Bias question

[12AX7]

Putting a 50k trimmer in my classic 3O and my questions are 1-the plates are 340 VDC, isn't that high for el84s? 2- the bias supply is at -16 VDC...i will likely set the soon to be bias trimmer to that same -16v  but do u think thats a good compromise for safety w/o effecting tone too much or where would u set it ?

[passaloutre]

I would measure the plate current and set the bias in for 70% of the max dissipation

[12AX7]

No way to do that tho i am considering 1 ohm cathode resistors if it can be done on this board. Haven't yet looked at it but if theres a easy accessible trace i can cut and solder a resistor over i'll do that. (circuit board mounted sockets)

[stratomaster]

No way to do that tho i am considering 1 ohm cathode resistors if it can be done on this board. Haven't yet looked at it but if theres a easy accessible trace i can cut and solder a resistor over i'll do that. (circuit board mounted sockets)

Look into the output transformer voltage drop method. No need to modify anything, and you'll get accurate measurements. The added pot will enable you to target the recommended dissipation.

[12AX7]

No way to do that tho i am considering 1 ohm cathode resistors if it can be done on this board. Haven't yet looked at it but if theres a easy accessible trace i can cut and solder a resistor over i'll do that. (circuit board mounted sockets)

Look into the output transformer voltage drop method. No need to modify anything, and you'll get accurate measurements. The added pot will enable you to target the recommended dissipation.

I will, hanks. Never heard of that b4. Dos the speaker need to be disconnected?

[pdf64]

Streuth no!
Never operate a valve power amp without a load.
Even if it's just idling.

[12AX7]

I haven't looked up this method so i didn't know if anything like measuring the OT must be done w/o a speaker, not when on. I know not to run it without a load. I'm no a tech to put it mildly but I'm not that clueless ! 

[acheld]

RobRob has a decent instruction for how to do this:

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

The 1 ohm cathode resistor method is easier IF one is present.  But the "Output Transformer Resistance Method" gives accurate results if you don't have one.

[12AX7]

tried it and it works fine, tho one thing has me confused. A smaller -bias voltage should be hotter, IE: -16 vdc bias should create LESS current draw and -15, yes? yet using the transformer method i did it with -16 vdc then again with 15 vdc and for -16 i got 23 ma and for -15 i got 18 ma. what am i missing?

[AlNewman]

Are you multiplying by the voltage to find wattage?
Voltage drops as current increases.

[12AX7]

Are you multiplying by the voltage to find wattage?
Voltage drops as current increases.

I used the output tranny resistance method as per the site linked below. I set the bias to -16vdc then measured the CT and plate voltages and did the math with that and the resistance from CT to plate as in the instructions. Then I readjusted the bias to -15vdc and did all the same steps. The results were more current draw at -16 volts than -15. https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

[AlNewman]

soo, what were the voltages?  How many watts were you dissipating at each setting?

[12AX7]

Don't know, i just know the current draw which seems to be opposite of what it;s supposed to do. Obviously a lack of understanding on my part not being a tech. But i'm just trying to learn why i'm seeing that,

[AlNewman]

I don't understand.  Current doesn't mean anything unless you cross reference it with voltage to find dissipation.
Power=Voltage x Current.

[12AX7]

Well, It should be about 7.7 with 16 v bias supply according the figures i posted above. 70% would be 8.4 so thats what i''ll shoot for when my JJs arrive. Still doesn't answer my question tho.

[pdf64]

Are you multiplying by the voltage to find wattage?
Voltage drops as current increases.

I used the output tranny resistance method as per the site linked below. I set the bias to -16vdc then measured the CT and plate voltages and did the math with that and the resistance from CT to plate as in the instructions. Then I readjusted the bias to -15vdc and did all the same steps. The results were more current draw at -16 volts than -15. https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Your results are anomalous, run counter to what's expected.
I suggest to repeat the experiment, noting the mains supply (wall outlet AC), OT CT and screen grid voltage for each bias setting.
Note that being regular copper, OT winding resistance will increase with its temperature.

