1960s Philips Tube Radio Converted to Guitar Amp

On an interesting tangent from our usual R&D work, I had the chance to pump some life back into an old 1960s tube radio, Gnome-style.

The whole thing started out when the owner of the radio said something like, ‘can you turn this old radio into a super awesome guitar amp and maybe hook up the controls on the front and make the tuning eye do stuff?’ I said yes, and here’s what happened:

Here is a shot of the amp prior to rework:

She’s certainly a pretty thing with the wood box and gold accents.

After finishing my sneeze-fest, I snapped a shot of the inside…

Interesting technical details:

The output stage is in the left portion, with the output transformer mounted on the inside of the chassis. There are radio tubes on either side of the elevated tag strip, which provides some electrostatic shielding between stages. The yellow electrolytic cap in the bottom center is flanked by the external antennae connector…. which had matchsticks shoved into it.

In addition to the fire hazard provided by the matches, the power transformer looked like it could fry a burger faster than a George Foreman.

The transformer got so hot at some point that its black coating bubbled and cracked. You can see lines on the surface where wires had melted in with the coating. Yikes..

After spending a bit of time looking at the output stage wiring and transformer, I discovered that it was a very interesting configuration; EL84 Single-Ended, Ultra-Linear, Cathode Biased. With this stock configuration, the output of the radio would have been around 5W.

Here is the output xfrmr:

You can see the standard primary connections on top (for A+ and anode of EL84), and a third connection on the bottom for the primary ultra-linear tap.

After pulling the transformers I decided to completely gut it and rework it with the following specs:

• Preamp: (1x) 12AX7 using both stages
• EQ: High shunt connected to front panel ‘tone’ control
• Gain: Connected as a shunt to reduce gain of second preamp stage
• Output Stage: (1x) EL84, single-ended, cathode biased, screen connected to B+
• Tuning Eye: (1x) EM81, connected to output of first preamp stage through a blocking cap + diode

My goal for this amp was to create a very direct signal path with no series EQ or Gain controls. This would allow for maximum gain with minimum tubes. Also, I decided to reuse the OT because it was matched appropriately to the stock speakers, which I would also be keeping.

Here is a photo progression from cleaned up chassis to finished product:

I was able to add the IEC mains connector, fuse, and input jack on the back panel and line them up with existing cutouts on the rear panel. Enough talk, give her a listen: