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Bush AC91 Radio and Micro AM Transmitter.

April 19, 2013

Bush AC91, 1947
This 1947 vintage radio resides above my workbench and I use it to listen to BBC radio re-transmitted from the internet using a two-tube AM micro-transmitter designed by Fred Nachbaur (deceased) that he named “The Bean Counter”. When the picture above was taken, it was playing BBC Radio 4 Extra from the Bean Counter.
I had this radio as a kid and around 1998 on a trip back to England (from Greensboro, NC, USA) discovered it in my Dad’s shed. It was almost complete except for the two grid caps but the case was scratched and dull and the works were filthy. Thanks to Ray at The Radio Workshop ( http://www.radio-workshop.co.uk ) for providing me with replacement grid caps that contain the correct 100k and 220R grid resistors. The speaker cone was torn and the pole piece was rusty so most of the work went into repairing the loudspeaker. I did this many years ago and no longer know where the pictures are. The repair entailed carefully separating the cone from the surround and frame. I then repaired the tear using toilet paper and paper paste on the back. This worked quite well! I then cleaned up the pole-piece and put it back together. Much time was then spent fiddling with both the diaphragm centering and the pole-piece centering to get it to speak clearly. It is necessary to ensure clarity at both low and high volumes. At low volume, if there is rubbing, it will suddenly break into action as the volume is raised while at high volume, the off-centre motion I noticed (probably due to non axi-symmetric stiffness resulting from the tear repair) can cause rubbing that is not present at low volumes. I diagnosed this movement by carefully touching around the cone at higher volumes when distortion was audible and I noticed that touching on just one region stopped the distortion. It was a tricky and time consuming process however, I was rewarded with success and the radio now plays clearly at all volumes.
Here is the schematic:
Bush AC91 Schematic
The difference between the AC and DAC models is that the AC model uses a auto-transformer rather than a dropping resistor for the heater chain. The advantage of the DAC model in the era of the radio was that it could run from AC or DC. The advantage of the auto-transformer model is much lower heat dissipation. The back is quite often scorched or missing from the DAC models!
Here is the back:1947 Bush AC91 Back
Here are the beautiful Mullard metalised valves (such splendid devices prefer to be called valves, I think):1947 Bush AC91 Inside
While performing IF alignment, I noticed that the gain would change a lot when I tapped on the chassis. After some fiddling, I isolated this issue to one IF slug and sighing a bit, removed and disassembled the faulty can. Sure enough once the offending screw-adjuster was removed from the transformer, bits of broken ferrite fell out. The slug had been molded around the end of the screw and it had crumbled apart leaving the actual slug to slide around inside the coil former. Here it is:Bush AC91 IF Transformer, Broken Slug
I had some heat-shrink tube that just barely would fit over the slug and knowing that the tube will shrink up to 3:1, I repaired it like this:
Bush AC91 IF, Repaired Slug
The increased diameter just fit into the former and so I had my repair and was free to complete the IF alignment.

