03-02-2017, 05:06 PM
Yes Terry, that would work but there are a few problems.
A triode connected 6F6 as a driver draws about 25-30 mA of plate current. Through the 5K, that would give a 125 V drop, so you now have only about 125 V left on the plate, limiting your swing. A better choice would be an audio choke, which would pass the full B+voltage and allow full output.
In fact John could use a resistor or choke to feed B+ to the 6F6 driver plate and bypass the open primary of his bad transformer, and then use the cap from the plate to feed signal to one side of the PP secondary. This would effectively give you a 1:1 ratio to the grids of the output 6F6's.
In any case, you really want a step down ratio to get full output from the PP 6F6's. As soon as your peak drive signal is greater than the negative grid bias, the output 6F6 grids begin to go positive and draw current in class B mode. The whole purpose of using a big tube like a 6F6 as a driver is that it can supply the current necessary to drive the grids positive. With a step down driver transformer, it is easy to supply the low impedance current drive necessary for the transition from class A to class B without distortion. The little 5W driver trans could not do this and your output would be limited to class A only.
Why Stromberg Carlson used a step up ratio is a mystery as it makes no sense. Maybe they already had this transformer design from another model, but it certainly would not be the best choice for low distortion and max power output from triode connected class AB 6F6's.
A triode connected 6F6 as a driver draws about 25-30 mA of plate current. Through the 5K, that would give a 125 V drop, so you now have only about 125 V left on the plate, limiting your swing. A better choice would be an audio choke, which would pass the full B+voltage and allow full output.
In fact John could use a resistor or choke to feed B+ to the 6F6 driver plate and bypass the open primary of his bad transformer, and then use the cap from the plate to feed signal to one side of the PP secondary. This would effectively give you a 1:1 ratio to the grids of the output 6F6's.
In any case, you really want a step down ratio to get full output from the PP 6F6's. As soon as your peak drive signal is greater than the negative grid bias, the output 6F6 grids begin to go positive and draw current in class B mode. The whole purpose of using a big tube like a 6F6 as a driver is that it can supply the current necessary to drive the grids positive. With a step down driver transformer, it is easy to supply the low impedance current drive necessary for the transition from class A to class B without distortion. The little 5W driver trans could not do this and your output would be limited to class A only.
Why Stromberg Carlson used a step up ratio is a mystery as it makes no sense. Maybe they already had this transformer design from another model, but it certainly would not be the best choice for low distortion and max power output from triode connected class AB 6F6's.
