I have been messing around with a hybrid amp using a transformer to couple the SS output stage to the tube section. Since I don't have a real interstage tranny, I used a very small tranny salvaged from a Muzak pa amp. It has three identical windings that was used to convert a single ended VAS to a push pull output stage using all npn type bjts. It was arranged in parafeed on the VAS side driving the emitter followers directly from the two secondary windings. The amp of course was rated 20-20khz. The problem using it with a tube for a voltage gain device was of course it's very small inductance in comparison to a standard interstage used with tubes. Measuring each winding using the series resonance method yeilded a measly 136mH. This is quite fine using bjts, but, the high plate resistances of tubes shoves the lower cutoff frequency a bit above 1khz. Unusable in a full range amp of course. Using it directly as the plate load directly is also out due to saturation of its tiny 1x1x1 core size, or is it. Now the intersting bit was the use of the 3 separate windings. First using the winding originally used in parafeed in the VAS to feed the output stage. Next I used the two windings that originally drove the output stage ,wired so that the dc current cancelled out with one winding as the plate load and the other as a cathode load. Now of course this makes the tube have only unity gain, expected. What wasn't expected was the now quite good low frequency response. It nearly matches my EL84 SE amp with proper gapped transformers. The tube I am using to drive it is a triode strapped 6aq5. Attached is the test circuit I am using this little transformer in. Disregard the two separate transformers since I don't have a model for a single with three separate windings. Also R8 and R14 depict the dcr of the windings.
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If you can stand to have an intersstage setup as a step down splitter, then mains toroidals in parafeed can work. I have used them as 1:1+1 but the capacitances created top end roll off. Since the front end is valve you may have gain to step down.
Unfortunately if you cant splash the cash then interstage transformers will ruin your amp. A LTP front end would do better.
Shoog
Unfortunately if you cant splash the cash then interstage transformers will ruin your amp. A LTP front end would do better.
Shoog
I didn't intend as a way to split or anything, more of a way to side step the need for the primary to have really high inductance to achieve a decent low frequency rolloff. A good interstage would say have 100H to achieve a good 20hz bottom end. Perhaps this way you could get away with only needing a fraction of that.
Point taken Shoog. This wonderful hobby is full of compromises, time, money, available parts, the list goes on. Plenty of people looking to adapt what they can into something they like and enjoy without going broke doing it. My case being just that, since the bills have to be paid I have very little to spend so I make and adapt what I can get my hands on, most with quite enjoyable results. Over the next couple of days I'll drop the SS output stage and add another 6aq5 triode strapped for an output section (curious to see if can be driven into A2 this way) and add a 12ax7 gain stage up front to see how this would sound in an all tube setup.
I am not trying to put a downer on your efforts. I admire the attempt to create great high end audio on a budget - its what has motivated me from the beginning. I have tried many things in my time which others would frown on. I have done a lot of research into using power toroidals as cheap valve transformers, and generally to great effect. The one area where they have failed to perform is as interstage transformers. They worked and sounded good, but I new the underlying cost was to great. Hence I can tell you that cutting corners on the most demanding transformer role will produce disapointing results.
Shoog
Shoog
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