AF Amplifier Modification for the Drake R-4A Receiver.
I have long wanted to replace the vacuum tubes in my Drakes. This is my first finished module.
It replaces the very hot 6EH5 amp tube.
Goals for the project:
1. Any thing that is not un-pluggable must be very easily removed at a later date with no trace.
2. Should allow quick replacement of the vacuum tube to allow performance comparisons.
3. Performance should be at least as good as the vacuum tube, does not necessarily have to be better. (meaning power out, distortion, frequency response, etc., of course the heat and power consumption should be better).
4. Cost should be reasonable.
I was able to find the perfect amp module on ebay from an overseas source. It uses a TDA2030 integrated circuit. I was able to mount it on a fabricated tube socket that plugs right in. Of course you could just solder it into place if you desire.
I had to add two wires to the R-4A/R-4B. One runs to the Phone jack, allowing headphones to disconnect the speaker and work as original. The second is a ground connection which is soldered to one of the tube socket ground lugs. Both of these added wires are passed from below chassis to the top using the hole in the center of the V7 tube socket. The wires are routed through the gap between pins 1 and 7 and up to the top of the module where they are attached to the modules screw terminals. Wires passing between the plug and socket means that the socket can not be fully pushed down, but is a minor annoyance.
I don’t have lab equipment to fully test the result but sounds identical to stock to my ears. The Drake output transformer is left in place to drive the anti-vox and receive the sidetone from the T-4X. The hum when using sensitive headphones is still there, just like R4’s always have. I use a cheap gaming headset that has a volume control built into the cable, by turning it down a little, the hum is eliminated but there is still plenty of volume available.
In all, I am quite happy with the result. The cabinet runs MUCH cooler for sure. This is much cheaper than buying a replacement vacuum tube too.
Originally Posted by Gary A. Winblad (WB6OGD)
Last Updated : 27th August 2024