Modifying a JMP1c to sound brighter. Noob questions

[bruzanhd]

I think what bruzanhd intends is a voltage divider to simulate the channel mix circuit in a plexi type design.

correct, here is a picture of the traces on the back. Any thoughts with this in mind? The ground from the volume pot is quite far on the board so i'd have to do some flying leads or something which i'd like to avoid if possible. they both end up grounding at the same pcb pad though

[Chuck H]

Seems ok to me. What ever resistance is on that short trace isn't going to cause an audible ground loop. Better than most vintage amps really.

[bruzanhd]

Seems ok to me. What ever resistance is on that short trace isn't going to cause an audible ground loop. Better than most vintage amps really.

Thanks for your insight, I just wanted to make sure I wasn't making an obvious mistake. I will try it out tomorrow and see how it goes.

[bruzanhd]

Well I did it, along with the bright cap and the presence mod. REALLY digging the sound I'm getting. I have a mild concern:

When the guitar volume is turned up and my hand is on the bridge of the guitar (or any grounded part) it still has some hum. When I roll the guitar volume to zero, the amp is silent. Is this expected behavior or am I experiencing a ground loop. As a side note, the amp has always done this although now that I'm hyper focused on it seems maybe louder? Although this may also be because I have made the amp allow more high end through.

Edit: Using my pedal board between the guitar and the amp lessens the hum. Does it seem like the noise is mostly the fault of my guitar I dont have another to try it with?

Edit 2: The hum peaks at 540hz. I wonder if I may just have low quality power in my home.

Here is some pictures of my work. Tell me if you see a problem or an improvement I should potentially consider.

Thanks again for all the expert advice you all have been giving me.

[Helmholtz]

Well I did it, along with the bright cap and the presence mod. REALLY digging the sound I'm getting.

Great.

Does it seem like the noise is mostly the fault of my guitar ?

Yes, because of this:

When I roll the guitar volume to zero, the amp is silent.

The hum peaks at 540hz. I wonder if I may just have low quality power in my home.

You might have a noisy environment.
I distrust sound analysis/measurement using a phone

[bruzanhd]

Great.



Yes, because of this:



You might have a noisy environment.
I distrust sound analysis/measurement using a phone

Borrowed and Tried another amp it and does something similar. I think it’s settled, guitar picking up environmental noise.

[bruzanhd]

Okay so now that I’m driving my self nuts about every little noise, is mechanical noise from the amp somewhat normal? My amp makes a low constant hum (240hz?) from the actual amp. Not through the speaker. So it’s a physical vibration somewhere. My guess is likely the power transformer but really I don’t know. It very well may have always done this and it’s a normal behaviour but I haven’t really paid attention before.

[Chuck H]

240Hz wouldn't sound like hum. More like a note just a tad sharp of A#3. I've never heard this phenomenon myself. It seems worth mentioning that it could be a harmonic of 120Hz. Does it dampen if you put your hand on the power transformer?

[bruzanhd]

240Hz wouldn't sound like hum. More like a note just a tad sharp of A#3. I've never heard this phenomenon myself. It seems worth mentioning that it could be a harmonic of 120Hz. Does it dampen if you put your hand on the power transformer?

Finally got around to tearing down the amp again and snugging up some screws. I just took everything apart and re seated and re snugged everything. Sound is mostly gone now. It still has a quick vibration when I switch the amp on but at idle it's a little quieter. Still couldn't pin point the source. When I put a small pressure on part of the chassis the vibration would change pitch. But i could apply that pressure anywhere and get the same effect. And just to clarify this sound does not come through the actual amplifier's output. It is purely physical vibration. If I run the amp with all knobs on zero and speaker disconnected the sound will be there.

[bruzanhd]

Hello folks I kinda finalized the mods, was fighting with getting the right amount of gain since I tightened up the response, so I ended up increase the gain in the second stage, and adding a shade more low end onto the bass control. Sounds great. Here's an annotated schematic. I have a question of best practice though, I have an unused triode in the middle of the amp, is it best to leave it floating or should I ground the plate, grid and cathode directly? The heaters are DC and the lines are run on the PCB so I can't disconnect it. Right now it is floating but maybe this is a bad idea? Maybe it doesn't matter. Thoughts and opinions appreciated.

[Chuck H]

Leave it floating

As it stands the mechanical parts in the tube are just that. No electrical significance. If you ground anything then you have parts that can present a "voltage differential". A differential can become part of a circuit and that is what you don't want from an usused triode.

[bruzanhd]

Leave it floating

As it stands the mechanical parts in the tube are just that. No electrical significance. If you ground anything then you have parts that can present a "voltage differential". A differential can become part of a circuit and that is what you don't want from an usused triode.

Thank you, this was my intuition but I started reading all kinds of crazy schemes and ideas about it.

[bruzanhd]

Well, for the first time a had a preamp tube go microphonic. It was my v1 and it was a tung sol 12ax7. I replaced it with a mesa 12ax7 and everything is now good. With this being my only modded amp, is it possible any of the mods I made may have caused this? Probably not but it's not like I have a point of reference. Any experience anyone would be able to share?

[g1]

Preamp tubes going microphonic is quite common, especially in sensitive spots, like V1 position. It's unlikely that any mods would contribute to the issue.
I think what stands out the most is that you have not had to deal with microphonic preamp tubes before this. Consider yourself lucky.
Sometimes with high-gain spots you will need to choose the 'least bad' from even a batch of brand new tubes.

[Chuck H]

Microphony happens. (<period) It wasn't anything you did or any circuit changes that caused it. All of my tube amp avocation time has included me marking and saving tubes for the V1 spot in higher gain circuits. Nearly impossible to find anymore from the regular sources. The Mesa rebranded Shuguang tubes have always been better than average for microphony. Sorry to say that second to that would be high end NOS. GE tubes are less expensive as NOS goes and are usually stable but sound a little flat IMHE. YMMV. But the message I intend is that good, stable tubes for V1 in high gainers are hard to come by. I tube roll with any new batch of preamp tubes I get just to mark and save the least microphonic for V1. That said, I'm not even sure I have any more tubes suitable for a V1 spot in my own stash unless I plug in one of three NOS Mullards I've been saving. Which I don't want to do. In other words, it's everyones dilema right now and your experience is unfortunately normal.