That's an element of what I am curious about and was asking myself...tubeswell wrote:The thing I was wondering about using a voltage divider to set screen voltages in an output stage is:
Allowing for the required amount screen current, wouldn't you need to set up the resistances in the divider so that they pass a relatively high amount of current through the divider - such that any changes in screen current do not hugely affect the voltage at the knee of the divider so much? In which case wouldn't the resistors in the divider need to be lowish resistance (and high-power) resistors, and in which case would they not use up a whole heap of B+ current?
I know of one guy (Steve/Hangman @ ampegv4.com) who used 20 Watt resistor pair of 2.2k/10k voltage divider, for a time, and said newcomb (sic?) amps used a similar configuration. Of course, the lower the overall resistance of the divider, the more heat yielded apparently, and this amp is already "a pretty good space heater" as is.
I assume the highest wattage/lowest resistance pair was used in order to minimize current. That said, the screens shouldn't be drawing (much) current anyhow, and while most people are aware that they do, it's evaluated in amps that run the standard configuration where the screens and plate supply are rather close in voltage. Likewise, the preamp valves shouldn't draw much current, but it's a design factor if you are 'tapping off' the B+ for the voltage divider and only using that for the screens and feeding the preamp's tubes separately through it's own tap, or if the preamp tubes are following the voltage divider/screen tap in series.
I know when I was screwing with a tube preamp, I wanted to mess with dropping voltage to hear tonal differences. Indeed, lower voltages sounded different than using higher voltages (think Soldano, Mesa Dual Rectifier) for preamp tubes. However, the 'sag' effect yielded by using very large resistors before a filter capacitor to ground, in order to effectively drop the preamp voltage incrementally became increasingly noticeable after a certain point of resistance, too. Hard to isolate the variables without a vast variety of parts and components, at your fingertips (including time and motivation), and know what difference one think's is being heard may best be attributed to...
Again though..my question is...does bypassing each resistor of the voltage divider with a capacitor effectively reduce/eliminate the 'seen' resistance in the voltage divider? Indeed, does significant stiffening and reduction of the screen supply cause the screens to draw significantly less current than the common (for guitar amps) configuration where each power tube screen is effectively isolated from the plate supply but at/just above/just below the voltage of the plates?
Has anyone analyzed an amp, such as an SVT with modded with proper screen resistors, 6550s run properly (with the screen voltage on it's own rectified transformer tap, at a voltage way below the plate voltage) for how much screen current they're putting out, and if it's noticeably lower than the more common, aforementioned configuration in guitar amps?