RCA Victor 641 into guitar tube amp

This beautiful RCA 641D radio was given to me to turn into a tube guitar amp. It had three knobs and enough space for a full-size amp.  I based the build on Fender Deluxe 5E3 circuit with a slight modification of a tone control section. I also left internal speaker connected, so you can opt for this one or external 8 Ohm speaker through a standard 1/4 jack.  With a 4:1 attenuator the small one sounds surprisingly good. Magic eye tube was replaced with a pilot LED lamp. 

As a push-pull on two 6V6 tubes the amp can generate about 10W of power. Additional master volume pot was installed on a phase inverter stage to get a famous "Fender" tone on a lower volume. 

Finally, I did my best to preserve a radio appearance, kept the dial indicator, dial capacitor, and some coils on the chassis. 

Chassis preparation

To start with I removed everything from the chassis and gave it a good cleanup. On this particular one I used Dremel abrasive buffs. Now I prefer mini metal brush, it works faster an provides cleaner results.

To unsolder massive element Propane Torch Kit helps a lot. I don't have powerful soldering iron, propane works just fine. Power transformer turned out dead, I had to find a similar one. Once cassis is ready I'm collecting all components to lay them out on the chassis.

Laying out the chassis

This is my fourth project so I learned the importance of proper chassis layout. All the components must be accessible for measurement and adjustment. Ground bus should be reachable and connected to the chassis in one point, this will prevent ground loops. For those components connected in series there must be terminal lugs for join points. 

Once the design is done you can start populating chassis with parts. I always start with the power section and move towards the input. 

Dial driver and control knobs

The original radio had doubled knobs with inner and outer shafts working separately. The decision was made to preserve original setup. The issue though is that only most left position had a double potentiometer, another two positions were switches as inner and dial drivers as outer shaft. Original pot was cleaned with WD-40 electric contact cleaner and tested ok. The radio had two dial drives - a common one in a middle position running a capacitor, and fine tunning drive in the right position driving a coil. I kept the common one just to save original radio appearance. The second one was re-wired to master volume pot, so the fine tune knob can adjust MV.

Master volume pot was installed on a small plate, equipped with a brass wheel, and corded to the shaft. Works as a charm!

Final assembles and test

Tone control

Fender Deluxe in 5E3 model has somewhat unusual volume and tone control. Volume pot is connected in reverse direction. When I followed Fender's design I got a very strong hum, particularly annoying at small or no signal. I think my issue is that my only pot was placed right below the power transformer and far from 12AX7 preamp section. Even though I got it connected with shielded cables, it collects too much hum. After trying several things, I just re-designed the tone section to the one Terry from D-LAB Electronics uses in a lot of his amps. What is depicted below are original Deluxe tone section and D-LAB's one (P.S. this tone section turned out to be from a Princeton 5E2)

       

Another challenge was having just a single channel, with no "bright" & "raw" section like in original Fender amp. I was looking for some brightness so I put a "bright" cap in series with a resistor across the volume pot. It doesn't give that much bright tone, but at least compensate a decrease in trebles on the low volume due to Miller effect.

Summary

The amp turns to be a beautiful thing and made a new owner happy! I spent about $150 to the parts, half of which is basically an output transformer.