ohm secondary: .6R
16ohm secondary: .9R
does this indicate a short to iron core? or am I interpreting the instructions incorrectly?
RobRob's notes also state that I should measure an open circuit (no connection) between the primary and secondary wires and between all wires to the transformer outer metal shell, but the transformers metal shell is grounded, so I get the same continuity as described above.
Lowerleftcoast
Poster Extraordinaire
The OT secondary has a wire to chassis ground. Yes the resistances are as expected.
Lowerleftcoast
Poster Extraordinaire
At this point I am thinking either it is tube related or a ground problem.
Jason McG
Tele-Meister
the way it stops when the NFB is out of circuit is what has me baffled. the amp is quiet otherwise, so doesn't seem to have a systemic grounding issue. tried all new tubes as well, so that seems to be ruled out
Lowerleftcoast
Poster Extraordinaire
What value is the feedback resistor and which OT tap is it attached to?
Jason McG
Tele-Meister
it's 27k, and attached to 16ohm tap
Lowerleftcoast
Poster Extraordinaire
Maybe try a 56k or 75k. Rob's site says the 27k on the 16ohm tap is *Heavy feedback*. 56k is moderate and 75k is light.
Of course with a pot placed in that spot you can dial in the NFB by ear.
Of course with a pot placed in that spot you can dial in the NFB by ear.
Jason McG
Tele-Meister
thanks, i'll give that a try!
Jason McG
Tele-Meister
would you use a 100k 2w pot for that?
Lowerleftcoast
Poster Extraordinaire
Low current there so low wattage pot is fine. Usually a pot is put in series with a resistor so NFB can't be turned to zero. Resistor of 10k to 27k and 50k to 100k pot. If you are just using the pot to *find* a sweet resistance the 100k pot is fine. Replace the pot with a *that* value resistor to get the pot out of the circuit. Some users like the adjutability. Your choice.
Jason McG
Tele-Meister
on the right track here! clipped in a 68k and it moved the problem to 5 on the presence dial. will keep going up to where it disappears. I think Marshall used a higher value here on the higher wattage amps, and I think this amp is more of a JTM50, with the EL34's. Thanks for steering me to this!
Jason McG
Tele-Meister
100k, still a touch of it, 220k, gone, so somewhere in between looks like the sweet spot.
Any downsides that could harm the amp, with those higher values here? I'll check and adjust bias, but other than that?
Any downsides that could harm the amp, with those higher values here? I'll check and adjust bias, but other than that?
Lowerleftcoast
Poster Extraordinaire
The larger the resistor the less NFB. It shouldn't harm anything.
I just wonder if this *fix* is just covering up something we are missing.
I don't think larger value EL34 grid stoppers have been tried.
IIRC EL34s like a larger grid stopper than 6L6. The value and wattage are not critical. Maybe try something in between 10k and 100K?
I just wonder if this *fix* is just covering up something we are missing.
I don't think larger value EL34 grid stoppers have been tried.
IIRC EL34s like a larger grid stopper than 6L6. The value and wattage are not critical. Maybe try something in between 10k and 100K?
Tom Kamphuys
Tele-Holic
Great minds think alike, though fools seldom differ.
What is happening is that one is limiting the feedback, just as with the presence knob below 5.
Lowerleftcoast
Poster Extraordinaire
So Tom any thoughts on what is causing the buzz type sound? I am grasping at straws.
Tom Kamphuys
Tele-Holic
No, unfortunately not. I'm lacking a lot of practical experience.
I've been following this thread, but have little to add unfortunately.
I've been following this thread, but have little to add unfortunately.
Tom Kamphuys
Tele-Holic
Do you have a scope? You might be able to find out where it starts. Or a signal tracer?
Have you chop sticked around?
Is the presence knob the only knob that influences the sound?
Does it matter which output jack you use? Or which impedance?
Have you chop sticked around?
Is the presence knob the only knob that influences the sound?
Does it matter which output jack you use? Or which impedance?
Last edited:
Jason McG
Tele-Meister
no scope...
yes, chopsticked heavily, tried all 3 impedance settings, all jacks, different cables, different guitars, different tubes, reflowed solder, changed out V3 socket, all three configurations on presence pot (5k, 25k, etc), different grounding groupings, and had the same outcome, where the buzz was there unless the NFB was out of circuit (presence maxed).
I've now found a 150k NFB resistor takes it out, and I still have good presence control. 120k and it disappears at 2 on the presence knob.
