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In Reply to: RE: Purpose of these Caps? posted by Triode_Kingdom on August 27, 2016 at 23:42:32
G.E.C. developed and patented this circuit (patent #487953) as a driver for Class B output stages. They called it a cathode coupled bridged transformer. The capacitors are there to short circuit the transformers leakage inductance which they claim is the main source of distortion and the bad rep. of Class B amps. In the book "An Approach to Audio Frequency Amplifier Design", originally published by GEC in 1957, there are a couple of Class B amp designs and a high power AB2 design that use this circuit.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
Follow Ups:
That's the sort of reference I was hoping for! I guess now I don't see this technique as being overly useful. Innovative in its day, but I don't like the caps in the signal path. Choke coupling off the cathodes can serve the same purpose, and like the IT, there's no risk to the outputs if the drivers open.Thanks for the info!
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Buy Chinese. Bury freedom.
Edits: 08/28/16
As an alternative they show that two separate inductors can be used (e.g. the primaries of two output transformers with their secondaries left unconnected). The claim in the book is that if the bridging caps are left off of the IT transformer transient oscillations often occur with the incidence of grid current.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
"The claim in the book is that if the bridging caps are left off of the IT transformer transient oscillations often occur with the incidence of grid current."
That doesn't surprise me. There's no dampening at the grids without the AC connection back to the cathodes. The more I look at it, the less I like this circuit. If I had this model Fisher, I'd be very tempted to eliminate the secondary connections altogether.
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Buy Chinese. Bury freedom.
So, in Cary's example of the 805, the secondary of the IT would be just to eat grid current from the output tube as the 0.22uf cap carries the full signal.
Yeah, that's standard thinking but it's been pointed out to me that the grid current isn't DC per se.
It's pulsating according to the signal that's driving the grid to current.
Doesn't that mean that it's the reactance of the winding, not the DCR of the winding, that comes into play?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I agree with this but it is also important to note that since it is a transformer, the Rp of the driver tube appears in parallel with the secondary inductance keeping it a low impedance path to ground and the Rp also damps any ringing that the grid current causes.
It is the case of just a grid choke where things get squirrelly with grid current since then you simply have the grid choke inductance in series with the DCR and this combo really rings out when it is excited.
I must add that this is just an observation of what really happens and not a judgement of the actual sonics since may people who have tried grid chokes praise them then go on to give an inaccurate rationale as to why it sounds better.
dave
"It is the case of just a grid choke where things get squirrelly with grid current since then you simply have the grid choke inductance in series with the DCR and this combo really rings out when it is excited."
With a cap coupled from the plate of the driver tube to a grid of the output tube and grid choke, isn't the Rp of the driver tube in parallel with the grid choke inductance in series with the grid choke DCR?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
It has been a number of years since I went through this but is do recall from my sims that a LCL circuit behaves quite differently to a 1:1 bridged cap when it came to grid current. The LCL rings and the bridged cap doesn't.
If I had to make a WAG as to why I would think it has to do with the parallel LC resonance of the LCL circuit. When grid current occurs in a LCL it sees the parallel combo of the inductance of the grid choke and the coupling cap in series wiht the tube Rp. With the bridged cap, the grid current sees the same thing with the Rp of the tube also being a shunt to ground through the transformer.
dave
did a quick revisit of the sims and here is the abridged version. Circuit is 801 biased at -30V with a 10K load. Voltages of the outputs taken at the 801 anode
Input signal to grid
Bridged cap response to grid current
Bridged cap grid current
LCL response to grid current
LCL grid current
dave
In the GEC book the recommended values for the bridging cap is 2uF-16uF. Dennis seems to have used a 2mF cap bypassed with a 0.22uF cap. Why so high? Got me. Then again, Dennis Had has been known to do some strange things. To answer your question: yeh, could be. Then again, I'm not an engineer. There are others here on this site more qualified than me to answer you.
"It is better to remain silent and thought a fool, then speak and remove all doubt." A. Lincoln
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