another dumb question.

[catnine]

 When P-P fixed bias amp is at idle and you measure a 4mA draw across the screen dropping resister and at idle the PI has no control grid signal are the two 6V6's both on at the same time? ie would the 4mA screen draw = 2mA screen draw for each 6V6 since they share the same 1K screen dropper?

[sluckey]

Just put a 1Ω resistor in series with each screen. Then all your questions can be answered by measuring the voltage across each of those 1Ω resistors.

[catnine]

I already have 470 ohm screen resisters in series and across both I have at idle  0.0005 amp on each 6V6.

 What I was looking for was when an amp is at idle and no signal just like when checking the mA draw to bias the 6V6's and since we are measuring DC and not AC and I can't measure 60HZ when DC is not alternating current is it correct to say that at idle the screens are a shared mA draw across the screen dropping resister?

[HotBluePlates]

HBP has left the building.

[catnine]

Measure voltage across the 470Ω resistor; black lead on one side, red lead on the other. Turn the amp off. Measure resistance of the 470Ω resistor. Divide measured voltage by measured resistance.

Because that 470Ω resistor is only connected to that one tube, the current flowing through it can only be for that one tube.

 Ok That's what I did . I also checked the droppers from each node to chassis ground and with a 2 volt drop after the 1K screen dropper I have 2 mA . Then I measured across the 1K like you said and got 4.5 volt drop across the 1K screen red on one side black on the other ended up with 4 mA yet there has to still be the preamps 3 mA draw always in the rail . If I measure across the two preamp droppers red on one side black on the other side  they are still  3 mA across each of them . There has to be something I'm missing here . I know if I read across one resister I should just get the drop across the one resister at least it seems so. If the 4 mA is correct  and the screen dropper is shared by both 6V6's that has to be 2mA screen current each 6V6 so that would mean that reading after the 1K to chassis ground is reading both 6V6's sharing the one  1K dropper.

[sluckey]

that would mean that reading after the 1K to chassis ground is reading both 6V6's sharing the one  1K dropper.

That's correct. However, the preamp current also flows thru that resistor (assuming you have a typical series node B+ rail).

[HotBluePlates]

HBP has left the building.

[catnine]

that would mean that reading after the 1K to chassis ground is reading both 6V6's sharing the one  1K dropper.

That's correct. However, the preamp current also flows thru that resistor (assuming you have a typical series node B+ rail).

 Yes I do have the typical series node B+ rail.

 When I check just across the 1K screen dropper and I see a 4.5volt drop the DVOM red and black leads on either side of the 1K I use ohms law and end up with 4 mA. Yet the DVOM leads must act as a path around that 1K allowing the preamp mA draw into the mix. It only makes sense because when I measure from each node to ground then I read a 2 volt drop across the 1K = 2mA and since the preamp draws 3mA whether I assume each triode draws 1mA each or measure  it's still 3 mA and if I subtract the questionable 2mA screen draw from the 3mA assumed or read preamp I then end up with 1 mA draw left for the screen and since the screens share the 1K dropper then I would have 0.5mA draw at idle for each 6V6 screen. I get that same reading if I measure across each 470  ohm screen resister on each 6V6 tube socket which are in serier with the 1K . There I read a .265 volt drop across 470 ohm = 0.5 mA  

 

[sluckey]

You will get a more accurate voltage reading DIRECTLY ACROSS the resistor than you will by measuring voltages on each side of the resistor with reference to ground and doing the math.

[catnine]

... If the 4 mA is correct  and the screen dropper is shared by both 6V6's that has to be 2mA screen current each 6V6 ...

Or 3mA and 1mA, or 3.5mA and 0.5mA, or 0mA and 4mA, or any other possible combination that totals 4mA.

Which is why I suggested finding each 6V6's screen current separately (if you were mainly concerned with the screen current of each tube).

If you're only trying to see/understand how the amp's current is pulled through each of the dropping resistors, it doesn't really matter how each tube divides up the current.

 I am really only concerned with the actual screen draw for each 6V6. If I really want to be exact instead of stating 3 mA preamp draw when the actual reading is 0.0035 that would be 3.5 mA preamp. Or pull the preamp tubes .

[catnine]

You will get a more accurate voltage reading DIRECTLY ACROSS the resistor than you will by measuring voltages on each side of the resistor with reference to ground and doing the math.

 I did that also . Now just to be clear the preamps are fed off two droppers in series with a 16uf cap between them and a 16 uf cap at the end of the rail . so if I measure across the 10K dropper there is a 33 volt drop = 3.3mA and across the 22K dropper 73 volt drop = 3.3 mA . If the 4.5 mA across the 1K which was a 4.5 volt drop across 1K is close enough 4.5mA - 3.3 mA = 1.2mA / 2 = 0.6 mA per 6V6 screen since the 6V6 screens share the 1K dropper. Pretty close to what I got across the 470 ohm screen grid resisters with a .265 V drop .

 I can't even say why I am so concered about the screen current draw . It's not I am designing or building amps for anyone. All I do know is this build has never sounded so good and I don't play gigs or even play the amp cranked because it's to loud . It's not like when I did play it cranked anything smoked . I can turn the amp up and dial the guitar vol down and get a nice smooth OD sound . I did use 3 watt droppers . I have the B+ fused after the rect tube pin 8 with a 1/2 amp slow blow which should be more like 150mA .