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Old 25th February 2013, 10:49 AM   #1
jazz is offline jazz  Netherlands
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Default screen grid resistor value

Hi,

I'm building a pp1-c as designed by Allen Wright. As I am a bit cash strapped at the moment I want to do this as cheaply as possible. Given that i don't have any 470R power resistors at the moment, what would the effect of going to 1.2K be?

I mean they are only there for stability and current control reasons, or am i wrong there?

thnks
Joris
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Old 25th February 2013, 10:53 AM   #2
DF96 is offline DF96  England
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The main effect would be that the resistor would need to be bigger, as it would be carrying a slightly smaller current but with a much bigger voltage drop. There might be a small reduction in output power and a small change in distortion.
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Old 25th February 2013, 10:58 AM   #3
roline is offline roline  United States
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I use 1k on most of my designs, 1/2 watt. This resistor with the miller capacitance form a low pass filter preventing high frequency oscillation.
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Old 25th February 2013, 04:23 PM   #4
Svein_B is offline Svein_B  Norway
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The only 470 ohm resitors on the PP-1C diagram are the EL34 cathode resistors, specified as 470 ohm 10 watt.
Diagrams here: http://www.vacuumstate.com/index.dna...289.6650079573

The resistor value should not be too much different from the 470 ohm.
You might get away with two 1200 in parallel for 600. Three for 400 ohm may run the tubes a bit hot.

5 watt rating might be OK instead of the 10 watt specified.

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Last edited by Svein_B; 25th February 2013 at 04:29 PM.
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Old 25th February 2013, 06:17 PM   #5
jazz is offline jazz  Netherlands
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Hi,

Thanks for the answers, the 1k2's will do for now.

Joris

ps Svein there's actually seven per channel in total...
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Old 26th February 2013, 02:06 AM   #6
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The PPC-1C schematic
Schematics

Clearly show screen grid resistors of 470 Ohms (triode strap resistors). Value is not super critical, 150 Ohms is minimum recommended for triode straps. 2 of your 1K2 in parallel to give 600 Ohms which also would be fine.

Cheers,
Ian
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