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Grounded Grid - The Pentode Experiment


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The saga of messing with my 2nd Carbon build continues. Since the circuit is very close to the Grounded Grid, I'd like to give it a try. Being hesitant to spend big bucks on a nice quad EL34, I have been on the lookouts for a cheaper substitute.

The curve of a pentode looks a lot like that of a SiC FET.  I need a pentode with the following properties:

  1. The plate curve should have low kinks or no kinks at the low Va range. The lower the 'knee' the better.
  2. Low Ig2. Ig2 should be much smaller than Ia (20mA), ideally 1mA or less under the operational Va range such that Ig2 doesn't interfere with cathode drive/ cathode degeneration.
  3. The amount of negative bias needed to get 20mA at 400V should be reasonably easy to handle.
  4. High rated Va(max) and Pa(max) for using a higher supply voltage and/or idle current in the future. That means I may need to look into transmitting tubes.

And the candidate is... (drum roll please) the FU-50/GU-50!

The linearity looks pretty good at around Ia=20mA, from Va=100V all the way to 1kV!
The bias voltage is between -20V and -25V, right around what Carbon has.fu-50_150vg2.jpg.e9f754a925463b073070c7a78e6347e1.jpg

The Ig2 is really low and changes very little from Va=100V to 1kV.


More importantly, the FU-50/GU-50 are relatively inexpensive and plentiful. A lot of them were made in the USSR and China during the cold war era. I read somewhere that those were designed for the comm gear used in the tanks and had very little success in commercial applications. I paid less than $3 a pop from Ukraine about 15 year ago. The going price for a NOS tube should be close to a SiC FET today. Well, any tube not designed for audio can be cheap these days. However, that wouldn't stop people from chasing after the Telefunken LS50 and the east-Germany SRS-552s, I guess 😉

Adapting those to the Carbon is surprisingly easy. I removed the SiC FETs and the 20k bias resistor, replaced the two 175k resistors with a 100V and a 130V 3W zener diode for G2 supply. The Ig2 is really small and the two tubes can share one set of the zener diodes. They drop 230V from GND and set Vg2 right at 150V, with about 22V left for the PZTA42 and the offset pot. 

The heaters are powered by a 12.6V filament trans with one side tied to B-. I could have tied the CT but there was very little hum to worry about.

Guess what, the GU-50s work right out of the box. I didn't even need to adjust the balance and offset!



The measured performance is pretty decent:


Although the distortion is low, the FFT does show some higher order 'pentode nastiness'. I guess the reasons being

  1. The pentode is not super linear to begin with.
  2. The transconductance of the GU50 is about 1/10th of the SiC FET. The PZT42 has to work much harder and the global NFB is less effective.
  3. Something else worth looking into


I'm not yet able to seriously listen to the sound, because I couldn't find another pair of tube sockets in my stash for the second channel 😂. If you want to know how it sounds, try it!

The GU50 with 400V PSU comfortably beats the KGST (below) on the frequency response and the output swing:


Next to try is to use the pentodes on the KGST, or should I call it KGSP then?

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Now we are talk'in 😉. The tube channel has become really close to the SiC FET channel, both at 100V output.


The performance is not yet at the Carbon level, but is pretty good for a tube amp.


What was the problem with the previous mod? The simplified G2 supply didn't work well. This tube is quite sensitive to Vg2 (which may make it a good candidate for G2-drive applications, given the low Ig2). The previous circuit has Vg1 as part of Vg2, so the Vg1 variations causes Vg2 changes. The solution is to use one 150V zener diode for each tube, put it across G2 and K, and feed <1mA of current from GND using a resistor or a current source. I also added a 10uF capacitor in parallel with the zener. Without it, the output clips at around 300V but the distortion still looks good before the clipping.

Now the circuit should deserve an audition. Since the main problem is the knee on the Ia curve and Ig2 curve below Va=100V, with higher B- (500V maybe?) and perhaps higher current, the tube circuit should perform even better. I'll leave that to the next episode. For now I'm waiting for the sockets from China to build the other channel. 


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  • 3 weeks later...

While waiting for the GU-50 sockets, I found a pair of reasonably matched EL34s, and decided to wire them in the original GG circuit for comparison. The TTC004Bs are used as drivers.


