Telefunken 512WL radio renovation

by mrgoodsound

In December, I took my first real vacation from work in 5 years and this was my project: a Telefunken 512WL radio from 1935.



The radio came to me in an operational state so I wouldn't call this a restoration or repair. It was more of a 'renovation'.

This is a very simple three tube radio in a fairly compact form factor. It sports an AF7 driver, RES964 output pentode and RGN1064 kenotron. My initial goal for the radio was to serve as a control path which I could use to evaluate the sound of various passive components by placing them in-between the plate of the AF7 and grid of the RES964. Due to the high input impedance of the RES964 grid, various components can be installed in this location to have their sound colouring evaluated prior to installation in another circuit.

I removed the RF section and simplified the AF section as much as possible. This mainly involved putting the AF7 into triode operation with an anode resistor instead of the native choke. The choke will be recycled in another project. Further, the AF7 is operating with an 'open' grid (no grid leak or stopper resistor) and therefore receives much less than the 140v anode voltage indicated on the schematic. The anode resistor value was chosen by ear to provide sufficient gain without distortion. I settled on 100kOhm, receiving a plate voltage of around 40-45 VDC.

All the remaining resistors were tested for value and in the case of capacitors, leakage. Thankfully, no modern substitutes had to be used. A photo of the original circuit is below. Only very high quality Siemens manufactured parts.



And after 'simplification':



In the original schematic volume control was done capacitively in the RF section before the grid of the AF7. Since I have installed a RCA line input which I will only use with digital sources that already have volume controls, a fixed 1Meg resistor was placed between the grid of RES964 and ground. The signal (again, from digital sources with 0 DC offset) is fed directly into the grid of AF7. The chassis has a service stamp from 1941, but I could not identify what was replaced other than the output tube, which is a later ST shape. I have purchased a globe mesh RES964 for the sake of completeness. Though only made a few years earlier, it provided an even clearer and more expressive sound.



Despite its bashful appearance (the front glass dial broke in shipping and I decided to just do without it), the final sound exceeded all expectations. It sports a 7" electromagnet speaker which is probably the best speaker I have in house. It simply sounds more interesting than the various 10-12" Telefunkens I have from the late thirties. The speaker surround has suffered a number of tears which cause it to wheeze and rustle when excited at its resonance frequency, but I decided to just leave it as is rather than risk glueing and worsening the sound.



I have long been moving towards serious monophonic reproduction at home, though in the back of my mind I was sure the 'special effects' of stereo were still necessary for modern recordings. Now I am not so sure, and may move to listening in mono full-time after further experiments.



Two sound demos are attached to get a taste of the sound. I draw the listener's attention to the striking spatial impression and depth in Cash's voice, as well as the comfortable sharpness of Luther Perkin's Telecaster, the edge of which is somewhat blunted on YouTube compared to reality. The sound is rich in overtones with an infectious bass drive. I have listened to the radio continuously for many hours without getting tired. This is a new level of sound quality for me, and I am eager to repeat the results in a larger system.