Plans to build a handwired JTM145 clone with added presence pot

[CandyRush]

Hi all,

I plan to build a handwired JTM145 clone with added presence and master volume pot. As foundation I took the plans of Matec from this forum. I'm sure that I probably did some mistakes in the design. But I want to ask you for early feedback to analyse if I plan in the right direction or need to recreate the DIYTC wiring diagram from scratch again. Or I ask myself if I did really basic mistakes when adding the Master or Presence controls.

I look forward for your feedback and thank you in advance for your help!

[martin manning]

...I ask myself if I did really basic mistakes when adding the Master or Presence controls.

Both of your mods will screw up the phase inverter bias. You can probably just move the presence nw to the right of C11, and do the MV as shown below.

[CandyRush]

thank you for that advice

[cdemike]

Not sure if it's what you're going for, but you could experiment with a post phase inverter master volume. Something lower volume like this could actually avoid a lot of the pitfalls associated with a cross-line ("type 3") MV, i.e. you'd be able to avoid some of the fizziness that some find with that style of MV. Someone more knowledgeable than I can probably comment on this, but I get the sense that, since the amp is cathode biased and therefore has a ground reference to the grids anyway, the "type 1" and "type 3" styles may be better suited here than on something like a big plexi with fixed bias. Could be off base on that, but definitely think it's worth checking out since you'll probably be using it at higher settings anyway, which would mean you'd be closer to a "non master volume" sound with a PPIMV.

Regarding lead dress, I think you might be able to get a lower noise floor by either simplifying the input stage to a traditional plexi/Fender style input stage or moving more components to the jacks. Migrating things off the board helps reduce overall lead length and also might enable you to use shielded wire. Here's a quick schematic of something you could do with a double-switched Cliff jack that'd keep all of the existing input circuit except for the 1K grid stop. I'm not convinced you'd need the grid stop, but you could also just include it as a flying lead at the end of your input lead. That'd also let you use shielded wire.

[CandyRush]

Did I add the two types of MV and the presence pot correctly?

[CandyRush]

I updated the plans again regarding the change in circuit and also changed MV Type 3 to linear pot. Is everything correct now?

[cdemike]

I updated the plans again regarding the change in circuit and also changed MV Type 3 to linear pot. Is everything correct now?

Input stage is looking much better to me! I'd suggest grounding your rectifier separately from the ground bus at the top of your board. That tends to be an extremely noisy ground and can introduce noise. In terms of the layout as the whole, may I ask what your general considerations were in terms of component locations? On most of the layouts I've seen and have done, I've tried to keep the components associated with each gain stage in sequential order, which helps keep leads short (and therefore noise low). The circuit styles I'm most familiar with are Marshall and Hiwatt, and they tended to follow that format, as well as to keep things ordered as cathode components -> grid components -> plate components if tracking gain stages on the board reading from input jack side to power control side. Seems to me that that would be out of noise considerations, but I'm sure there are additional reasons that I'm not familiar with.

Only thing I'd change about the MV is moving it to the other side of the grid stops. So rather than connected right at the grids, they're where the "grid leaks" go (i.e., at the junctions of R3 and R5 on one side and R4 and R6 on the other). Probably easier to un-solder there also if you decide you don't like it (possible since the JTM145 uses a cathodyne rather than a LTP phase inverter, but you never know). I'm eager to see where this leads.

[cdemike]

Did I add the two types of MV and the presence pot correctly?

One thing I'd suggest before you "wrap up" the build and you still have it on a bench is trying out different NFB resistor levels. 1Meg is pretty high, and I'm not sure you'd get much authority with your presence pot in that configuration. If you're considering an upper bound of 1Meg, try wiring a 1ML pot as a variable resistor in place of R15 and play through it with the pot set to different values. Once you found one you like, you could replace the pot with a fixed resistor at that value.

[CandyRush]

Thank you for your opinion! How do i add two different grounds? Arent they connected all to each other normally? I didn't have a general consideration about the design layout. I just wanted to make it as clean and the connections as close as possible. In the circuit there are many connections with up to 5 components connected at one side. Its beyond my current imagination how to further clean that up. "at the junctions of R3 and R5" good idea.

[cdemike]

Well I just reviewed Merlin's grounding article to try to write a well thought-out response (https://valvewizard.co.uk/Grounding.pdf), and I seem to have been full of it. I've seen the rectifier and reservoir cap usually referenced to a separate grounding lug from other chassis grounding points in almost all of the layouts I've seen, but those haven't strictly a star grounding scheme. I understand the star ground to be the "ideal" case for a quiet grounding scheme, but that's definitely not to say that other grounding schemes can't make for quiet amps (think Hiwatts, which do not follow star grounding schemes). In any event, a bus ground setup like you have should be fine, provided you rework the leads to connect in the correct order (generally, rectifier ground > reservoir cap ground > output section cathode grounds > screens smoothing cap ground > output jack sleeve > PI smoothing cap ground > PI cathode ground > preamp smoothing cap ground > gain stage cathode ground, and so on). The valvewizard site is full of other great tips and tech articles.

In the FAQ section, there's a reposted write-up from R.G. Keen, who wrote another article on lead dress and general considerations for making a quiet amp.

Apologies about the bad info about grounds!

[CandyRush]

Thanks for the advice about the star grounding! The longer I look at the shematics the more i ask myself why C19 cap is connecting the positive high voltage with a realtivly small capacitor to the cathode? Also i ask myself, why so many electronic elements have been added to the input, cathode resistor and coupling cap at the first stage. Can someone explain to me why the jtm145 shematic designer chose to add these additional electronic elements?