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Uses for the Bogen T-725 Audio Transformer

The Bogen T725 Transformer

Have we found a gold mine here?

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Hi folks. You crystal set fans will recognize this little part pictured above right away. It is a Bogen T725 audio matching transformer. It's original purpose is a matching transformer for PA system multi-speaker arrangements. Each speaker gets its own transformer. The power that the speaker receives is determined by the selection of the primary wires. You add up all the power levels and that is supposed to come close to matching the amplifiers output power. I have been selling these transformers for decades where I work.

The builders of quality crystal radios have found them useful as a matching auto transformer between the diode output and headphones. They are reasonably priced and readily available. What a deal!

The other night while I was sitting around, between projects and wondering if my house really needed cleaning, I had a brainstorm (well perhaps more of a drizzle). Why can't these transformers be used in tube circuits? Before continuing further, I am disclosing that I have not tried all of the ideas I am presenting here. Please test before drilling the holes in that nice new chassis! If you have other ideas, please let me know. I would like to add them to this page.

These transformers are available from a variety of places.

Note: Some of the applications shown below may or may not work so well. In inspecting the transformer, I found that the latest ones are wound on a laminated core. This means that this transformer might stand moderate dc bias that a class A audio stage would exhibit in the plate circuit. I never took apart the older T725 transformers so I am not sure of the core that it uses. But it depends on if there is an air gap on the transformer sections. This may not be. If not, then the core will saturate fairly easy.

Comments? More ideas? Did I mess something up? Let me know by e-mailing me. Thanks.

The Transformer

Bogen T725 Schematic

Bogen Specs. All values are relative to the black (blk) tap

     Color      Resistance     Inductance       XL @ 300 hz      Rounded Value
   White  (WH)  1424.3 ohms      24     H       45.239k  ohms      40k    ohms 
   Gray   (GRY)  886.4 ohms      12.04  H       22.694k  ohms      20k    ohms
   Violet (VIO)  516.5 ohms       6.06  H       11.423k  ohms      10k    ohms   
   Blue   (BLU)  260.1 ohms       3.04  H        5.730k  ohms       5k    ohms  
   Green  (GRN)   81.8 ohms       1.565 H        2.950k  ohms       2.5k  ohms    
   Yellow (YEL)   56   ohms         787 mH       1.483k  ohms       1.2k  ohms
   Orange (OR)    38.2 ohms         398 mH         750.2 ohms         600 ohms
   Red    (RED)   26   ohms         197 mH         371.3 ohms         300 ohms
   Brown  (BRN)   18.2 ohms          98 mH         184.7 ohms         150 ohms

   Pink to Pink    0.5 ohms        5.23 mH          9.86 ohms           8 ohms 

Transformer closeup showing two pink wires

All impedances calculated at 300Hz. This seems to be the accepted lower 3dB frequency. Notice the effect on the brown - yellow taps when the pink secondary is added in series to the main winding. This could prove helpful towards achieving the exact match to your headphones.

Ramon Vargas of Lima Peru has kindly measured and calculated the specs of the Bogen T725. I am sure this information will be helpful to you has it has to me. For a more in depth discussion on the Bogen T725, please download this doc file (134k), also written by Ramon.

There is a page devoted to using the Bogen for crystal radio audio matching,. This page will be devoted to other uses of the Bogen.

In the old days a 1:3 transformer refereed to the ratio of the windings, not the impedance. Now days we talk about impedance ratios. To figure this, the impedance ratio is the turns ratio squared. The Bogen wired with the black wire as the common, the blue wire (5000 ohms) as the input and the white wire (40000 ohms) as the output is an 8 to 1 impedance ratio, or a 1:2.82 turns ratio. Pretty close to 1:3 and a good plate match for many tubes too. If you connect the green wire (2500 ohms) and the white, the turns ratio is 1:4.

Audio Output Transformer for Single Ended Tube Amplifiers

Single Ended Output With A Bogen T-725

This is a circuit that came to my mind first. This would be good for a few watts at best. I am guessing a 6AQ5, 6AK6, 50C5, 3V4 and such tubes could be used with the Bogen. The tap I selected is only an example. Use the tube data to pick the tap closest to what you need. I would keep also want to keep the plate voltage below 200 volts too. There are no specs from the windings to ground on these.

Push Pull Output

Push-Pull With A Bogen T-725 Transformer

If you are building a small radio with a push-pull output or repairing same, this might get you by. To be honest with you, I haven't tried this yet. If you do, let me know how it sounds. You don't get much of a choice of an impedance, but it might help until you find the right transformer.

Using The Bogen As An Audio Choke

Using The Bogen T-725 As An Audio Choke.

This is kind of like the circuit above. If you don't want to run the B+ through your headphones, this is an excellent way to couple. I measured nearly 10 henries most of the way up the transformer, before the meter ran out of range. Another use is a B+ supply choke if you need to highly filter a very low current (a few milliamps).

Using The Bogen As An Interstage Auto-Transformer

Using The Bogen T725 As An Interstage Auto-Transformer

I saw this in a 1926 radio circuits book recently. It isn't a true interstage transformer as there aren't two independent windings. But it should work as a 1:? transformer. The biasing can be done at the bottom of the grid resistor.

Using The Bogen T725 As An Interstage Auto-Transformer, Version 2

This variation was submitted by my friend, Fred Wise in Glen Burnie, MD. This would solve the problem of coupling to high voltage circuits if the transformer voltage rating seemed to be a problem. Also you can pick your own plate load resistor. The grid bias can be applied at the bottom of the transformer (black wire).