The design of “The Bean Counter” AM Micro-transmitter” is published at http://www.dogstar.dantimax.dk/tubestuf/amtx-2.htm
Read on before you click the link though because the information that Fred gave on the tank coil isn’t complete.
While Fred is deceased, I believe that somebody is maintaining his work on the internet, whoever you are, THANK YOU!
Here is my version:
Nachbaur 2 tube AM transmitter 1
Nachbaur 2 tube AM transmitter
The unit is built on a RadioShack box/chassis and I used a Hammond P-T442 power transformer.
I found Fred’s description of the 350µH tank coil a bit puzzling so I searched for an on-line air-cored inductor design tool and found Martin Meserve’s page at: http://www.k7mem.com/Electronic_Notebook/inductors/coildsgn.html and using his program designed a 350µH tank. I used a standard prescription drug container having an outside diameter of approximately 1.27″ (it is tapered) as a former and was just able to get 156 turns of 28SWG enamelled wire onto the 2.3″ length. Using my GR 1650A impedance bridge, it measured at 340µH, close enough. Here is my coil winder (that I built years ago to wind audio output transformers) with the tank coil in place:
Coil Winder
Ex RAE mechanical apprentices may recognise the live centre!
Fred’s data for the tank coil states 88 #29 turns on 1″ diameter and 15 #20 secondary turns. The issue that puzzled me was that 88 #29 turns on 1″ theoretically results in an inductance of around 100µH. Doing the math, neglecting the 6BE6 plate capacitance and stray capacitances, Fred’s value of 350µH and the tuning capacitance of 50pF gives a resonant frequency of 1.2MHz. This is 20% high which is most likely “corrected” by stray capacitances, also the capacitance between the tank and the secondary coils. This was enough evidence for me to work from Fred’s stated inductance value rather than his winding data.
Here is the resulting tank coil and secondary:
250uH tank and secondary
I used Fred’s winding ratio of 88:15 turns to come up with 26 turns for the secondary which I wound using 20AWG onto a former cut from a Radioshack copper wire reel. The resulting diameter was a nice tight fit into the tank coil. I placed it into the “earthy” end to minimise shunt capacitance at the plate end.
Here is the assembled transformer:
250uH tank and secondary assembled
I put a hole in the base of the container to pull the wires through and mounted the transformer by fitting the lid to the chassis using a screw and simply inserting the coil in the lid, twist-lock fashion. The chassis end of the secondary is connected to the earthy end of the coils (tank, B+ and secondary, ground).
I actually found that with the cover on the unit, the oscillator peaked at 50pF (plus strays including the cover). With regard to the design of the transmitter, I did not deviate much from Fred’s design, the only changes being to
1/ Insert a RF choke I had on hand (450uH) and a 22nF capacitor after the full-wave detector that provides audio feedback to the input tube so that I could see the audio feedback signal clearly on the scope.
2/ Reduce the oscillator / mixer cathode resistor to 100R.
3/ Fit a 10nF cap from the 6AU6 screen grid to ground right at the pin, as I had noted a tendency to “sqeg” (blocking oscillation that breaks in and out as the audio signal modulates the Gm of the tube).
I doubt that the RF detector filter makes any difference to the operation of the unit. Initially, I had 1N277 germanium diodes on hand, Fred recommended 1N34 devices. I obtained some 1N34s and sure enough, the linearity of the feedback detector improved. Displayed on a Tektronix 547 (with 1A4 plug-in) is the signal at the antenna (lower trace) and the resulting clean audio signal at the secondary of the AC91 output transformer:
Output at Antenna and Bush AC91 Speaker.
You may notice that the two traces are displayed at different sweep rates, this is a perhaps unique feature of the type 547, the ability to alternate the two timebases.
I did find it necessary to limit the modulation to 50%.
With regard to the setup, I found that the longer the transmit antenna is the better, this is not surprising given the relatively low carrier frequency of 1MHz. I am using around 12′. At the radio end, a short antenna or even no antenna at all works best, I am using around 18″ into the “selectivity” input on the AC91. All this is fed by a Squeezebox internet radio which has variable output allowing the modulation depth to be varied for the best sound and noise compromise. The result is extremely clear and free from distortion, most satisfying after much frustration especially after discovering that I needed to play with the speaker voicecoil alignment again, it has been around 14 years since I initially restored this lovely post WW2 radio. It is just a shame that the BBC Home Service, Light Programme and Third Program are no longer available……

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11 Comments
  1. Lars permalink

    Impressive work and history.
    Thank you very much for sharing!

    • Thanks Lars.
      I love this stuff however, it is esoteric and can be a bit lonely. It helps to know that other get pleasure from reading my notes.

  2. Bruce Baur permalink

    Glad to see you are still playing with tubes. Nice write up.
    Bruce

  3. Matthew Barton-Hawkins permalink

    Thank you Richard, I was planning on building Fred’s AM transmitter to play stuff through an old Roberts my wife got me as a gift. Sadly here in the UK medium wave and long wave are in decline. Droitwich will soon be turned off for good!!
    I too was puzzled by Frank’s coil. You have done the leg work for me.
    Excellent web site by the way. I’m a bit of a collector of test equipment as well.
    Cheers
    Matt.

  4. Bruce Baur permalink

    FYI, I am working my way through a Bush PB12 when I get time. It is full of leaky wax caps. I have never seen leaky caps in the 300pf range as on this side of the pond we use mica’s not wax in that capacitance range.

    Bruce

    • Hi Bruce:

      This is a reply to your comments on my AC91 post.
      On the low value caps, I don’t think I have ever encountered paper insulation, usually mica, this is true of both US and UK gear. However, I have encountered several mica caps that would no longer support operation of the circuit again, both US and UK. The older UK caps tend to be wax “sealed”, both countries used “Micamolds” and they do become leaky. I don’t know what the leakage mechanism is since I imagine that mica is impervious to H2O molecules.

  5. Excellent Sharing. I will come back often!

  6. It is really a great and useful piece of information. I’m glad that you shared this useful information with us. Please keep us up to date like this. Thank you for sharing.

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