Switching Screen Node Dropping Resistor

[Fresh_Start]

I'd like to experiment with different power tube screen grid voltages, particularly in conjunction with switching from fixed bias to cathode bias.  My thinking is that I'd like to get the screen voltage close to the plate-to-cathode voltage drop when in cathode bias mode.  My guess-timate is a 20-25 volt drop, requiring an additional 1K of resistance.  (B+ = 350, EL-34 power tube, single ended Class A, 3 12AX7s downstream of the screen grid node)

Two options have occurred to me:
1) Have larger resistor permanently in place and switch in parallel resistor to get lower value using SP/ST switch; or
2) Swap resistors using an on/off/on SP/TT switch where "off" cuts B+ from everything except the power tube plate

The first option worries me in terms of a big "pop".  Perhaps having a high value (100K) resistor bridging the switch would solve that.

The second option seems safer, especially using a DP/TT switch where the other pole switches from cathode bias to fixed bias.  No worries about NOT having any bias voltage even for a moment.  Except I don't know what happens to a power pentode if you remove the screen grid voltage entirely but maintain (actually increase slightly) the plate voltage.  Is that "BAD"?

Thanks in advance for your help,

Chip

[HotBluePlates]

Two options have occurred to me: ...

Option 3: Place the amp in Standby, then switch, then place the amp in Play.

I'm no switching expert, but one thing occurs to me: If you make a change that causes a d.c. shift, there will be a thump no matter what. Maybe not in this case, because there is a filter cap that could soften the transient.

Okay, okay... the real Option 3: Whatever your switched parallel resistor is, place a real big resistor in series. Say, 220k-470k. Place your switch across that big resistor so that when the switch is closed is shorts out that resistor. Open, you have a big honkin' resistor in parallel with your existing dropping resistor, so almost zero resistance change. Closed, your added resistor is in parallel (big resistor shorted) and the total resistance is reduced as expected.

About the desired total resistance value (for changed screen voltage):
Simple rule is that kilo- means 1,000 and milli- means 1/1,000. This can be used to make mental math easy. If you wanted a 25v drop and your screen plus preamp draws 25mA, you could use a 1k dropping resistor. If your screen plus preamp really only draws 2.5mA, you need a 10k dropping resistor to get a 25v drop. Consider this to zero in on your resistor value.

And one question. What exactly are you trying to do, and what do you hope to achieve with the change? I'm guessing you want to reduce the screen voltage while in fixed-bias mode to compensate for any difference in screen/plate-to-cathode voltage reduction when you're in cathode bias mode (or maybe you want higher screen voltage in cathode bias mode).

I wonder (and don't know for sure without experimentation) that a 25v screen voltage change will make an audible difference. At least, I don't know that you could hear the effect when it is steady and not due to screen voltage compression as the tube tries to make maximum power.

[Fresh_Start]

Option 3 = Option 1 but with a bigger shunt resistor (or whatever we should call it) across the switch

My muddled thinking was that in fixed bias mode, the screen grid and plate are at approximately the same voltage relative to ground.  The cathode is also at ground.  However, in cathode bias mode, the voltage difference between the plate & the cathode is lower because the cathode is no longer at 0 volts/ground.  If I lower the screen grid voltage by the voltage across the cathode resistor, the screen grid voltage relative to ground would be about the same as the voltage drop between the plate and the cathode.

Why is this important?  Damned if I know!  Somehow I got it in my head that screen grid voltage has a direct impact on power output, so this is supposed to further lower power output when the amp's in cathode bias mode.  (I'm assuming that the fixed bias mode will have greater power output and more headroom... please don't ask me to embarrass myself with the thinking behind that belief.)

I don't know enough to do the math or simulations, so I'll just have to do experiments.  At least the upcoming build's layout is designed for changes from the start.

Wish me luck!

Chip

[HotBluePlates]

Somehow I got it in my head that screen grid voltage has a direct impact on power output ...

Yes, it sure does. Well, the screen determines how much plate current the tube is capable of passing, and that current determines possible output power. So you're good on that front.

However, in cathode bias mode, the voltage difference between the plate & the cathode is lower because the cathode is no longer at 0 volts/ground.  If I lower the screen grid voltage by the voltage across the cathode resistor, the screen grid voltage relative to ground would be about the same as the voltage drop between the plate and the cathode.

I see.... Don't bother. 

The plate-to-cathode voltage decreased because the cathode voltage is now higher than your reference voltage (ground). But your tube also only feels its screen-to-cathode voltage, so that has decreased by the exact same amount as the plate to cathode voltage.

Your tube doesn't know what ground is or care what you choose to call "0v". If you place your meter's black lead on the B+ rail and say the cathode is at -385vdc, it's all the same to the tube.

So, you see that you don't need to make the change you were contemplating, because the tube already feels it.