Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Champ_49 on August 07, 2023, 02:39:37 pm
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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.
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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.
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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.
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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
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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
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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.
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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.
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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.
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Now give him the formula to convert peak power to average power and express in dbm. :icon_biggrin:
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:l2:
it's 5am