Above the THD+N curve turning point at around 80V (multiply the X axis by 10 for actual voltage), the majority of distortion is 3rd harmonic. So I think the tube matching is not the major contributing factor here. The EL34 is biased at -37V, which means the driver transistors have to work really hard to push the tubes to make the output swing. The Pentode could be a better choice in a simple circuit such as the Grounded Grid. A more complex BH-style final stage would be needed to make the EL34 really shine.

The triode-connected EL34 also lost on the high frequency distortion (100Vrms)




Edited by simmconn
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  • 2 weeks later...

I'm finally able to put a closure on this pentode experiment, sorta. What finally improved the THD figure between 300V and 500Vrms output is the driver transistor. I replaced the PZTA42s with TTC004Bs when trying the original GG circuit with EL34s, while the PZTA42s remained in the left channel. With SiC FET as the output device, the driver transistor has a pretty steep load line, so the nonlinearities on the upper left portion of the curve didn't matter that much, as long as the idle Vce is chosen to be high enough to avoid that area.

With the GU50 tube as output device, the driver transistors not only need to swing current, but also swing voltage (about 10Vpp), so the load line is more flat. The nonlinearities of the transistor becomes more problematic. The PZTA42s have to go. The KSC2690s I have on the left channel have higher hFE than the TTC004B on the right. The two channels ended up having very close THD+N curves. The 2nd harmonic is more prominent than the 3rd in the left channel with the KSC2690s, not sure if that's consequential.


The hum is 110dB below the 100Vrms mark, with the 12.6V filament transformer CT simply tied to B-. I'm quite happy with the sound on my SR-009. It has the basic characteristics of the Carbon with a very slight touch of tube softness, yet not losing resolution or immediacy. I searched up and down but didn't find another pentode more suitable for this role, at least on paper. If some day I manage to put this into an enclosure, the ugly 'hats' would surely be sticking out.


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I found that GU50/FU50 has been put on spotlight in these years in speaker amplifier world and sometimes called as poor man's 300B.   Although its unique appearance and rare metal socket made many DIYer hesitate, its characteristics advantage is being recognized and Chinese cheap sockets are now available.   I think you're the first to put them into E-stat amplifier and it's nice challenge considering the high voltage usage.   Your way of try and improvement was quite interesting.   Adjustment of Vg2 story was exciting.  The resulted performance is impressive.   I couldn't stop placing order for some GU50s on eBay as they are such cheap (20-30 EUR for brand new Quad incl. shipping) 🙂  Thank you very much for your sharing this great experiment with us.

Edited by ibuski
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Thanks! Speaking of low-cost FU50 sockets from China, there are different kinds.

The socket on the left was removed from a NOS socket set with the cast aluminum frame. The frame helps the tube to align properly, and also works as a mechanical shield of some sort. It is unnecessary for a DIY amp IMHO. The ceramic socket base is pretty well made with good tolerance. The pin contacts have large flared openings and two holes on the soldering tab. The entire socket set including the aluminum frame can be had for about $3 each in the Chinese domestic market.

The one on the right is the 'new production'. I got them for less than $1 a piece. Some with tin-plated contacts can be even cheaper. However the contact has a much smaller opening on the top, and a single oblong hole on the soldering tab. There is a risk of pins getting plugged into the well but outside the contact. The ceramic base are often poorly made with an uneven mounting surface. What's more, the pin 1 location is different from the NOS socket. Do not mix them!

I would say go get the NOS socket set, spend a few minutes to remove the frame and end up with a socket in better quality.



The socket mounting hole pattern is somewhat unique. A caliper and some middle-school trigonometry will be useful to figure out the exact angles of those mounting holes. 


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Hi Simmconn,

Thank you for your suggestion about the sockets for GU50/FU50.   Problem of decent sockets for them is, their price is usually higher than the tube itself.   And when I see NOS metal sockets, their connectors don't look good as yours, see picture for example.   Is that type of better quality sockets available in China local market only?   I made research on Internet incl. Aliexpress but haven't found that type.   Yes, the risk of putting pins into the gap between ceramic folder and metal connector was mentioned, sometimes even possible to damage the tube.  We have to be careful to use those loose holding sockets....


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Just as a sanity check, with the GU50 substitution you don't really need offboard heatsinks anymore right? the KGST only needed those PCB mounted ones. 

My idea was just to fix the 10m90s devices to the sides of those hifi2000 cases with the quasi heatsinks, seems like a nice little project to use some of the older boards I have lying around. 

