I just noticed that the Norman Crowhurst Twin Coupled design works for SE use too!
Doesn't seem to have been noticed before, almost shocking really. So simple, yet quite useful too.
Since 10K Ohm and above SE (or P-P) OTs are hard to get good results with typically, this provides a means of using lower primary Z, off the shelf OTs to get there. (getting 2X the primary Z) No more custom OTs required. The Watt ratings of the two SE OTs add up too. You need 16 Ohm secondaries, to parallel together, to get an 8 Ohm secondary for the load.
To get a 16K Ohm SE OT say, you would just use two 8K Ohm SE OTs, one for the tube cathode circuit and one for the tube plate circuit.
If you don't want the 50% CFB inherent in the N. Crowhurst Twin, you use the cathode circuit to provide a cap coupled bootstrap boost to the driver stage plate load resistor. A HV pentode or Beamer would be best for driver duty, but not essential. A HV triode driver would leave some CFB effect in place and lower the output impedance too (for improved damping factor and bass).
The little 6GF5 TV Sweep tube (9 Watt) is rated up to 770 V B+, but many others will work too for a driver. (6GF5 is on the $1 list at ESRC)
6S4/A triode is rated 550V B+, and 6SN7 triode is rated 450 V.
Schematic below of the usual Push-Pull Norman CrowHurst Twin Coupled design for some reference. SE is similar, but just half of the P-P circuit.
I'll have to get another SE OT to try this out now.
Just thought y'all would want to be in on the BIG SECRET! No need to order custom Hi-Z SE OTs anymore.
With a Hammond 30 Watt SE OT (for example) you can make a 60 Watt SE OT now for those really big projects. Their 75 Watt SE can do 150 Watt SE now!
Don
Doesn't seem to have been noticed before, almost shocking really. So simple, yet quite useful too.
Since 10K Ohm and above SE (or P-P) OTs are hard to get good results with typically, this provides a means of using lower primary Z, off the shelf OTs to get there. (getting 2X the primary Z) No more custom OTs required. The Watt ratings of the two SE OTs add up too. You need 16 Ohm secondaries, to parallel together, to get an 8 Ohm secondary for the load.
To get a 16K Ohm SE OT say, you would just use two 8K Ohm SE OTs, one for the tube cathode circuit and one for the tube plate circuit.
If you don't want the 50% CFB inherent in the N. Crowhurst Twin, you use the cathode circuit to provide a cap coupled bootstrap boost to the driver stage plate load resistor. A HV pentode or Beamer would be best for driver duty, but not essential. A HV triode driver would leave some CFB effect in place and lower the output impedance too (for improved damping factor and bass).
The little 6GF5 TV Sweep tube (9 Watt) is rated up to 770 V B+, but many others will work too for a driver. (6GF5 is on the $1 list at ESRC)
6S4/A triode is rated 550V B+, and 6SN7 triode is rated 450 V.
Schematic below of the usual Push-Pull Norman CrowHurst Twin Coupled design for some reference. SE is similar, but just half of the P-P circuit.
I'll have to get another SE OT to try this out now.
Just thought y'all would want to be in on the BIG SECRET! No need to order custom Hi-Z SE OTs anymore.
With a Hammond 30 Watt SE OT (for example) you can make a 60 Watt SE OT now for those really big projects. Their 75 Watt SE can do 150 Watt SE now!
Don
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