Just curious if anyone has built/analyzed anything like the original push-pull series feedback IT circuit.
Doing a bunch of spice modelling with some different pentodes, the surface level analysis looks good, but two troublesome issues stand out:
1) The 0.1uF capacitors in the feedback path cause a huge increase in gain at low frequency as the feedback trails off. If the interstage can pass frequencies below 10 hz, feels like things could be problematic. I had to go up as high as 3.3uF in some cases to kill the hump down in the 3-5 Hz range. Certainly you can get caps that size but it feels a little ugly to have both an interstage and also caps large enough that they probably have to be metallized.
2) I have some bifilar interstages I was thinking about using if I did try to build it, but it looks like this circuit is not going to be ideal for inter-winding capacitance no matter what, as the lowest AC voltage you could put on the wires that are physically at the center of the winding would be the voltage grid-to-grid for the drivers. The voltage swing on the feedback dividers is very large so you'd essentially setup the secondary to 'drive' the feedback path while the grids are the ground side of the windings.
Probably the answer is 'Don't do it, this is not worth it.' Since you have really high drive voltage through the IT, you'd basically have to build a driver setup powerful enough to drive an IT at low distortion and enough voltage for big power triodes anyway. You'll have basically built a monster PP driver that could just drive PP 300B's anyway. In theory the series connection doesn't present much load to the IT, but Rp of the driver is still going to dominate distortion through the IT, plus whatever the parasitics I mentioned above are doing.
Doing a bunch of spice modelling with some different pentodes, the surface level analysis looks good, but two troublesome issues stand out:
1) The 0.1uF capacitors in the feedback path cause a huge increase in gain at low frequency as the feedback trails off. If the interstage can pass frequencies below 10 hz, feels like things could be problematic. I had to go up as high as 3.3uF in some cases to kill the hump down in the 3-5 Hz range. Certainly you can get caps that size but it feels a little ugly to have both an interstage and also caps large enough that they probably have to be metallized.
2) I have some bifilar interstages I was thinking about using if I did try to build it, but it looks like this circuit is not going to be ideal for inter-winding capacitance no matter what, as the lowest AC voltage you could put on the wires that are physically at the center of the winding would be the voltage grid-to-grid for the drivers. The voltage swing on the feedback dividers is very large so you'd essentially setup the secondary to 'drive' the feedback path while the grids are the ground side of the windings.
Probably the answer is 'Don't do it, this is not worth it.' Since you have really high drive voltage through the IT, you'd basically have to build a driver setup powerful enough to drive an IT at low distortion and enough voltage for big power triodes anyway. You'll have basically built a monster PP driver that could just drive PP 300B's anyway. In theory the series connection doesn't present much load to the IT, but Rp of the driver is still going to dominate distortion through the IT, plus whatever the parasitics I mentioned above are doing.
Since C is part of the NFB loop, there will be peaking. Instead of increasing the value of C,
the values of R1 and R2 could be increased by the same factor, keeping their ratio constant.
the values of R1 and R2 could be increased by the same factor, keeping their ratio constant.
Fair enough on the capacitor and resistor divider values. I'm curious about how that will interact with the various parasitics. I need a better model of a PP interstage transformer.
I've had astounding success by doing series feedback to the previous stage cathode. It's so simple and just so good. You can do much more feedback if you want to.
Schade's circuit was interesting for showing some curves in a white paper, but it's just pretty complex for what it does. But it could be fun to build too.
Schade's circuit was interesting for showing some curves in a white paper, but it's just pretty complex for what it does. But it could be fun to build too.
I did it just like OA's paper in an amp with IT's. Worked just fine but I went back to triode connected output tubes and took out the "Schade" feedback. Less power but I liked it.
I have done some tyest without trafos.
In the original circuit the explanation is very general; so who want to built one similar need to have a good test set to check in deep the performance.
With some triode as driver, without OT it seems a bad solution for output stage. Also using different types.
I would like to test with pentode but not sure.
There is a good notes on one Philisp book.
Then with test to maximize the circuit I decided to use.
Using some strange kind of mixture with sand and tube adding three or four type of feedback is not my goal.
One example of what I mean is here:
This is a freq. answer of an amp with 4 x 300B in p-p
With and without FB; 10 dB of FB that I normally use.
Red is without and blue is FB
The BW is...
post nr. 46
""""""""""""""
As Robert Gribnau wrote in another thread:
See for an explanation of this effect pages 396 to 399 of this book:
Fundamentals of Radio Valve-Technique, Deketh, Philips, 1949
"""""""""""""""""""""""""""""
IF in the 80 years nobody of the greatest manifacturer built something similar one reason exist.
In the original circuit the explanation is very general; so who want to built one similar need to have a good test set to check in deep the performance.
With some triode as driver, without OT it seems a bad solution for output stage. Also using different types.
I would like to test with pentode but not sure.
There is a good notes on one Philisp book.
In general, I always search to get the best results using the best components available and, for power amp, the first one is OT
Then with test to maximize the circuit I decided to use.
Using some strange kind of mixture with sand and tube adding three or four type of feedback is not my goal.
One example of what I mean is here:
This is a freq. answer of an amp with 4 x 300B in p-p
With and without FB; 10 dB of FB that I normally use.
Red is without and blue is FB
The BW is...
post nr. 46
""""""""""""""
As Robert Gribnau wrote in another thread:
See for an explanation of this effect pages 396 to 399 of this book:
Fundamentals of Radio Valve-Technique, Deketh, Philips, 1949
"""""""""""""""""""""""""""""
IF in the 80 years nobody of the greatest manifacturer built something similar one reason exist.