Where to purchase Brook 2a3 interstage transformer

First, it is in fact a center-tapped choke, and not a transformer. B+ is applied to the center tap, and the ends of the choke are connected to the plates of the two 6J5 driver tubes. The choke thus serves in place of the plate resistors that are used in more conventional designs.

I have no idea where to find a replacement, but can you describe what is wrong with the existing one? If the problem is simply a winding that has opened due to corrosion (as opposed to the choke having been burned out or severely damaged as a consequence of some other failure), it might be repairable by zapping it with high voltage. That technique is commonly used on antique radios which have audio transformer windings that have opened due to corrosion.

If you think that might be applicable, post back and I can point you to someone who might be able to do it for you.

Good luck! As you no doubt realize, that amp is a rare treasure.

Regards,
-- Al

Al: I am building a pair of these , but this choke has been the holdup....I know the DC resistance, so I may use a couple resistors to get it going and by then I can get someone to wind me a couple of these chokes.....I could see that it was a choke but I couldn't figure how it worked in that circuit....I have all the other stuff, plus a friend is making the chassis for me.....Thanks for your help on this.........Will

Hi Will,

If you want to temporarily substitute resistors for the choke, the value should be considerably higher than the choke's dc resistance. Otherwise the gain of the stage will be reduced. But it shouldn't be too high, or the dc plate voltage (the quiescent, or zero signal plate voltage) will be reduced significantly, which will also lower the gain.

Essentially the choke allows B+ to be applied to the plates through a minimal impedance (1.8K if the 3.6K you mentioned is the end-to-end resistance of the choke), while presenting a much higher impedance at audio frequencies. That maximizes gain at audio frequencies, without reducing the dc plate voltage to the extent that a resistor would.

Another approach to consider, as a temporary measure, would be to use an audio transformer with a resistor connected across its secondary. You would need one transformer for each of the two 6J5's, having a dc resistance in its primary of 1.8K (assuming the choke is 1.8K from center tap to each end). You could then experiment with various resistor values on the secondary.

Looking at the plate characteristic curves for the 6J5 in my older RCA tube manual, and at the operating voltages indicated on the schematic, my very rough ballpark guess would be 20K or 30K for the plate resistor. If the resistor is on the secondary side of a transformer, factor that by the square of the turns ratio, of course.

Best of luck with your project!

-- Al

Al: Maybe I should dig up one of my cans of money and pay the Magnequest 125.00 each for these transformers and call it good.......You have been quite a lot of help on this ....Thanks very much Will Vincent 1-208-746-9650

Sounds like a plan, assuming that they understand the circuit application and indicate that the parameters of the particular choke will be suitable.

Best of luck with it!
-- Al