After butchering the poor old Telefunken reel-to-reel I wanted to build a Deacy-style using the salvaged transistors and transformers and give it a new life. For the most part, I used Mullard’s amp schematic from below and tweaked few component values.
Comparing the Mullard schematic to various Deacy schematics that are floating around, I made the following changes to the Mullard amp:
- Instead of 39 ohm resistor that biases the output stage, I used a 56 ohm resistor in parallel with the 350 ohm thermistor that was originally mounted on the Telefunken transformer. It will provide temperature compensation and make the amp run more stable.
- Instead of two 10uF capacitors in the signal path I used 1uF cap at the input and 2.2uF poly film cap to couple the first and the second stage.
- Omitted the 390K input resistor and input volume pot.
- Added a 2.2K resistor downstream the coupling capacitor.
- Omitted input volume pot.
I’ve had a cool looking, compact tag board for years and couldn’t find a good use for it, as it’s often too small for tube projects. But it turned out to be perfect size for this project. All components fit on the board (except for the output transistors that are mounted on the transformer) and it fits perfectly inside the aluminum project box I got from Radio Shack years ago. To make it possible to try other transistors, I used individual IC socket pins soldered on the board.
Fast forward to the completed board. I didn’t have a 220nF capacitor for the output transformer zobel network, so I replaced it with two Panasonic 100nF capacitors in parallel. I did not have any axial electrolytic capacitors, so I faked them using radial capacitors mounted horizontally.