Princeton Reverb, LTP PI, No Trem

[sgbrewing]

Hi guys, hope you've had a good Christmas! Diving in to my second amp build and I would really appreciate some experienced eyes checking over my work

During the first lockdown in the UK I built my first amp, a JTM45, and it went really well. Sounds great and was fun to build and research tweaks for the tone.

Now I'm ready to build my second amp and am looking to be a bit more ambitious! I'm going to build a blackface Princeton (well initially based on that) but with quite a few changes. I've been reading around lots and have combined quite a few ideas into one design. I've taken a few of the recommended upgrades from RobRobinette.

I don't use tremolo so I'm going to drop that and use that half of the tube for a LTP PI for a smoother overdrive. With the 2 vacant holes I will use a 10K mid pot (still with the 6.8K resistor to ground, so I can use this on 0 for a stock sound and to boost the mids slightly), and a PPIMV for the second hole (I used the Rich Mod II PPIMV in my Marshall with the PEC 500K dual pot and it sounds great at lowering the sound for home use).

Im going to use this amp in 2 ways, clean with a zen drive for the D sound and turned up to overdrive for some lovely smooth blues lead tones.
I would really like the overdrive to be as smooth as possible, I'm not too sure on how to bias the preamp (but I do want to use 5uF cathode caps, I'm just not too sure on the resistor values), on the current layout I have it set as a super reverb with a few 820ohms. Im just wondering what the difference would be with:
1) These super reverb resistor values
2) All 1.5Ks, stock Princeton Reverb
3) Higher values at 2.2k and 3.3K Like a Dumble
I would like the amp to be clean at 3-4ish and when pushed to 6+ the smoothest drive possible from this amp.

I would really appreciate any help and checking of my layout.
Once the layout is all sorted I will upload the .DIY files for the layout and for the board to be made by Hoffman for anyone looking to build an amp like this to use.

Thanks again for your help!
Shawn.

[sluckey]

The bias board does not have a ground connection. It also does not have a bias voltage output. I quit looking after spotting these errors right off the bat. Your circuit is quite a deviation from an original PR. That makes seeing a schematic even more important for evaluating your layout.

[sgbrewing]

The bias board does not have a ground connection. It also does not have a bias voltage output. I quit looking after spotting these errors right off the bat. Your circuit is quite a deviation from an original PR. That makes seeing a schematic even more important for evaluating your layout.

Thank you for your reply and apologies for those omissions, I've corrected those below.

Sorry I don't have the electrical knowledge to be able to draw up a schematic. I've used a quite a few layouts that I've seen and i've pieced them together.
It's basically a PR with the PI replaced, I added a 220K to ground off the PI to emulate the second channel of an amp it would be used in to prevent it from being overdriven too far.

What cathode resistor values would you use with the cathode caps of 5uF to achieve a smooth distortion sound?

Thanks for your help,
Shawn.

[sluckey]

I would use 1.5K cathode resistors for proper biasing. The only time Fender used 820Ω cathode resistors was when two triodes were sharing the same cathode resistor.

[sgbrewing]

I would use 1.5K cathode resistors for proper biasing. The only time Fender used 820Ω cathode resistors was when two triodes were sharing the same cathode resistor.

Thanks, what would the difference be if I used 2.2k or 3.3k cathode resistors?

[sluckey]

Lower gain.

[didit]

Hello --

Lower gain.

Slightly more complicated as posed initially:

3) Higher values at 2.2k and 3.3K Like a Dumble

Perhaps this older thread sheds a bit of light --
https://ampgarage.com/forum/viewtopic.php?t=31061


Learning to draw schematics is good advice. Building a prototype and experimentation for a few hours with a few dollars in an assortment of component values would be truly enlightening.

Best .. Ian