Eurotubes biasing vid and plate voltage question

[stokes]

I think you are looking at this inside out,kind of."So a higher plate voltage will cause the maximum current draw in mA to be lower (i.e. indirectly proportional)".Your plate voltage in a given amp will vary as you change the current draw.The plate voltage isnt "adjustable",your bias voltage or current draw are.Lets take your MKIV for example.Mesa tells you it is not adjustable,that is not the case.For years many amps were made like Mesa with a fixed resistor setting the bias voltage,when you changed tubes,you changed that resistor to adjust your bias.But in the case of a modern MKIV if you have 470volts on the plate with a set of tubes that are drawing lets say 24ma's and you change to a set of tubes that are drawing 35ma's your plate volts will drop some.Or if you were to keep the original tubes and adjusted your bias voltage so the tubes would now draw 35ma's the plate volts would drop as well,see where we are going here?To use your Bias Rite,without using an adjustable bias mod,what you want to do is,get a set of tubes with an unknown "grading" lets say a random matched pair of NOS RCA 6L6GC's.You would install the tubes,with your bias rite set up,see what the plate volts are and the current draw.You multiply your plate volts times the current draw,using the above numbers 470v times .024 amps equals 11.2 idle watts that is a "safe" idle for those tubes,somewhat cold though,class AB1 you would want to see about 20 idle watts.If you are not going to get into adjusting the bias,the best you can do with your bias rite is to check to verify that whatever tubes you are using are "safe",which in my opinion is not always the best.You will never see a stock Mesa amp idling 6L6GC's at 20watts.If they set the amp to idle that hot and then sold you a set of their "pre-graded tubes" that turned out to be hotter than expected you could fry something,not very inducive to selling tubes if they have users frying their amps,eh?

 

[stokes]

"what effect does it have on the amp/volume/tone when the plate voltage does drop? increasing the current draw affects the plate voltage. apparently percentage-wise it's not a big drop according to that video."
The amount the plate volts drop/raise when adjusting the bias isnt going to affect the output volume much at all.Tone will be affected more thru the change in the operating parameter of the tube at different current draw.The power out (watts) is a product of many different factors whithin the circuit,the amount of power the PT can deliver before it begins to sag,the voltage swing when the signal hits the power tubes which is a direct product of how the signal is amplified thru the preamp on the way to the PI and of course how big the signal at the input is.If you measure the output of an amp with a Strat in the input it will be smaller than if you put a pair of high output humbuckers in.Manufacturers design for a plate voltage that will fit their need according to the tubes they will use and the target output.
so what if a mark iv plate voltage was 370volts or 570volts (if possible)? what effect would that have?"
You wouldnt likely see a voltage swing that wide,but generally speaking if you had 2 different PT's putting out those stated volts the 570v scenario would be capable of putting out much more watts than a PT putting out 370v's.

 

[dethrattle]

\m/ :twisted: \m/ BOB rules

 

[stokes]

Higher plate voltages will generally give more power and more headroom,unless your tubes are biased very hot,which will reduce headroom and cause the tube to distort sooner.So it is really a combination of both.In a preamp if you adjust the resistors in the B+ rail to drop/raise the volts to the preamp tube plates,lower volts will result in a warmer,browner tone,higher volts will stiffen things up edging towards a more brittle,shriller tone.