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 No finished schematic that I know of. Kevin just did a quickie concept and used bis Carbon as the basis for the GG PCB, by directly substituting triode connected EL-34s for the Carbon's SiC outputs. Also, Q11&12 in the tail of the Carbon output  were replaced with a pair of KSC2690A. 

So use the Carbon schematic (but think Triode for Q9-10) to understand the functioning of the GG circuit. 

 

Edited by Laowei
Typo on SiC schematic numbers.
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Just like Carbon we built. Same box, same PS. Relay=120s. Separated filament transformer.      

Another Grounded Grid lives.  Have been listening to it for about a week. Powered by the same GRHV/GRLV PSU I use to power my KGSSHV, Carbon and Blue Hawaii. 400VDC rails and 20mA plate current.

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Ok I'm listening to my new "Fat Bottom Girls"  :D

eX8hRlF.jpg

 

I recognize that the sound is much better than the EL34. I had never used 6CA7 before but I'm very surprised.

 

 

Edited by jose
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13 hours ago, eggil said:

Jose.WDYT of the sound know with the fat girls as opposed as the Carbon ?

I will try to explain myself with my limited knowledge of English. Please don't taunt me  :D

Although the GG is now to my liking, IMO there is still a big difference between both:

The Cabon sound is much flatter, more precise and detailed. 
The problem is that he doesn´t forgive the bad recordings or sources. You can hear all defects and goodness of a recording. Sometimes this is good... or not. 

The GG with the "fat bottomed girl" :D he has a nice bass and a midrange colored and more balanced between both, IMO this makes it less precise (a bit), but at the same time make it more musical and relaxed.

With my EL34 quads (JJ, Mullard and Tung-sol) I think that the GG sound is a disaster because you have U-shaped equalization and I don´t like this kind of sound.

IMO both amps have an opposite character and you can determine perfectly which of the two you are listening to. If you have an aggressive source (for example DAC with many treble or bad records), you probably would appreciate having the GG. For other hand if you prefer a sound as faithful as possible, the Carbon is your amplifier.

PD. I haven´t any "sacred cow" yet (007/009), I only have Lambda, 202 and 307. :(

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You're not really missing out on that much with the 007 and 009. They all have their strengths and weaknesses, and it sounds like you value a flat/uncolored sound above all else which the Lambda and 202 get right more than either the 007 or 009. 

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  • 1 month later...

Just plugged in 2 matched pairs of used/tested above new, Valvo labeled Mullard xf3 tubes in my slightly modified GG.

Now we're talking. That's how I remember real tube magic.... :D

The only other tubes tried in this amp: 2000 vintage Svetlana Winged C EL34, NP JJ 6CA7, NP Gold Lion KT77. Not a drastic SQ difference with the OS Mullards, but the music simply flows with the xf3s, reproduced with a calm ease, clarity, presence and majesty absent in those other tubes. Everything sounds so balanced and right with these old guys. 

 

 

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A couple little things for now.

Replaced the originally built Dale RN60 resistors in the global NFB circuit with ultra low noise and TCR TX2575 naked Z foil Vishay resistors (400VDC PS only). 

Also added a 1N4007 diode in series to the screen current limiting resistors that wire the pentode into triode operation. Similar as have done to my amps in the past. Will replace with Schottkys soon. 

 

IMG_2333.GIF

Edited by Laowei
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With G2 connected to the plate with a resistor, as the screen passes current across the resistor, the screen will have a (slightly) different potential than the plate. With a CCS and DC, this relationship will be stable/constant. 

Varying AC load will vary the current across the resistor, and vary the screen potential relative to the plate..The varying potential on the screen can to an extent control current collection of the plate. 

Adding the diode, AC load is blocked from modulating the potential of the screen. The voltage drop across the diode is constant so should not be of consequence. 

 

5 minutes ago, kevin gilmore said:

in a push pull circuit with a transformer output, this will keep G3 from going negative.

for the electrostatic circuit pretty sure it does absolutely nothing

Follow the AC loop in the differential circuit. 

7 hours ago, jose said:

Ummm I'm interested. Only for 400v Psu?

200Vrms  (300Vpeak) voltage limit on TX2575. Two 100K wired in series in the GG.

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I see the effective AC loop in the GG as this: ( Carbon shown) 

IMG_2715.thumb.PNG.e70ede2f649f82de300167e75f33e118.PNG

 

In a pentode, the screen grid electrostatic potential effects the electron flow to the plate. One can even use the screen to drive the plate if so inclined.

Connecting G2 directly to the plate to simulate a triode, they would share the same potential, but G2 would fail due to excessive current draw. So to limit current, a resistor is usually added, which separates the screen from the plate potential depending on current across it. The higher the resistor value, the more difference. At DC, things look stable. Add in hundred of volts AC swing and the voltage across the resistor will modulate the screen potential. Adding the diode stops AC thru the screens (stabilizing the potential)  and leaves it to pass only through the plates. 

I've found my GG to be able to play both a little louder and a little cleaner into my SR-009 after doing this addition. 

Edited by Laowei
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3 hours ago, Laowei said:

Adding the diode stops AC thru the screens (stabilizing the potential)  and leaves it to pass only through the plates. 

how? Isn't the plate always positive in rgrds to the screen. Have you tried to increase the resistor value x2 to see if you got the same change in sound?

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16 minutes ago, sorenb said:

how? Isn't the plate always positive in rgrds to the screen. Have you tried to increase the resistor value x2 to see if you got the same change in sound?

Not sure if that's true in a differential circuit. 

In other amps, I have increased the resistor and made the amp sound worse. Same current thru higher resistance equals larger voltage drop. Add AC and it would make things dynamically worse; higher voltage modulation on the screen. 

 

Edited by Laowei
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