JTMP50 Build

[rgorke]

So, was a bit undecided on where to post this build so here it is. I have been wanting to build a JTM50 for a long time. Was sourcing some part here and there and trying to repurpose (recycle) some parts from other project or what have you. I started with a JTM50 spec but ended up with a hybrid between a JTM and JMP 50, thus the name.

The board is from an old Hammond organ. I loved the look and the shape was a perfect fit. So I pulled the old turrets off and drilled new holes and installed new turrets. Problem is that either I miscounted and/or mis-allign the power stage so the components don't line up perfect and pretty but it is working. I had an issue early on in that I was using a dual 32uf + 32uf preamp cap that seemed to have one bad side that kept voltage from reaching V1 and V2. You can also see how I moved the wire that was under the board to the plates to on top of the board. That was during my phase of investigating why there was no voltage on V1 before discovering the cap issue.

I have run into a very frustrating issue. My "normal" channel (#2) is dead quiet but the Bright channel (#1) has some noise in it from both inputs, hi and lo. The noise is greatly reduced and almost gone when I run through my Echoplex into the amp. Guitar straight into amp is noisy, guitar > echoplex > amp is quiet. I have switched all components (inputs, volume pots, input resistors) between the normal and bright channel and noise continues. I thought I had it licked but then it came back.

What also makes me crazy is that in the evening, the noise appears to be more pronounced. I have also discovered that our iPad seems to play a bit of a role as well in exacerbating the noise.

I have some new input wire coming and I think that is where the source of the noise may be in terms of solder joints under the board. I have been looking at how old Marshall inputs are wired and see how they brought the wires up through the board prior to attaching to the turrets.

Here are some pictures.

Thoughts

[catalin gramada]

Hellooo
man , don't change nothing , please. You layout looks more vintage like original in terms of handcrafting, really
Regards the noise, could possible be a interference issue. you can simple check just touching the chassis with finger-if the noise level changed, it is for sure. cover the chassis bottom with a piece of metal sheet and check again. you must not record any changes in noise level/modulation with this simple test, otherwise a grounding inadvertence could be. If you pass this and nothin gonna happen , a wiring diagnose will be next.congrats

[maxwedge572]

Make sure that those grid wires are well away from the heater and plate wires. Are those sockets new?
What value bright channel by-pass cap and the cap across the pot did you use?
You tried a different tube in V1?

[neikeel]

What sort of noise - do you mean hiss?

That will depend on size of that big ceramic cap on the pot.

Personally I think that the whole point of a JTM50 is the organic feel that the shared cathode spec with small bright cap and tube rectifier gives you, you will lose that with your hybrid spilt cathode, lead sort of amp. I have an original 68 split cathode and my clones and the JTM is the sweetest of them all - more oomph than a 45 too.

ps - have you tried it in the head shell with the flashing to complete the faraday cage shielding?

[rgorke]

First, the spec is a 2.7k/.68 uf shared cathode on V1. V1b plate is 0.0022. I also just put a 0.068 on V2 for giggles.

Second, the noise is a buzz that gets worse when my hands are off the strings. Maybe I can try to get a recording of it. I don't have a head box yet for this so I haven't tried that helping. I have taken the 5000pf cap off of the bright volume and that did not seem to help things. Last night I tried putting the input resistors right on the inputs and running a shielded cable to the grid and that seemed to help a great deal ala Metro-Friedman. My thinking is that 1) the input wires to the resistors are/were too long and 2) with switching the resistors in and out, solder leaked out the bottom a bit both at the input wires and at where they come together to go to the grid. Essetially having an unshielded glob of solder picking up all the 60 year old house can put out in terms of noise.

Does that make sense?

Finally, I blew something last night. I had the guitar straight into the amp, I pulled out the cable and plugged it into my echoplex (because usually the Echoplex reduces the noise) and when I plugged into the amp again I had NO sound. Nothing, no hum or anything out of any of the 4 inputs. It was late so I just shut everything down and took the kids to bed. I did check the HT and Mains fuses and they are ok.

Ain't this fun????!!!!

[neikeel]

Most likely think is a fuse - check it with a meter - not just by eye.

The noise you describe sounds like a grounding issue, could be the guitar, could be the cable, more likely the ground that you have off the input jacks, maybe a dry joint or if you are using a star washer and the chassis is plated not digging in and grounding well enough. That might be evident if the noise goes when you touch the chassis too.

If it does not do it on the normal channel at all that really pins it down to the input jack wiring.

It might be just the angle of the pic but that off board 10k has wires much too long and very close to the cathode follower 100k resistor.

Did you use a 0.068uF or a 0.68uF over V2a?

What made you use 2k7/0.68uF as shared V1 cathode values? Does it sound good, when you are playing that is?!!

[rgorke]

Well, it appears a tube went bad. I used the 2.7k/.68, well for no real good reason other than to try. It works really well. I can dial in bass using V1. I really, really like it.

That 10k in the first picture was replace and I now have a 10k metal oxide there.

Yeah, it sounds like a grounding issue but I switched the sets of inputs leaving the grounds alone, so the grounds stayed the same. I think it is a noise from the inputs issue. very weird. I shall over come.

[rgorke]

So, it appears my V1 tube blew. Too bad, it was an old GE Mullard I61 or I63.

I wired the #1 inputs with the resistors right at the inputs makes a big difference in the noise. Attached is a recording of "the noise". it is only on volume #1, not volume #1. It appears, as demonstrated in the audio that the treble and middle controls affect the noise. Turned up, it is louder, turned down, it goes away. Again, ONLY on the bright channel. Also, it may be a nighttime thing at my house. I can double check during the day.

Here is the noise:

noise.mp3

(1011.95 KiB) Downloaded 402 times

[maxwedge572]

I was going to say, but forgot, that you could have some noise coming in over your AC line. I had all kinds of strange noises until I plugged my amps into my home theater power strip, and a quite expensive one at that! The bright channel will really amplify that shit too.

[rgorke]

I was going to say, but forgot, that you could have some noise coming in over your AC line. I had all kinds of strange noises until I plugged my amps into my home theater power strip, and a quite expensive one at that! The bright channel will really amplify that shit too.

Thanks, with the shielded input "mod" things are pretty quiet. Kinda bummed it takes away from the vintage look but oh well.

Also bummed I blew that tube. I threw a china 12ax7b in there and not the same.

[rgorke]

So an update. Just for Neil, I put in an 820 with a 22uf for V1 and I like it. I still don't have a bright cap on it. It just seems to make it too thin. I have tried 100pf, 430pf, and 5000pf and they just don't sound right. It kinda looses its balls and thickness, if that makes any sense. I need to turn in it up to get a bit more gain say from instead of 4 to 6 to get that good grind.

What else? The tube in V4 seems to glow, not red plate, just glow brighter when I turn up the volume. I tried switching tubes from one socket to the other and still happens. I now have a sort of mismatched set of tubes in there an I have the "colder" of the two tubes in V4 and that seems to be better. So V4 is biased at -20 and V5 is at -34 or so.

Other than that, the amp is super fun to play with any one of several guitars. Cleans up great with the guitar volume.