Hi guys, been a while (thankfully? lol)
A friend of mine is trying to figure out a strange issue that's going on with his JTM45 clone by mojotone (schematic link below). He bought it pre-built online, second-hand. I've linked to an audio file which he provided, the problem definitely disappears with lower tones being played.
Here's his take:
Right away I noticed some prominent, not so nice harmonics/overtones on certain notes. They become noticeable with the volume around half, and present themselves more as I turn up the volume from there. I've attached a short clip (volume at 10) demonstrating what I mean. The higher notes at the beginning are the ones with the stronger overtones. As I come back down the neck, the overtones (to my ears) disappear. Also, I only really notice it if I'm doing single note stuff, which could still be problematic if I'm getting a G# overtone ruining my E-minor solo.
I've asked him to run the amp head removed from the cabinet to rule out sympathetic vibrations, no go. Lastly, when it comes to odd noises my first go-to is swapping out the tubes, as I've found them to have the highest likelihood of microphonics. Also no go. The company has written to him and mentioned lead-dress as the major factor for 'ghost notes' - but as far as I can tell (photo attached), this amp was built by someone with a fair bit of experience.
I'm stumped. I'm now suspecting those carbon-comp resistors but I feel that's grasping at straws. Hopefully someone knows what this is all about...
Audio file link
Schematic link
Thanks!
A friend of mine is trying to figure out a strange issue that's going on with his JTM45 clone by mojotone (schematic link below). He bought it pre-built online, second-hand. I've linked to an audio file which he provided, the problem definitely disappears with lower tones being played.
Here's his take:
Right away I noticed some prominent, not so nice harmonics/overtones on certain notes. They become noticeable with the volume around half, and present themselves more as I turn up the volume from there. I've attached a short clip (volume at 10) demonstrating what I mean. The higher notes at the beginning are the ones with the stronger overtones. As I come back down the neck, the overtones (to my ears) disappear. Also, I only really notice it if I'm doing single note stuff, which could still be problematic if I'm getting a G# overtone ruining my E-minor solo.
I've asked him to run the amp head removed from the cabinet to rule out sympathetic vibrations, no go. Lastly, when it comes to odd noises my first go-to is swapping out the tubes, as I've found them to have the highest likelihood of microphonics. Also no go. The company has written to him and mentioned lead-dress as the major factor for 'ghost notes' - but as far as I can tell (photo attached), this amp was built by someone with a fair bit of experience.
I'm stumped. I'm now suspecting those carbon-comp resistors but I feel that's grasping at straws. Hopefully someone knows what this is all about...
Audio file link
Schematic link
Thanks!
Attachments
Do I see a rheostat next to the speaker jacks? Maybe something is awry with that. Impedance mismatch etc.
Otherwise it does sound like a Marshall.
Otherwise it does sound like a Marshall.
It's just the impedance switch. No change in overtones between the different output selections (just in case it was wired wrong?).
What kind of capacitors are those, in yellow? Film caps? Maybe replacing those with PIO would change the overall response of the amp to "higher notes"...
Yet another straw grasp, I know. Your included picture (with the nice looking wiring) maybe worth a lot of words!
Yet another straw grasp, I know. Your included picture (with the nice looking wiring) maybe worth a lot of words!
Rather easy is to test the effect of Feedback first detaching it from the 16 ohm output. Sometimes transformer wires are not what they should. If sound improves test FB connected to 8 ohm output.
Also using an ohm meter measure that there are more resistance between 0 and 16 than 0 and 4 ohm outputs. Some ohm meters produce so little current that it reads 0 or 0,1 on thich output coil and it does not reveal if outputs are connected right.
You can measure and compare output transformer voltages from different outputs using multimeter VAC playing low E keeping volume settings the same. Naturally be cautious of dangererous voltages if you co inside the amp and have mains cable connected. If amp has multiple output jacks perhaps you can measure voltages using a cable plug.
Esa
Also using an ohm meter measure that there are more resistance between 0 and 16 than 0 and 4 ohm outputs. Some ohm meters produce so little current that it reads 0 or 0,1 on thich output coil and it does not reveal if outputs are connected right.
You can measure and compare output transformer voltages from different outputs using multimeter VAC playing low E keeping volume settings the same. Naturally be cautious of dangererous voltages if you co inside the amp and have mains cable connected. If amp has multiple output jacks perhaps you can measure voltages using a cable plug.
Esa
