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GG preamp, why is it uncommon?

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I'm spending my evenings trying out every idea for using a 4HA7 twin triode in a pre-amp, basically an "AU" and "AX" in the same bottle.

I ran across the attached circuit at:

Real Grounded-Grid Aplifier

With the values of B+ and resistors it came in at 20dB gain, vs 19dB as stated in the article, but I'd consider this pretty much spot on equal given the different tube and voltages.

It's a wonder. The low-ish (for a tube amp) input impedance matches the intended application as an "AUX IN" level amp, and reduces hum pickup. No signs of oscillation, and it actually produces usable gain (from an RF standpoint I guess ;-) at 600Khz. Insane bandwidth basically. Since the forward coupling and feedback are both DC there's no phase shift at all, the input and output lie square on top each other. Basically lot's to like.

So why isn't it more common, or am I missing something when I search for similar circuits?
 

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Low input impedance and lack of direct connected input (at least, without special circuitry or consideration) are my opinion the main things keeping people away.

I've used them at the output of current output DAC circuitry (PCM1794) using submini tubes in the past and it was a very simple yet fantastic solution, even if a bit unorthodox.


Fyi that diode does nothing once the tubes are warm, but is worth leaving in circuit, or replacing with a neon lamp.
 
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Up to now, the discussion has been about a true grounded grid stage. Then there is what Bruce Rozenblit called grounded grid, which is "really" a buffered diff. amp. I'm providing my take on that sort of circuitry. With zero loop NFB, the setup is HIFI, and then some, linear.
 

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Some discoveries:

Mostly looked at the input impedance tonight. Measured (very) roughly as the value of the series resistance that halves the output with a constant input V, the amp as-is has 7K input impedance.

If you split the anode load on the gg side and bootstrap it from the output of the cathode follower side with a large cap (I used 11.5uF) it doubles the input impedance to 14K, and increases the gain about 10%.

On the other side of things, if you return the cathode resistors on both sides to -17V the input is pulled down to 0V and can be directly coupled. I amused myself for a couple of minutes looking at perfectly square 1 Hz square waves, but can't see the practical utility.

So far so good, I'm liking this little amp more and more...
 
Yes, bootstrapping the GG anode will help. Not only does it raise input impedance (to nearly reach the value of the cathode resistor) but more importantly it also linearises the input impedance. Not an issue if you are driving it from a headphone socket, but could matter if the source has higher impedance.

The downside of bootstrapping is that it raises output impedance too.
 
When the 12AX7 is warming up, it puts current back into the signal source (10V charges the 3 uF cap).

When the 12AU7 is warming up, it puts current into the output. 103V charges the 1 uF cap. Without any other DC path to ground, the time constant is 1uF x 1 meg Ohm = 1 second. That means that the output will be at about +5V after 5 time constants (5 seconds).
 
6A3sUMMER said:
When the 12AX7 is warming up, it puts current back into the signal source (10V charges the 3 uF cap).
Yes. The input ground leak resistor may need to be a bit smaller.

When the 12AU7 is warming up, it puts current into the output. 103V charges the 1 uF cap. Without any other DC path to ground, the time constant is 1uF x 1 meg Ohm = 1 second. That means that the output will be at about +5V after 5 time constants (5 seconds).
No. The diode ensures that the voltage is set before the 12AU7 warms up. The instantaneous output voltage on switch on could be 100V or more. It will then decay. By 5 time constants it will be around 700mV or maybe slightly more. A smaller ground leak, combined with a slower HT/B+ rise will avoid this problem.
 
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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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.
 
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...
 
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 :) :)

..... 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!
 
Johan Potgieter said:
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.
 
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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