Grounded Grid Tube Gain Stage

No, no, no, I’m still alive! Sorry for not updating my website more frequently but my disappearance the last 3 months can be traced to these reasons.

1. I’ve been abducted by aliens. Apparently they wanted to clone me, seeing me as the perfect human specimen. Man! Must be my awesome good looks! That’s what Monica said! I guess life is unfair… For you! [Insert sinister laughter here]

2. I finally found my twin brother! He rules a tiny kingdom far away from all forms of modern civilization. It’s paradise here! No traffic jams, no pollution, it’s like a step back in time! Curiously everyone has a sound system and boy do I mean ‘sound’ system! Everyone listens to non-oversamping dacs, class-T amps and horn speakers! Evidently my twin brother has good taste! When I told him about monica and charlize, he reprimanded me for taking a piece of heaven away. And he banished me from his kingdom! Well, he aint heavy, he’s my brother!

3. I’ve been busy working on a tube gain stage for monica. This is the most difficult part, a gain stage that does not colour monica, a gain stage that enhances monica’s beauty. Not easy! Many tubes have come and many have failed. I love 12b4 but its microphonics!

Well, out of the 3 above, the last one is a lie. I was supposed to, but in the end didn’t, develop the gain stage. But the good news is, someone did the gain stage! And none other than the famous `DAC Man’ Liew!

Not contented with building the only out-of-this-world DAC, our solder happy Diyer was keen to get more of Monica in his system (greedy boy!). So he set forth on the search for such a gain stage.

And he zeroed in on the oft-overlooked, vastly under-appreciated grounded grid circuitry for it offers low input impedance to the DAC and if the right tubes are used, low output impedance as well. (Sorry you folks with 12A*7 tubes need not even enquire)

`DAC Man’ at first tried with 5687 and found good sonics with Monica but once he tried 5842…Eureka!

CY Liew’s “5842 GG”

It’s easy to understand why. Don’t let the tiny 5842 fool you. Among its tube brethrens, it has one of the highest tranconductance around! Meaning it will swing a lot of volts for a tiny bit of current. Or is it swing a lot of current for a bit of volts? Never mind! I guess you could say this tube is very responsive! No wonder 5842 excels in phono stages as well huh? But you don’t often see them in commercial designs. Told you most of those guys got their heads screwed on wrongly….

If you are adventurous, or if miniatures aren’t your fancy, triode-wired 7788 with its high conductance should be a wonderful candidate. vt4c suggested D3a, another mojo tube! That said, try this with other tubes you have in hand. There’s more than one way you could please Monica okay?

Now GG works better than common cathode gain stages as its low input impedance is easier on Monica’s TDA1545A current output stage. If your DAC chip is voltage output, then you would prefer a high input impedance gain stage. As GG has low input impedance, don’t be daft and build one as a preamp. Unless you are REALLY DAFT.

I want more mojo
Typical of me, I only completed my GG much much later. But with slight mods. (Narcissicm is well alive!)

As I built my Monica2 inside, I omitted the 100uF (C2/C4), 100ohm (R2/R6), 1Meg(R3/R7). On the power supply side, I opted for choke filtering but these are just my changes. You can do it any way you want!

Now there’s more Monica in my system! Mama mia! : )

Update on April 2006!

Wow! I finally got my plate chokes ready. A fantastic 120H/16mA!

Schematic below shows a R and C across the 10H choke. This is to “tune” the choke to 100Hz power supply noise. 5842 cathode resistor is still 160ohms.

With plate chokes, it stores energy and thus able to supply instantaneous “juice” as required. This is why along with the much higher loading on the tube (at 20Hz, a 120H choke is about 15kohm load!), plate chokes REALLY REALLY SING! Transients are so much better, the “MUSICSIANS ARE HERE” quality just jumps out! Of course, the higher loading gives even smoother mids and highs. The realism is simply outstanding. Anyway, this is how my simple mind understands this. Let’s play some music.

Wait! There’s more to it!

She’s gyrating to the music! Hands on her hip, swaying gently to the rhythm, seducing me with every note!

Ohhhhhh that lowwwwwwww cuttttttt blouuuuuuuuuuuuse!

That pouttttttttttttty lipsssssssss!

Man, am I on drugs?

Drug or no drug, this is awesome!

Will leave you pathetic folks out there!

Mama mia! Here I come!

Update on Aug 2006!

Rewired gg for 6688 in place of 5842. EVERYTHING IS BETTER!!!!!

Refer Downloads page for GG wiring.

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Experimenter 45

This is by Matthew Reidsma in a past issue of
Valves magazine. Circuit redrawn here.

Notice how simple the circuit is. The initial
circuit calls for a cell of 1.2V but Matthew later revised it to 2.4V. You can
budget the B+ at around 380V or more, depending on the DCR of the output
transformer. By the way, Matthew used the Hammond 125E at 3k.

The 0D3 or 0A2 can both be used. Any gas regulator
will work as long as it gives 150V at the plate. You might prefer the 0D3 over
the 0A2 for aesthetics reason.

Note the idea of “tapping” or
“recycling” the output tube plate current to drive the input tube is
not new here. Refer to similar circuits like Jeremy Epstein’s “Free
Lunch”, “Monkey” and also “Direct Reactance”. Do a
Google and find out more. 

45? Do you have any idea how expensive
this tube is?

Yes, I’m aware of that. You don’t have
use a 45 here, Can just substitute with a 2A3. Read other pages here to find
where you can get good value Sovtek 2A3s. Just be aware that filaments for 2A3s
and 45s are different so don’t go swapping them here and forth.

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5842-2A3 integrated/power amp

As simple as it gets!

I haven’t built this yet but let me know if you are interested.
Operating points were taken off from various user’s input in Joelist.

Note that if you want to use it as power amp, simply take away the volume pot and replace with a 100k resistor OR just turn the volume pot to max and use your preamp to control volume.

Latest update! Mr. Chang here has built the above amp and found the voltages exactly as shown!

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