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In Reply to: RE: Purpose of these Caps? posted by Chip647 on August 28, 2016 at 14:43:24
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"
Follow Ups:
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
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