interstage transformers.

[Ned Sheats]

No fool like an old fool. I recently purchased two THordarsn 20A25 Interstage transformers for a all triode PPP amp I am building. When I received the xfmrs I immediately hooked the primary to my 50 ohm Leader generator and the secondary to my scope with an appropriate 10K load resistor in parallel with the probe. I was amazed to find the freq response was from nearly 20 to over 20KHz.. So I built a nice Push pull citcuit only t o find that with or without DC through the primaries or secondaries the best I can do is 400 to almost 10Hz. Can someone clue me in as to what I have forgotten about transformers in 53 years and also let me know if there are any transfromers around that will meet my specs

 

[Palustris]

The Thordarson 20A25 is a good transformer that I used many years ago for a PP 6B4G amplifier. It is parallel feed and that may be your problem. BTW: I still use this amp every day in my mono system using a Tannoy 12" speaker. Make sure the parallel feed capacitor is a high quality poly cap.

 

[Tom Bavis]

What's the driving stage? The low frequency end needs a low-impedance driving stage, The high end may be limited by the Miller capacitance (and strays) of the outputs - and a low driving impedance will help here too. A resistor across the secondary may help, hard to say without measuring the transformer. BTW, the 20A25 is specified as - 5 dB at 40 Hz, though driving impedance isn't given.

 

[BinaryMike]

Tom's right; the source impedance seen by the primary is a big deal. You say the amp design is all-triode, which ought to yield relatively low driving Z, but this issue should examined in more detail. You could confirm the cause of the problem by driving the primary from your audio generator again, but with its source Z built out to the output Z of the stage that drives the transformer primary in situ.

 

[Palustris]

The specification sheet for this transformer shows that it is down 3dB at 40Hz in parallel feed and much more with DC in the primary. The implementation I show above is based upon a Thordarson application note.

 

[Ned Sheats]

There are at least three items I had not considered here since this is my first time with transformer coupled stages. THanks a bunch, I noticed all the coupling caps are very large. Is this due to the inherit low impedence of the transformers. The transformer i s being capacitor driven by a 12AU7 driving a 6S4 driving PPP 6SA7

 

[Palustris]

The .47uF coupling caps to the output tubes could have been smaller; the -3dB point for the .47uF is 1.5Hz. A .1uF cap would have been fine at 7Hz. The parafeed cap needs to be large because, as you point out, the impedance is lower and should be as high quality as the coupling caps. The 12AU7 should be perfect for the transformer since it was intended to be used with the 56/76 which are the progenitor of the 6J5/6SN7 which is the progenitor of the 12AU7. I think the 6S4 is the perfect tube to drive the 6SA7 and I used it to drive PPP EL34.

Note the 10k resistors to ground on the secondary side of the IT. I installed those after seeing the IT used that way in a Thordarson schematic. I built the amp a decade ago so don't remember specifics, so you might try testing with them installed or install a pot instead and see what difference loading the transformer makes. Also note that Thordarson shows the plate resistor for the driver tube to be in parallel with the output transformer. I don't remember if I tried that.