Two Rock JM Preamp Pedal - No signal, just hum

[RyanFromQA]

Hello all, this is my first post on here.... really blown away by the wealth of information on here. I'm hoping someone can help me out. Last year I saw the Peace Hill preamp pedals and the MasonFX Mayerland, and decided to try to take on a build myself.

I worked off the schematics and info in this thread:
https://ampgarage.com/forum/viewtopic.php?f=4&t=34715

Taking a look at the two rock schematics, I decided to try to incorporate the additional gain stage that the amp has, using half of an additional 12ax7.

The thought of doing it point-to-point like that overwhelmed me, so I took the PCB approach. Due to the space constraints in the enclosure and the fact that I work with the free version of Eagle, I decided to split the circuit into two boards: the high voltage components and tubes on one board, and the eq components and pots on another board.

I used the FrogPedals SMPS as the power supply and it seems to work well.

Here are the schematics and boards that I ended up with:

• Combined Reference Schematic

• Schematic for EQ Board

• Schematic for Tube Board

• EQ Board layout

• Tube Board layout

After double and triple checking them over and over, I got a few made by JLCPCB and wired it up. Here’s what I ended up with. Additional stuff not in the schematics are:

• Bypass Switching

• 9V cutoff to kill power when not using it

• Outputting through an off-board 1MA level pot which shorts signal to ground (seen on the combined schematic)

Now to the issue: When I turn it on and run signal through it, all I get out of it is a fairly loud, low hum. Not 60hz buzz, but an actual hum. I can’t hear any actual signal even with everything cranked. The hum changes volume with the level pot.

Here’s what I’ve checked so far:

• The B+ is what I expected: 250 without tubes, 170 with tubes.

• Heater voltage is good at 6v and the tubes are heating up

• I can’t find anything shorting to ground

• The preamp “warms up” like you’d expect a tube amp to. When turning it on unbypassed, it’s quiet and the “hum” fades in as the tubes warm up.

In the gut shot it kind of looks like the 9v pad on the 3pdt board is shorting to the ground pad but it’s an optical illusion (I checked)

Anyway I’m hoping someone can spot the (probably many, glaring) mistakes I made

[Synchu]

Not sure whether I’m missing something being on my phone, but I don’t see any plate load resistors? Besides, heat dissipation of the mosfet and 7806 regulator is not optimal that way - heat sinks should cover the whole tabs.
Another suggestion, coming from what I typically mess, is footswitch wiring, so I make sure everything works without it wired.
Niki

[RyanFromQA]

I don’t see any plate load resistors?

That's cause they aren't there! Someone else also pointed that out to me and that's absolutely correct. I totally missed it when transcribing from the full schematic. Last night just to test the theory I soldered a 100k resistor inline with the B+ supply coming out of the SMPS and suddenly it passed signal. Of course now all 3 triodes are in parallel and nothing about the circuit actually works but at least it's progress. And it didn't sound half bad when pushed with a Klon! I'll need to fix the tube PCB to accommodate plate resistors for each stage and build it all over again, but I'm relieved to be on the right track.

Besides, heat dissipation of the mosfet and 7806 regulator is not optimal that way - heat sinks should cover the whole tabs.

Thanks for this tip, I will fix that when I rip out the tube board

[rccolgan]

super cool project! Congrats on the build!

[Synchu]

True. Pretty inspirational it is I roughly breadboarded a Rocket/ZWreckish pre yesterday, inspired by this, but will have the time to test it on Sunday… may even throw a jfet stage in front for these higher gain days.

Niki

[the_jessmo7]

Love this idea! Did you make any more progress on it? Would love to hear more.

[Synchu]

If that was at me - it’s done (the breadboard I mean) and sounds great to me at least
Will not be hijacking the thread though.
Once my finger tip heals from the rather nasty cut I managed to produce, as well, I’ll make another thread with the design, some sounds and the breadboard view.
Will take some time to build it and box it.

Niki

[the_jessmo7]

Oops! Sorry, my question was directed at the OP. Your build sounds fantastic too, friend!

[the_jessmo7]

Hey Ryan! Did you ever get the preamp up and running?