Hi Guys,
I'm missing some knowledge.
I can't figger out how to calculate the optimum screengrid stopper for a guitaramp with EL84 class A (cathode biased).
Till now I choose a resistor by looking at the screencurrent (~ 5 mA) at iddle.
Does anyone know how to calculate this resistor to get the screens in a tube save area and having a good sound (blues)?
What I've learned till now is:
- it depents on the actual platevoltage, the level of the screenvoltage and the max screen dissipation.
Also:
- if the screen resistor is to low and the screenvoltage is to high it can burn up the screen. On the other hand, if the screenresistor is to high, the screen voltage fluctuates to much and this influences the sound (linearity).
I once read somewhere that Philips suggested to use a resistor in the range 500 - 1000 Ohm in each screen grid supply line.
So does anyone know how to calculate this resistor and what if the calculated optimum screen resister is way to high? (well over 1000 ohm)
regards
Guss
Again question about screens
Moderators: pompeiisneaks, Colossal
Again question about screens
Last edited by Guus on Thu Oct 03, 2013 12:49 pm, edited 3 times in total.
Re: Again question about screens
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Re: Again question about screens
What's important is to measure screen current under signal conditions. Unless voltages are really high, you won't be exceeding screen dissipation at idle.
For example, I have recently posted about an EL84 amp I built with 365vdc plate supply (wrt cathode), and 3.3K/10W resistor in place of a choke, then just 100R screen resistors. under max signal conditions screen dissipation is just at 2W, which is the max, and I can see no screen glow through the plate structure, which is a clear indicator of over-dissipation.
Now, if I had a choke in there, I would definitely be using bigger screen resistors! I have found that even at specified voltages, 310vdc on plates and 305 on screens (again, wrt cathode) that I can still get the screens to glow and over-dissipate (up to 4W) with 1K screen resistors. 1K resistors yield a slightly squishier compressive tone, and limit output power.
If you are close to specs a little screen glow won't instantly kill your tubes, but of course it should be avoided. I don't trust the old specs with new tubes.
For example, I have recently posted about an EL84 amp I built with 365vdc plate supply (wrt cathode), and 3.3K/10W resistor in place of a choke, then just 100R screen resistors. under max signal conditions screen dissipation is just at 2W, which is the max, and I can see no screen glow through the plate structure, which is a clear indicator of over-dissipation.
Now, if I had a choke in there, I would definitely be using bigger screen resistors! I have found that even at specified voltages, 310vdc on plates and 305 on screens (again, wrt cathode) that I can still get the screens to glow and over-dissipate (up to 4W) with 1K screen resistors. 1K resistors yield a slightly squishier compressive tone, and limit output power.
If you are close to specs a little screen glow won't instantly kill your tubes, but of course it should be avoided. I don't trust the old specs with new tubes.