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GG preamp, why is it uncommon?
GG preamp, why is it uncommon?
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Old 14th January 2018, 04:33 AM   #11
geezertron is offline geezertron  United States
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Attached is the latest schematic and measurements. It's tweaked a little to raise the input impedance, but this is kind of a "diminishing returns" exercise I found.

15K input impedance is not all that low, and should suit the applications I have in mind just fine. Mainly I like it because it's simple and not fussy...
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Old 14th January 2018, 06:13 AM   #12
Wavebourn is offline Wavebourn  United States
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GG preamp, why is it uncommon?
15k input impedance is more than plenty for modern signal sources.
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Old 14th January 2018, 06:30 AM   #13
Wavebourn is offline Wavebourn  United States
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GG preamp, why is it uncommon?
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Originally Posted by Wavebourn View Post
15k input impedance is more than plenty for modern signal sources.
...

But the dark side of the moon is, how linear is it...
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Old 14th January 2018, 12:07 PM   #14
Johan Potgieter is offline Johan Potgieter  South Africa
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I am somewhat surprised that the input impedance is that high. The cathode resistor is in the feedback path, and the cathode usually quite a low impedance point. (I did not 'Spice' it myself.)

By the same token, the feeding impedance becomes part of the equivalent circuit (in parallel with the 18K). Feeding from a lower impedance will change parameters. Current feed again diminishes gain.
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Old 14th January 2018, 03:57 PM   #15
Lingwendil is offline Lingwendil  United States
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GG preamp, why is it uncommon?
Personally, I would replace the cathode resistor on that first triode with a Jfet, and do a little fooling around to get you a really cool little rig. I'm not even a hybrid design guy (other than rectification, current sources, and voltage followers, I tend to try to keep all active devices tube) but it seems like a good idea. I seem to remember that trick from an Allen Wright design, and I think Sy has mentioned it before too.

Unless of course, you're trying to keep it all tube. No worries there, but it seems that the gain wouldn't be perfectly consistent, with the input signal modulating the current through the cathode it seems like it can cause a little nonlinear action...
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Old 15th January 2018, 03:28 AM   #16
Johan Potgieter is offline Johan Potgieter  South Africa
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Still has me thinking. With a c.c. element in the cathode (input) circuit, gain would be proportional to the output impedance of whatever is feeding. I am still wondering about the equivalency of a normal triode amplifier (with or without bootstrapping) with grid input plus cathode follower. Why specifically a g.g.? But I must hasten to add that I have not yet read the references provided.

To be a little naughty Lingwendil

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Originally Posted by Lingwendil View Post
..... with the input signal modulating the current through the cathode it seems like it can cause a little nonlinear action...
Any input signal in whatever topology will modulate the cathode current, otherwise no amplification .... but I think I see what you mean!
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Old 15th January 2018, 05:23 AM   #17
Wavebourn is offline Wavebourn  United States
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GG preamp, why is it uncommon?
Quote:
Originally Posted by Johan Potgieter View Post
I am somewhat surprised that the input impedance is that high. The cathode resistor is in the feedback path, and the cathode usually quite a low impedance point. (I did not 'Spice' it myself.)
Because of feedback to the control grid.
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Old 15th January 2018, 10:54 AM   #18
DF96 is offline DF96  England
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Quote:
Originally Posted by Johan Potgieter
I am somewhat surprised that the input impedance is that high. The cathode resistor is in the feedback path, and the cathode usually quite a low impedance point. (I did not 'Spice' it myself.)
Series feedback (which is what this circuit has) raises input impedance. It cannot raise it higher than 18k, as the cathode resistor is outside the feedback loop.

The cathode is often a low impedance point, but in this case it is looking up to a bootstrapped anode load so it becomes a high impedance point even without the overall feedback.
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Old 19th January 2018, 06:44 PM   #19
geezertron is offline geezertron  United States
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Quote:
Originally Posted by DF96 View Post
Series feedback (which is what this circuit has) raises input impedance. It cannot raise it higher than 18k, as the cathode resistor is outside the feedback loop.

The cathode is often a low impedance point, but in this case it is looking up to a bootstrapped anode load so it becomes a high impedance point even without the overall feedback.
I've been playing around with this circuit for a few days and basically it boils down to trading off input impedance and gain, while keeping the tubes both biased correctly. Raise the cathode resistor on the GG input side and you need to adjust the feedback to the grid from the divider on the cathode follower to up the DC voltage on the grid, which necessarily raises the feedback and lowers the gain (and improves the distortion a bit too I guess).

With a 27K cathode resistor and appropriate adjustments elsewhere the gain goes down to 8, but the input impedance goes up to 24K. After realizing that I was going to need to put a passive attenuator ahead of this thing for line-in connections it became apparent that I needed a bit more impedance here to not load the attenuator. The drop in gain does not matter, anything over 5 is enough. What might matter is that this raises the voltage on the input port, but I'm using a 525V cap here for safety in any case.

The other thing I've noticed is that it seems you can put quite a bit larger anode load on a tube than normal if it's split and bootstrapped. Intuitively this makes sense, but I've not found a good analysis of the topic, if someone has a pointer I'd appreciate it.
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Old 19th January 2018, 06:52 PM   #20
DF96 is offline DF96  England
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Quote:
Originally Posted by geezertron
The other thing I've noticed is that it seems you can put quite a bit larger anode load on a tube than normal if it's split and bootstrapped.
Could you clarify what you mean?
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