Modifications
I simplified the original 5F2-A circuit but I also added some extra controls. I was interested in hearing the effect of negative feedback and preamp/power tube distortion. I made the following changes to the original circuit:
A beefier Hammond 125E universal output transformer is substituted for the Fender OT The Mic input was eliminated. The power supply is different. A pair of 1N4007 solid state diodes are used instead of a 5Y3. Because SS diodes are used, a lower voltage Hammond 270DX power transformer (275-0-275) is used in place of the (300-0-300) Fender PT, a 33uF cap and a 120 ohm/5W resistor were added between the diodes and the first filter cap, and a standby switch was added. Even with the lower voltage PT the plate voltage is still 373 V. A Variable Negative Feedback control was added.The 22K negative feedback resistor was replaced with a 15K resistor in series with a 100K pot. A Master Volume was added. I replaced the 220K resistor at the grid of the 6V6 with a 250K pot. A 1K grid stopper resistor was added to the 6V6 grid to eliminate a problem with oscillations. Star grounding was used and the filament was referenced to ground with a pair of 100 ohm resistors to reduce hum.
Views
I plan to build a cabinet and make it into a head, but for now it is just a chassis. I play it through the Mojo MP10R speaker in my AX84 combo. The chassis is a Hammond 10x6x2 steel chassis. From left to right on the front panel are the input jack, volume, tone, master volume, standby switch, power switch and indicator light. On top are the OT, the 12AX7, the 6V6 and the PT.
Here is the back side. From left to right you see the fuse, AC jack, Negative Feedback control and speaker jack.
Here's the underside of the chassis. Eliminating the rectifier tube and building the amp as a head made the layout very compact. As an experiment I used Vectorboard to mount the components. It made assembly quick and easy. However, it is not very rugged and the pins tend to pull loose when unsoldering components.
Conclusions
The variable negative feedback is a useful feature. Decrease the feedback and the sound "loosens up" - it gets louder and brighter, the bass is reduced and it distorts more easily. It's nice when you're playing at a lower volume but still want the sound to break up. With the feedback enabled you get a cleaner, more hi-fi sound. Using a potentiometer for the feedback control was overkill. A simple on/off switch would have been sufficient.
The Master Volume is a nice feature. If you set it to 10 it's equivalent to having the original fixed resistor in the circuit. Turn it down and you get mostly preamp distortion. It's fun to use when you want some distortion and don't care too much about the tone.
Do It Yourself
Here is the schematic and Bill of Materials. The BOM includes Mouser Part numbers and prices circa 1999. The BOM is also available in the form of a spreadsheet. Click here for an Excel spreadsheet compressed into a ZIP file
Links
The Angela Instruments website has a 6V6 amp project that is similar to my amp. It's also a Tweed Princeton clone, but it's truer to the the original design. The Fender Amp Field Guide has descriptions, pictures, schematics and layouts for Princetons and other older Fender amps. For something slightly different, Runoffgroove.com has a Tweed Princeton Pedal. It's nearly the same circuit but with JFETs in place of the 12AX7 and 6V6.
Feel free to contact me if you have any questions or comments. My other DIY amp page can be found here