surfsup wrote:Couple questions as I follow along:
The upper leg of the voltage divider drops 20V across the 500R.
Where do you get the 20V? half the 40 volts total drop on the schematic? I dont think that is correct because the first 500R has all the preamp current through it plus the screen current, the screen 500R only has the screen current so it wouldnt be 50/50. I guess it would only be a few volts difference but just curious on the clarification on the 20V.
The preamp stages are only drawing ~4mA altogether. And so yes, basically I crudely cut the 40V difference (between 590 and 550) in half between the two 500R resistors, which is 40mA passing through each resistor on average. But if you add 4mA for the pre-amp current through the first resistor, then that will increase the voltage drop across that first 500R a wee bit and decrease the voltage drop across the 500R at the input for the screen voltage divider a wee bit. Nevertheless, you still end up with 550V dropped across 22k, being 25mA through the bottom of the divider, and relative to screen current (say 5-6mA per screen for 6L6s or more for EL34s), then that is somewhere between about 10mA to 12mA going to the screens, which would still be about 1/3 as much as the whole divider is dropping.
The only consolation is that while one screen is sucking more current, the other one should be sucking only the idle amount (in Class AB1 - in the part of the cycle when the 'non-active' tube's screen voltage is, I think?, quiescent - but I stand to be corrected about that) .
Nevertheless this form of regulation is still subject to limitations of the effect of varying screen current on the voltage at the output of the voltage divider, and the higher the proportion of screen current to total current in the voltage divider,the bigger the potential output voltage fluctuation will be.
surfsup wrote:That is 40mA. The remaining 550V is dropped across the 22000R, which is 25mA. So 15mA is available to go to the screens (7.5mA per screen).
Aren't you also assuming here the max current is 40mA through the divider network when really the max current is if the screen went to zero, which would be 550v across the 500+200 = 785mA (assuming the screen went to zero (unlikely) and the resistors didn't fry - which they would). My point is I don't think 40mA is a "limiting" current value for the screens. It is only valid across the 220k.
I was thinking about screen current increase (not screen current decrease) affecting the current through the upper leg of the divider, as a proportion of the total current through the divider (i.e.: from input to ground), which therefore affects the voltage at the output (knee) of the voltage divider.
He who dies with the most tubes... wins