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It looks like an used USSR-style socket. I would prefer that type of socket more than the NOS socket from China. I don't think the variations in design matter that much. As long as they were originally made for the military/industrial use, the quality should be trustworthy.


The 10M90s still burn 7 to 9W each depending on the B+ voltage and the idle current. Using the enclosure side panel as heat sink should be doable, but it could get pretty hot. I would put some warning sign stickers on the chassis.

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  • 2 months later...

The pentode experiment gets a little twist - with a beam tetrode. In my search for an acceptable output device, I went through the popular low-power transmitting tubes. The 807/6L6 family has such a wide, kinky curve in the low current region with typical Vg2 (250V) and was rejected as a candidate at the first glance. 


However, the curves around zero Vg1 looks quite linear. Lowering the Vg2 could sort of 'bring them down' to the low current region. With about 60V on the G2, the kinks are largely gone, and the linearity looks pretty good (pictured below X= 100V/div, Y= 5mA/div, Vg1= 0.5V/step). Another benefit is that Ig2 comes down considerably with such a low Vg2, which allows me to use the same simple G2 supply used with the GU-50 (zener diode paralleled with a 10uF cap).


I compared quite a few tubes on the curve tracer. The National Union 807s (shown above) from WWII era seem to have the best linearity and are the most stable. The RCA and Sylvania tend to oscillate on my test jig. 1625s are about the same. Unfortunately I don't have enough National Union for a matched quad. The earlier 6L6/6L6-G are not officially rated for 400V plate voltage and have to be excluded. The Tung-sol 6L6WGB/5881 (the real deal, not the 'reissue') looks much more linear compared to the 6L6GC. The Sylvania 5881 (shown above) although has the same internal structure as the Tung-sol, doesn't seem to be as linear. The 1624 would be an interesting direct-heating variant that has a bit higher gain than the 807. The 2E26/2E24 brothers are suitable, too.

Here are the 807s on the test bed:


The result is convincing:


The 100Vrms output FFT shows harmonics -115dB below the fundamental:


The 1624 turned out to be a disaster. It was hard to get a clean, low capacitance filament supply. When attached to the signal path, the common-mode noise would kill the S/N and the capacitance would mess up the frequency response and THD. I have to admit I don't have much experience with direct-heating tubes in circuits where every uV counts. It would be nice if the few exotic direct-heating tube eStat amp owners can get their units measured and share their secret to success...

The grounded-grid cascode output stage is such a nice thing that it allows us to play with many tube types, as long as they have enough plate voltage and power ratings and are reasonably linear. We could end up having unique amps that still have top-notch performance.

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  • 2 months later...
On 8/3/2022 at 11:28 AM, luvdunhill said:

Nice work!

Thanks! I've got to bump this thread to page one with some real stuff 😀.

The GGPT (Grounded-Grid Pentode/Tetrode) now gets a proper PCB laid out:


I made a few deviations to the common choice of parts:

1) I found some 2SK332, could be Sanyo's answer to the low noise dual N-JFET market back in the day. They cost me about 60 cents a pop, so I ditched the LSK389. The footprint is also usable for a pair of 2SK170s. SOT-23 footprints are added for the 2SK209, and a few others that share the same pinout.

2) Through-holes on the SMD pad for the SOT223s are a big no-no for machine assembly due to solder wicking, but works okay for hand soldering. It allows the use of A970/C2240s. I found some from a Chinese manufacturer Foshan Blue Rocket Electronics, pretty decent build quality, good low-current linearity, and cheap (14 cents a pair). 

3) All through-hole RCs. I know the world has moved to SMD but I have quite some through-hole resistors accumulated over the years, and personally prefer leaded components on tube PCBs due to the thermal and mechanical stress they have to endure.

The FU50 tube sockets are so big and everything else have to dance around them. I chose not to use slotted pads at the exact socket pin locations, instead having soldering pads on the side allows flexible or semi-rigid connections from PCB to the tube socket contacts, which can absorb the strains. The scheme is similar to what Stax had on the original T2 PCB. A couple more mounting holes are added to allow other types of tube sockets to be used.

The board mounting holes have the same distance compared to the Carbon board. This board can overlay on top of the Carbon board in the chassis where I have enough Z space left. They can share the heat sink and the power supplies. The goal is to be able to switch between them with a flip of a switch.

The GG Pentode with GU-50s and the GG Tetrode with 807s, side by side:


The 807s have slightly better measured performance, at the voltage level that I probably never going to drive my phones at (when I'm wearing them).

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