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Old 12th April 2011, 12:31 AM   #1
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Default Grounded grid Zout

Read somewhere recently that grounded grid (cathode drive) has a rather high Zout. Is this effect greater than the increase in zout due to unbypassed cathode resistor (which is of course necessary for GG)? Can anyone point me to a good write up on the derivation of impedances in the GG amplifier stage?
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Old 12th April 2011, 12:49 AM   #2
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google for "How to gain gain_A Reference Book on Triodes in Audio Pre-Amps"
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Old 12th April 2011, 01:43 AM   #3
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RDH4 should have some info on grounded grid as well. It wouldn't surprise me if that configuration was used for RF back in the day just like grounded base/gate is today.

~Tom
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Old 12th April 2011, 02:23 AM   #4
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Originally Posted by mashaffer View Post
Read somewhere recently that grounded grid (cathode drive) has a rather high Zout. Is this effect greater than the increase in zout due to unbypassed cathode resistor (which is of course necessary for GG)?
No. It's the same, though it can be influenced by the source impedance. Accounts for why cascoded triodes (GK stage DC coupled to GG stage) have such high effective plate resistance. Typically makes broadband response a good deal harder to come by when using VTs than it does with transistors.

See: Vacuum Tube Amplifier Circuits and Equations
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Old 12th April 2011, 02:32 AM   #5
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Originally Posted by tomchr View Post
RDH4 should have some info on grounded grid as well. It wouldn't surprise me if that configuration was used for RF back in the day just like grounded base/gate is today.

~Tom
ABSOLUTELY! An untuned I/P connected to the antenna. That's exactly how the 6С45П (6s45p) and 5842 were intended to be used. Examine the data sheets.

An interesting "spud" FM receiver can be found here. Isolating the antenna from the oscillating detector is a least as important as the gain. An interesting thing about the design is the triode working as a GG RF amp is also working as a reflex common cathode audio amp. Slick!
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Old 12th April 2011, 03:47 AM   #6
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Originally Posted by tomchr View Post
RDH4 should have some info on grounded grid as well. It wouldn't surprise me if that configuration was used for RF back in the day just like grounded base/gate is today.

~Tom
I grew up in ham radio "back in the day" -- most of the high power amps I used were grounded grid -- for SSB and CW.
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Old 12th April 2011, 11:31 PM   #7
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Thanks guys I will check those sources out. Of course high Zout is not necessarily a bad thing for full range drivers on open baffles but I suspect that gNFB would solve the problem if it arose.
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Old 13th April 2011, 12:56 AM   #8
tomchr is offline tomchr  United States
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Originally Posted by Eli Duttman View Post
An interesting "spud" FM receiver can be found here. Isolating the antenna from the oscillating detector is a least as important as the gain. An interesting thing about the design is the triode working as a GG RF amp is also working as a reflex common cathode audio amp. Slick!
Yep. I *love* how the tube radios and early transistor radios used one device for multiple circuit functions. That's real electrical engineering.

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Old 16th April 2011, 01:22 AM   #9
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If you guys don't mind going a little further with me on this one...

The first most obvious way to drive the cathode was (at least to me) to use a stout cathode follower coupled by a large film capacitor such as are used in speaker crossovers. The value needed would seem to be about the same as one would use for a cathode bypass.

In order to avoid the cap it occurred to me that one could direct couple a CF as long as one took into account the bias current of the CF in the selection of the shared cathode resistor. However in the particular project I have in mind I would want to be able to drive the GG stage both solidly into cut off and well into grid current. It seems that direct coupling would not allow that kind of swing. Possibly if one added a negative rail only for the CF driver one could make a driver that could swing well below ground...

In searching around for alternatives I came across Crowhurst's short write up on the GG stage and it seems that he favors transformer coupling.

Input to the Grounded-Grid Amplifier

As usual I find it hard to follow his articles (not sure why) but this would seem to allow for the kind of swing I envision and removes the need for the cap (at the cost of an expensive tranny). The bias is set via. the DCR of the secondary and seems a nice elegant solution.

Toward the end he claims an improvement in performance (he doesn't say in what respect the performance is increased) by using a series resistor for part of the bias and bypassing it with a cap. At first I though that the cap would short out some of the input signal but I realized that the actual signal is all "above" the cap/resistor combination. But it still looks like a tank circuit to me. Is there a danger of ending up with a tuned circuit here if the R and C are not chosen carefully? Or is the value of C so high that it is incapable of leading to instability?

OK, so that was a lot further in reality but I appreciate your patient tutelage.
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Old 16th April 2011, 12:32 PM   #10
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I normally find Crowhurst easy to read, but this time he is confusing. He is not actually adding a resistance to the cathode circuit, he is just making it more obvious. At first he has the resistance hidden inside the transformer secondary. This reduces gain as it can't be bypassed. So he comes up with the brilliant idea of winding the transformer with ordinary wire (instead of resistance wire) and putting a resistor in, which he then bypasses. I am baffled as to why he makes such heavy weather of what should be a fairly simple circuit.

Little risk of a tank circuit formed by the transformer leakage inductance and the bypass C because the resistor and cathode impedance will damp it.
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