SE Pentode, regulated screen voltage?
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 7th November 2017, 06:50 AM #1 glina   diyAudio Member   Join Date: Oct 2003 Location: Warsaw SE Pentode, regulated screen voltage? My amp is a parallel single ended class A pentode based o 2x 6L6GC per channel. Cathode bias based on resistor plus bypass capacitor for 60mA bias current per tube. B+ is 380V, screen 280V, cathode 18V. This is the typical point of operation as outlined in the 6L6GC datasheet. I've read numerous threads on importance of screen voltage in P-P pentodes. My screen supply is a separate transformer tap with a CRC filter (100uf-1k-100uf). According to the 6L6GC datasheet for my point of operation, the screen currents are: Zero signal screen current 7mA Maximum signal screen current 2.5mA According to PSUD simulation of my screen supply this results in a voltage swing of up to +20V on the screen. It is my understanding that Class A draws 100% current with no signal, and minimum of 75% with peak signal (is that correct?). This peak signal condition, according to PSUD, would generate up to +10V on the plate voltage. My questions are 1. How important is the screen voltage in SE operation? 2. Which is the condition for lowest distortion, best linearity: a) (Vplate-Vcathode)-(Vscreen-Vcathode)=constant b) (Vplate-Vcathode)/(Vscreen-Vcathode)=constant c) other? 3. I'm considering building a Mosfet regulated supply for the screens. Would a regulated screen supply be beneficial? 4. Is there a way to correlate the sag of relevant voltages for minimum distortion? Perhaps a regulated screen supply + additional resistor for sag? How to calculate? Thanks
 7th November 2017, 08:25 AM #2 6A3sUMMER   diyAudio Member   Join Date: Jun 2016 The maximum signal screen current is more likely to be 25mA, not the 2.5mA you wrote.
 7th November 2017, 08:34 AM #3 glina   diyAudio Member   Join Date: Oct 2003 Location: Warsaw Thanks. Seems I mixed up. Zero signal current is 2.5mA, maximum signal current is 7mA. http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf This would imply that plate and screen voltages swing in opposite directions as signal increases.
DF96
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by glina It is my understanding that Class A draws 100% current with no signal, and minimum of 75% with peak signal (is that correct?).
No. Class A draws the same average current whatever the signal level.

Quote:
 1. How important is the screen voltage in SE operation?
The issue is not SE operation, but Class A pentode operation. For minimum distortion the screen voltage should not vary with signal, which means a low impedance screen supply. However, this primarily means low impedance at audio frequencies. It is less important that the screen supply has low impedance at DC, although this still matters.

Quote:
 2. Which is the condition for lowest distortion, best linearity: a) (Vplate-Vcathode)-(Vscreen-Vcathode)=constant b) (Vplate-Vcathode)/(Vscreen-Vcathode)=constant c) other?
You need (Vscreen-Vcathode)=constant, and Vcathode=constant. If you cannot achieve that, then try for (Vscreen-Vcathode)/(Vcathode)=constant.

glina
diyAudio Member

Join Date: Oct 2003
Location: Warsaw
Quote:
 Originally Posted by DF96 No. Class A draws the same average current whatever the signal level.
Zero signal plate current 54mA
Maximum signal plate current 66mA.
For Class A operation.

How should I interpret that for power supply load?

Quote:
 Originally Posted by DF96 The issue is not SE operation, but Class A pentode operation. For minimum distortion the screen voltage should not vary with signal, which means a low impedance screen supply. However, this primarily means low impedance at audio frequencies. It is less important that the screen supply has low impedance at DC, although this still matters.
I intend to build a regulator based on the circuit from Pete Millet:
DCPP Amp

Does a large enough output capacitor satisfy the low AC impedance requirement?

DF96
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by glina The datasheet linked above quotes Zero signal plate current 54mA Maximum signal plate current 66mA. For Class A operation.
That is because of second-order distortion. This generate harmonics and intermodulation; it also causes a DC shift.

Quote:
 Does a large enough output capacitor satisfy the low AC impedance requirement?
Only if the regulator is designed to drive a big cap. Many regulators have an inductive output impedance so adding a cap can increase output impedance.

