Fender Princeton from RCA tube radio

The story of the RCA 641D radio I turned into a Fender Deluxe guitar amp was quite funny. I bought it on a FB market as a cheap radio to scavenge some tubes and parts for my amp project. During a conversation, it came to light that the seller is a guitar player and songwriter who even runs his own recording studio. What’s more, he had a twin brother to this radio he bought some time ago to be transformed into a guitar amp. I was happy to offer my service. 

When I built the amp from the first one and handed it over to the seller, I realized that I'd like to build another one for myself from the second radio.

Please welcome the RCA 641D Fender Princeton amplifier, the twin brother of RCA 641D Deluxe.

It's a fantastic amp with versatile tone and volume controls, and it can provide up to 15W of power to the external 8-ohm speaker. In the video, my friend Dmitriy is trying it out.

Chassis and layout

Keeping a point-to-point design, I rearranged the tube and pot positions. I installed brand-new pots instead of the original double pots with independent shafts, which are hard to find if needed. After I had a severe parasitic oscillation issue, I moved the second stage to the phase inverter tube in the V2 position. With the power transformer on the far left side of the chassis, I put the Master Volume first so it is less prone to hum. Following is a Gain pot in a middle position. The most right is a tone, which is attached to the dial so the dial pointer is moving across the scale when control the tone.

The amp is built following the classic 6G2 circuit with no Tremolo. Half of 12AX7 in the first position works a preamp. The V2 12AX7 is a second stage pre-amp/driver and a cathodyne phase inverter. Two 6V6GT with a fixed bias form an output stage with a Hammond 1750E Output Transformer. Original power transformer supplies enough power for this kind of amp. As there is no dedicated 5V AC rectifier heater winding, I'm using a solid state rectifier.

Internal AlNiCo speaker

In the first amp, I kept the built-in speaker just to preserve the radio's appearance and didn't expect much from it. To my surprise, I got feedback from the client that it sounds way better than any other speakers he tried. An L-pad 1/4 attenuator saves this little speaker from excessive power. A 300uF capacitor forms a high-pass filter to reduce any "farting" sounds. The output jack has a switch that sends the signal to the internal speaker when an external one is not connected.

Tone stack and Marshall gain booster

What I’ve learned so far working on the Princeton/Deluxe schematic is that a tone stack positioned after the first stage—along with a volume control—can eat a lot of gain. This makes it quite difficult to overdrive the second tube.
The signal loss becomes even more significant if you use a 2-band EQ instead of a single tone pot, and it's even worse with a 3-band TMB (Treble, Middle, Bass) tone stack. Some builders recommend adding a "raw" switch to bypass the stack and preserve the signal.
I chose to stick with a single tone pot. To provide a gain boost, I added a switch that engages a 0.68μF capacitor across the second tube's cathode resistor. I call this the "Tweedy-Plexi" switch, as it reflects the tonal differences between Fender and Marshall designs.
Overall the amp looks fabulous and sounds awesome. Watch out for those golden stickers lol!