Importance of Screen Voltage of Pentode??? - diyAudio
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Old 2nd March 2012, 03:44 PM   #1
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Default Importance of Screen Voltage of Pentode???

Hey everyone, just a general question on power tubes...most of the power tube data sheets I see tend to have the screen voltage a good 50 volts below the plate voltage yet 99% of the time I see tubes in real applications they're only running a couple volts below the plate...how critical is this voltage? Would performance/efficiency/tube life be increased at all by getting these voltages closer to the theoretical value or is "a little bit less than plate voltage" good enough. Thanks.
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Old 2nd March 2012, 05:10 PM   #2
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In a pentode, anode current is most defined by screen voltage, the anode voltage is less important defining the plate current. Such important is the screen voltage, that this value divided voltage at grid #1 (Signal) defines the cut off current by a coefficient called g2/g1 mu value. Anode voltage defines few % of the cut off current. Normally, the screen must be at or under plate voltage, but practically because of transformer primary resistance, in turn it is plate voltage less than screen.
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Old 2nd March 2012, 05:17 PM   #3
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Screen voltage doesn't need to be far below the plate voltage, as long as you're not above the maximum. The datasheets likely also tell you that max plate voltage is typically higher than screen grid voltage. If you tie the screen grid directly to the plate then obviously you'll be putting the pentode into triode mode, but a fixed voltage slightly lower than the plate is normal, as you've seen. Tube life would be a question. Screen grids have a maximum dissipation wattage, just like plates that you need to be careful with.

I've been contemplating a new pentode design and would like to add to your question (hopefully not a highjack) and ask, what effect on tone would there be with different plate to screen grid voltage differentials? ie, how would the amp sound with screen = plate - 10V, vs screen = plate - 100V?
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Old 2nd March 2012, 05:23 PM   #4
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doozerdave: I donīt know about sounding because iīm not a musician, but surely the screen voltage affects directly to plate current, and transconductance. So gain will be different, possibly lower than with normal screen voltages approximate equal to plate voltages. I donīt know how it can modify sound quality.
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Old 2nd March 2012, 05:58 PM   #5
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Perhaps I'll experiment with it then. I have a spare power transformer that I want to use in a single ended pentode design, but it's 700Vrms (at 115V primary), which is giving me 500Vdc because I have 123V mains in my house. I want to use a 6550a or EL34 (interchangable), so I need to be careful how I design this.

Anyway, I don't want to high jack with that topic.
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Old 2nd March 2012, 06:24 PM   #6
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Difference in screen voltages changes shape of plate curves. For higher currents on lover anode voltages of course you want higher G2 voltages. But the drawback is, when anode voltage goes down with current G2 current starts going up sharply. G2 can be overheated and damaged. What matters, peak and average power dissipation of screen grids. Max screen grid current and max power dissipation are usually specified by manufacturers. Voltage does not matter, it is up to you to decide what is the best compromise between shapes of curves and reliability. Lower G2 voltage can be good as well for high swing linearity when the tube is loaded on high resistance. I usually stabilize G2 and G1 voltages of output tubes by solid state regulators.
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Last edited by Wavebourn; 2nd March 2012 at 06:26 PM.
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Old 2nd March 2012, 07:10 PM   #7
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The important thing is that whatever g2 voltage you choose you maintain it. An unstable g2 voltage creates intermodulation, and could be part of the reason why pentode output stages are unpopular.
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Old 2nd March 2012, 11:27 PM   #8
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i found out that regulating the g2 voltage really makes for a good sounding amp, in some cases, my 6LU8 pp amp has better bass and overall sound than my 6P3C ultralinear pp amp.......
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