Measuring V+(plate voltage)

[Champ_49]

I hope I posted this on the right thread. I apologize if not. I'm still confused what plate voltage is. Let me explain what I mean.  This is to bias the tubes. On the web you would need this value to calculate the current draw to bias the power tubes at let's say 70% of max plate dissapation.

My question is when you bias the tubes or current draw it CHANGES the plate voltage. So the plate voltage is not a constant value but VARIES. So what exactly is the plate voltage that you use to calculate the bias current draw?? I apologize for the dumb question if it is a dumb question.

[PRR]

It varies a little. NOT a lot. Unless your power supply is lame.

Get some numbers from a trial bias. Adjust. Check. One or two rounds should be plenty close.

[Champ_49]

It varies a little. NOT a lot. Unless your power supply is lame.

Get some numbers from a trial bias. Adjust. Check. One or two rounds should be plenty close.

This is what I measured. I have put in seperate bias pots for each tube. Roughly the current draws are as follows and the plate voltage that corresponds to that current draw for each tube.

14mA - 471V
15.8mA - 468V
26mA - 461V
41.5mA - 451V
55mA - 443V

From 14mA to 55mA that is a 28V difference. Would this be considered alot? Or negligible in this case.

[Esquirefreak]

Isn't the whole point of each tube having a dedicated bias pot to balance the bias? Or why else would you do it that way?

/Max

[shooter]

take those #'s and do some math to get Watts, you should see a correlation.


all that goes out the window when you start jammin some heavy medal, followed by lounge music, followed by.....


if you have 1 ohm resistors cathode to ground on the power tube(s) you can "monitor" the real-time effects of the AC signal effecting the DC characteristics

[sluckey]

From 14mA to 55mA that is a 28V difference. Would this be considered alot? Or negligible in this case.

I would consider that typical, nothing to get up about.

[Champ_49]

take those #'s and do some math to get Watts, you should see a correlation.


all that goes out the window when you start jammin some heavy medal, followed by lounge music, followed by.....


if you have 1 ohm resistors cathode to ground on the power tube(s) you can "monitor" the real-time effects of the AC signal effecting the DC characteristics

Yes. I have 1ohm resistors to ground and then to test points.  I've seen the fluctuations with the multimeter when injecting an AC from the guitar. When I first saw it the readings on the multimeter jumped quite high. Freaked me out a bit but realized it's normal and it's AC so there are lows and high peaks which balance out.

What matters would be the bias at idle.

[shooter]

it's normal and it's AC so there are lows and high peaks which balance out.

In my world we had Peak power and Average power.  I could get 25Kw of peak power with an average power of 500W.
Time is the factor to consider, duty cycle.  So a long hard note feeding back through the speaker is gonna make the tube work overtime, where a quick pick note that's hand muted will let the tube lounge around n cool off.

[sluckey]

Now give him the formula to convert peak power to average power and express in dbm.   

[shooter]

 
it's 5am