Telefunken 512WL radio renovation

Discussion in 'Speakers' started by k4rstar, Jan 15, 2023.

  1. k4rstar

    k4rstar Britney fan club president

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

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    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.

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    And after 'simplification':

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    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.

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    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.

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    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.

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    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.



     
  2. k4rstar

    k4rstar Britney fan club president

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    [​IMG]

    Half a year has passed since I rebuilt this radio. It was initially supposed to be a test bed; the theory being that the very high input impedance of the RES964 output tube would allow placing various passive components between the AF7 plate and RES964 grid and checking how they affected the sound before installing them in another circuit.

    That never happened because I became so enamoured with the sound of the radio that I dropped all of my other projects, and interest in audio advances in general. It's difficult to convey but there is something hypnotic about the sound. The recording linked below I took shortly after finishing the project seems to do the best job of conveying that. A lot is lost over YouTube but it's interesting how you can sort of hear what's going on even through an iPhone mic, compression and many web servers.

    There's nothing hi-fi about it, but surprisingly I am mostly satisfied listening to modern recordings on it. It's far from perfect, the speaker still wheezes when excited near its resonant frequency, but I can listen to Ladytron and Goldfrapp without much complaint. The AF7 socket has a loose connection which causes some intermittent volume problems but usually some percussive maintenance solves it.

    I purchased some very nice drivers, tubes and transformers for my stereo system but they've sat unused for months. At this rate I think I will sell them and convert to mono full-time. The minimalist in me feels I lost very little yet gained a lot.

    I think more people should try playing around with mono systems (once you've gotten some stereo experience). To be honest most normal people (now myself included) are perfectly satisfied listening to tiny single-channel Bluetooth speakers without thinking too hard about it. This is like an esoteric vintage nerd version of that. The experience has been enlightening and healing. Over the past 6 months my relationship with music has changed, not unlike recovering from a junk food addiction and learning to appreciate normal food again.

    Unfortunately most conventional speaker designs do not translate well to single channel listening. I wouldn't be able to elaborate on the physics of it but it seems bass reflex cabinets suffer a lot when you play with just one, the sound gets 'stuck' to the box and doesn't load the room satisfactorily. Open baffle, open-backed cabinets or horn-loading is recommended for best mono results. Obviously in the 1930s they had it figured out.

     
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    Last edited: Jul 11, 2023
  3. caute

    caute Lana Del Gayer than you

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    ur very own goebbels radio lol
     
  4. JK47

    JK47 Friend

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  5. k4rstar

    k4rstar Britney fan club president

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    My attempt at a mono system in a larger form factor

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    An open backed cabinet loaded with a single 10" Makrophon speaker from 1939-ish. It makes enough bass and treble by itself.

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    The amplifier is a SE 2A3 driven by a 24a tetrode. Two kenotrons are used to supply separate half-wave rectification to the driver and output tube.

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    The sound is very good but not better than the Telefunken radio. More fine tuning required
     
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