 7th November 2017, 06:12 PM #7 6A3sUMMER   diyAudio Member   Join Date: Jun 2016 Quote: This would imply that plate and screen voltages swing in opposite directions as signal increases. No, the above is not true. Lets look at Pentodes and Beam Power Tubes Pentode Connected Mode: As the control grid voltage goes less negative with respect to the cathode voltage, then both the plate current increases and the screen current increases. Suppose the plate is driving a 5K output transformer. And suppose the screen voltage is fed by a series resistor coming from a regulated voltage supply. Then both the plate voltage and the screen voltage will decrease (voltage drop = I * R). The plate voltage and screen voltage move in the same direction. Caution, when the screen voltage is connected to a very well regulated stiff supply voltage: If the plate voltage swings much lower than the screen voltage, then the screen current will increase to a very large value (sometimes not a safe value). Look at pentode curves that show the plate voltage, screen voltage, and screen current all on one graph. As the plate voltage drops far below the screen voltage, the electron cloud only “sees” the screen, because the plate is physically on the other side of the screen, but the plate voltage is “closer” to the cathode voltage. The plate impedance is very high in pentode mode, the damping factor without negative feedback is very low. Ultra Linear Connected mode: Ultra Linear mode causes the screen voltage to decrease at the same time the plate voltage is decreasing, often at a 40% rate (i.e., 100V plate swing, 40V screen swing). Ultra Linear is a form of local negative feedback (not global, that would require additional circuitry). The plate impedance is high to medium in ultra linear mode, the damping factor without even using (global) negative feedback is low to medium. Triode Wired Mode: The plate impedance is lower in triode wired mode, the damping factor without even using negative feedback is medium to high. Triode wired mode is a form of local negative feedback (not global, that would require additional circuitry).
 7th November 2017, 11:37 PM #8 PRR   diyAudio Member     Join Date: Jun 2003 Location: Maine USA G2 current does change. However 90+% of hi-fi plays under 10% maximum power. Peaks are a few milliSeconds, rarely ever 100mS. So do you have a big cap on the G2 supply? A few mA change at 280V, with a 40uFd cap, will take *2.5 seconds* (2500mS) to shift. Much longer than hi-fi peaks last. Don't listen to me. Don't listen to PSUD. Put voltmeters on the amplifier (carefully!!) and play it. Most listening there will be "no change". I've done this for hours (testing repairs and new builds). If you are not grossly clipping, the voltage dips are very-very small. Small change of Vg2 will not upset the amplifier. In ideal class A, the Plate current is constant, yes. In real class A audio amplifiers, it depends. If the load impedance is correct for the tube idle operating point the current stays the same up-to and past clipping. But the makers' data-sheet often cheats the load down, so at clipping the current rises, because this gives a higher "Max" power output without exceeding the idle dissipation limit. I would guess that the majority of DIY amps don't have an exact-right load on test bench. And when driving a speaker, the load is "wrong" and high over most of the audio band. Driven into clipping, current will tend to fall. Of course this is far past the point of clean "quality". That's for hi-fi which may touch but never stays in clipping. Guitar amplifiers driven to CLIP!! are a further study (not simple) but not on-topic here.
audiowize
diyAudio Member

Join Date: Aug 2005
Location: Seattle
Quote:
 Originally Posted by PRR Don't listen to me. Don't listen to PSUD. Put voltmeters on the amplifier (carefully!!) and play it. Most listening there will be "no change". I've done this for hours (testing repairs and new builds). If you are not grossly clipping, the voltage dips are very-very small. Small change of Vg2 will not upset the amplifier.
I made this same observations some years ago with a single ended EL84 amp. It was quite surprising.

GoatGuy
diyAudio Member

Join Date: Dec 2012
Location: SF Bay Area
Quote:
 Originally Posted by DF96 That is because of second-order distortion. This generate harmonics and intermodulation; it also causes a DC shift. Only if the regulator is designed to drive a big cap. Many regulators have an inductive output impedance so adding a cap can increase output impedance.
I tip my hat to you, ol' debating partner. You're THE man.
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