[12AX7]

Are you multiplying by the voltage to find wattage?
Voltage drops as current increases.

I used the output tranny resistance method as per the site linked below. I set the bias to -16vdc then measured the CT and plate voltages and did the math with that and the resistance from CT to plate as in the instructions. Then I readjusted the bias to -15vdc and did all the same steps. The results were more current draw at -16 volts than -15. https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

Your results are anomalous, run counter to what's expected.
I suggest to repeat the experiment, noting the mains supply (wall outlet AC), OT CT and screen grid voltage for each bias setting.
Note that being regular copper, OT winding resistance will increase with its temperature.

I did this many times and trust me, the results aren't going to change. no matter how many times i did it, even if i did something wrong the fact is both bias setting always yielded the same relative difference....the lower the bias the lower the draw. As to the resistance, i did consider that but it should be done when hot since thats where it will be when in use, and that what i did. i do however question whether the change between turning  off the amp n when u measure it will wrongly affect the results. So i tried to do it as fast as i could after turning off the amp. But again, thats not the reason for the anomalous results with the different  bias voltages always showing the odd current draw relative to each other.

[AlNewman]

Where are you measuring the bias voltage at?  Directly at the tube pin?

[12AX7]

At the bias supply where i installed a trimmer.

[AlNewman]

Maybe somehow you're referencing somewhere along the voltage divider.
Try measuring at the tube, or grid stopper and make sure your ground reference is at chassis.  Be cautious if measuring at the tube, a slip could cause a short.

[pdf64]

... Your results are anomalous, run counter to what's expected.
I suggest to repeat the experiment, noting the mains supply (wall outlet AC), OT CT and screen grid voltage for each bias setting.
Note that being regular copper, OT winding resistance will increase with its temperature.

I did this many times and trust me, the results aren't going to change. no matter how many times i did it, even if i did something wrong the fact is both bias setting always yielded the same relative difference....the lower the bias the lower the draw. ...

The bottom line is that you're doing something wrong, making a mistake somewhere.
If you provide sufficient, high quality info, that error can be identified sooner.
If you're not willing to do so, why ask in the first place?

At the bias supply where i installed a trimmer.

Please supply a schematic showing exactly how you've done this, and showing what your test point is.
Please don't attempt to describe what you've with words, there's too much room for misunderstanding / misinterpretation, use a schematic.

[12AX7]

I'll post the full schematic again in a few once i identify the points i used to get center tap and plate voltages and resistance. In this crop i tested the voltage where the "bias" point is shown and replaced the 47k with a trimmer and the 33k with a 22k.

[12AX7]

Ok, heres the full schematic again.

1. First i got the CT voltage by measuring from ground to P15
2. then measured plate voltage at one of the el84s pin 7 and ground
3. subtracted the plate from the CT and usually depending on where i set the bias got around 4v.
4. set the meter to ohms and checked the resistance between p15 and pin 7 immediately after turning the amp off
5. divided that resistance reading by the voltage drop (lets say 4 v) to get current
https://el34world.com/Forum/index.php?action=dlattach;topic=33458.0;attach=124838

[jbrew73]

Did you measure the center tap voltage each time you adjusted the bias -voltage or did you measure it just one time? 


Your B+ at the center tap could be changing because of higher current draw when testing at -15v. 


I would suggest measuring your voltage drop directly from center tap to plate.  I think the way described in the link provided could allow unneeded variables to enter into the equation.

[12AX7]

Did you measure the center tap voltage each time you adjusted the bias -voltage or did you measure it just one time? 


Your B+ at the center tap could be changing because of higher current draw when testing at -15v. 


I would suggest measuring your voltage drop directly from center tap to plate.  I think the way described in the link provided could allow unneeded variables to enter into the equation.

Yeah, each time i set the bias differently i did every step over again except measuring the CT to plate resistance.