I finally have the critical parts, to move ahead a bit more with my idea on using the Leslie/Triad 100-12 output transformers.
These transformers have been a bit of a pickle- they have great specs, but they're made for SIXTEEN ohm speakers only. And they lose a LOT of power when loaded super-hard, into lower speaker impedance.
Well- I took a cue from some really high-end amps (VAC Statement, etc) and decided to do something about it. Instead of one transformer (with one pair of output tubes) per channel- TWO output transformers (each with a pair of output tubes) per channel, with the secondaries wired in parallel- voila, EIGHT ohm output.
The first problem, naturally, is packaging. That's a LOT of transformers and tubes to fit onto a chassis. I think I came up with something that will work well (17"w x 8"d x 2"h chassis):
Two output transformers and four EL84s per side, two 12AU7 per side for pre-amp, driver and phase inverter, and two 12AX7s for phono stage. That's fourteen tubes!
According to the sims (and I've done a LOT of them), it should easily be able to do 30w/ch- maybe as high as 35 or 40 watts per channel. And by the measurements that have been done on these transformers- it should be able to do this from 20Hz to above 50KHz, EASY.
I found a couple of chassis, that were made for the "Trainwreck clone" guitar amp, that are exactly the right size, and built like a tank. Unless I find something better soon, that's what I probably will be going with.
Now, I just gotta get a suitable power transformer. Currently, I'm looking at an Antek toroid, with 230VAC secondaries. That should give me right at 300v on the plates and screens.
The circuit will likely wind up being my modified Pilot AA902 layout that I've used so many times before, with a Dynaco phono stage. Should be plenty of gain- in fact, I may wind up having to run things at minimal gain on the preamp and driver stage, to keep the gain from getting too high. But, that's a better problem than not having ENOUGH gain, naturally...
Regards,
Gordon.
These transformers have been a bit of a pickle- they have great specs, but they're made for SIXTEEN ohm speakers only. And they lose a LOT of power when loaded super-hard, into lower speaker impedance.
Well- I took a cue from some really high-end amps (VAC Statement, etc) and decided to do something about it. Instead of one transformer (with one pair of output tubes) per channel- TWO output transformers (each with a pair of output tubes) per channel, with the secondaries wired in parallel- voila, EIGHT ohm output.
The first problem, naturally, is packaging. That's a LOT of transformers and tubes to fit onto a chassis. I think I came up with something that will work well (17"w x 8"d x 2"h chassis):
Two output transformers and four EL84s per side, two 12AU7 per side for pre-amp, driver and phase inverter, and two 12AX7s for phono stage. That's fourteen tubes!
According to the sims (and I've done a LOT of them), it should easily be able to do 30w/ch- maybe as high as 35 or 40 watts per channel. And by the measurements that have been done on these transformers- it should be able to do this from 20Hz to above 50KHz, EASY.
I found a couple of chassis, that were made for the "Trainwreck clone" guitar amp, that are exactly the right size, and built like a tank. Unless I find something better soon, that's what I probably will be going with.
Now, I just gotta get a suitable power transformer. Currently, I'm looking at an Antek toroid, with 230VAC secondaries. That should give me right at 300v on the plates and screens.
The circuit will likely wind up being my modified Pilot AA902 layout that I've used so many times before, with a Dynaco phono stage. Should be plenty of gain- in fact, I may wind up having to run things at minimal gain on the preamp and driver stage, to keep the gain from getting too high. But, that's a better problem than not having ENOUGH gain, naturally...
Regards,
Gordon.