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Plate vs grid 2 voltage


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7/10/1998 12:08 AM
Nils
Plate vs grid 2 voltage
What's the theory behind the grid 2 voltage  
Usually there is B+, a filter choke and then  
B+1 for grid 2.  
Mostly the B+1 is a few volts lower but then as in the concert schematic the voltages are the same.  
I think I understand about the need for filtering but how about the voltage, are there any rules for this?  
Can anyone explain please
 
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7/14/1998 6:52 AM
JR

It is for collecting the socalled "secondary emission electrons" which are coming off from the plate when the main flow of the electrons are hitting to the plate.  
In other words, the effect of this is that, you're getting more gain with the power penthode/tetrode when connected properly  
 
The Grid 2 (screen) is connected to the slightly lower voltage than the plate .  
If it operates on same voltage as the plate  
it's going to act like the plate/anode ,and that's no good.  
Are those voltages in the concert schematic really the same ?  
 
JR
 
7/14/1998 8:39 AM
Doc

You may be confusing the purpose of Grid 3, or Suppressor grid, with Grid 2 (screen grid).  
 
The screen grid is an accelerating electrode, and helps draw electrons from the cathode toward the plate. Most of these streaming electrons pass right between the thin grid wire helix and hit the plate. A small percentage of the electrons do collide with the wire in their path and are conducted out to the positive screen supply without passing through the transformer load.  
 
The idea is for the screen to attract the electrons toward the plate, but detour as few as possible from their intended destination. The screen voltage should be less than or equal to the plate voltage, certainly no more than. It also depends on the tube design and under what plate voltage your circuit is operating, how high you can run the screen. Some tubes can operate under very high plate voltages, but have a much lower limit on the screen.  
 
The other night I was messing with a silverface Champ. The stock power supply setup had the screen voltage higher than the plate, which is unsafe from a tube life standpoint. I added a 470 ohm resistor in series with the screen (missing from most, or all, Champs), but that didn't lower the voltage. So I changed the 1k series dropping resistor between plate B+ and screen B+, finally to 5.6k (2.2k wasn't enough) which placed the screen voltage in a desireable relationship with the plate. This amp, by the way, is running a 5881 in place of the 6V6. It is probable that the higher current draw of the 6L6 type was causing a higher than normal voltage drop through the output transformer (although it's not getting hot) bringing the plate voltage lower than the screen. Something no one mentions as a caution when converting a Champ to 6L6.
 
7/14/1998 10:46 AM
Jack Orman

quote:
"The idea is for the screen to attract the  
electrons toward the plate, but detour  
as few as possible from their intended  
destination. The screen voltage should  
be less than or equal to the plate  
voltage, certainly no more than."
 
 
And I guess this is the mechanism by which a pentode generates more gain than a triode? Would it be possible to control the screen voltage to vary the gain of the pentode? i.e. make a lower powered amp by cranking down on the screen voltage, or even an output power control by switching the screen voltage between a high and low value.  
 
regards, Jack
 
7/14/1998 12:41 PM
Doc

Right, Jack. That's the end result, the tetrode & pentode have higher gain because of the placement of the screen grid. The mechanism is more complex, electron space charge clouds and all that high-brow electron physics stuff.  
 
I've always thought that the gain of an output stage could be varied by varying the screen voltage while holding the plate voltage constant, but I haven't actually run a meaningful experiment. Since the screen doesn't draw a high current, the supply could be from a variable voltage divider between B+ and gnd. My guess is the resulting tube output vs. screen voltage won't be a linear relationship, and may change the operating point enough to be close to the knee of the curve. (Look at some 6L6 plate curves and where the load line falls.) The resulting sound may not be pleasing. It'll take some experimentation to determine this method's usefulness. Sure would be nice if it works. I wonder if some innovative circuit designer like Kevin O'Connor has tried this.  
 
Doc
 
7/14/1998 7:03 PM
Jack Orman

quote:
"I've always thought that the gain of an output stage could be varied by varying the screen voltage while holding the plate voltage constant, but I haven't actually run a meaningful experiment. Since the screen doesn't draw a high current, the supply could be from a variable voltage divider between B+ and gnd. "
 
 
My thoughts exactly! You could make a power output control by varying the screen voltage of a pentode. I could only find 2 curves for screen voltage vs. gain but both seem to indicate the useable range is from 0 to 10v or 20v or so.  
 
Use a big resistor in series with a pot to drop B+ so that 20v is across the pot (bypass the pot with a cap for noise filtering). Tie the screen to the wiper of the pot. As the pot is varied from 0v to 20v, the gain will go from 0 to near max. The response looks semi-log so maybe a log taper pot (or is that anti-log... I'll have to ponder that).  
 
I'm designing a small all-tube practice amp and I may try to include this feature.  
 
 
regards, Jack
 
7/15/1998 5:57 AM
Graywater

Doc: I have to scrape up the details but a close friend made an amp whose screen voltages varied inversely with signal amplitude - a compressor. I have fooled around with using a 6L7 pentagrid amplifier (not a converter, but could be used as one) as a compressor/expander - this tube was popular for this use in the 1940. So, anyway, George, decided he'd "borrow" my variable grid bias for gain control and adapt it to the output stage of an old Super Reverb attempting to get the saturated output tranny type distortion at lower volumes. I'll call him up but if my memory serves the circuit worked as a compressor, with some R/C time constant clicks and such, but the desired tone was never achieved.
 

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