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Saurav 25th May 2004 06:29 PM

Trying to understand preamp schematic...

Here's the ASL AQ-2004:

It's a pretty bad scan, but it's all I've got. The first and third stages look like simple cathode followers. It's the stage in the middle that confuses me. That's a grounded grid, right? And it looks like the cathode bias circuit for the second tube includes the resistor under the first tube, so you'd have to calculate currents for both. Why is this done? I assume to set the DC voltage at the cathode/grid of the second tube?

Also, there's a resistor between the cathode of the first tube and the grid of the second tube, and what looks like a capacitor from the grid of the second tube to ground. What's the purpose of these? Just to roll off the high frequencies? It's really hard to read values, so I'd assume that the corner is above 20kHz.

Thanks in advance for any help,


Jax 25th May 2004 06:40 PM

It indeed looks like a grounded grid. I think the cutoff frequency of the RC to the grid of the second tube is well below 20Hz.

Very difficult to see though.

SY 25th May 2004 06:59 PM

The cap, C2 (it looks like), sets the grid at AC ground. The resistor that runs from the grid of the second tube back to the cathode (I can't read the designator, R4 maybe?) sets the DC level of the second tube's grid to be equal to the DC level of the cathode of tube 1. That makes sense to do so that the grounded grid tube's operating point can be set by its cathode resistor (R5?).

Saurav 25th May 2004 07:11 PM

Ah, I see. So the one resistor sets the second grid to the first cathode, then the other resistor raises the second cathode up above its grid. Neat. That makes sense now.

Questions. Could the second grid have been connected directly to the first cathode? Also, why do we need the capacitor, why does the grid need to be at AC ground? Or is that a stupid question? It looks like C2, which makes it 0.47uF, and the resistor beside that looks like R4, which is 470k. So it's certainly not a high frequency rolloff like I though. OK, now I'm confused again. If the grid is at AC ground... ah, never mind, it's a grounded grid circuit, the signal goes up through the cathode, so we do want the grid to be at AC ground. OK, I think I understand it now, I was trying to relate that to a common cathode stage, which was my mistake. We wouldn't want the grid at AC ground if the signal were coming in through the grid, and that's what I was thinking about.

Thanks a lot :)

cantskienuf 25th May 2004 07:18 PM

I believe that this was a prototype schematic because I seem to remember Mr. Lau posting on Audioasylum when they initially made the unit available. While the production model of this pre-amp does sound very good, I'd have to warn you that the model I bought has differing values for just about every component that you see listed here. I have never seen a schematic with the updated component values.

SY 25th May 2004 07:21 PM

The designer wanted the second stage to be a grounded grid, so that's why he didn't connect the grid directly to the cathode of the previous stage. And that's why there's a cap tying that grid to ground- otherwise it ain't a grounded grid.

Oh, yeah, you figured that out! ;)

Now you can ask why someone would want to do a grounded grid circuit. Go ahead, ask, I dare you!

Saurav 25th May 2004 07:32 PM


While the production model of this pre-amp does sound very good, I'd have to warn you that the model I bought has differing values for just about every component that you see listed here.
Thanks, that's good to know. Do you know if the basic layout has changed? I just found this on a separate discussion, and thought the design looked odd. At least, different from stuff I'd seen before. And, I couldn't understand how it worked.


Now you can ask why someone would want to do a grounded grid circuit. Go ahead, ask, I dare you!
OK, why would anyone want to do a grounded grid? :)

I guess I would find some answers if I read Rozenblit's (sp?) book. I've also read that they're the most stable at RF, but I'm not sure if that's true, and if it even applies to audio.

This is mostly idle curiosity on my part, sice I have no plans of changing my autoformers for anything else in the foreseeable future. This is unless it can be easily adapted to improve the driver stage of my amp, and then I'll be more interested :)

SY 25th May 2004 07:42 PM

Q: Why did the designer use a grounded grid circuit?

A: Millerophobia.

David Berning's solution always looked more elegant to me- he used a pFET source follower to drive the cathode of a tube with its grid tied directly to ground. Very, very simple and linear. Check out the schematic for the TF-10 on Berning's website.

Saurav 25th May 2004 07:50 PM

Hmm. But the two CF stages are still common cathode, so they'd have the same Miller C as a plate loaded common cathode stage, right? Does it really make that much of a difference to make one stage out of 3 a grounded grid? Ah, I think I get it, the Miller C will be worst in the voltage gain stage, so that's the one you want to address, and the CFs form buffers before and after it.

I have an ECC99 with a Bottlehead CCS (20mA) on top and a NiMH battery (4V when charged up) below. I'm using just the single triode in each channel, which gives me a free triode to play with if I want to. Any suggestions? Miller C would have been a bigger issue for me when I had the autoformers directly driving the 2A3 amp, now there's an active XO in the middle so it's opamps doing the driving. I wonder if I should add a buffer stage at the end of my XO, right now the last stage is the last stage of the high-pass filters, followed by a series cap to block DC.

Speaking of caps, I picked up a bunch of Wima MKP4 1uF 100V caps for 28c each. I was pretty kicked when I found them, and my friend was impressed that they were red. I couldn't find any violet caps in values that I needed, she really tried to get me to buy some violet caps.

I'll take a look at the Berning circuit. Thanks for the tip.

Jax 25th May 2004 08:00 PM

Grounded grid is indeed intended for RF amplifiers at frequencies where the transit time in the tube becomes significant. The grounded grid acts like a screen between input and output much like the screen grid in a pentode.

Triodes for GG RF applications often have several pins connected to the grid to achieve low impedance to ground.

GG has low gain and may be suitable in a preamplifier though.

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