... Still debating which way to go with the voltage regulation, I may do a switch with both.
Question.....
How do 5881's respond when they are not sliding screens?
I read this as "voltage-regulated screen supply" (which might be wrong). What is implied is a stiff screen supply and possibly no screen resistance. That will give you every last watt for your supply voltage, and will have no compression.
If you have series screen resistance, you might as well skip regulating the screen voltage, as changing screen current through the series resistance will cause the voltage right at the screen pin to vary. It will vary less with smallish screen resistance (100-470Ω), more with high screen resistance (2k-3.3kΩ).
High series resistance can be used to infuse sag in an amp which doesn't get it from other sources (like a tube rectifier, high PT resistance, etc). The screen voltage dictates plate current more than plate voltage, so if the screen voltage can sag, it will reduce maximum power output somewhat and result in a compressed sound. Note that "sag" or "compression" derived in this way (or with a rectifier tube) really only kicks in when the tube has run out of clean power; i.e., it happens at high volume. You can select the series screen resistance to get this to happen at a somewhat lower volume, within reason. The only cost is reduced power output & volume (and it's easily reversible).
The above approach is handy when you need (or want) to use silicon diode rectification, but the amp would otherwise be too stiff sounding.
If the above sounds appealing, you might as well skip voltage regulation altogether. Regulation will mainly provide an un-sagging supply voltage, and would be good if you needed every last watt and a stiffer, more solid feel. If you undo the effect of regulation on the screen, then it is pretty pointless to regulate the plate supply voltage (except if you plan on using powerscaling or VVR, which uses a variable output voltage from a regulator to dial down output power).
How do 5881's respond when they are not sliding screens?
Ie 360v at the plates and 270 or so on the screens. Lower output is ok in this case.
If the question was really only "what does 360v plate and 270v screen sound like?" then the answer is same as any other condition, except power output might be reduced compared to 360v plate and screen. That's because the tube then has a lower peak plate current possible, because the screen voltage is lower than if you had 360v screen.
But actual output power is a function of supply voltages and load impedance. Scroll to the bottom of page 2 of the
Tung Sol 5881 data sheet. Two conditions with 360v plate and 270v screen are given, with the exact same bias voltage, idle currents and input signal level. One is loaded with 6.6kΩ and gives 26.5w output while the other is loaded with 3.8kΩ and gives 18w output. The lower load impedance results in a little more maximum output plate current though less power drives the speaker.
... 360v at the plates and 270 or so on the screens. ... What should I be shooting for current wise for the PT? My guess is approx. 200ma.
If you actually use 360v plate and 270v screen, the data sheet tells you exactly:
Assume the 6.6kΩ load case; at full output you need 132mA for the plates, 15mA for the screens, and you might guess 1mA per triode for the small tubes (say 4mA for the 2 octal tubes you said you'd be using). Grand total of 151mA for the B+.
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