is this a dumb question?

[jerrydyer]

any pro' / cons for using a standbye switch on pins 1-8 instead of the so called conventional way?
also if the -mV   number coincides with the mA number on pins 1/8 with a 1ohm res,  why bother doing all the extra work. why not just measure at pin 5...
 i measure my plate voltage (520 by the way)  , then my 70% of my 5881's are  16.1, comes to about 32 32 mA.. if pin 5 reads -32 or -33 am I not there.

amp sounds great just like all the other ones but every time I try to wrap my head around all the math, someone throughs a new wrench in it..

thanks in advance. 

[phsyconoodler]

Not sure what you are asking about the standby switch??Do you mean lift the cathode ground?Why would you want to do that?

 With the SAME tubes in the amp,the negative voltage will be fine,but as soon as you change tubes or they start to age,they may draw less or more current at the same negative voltage reading,so it is not the best way to check bias.It's simply a ballpark way to check to see if the bias supply is actually working.
  No two tubes are the same from one brand to the next.Mesa tries to make people buy their tubes by matching them and putting them in a range that draw the same current.

[jjasilli]

It has recently been pointed out that there are no dumb questions here; so don't worry about that!

1.  pins 1-8:  Check your tube charts.  Most octal power tubes do not use pin 1; but some use it for a internal connection!  The 5881 does not use pin 1.  Some versions may not even have a physical pin present.  Anyway, pin 1 is irrelevant here.

2.  Yes a standby SW can properly be placed between pin 8 (power tube cathode) & ground.  Unless you do the same to the PI & preamp tubes, then they will have no standby mode; but this is OK.  

3. Your math is OK, but note that the 5881 max's out at 400 plate volts:  http://www.mif.pg.gda.pl/homepages/frank/sheets/127/5/5881.pdf  At 360VDC, Class AB, -22.5 bias volts yields 44mA per tube.  The point is that the value of bias voltage does not equal the value of current draw.  

[jerrydyer]

I should have been cleare about 1 and 8 .. using 1 as a solder point for the 1ohm res on its way to ground. I like the standby like this .  My plate voltage right now is 517.... 5881 's cant do this ????? hhhhmm. so I see accrding to the numbers you posted, -MV and MA  numbers do not coincide or move at the same rate when different plate voltage is used?

for example  520 plate voltage , 5881 70% is 16.1 equals  32 33 ma...... does not necessarily equal  -32mV 

[jjasilli]

-MV and MA  numbers do not coincide or move at the same rate when different plate voltage is used?  for example  520 plate voltage , 5881 70% is 16.1 equals  32 33 ma...... does not necessarily equal  -32mV

Yes, note emphasis added to your quote.  If they do happen to match it is an accident of fate in that case.  E.g., checkout the 6L6GC in AB1 operation:  http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6L6GC.pdf  @ 450V , -37 bias volts yields 58mA per tube.  Clearly the bias voltage does not equal the current draw. 

They are not equal, but they are related.  As bias voltage increases, tube current decreases; i.e., higher bias voltage = colder bias, and vice-versa.  So what's the right bias voltage?  It is not the same number as the desired current.  For a guitar amp you probably want to run  at or near max plate diss.  You already nailed the formula for that.  So what fixed bias voltage to use? Go to the tube charts to get the value or at least a guesstimate based on the tube table.  Tweak bias voltage from there.

[panhead]

I like the idea of putting the standby switch in the power stage cathode circuit. I've never had a problem putting the switch in the plate supply circuit using the same type of switch as the AC power switch, but I can't help but wonder if the high voltage will someday arc the switch contacts. Your idea gets the switch it in a low voltage spot.

[jerrydyer]

works great. I can check other voltages in the simulated standby. You think there ouwld be any prod keeping this switch but still using the 1 ohm res between the tubes and the ground plane. I dont see why it would but there is so much I dont know.