So as a general statement if I design an output stage with a particular XFMR matched to a pushpull pair of tubes, if I then want to double up those tubes to 2 pair of PP otputs to distribute the power does my XFMR now need to change? Like something with half the primary impedance than the original?
That's generally how you would do it if building from scratch and trying to get the right primary Z. If you are modifying an existing amp there are many other design considerations you need to account for. The PS has to be able to accomodate the new load on it or you have to reduce the power draw of all the tubes... etc. Can the OPT's stand more...? Have fun.
So what I looking at is 100W/ch tube amp. My output tubes are KT-90 and will achieve 90-95 W/ch now. However I was just thinking that maybe splitting that same load between 2 pair vs 1 pair that it would be easier on the tube. It is class AB UL design
Do you have the specs for your OT and operating conditions of the KT-90s. One can’t speculate before knowing a bit more.
But first: what do you want to achieve by “splitting the load”? Longer lasting tubes? But now you need twice as many!
But first: what do you want to achieve by “splitting the load”? Longer lasting tubes? But now you need twice as many!
In order to split the load into the same impedance you would need to drop the screen voltage. If you don’t you reduce plate current/dissipation but increase it in the screen - perhaps beyond rating. Relatively easy to do in pentode, not so easy in UL since it would require different taps.
Example: With 1 pair, 50-60 clean Watts should be possible with 500V anode voltage, 50-60 mA per tube (bias should be some -52V to -55V) and the classic 6.6K plate-to-plate. So you just need to drop to 3.3K plate-to-plate with the parallel push-pull and make sure the power supply can deliver 2x the anode current at the same anode voltage.
Personally, I wouldn't do it. The difference between 50-60W and 100-120W is not worth the trouble and cost. It would be different if it were 10W instead of 5W.
If I really wanted 100W I would still use one pair in pentode mode with local feedback instead of UL. Pretty "easy" to do with KT88, so I guess it should be possible with the KT90 too. That would not stress the tubes and I would keep the other pair for replacement.
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