V+ Plate Voltage measurement

[Champ81]

I am a bit confused still on how plate voltage is measured
 I hope I have the terminology correct first of all (V+ is B+ is plate voltage).
I hear on forums and websites to measure the plate voltage to calculate the desirable current draw (around 70%).

Referring to TUT 2 p3-27 dissapation bias set procedure it says to measure the V+  with amp under test. The previous step says to have the bias pots to maximum negative voltage to check presence of voltage on pins 5 of the power tubes.  Amp is unplugged. Tubes are out back in and then amp is powered back on to measure the plate voltage.  

My question is. Plate Voltage varies as you bias the tubes.  So then what is the plate voltage since it is not constant with pot adjustment. 
In TUT 2 it doesn't say to turn the bias pots to reduce current draw before plugging the power tubes back  So in that state the tubes would be conducting maximum limit based on R4 (currently at 6.K81) of the RBX.  Mine is drawing past 190mA in that state. Is this where you measure the V+?


Edit:
I realized that the voltage does vary but when you plug the numbers in to get current the voltage differences are in fact negligible.

[K O'Connor]

Hi Guys

Note that this discussion is with respect to fixed biased amps.

It is safest to use the unloaded supply voltage for the calculation of maximum idle current for the tube type you want to use.

It is easiest to calculate the maximum current and then the related voltage that will appear across the cathode current-sense resistor. If we assume these are 1-ohm each, then numerically the milliamp goal is the same as the millivolt reading.

Why are we calculating maximum values for the tube instead of arbitrary 50% or 70% values? The maximum dissipation of the tube sets the limit to the maximum current IN THIS SPECIFIC AMPLIFIER and that is what we need to know.

As I stated in TUTs and elsewhere on this forum, the idle current for the tube can be set to any value between zero and the maximum calculated for the specific voltage environment of the amp in question.

Yes, as you dial the tube current up from zero, the plate voltage will drop, since PT regulation in most guitar amps is about 20%. Still, continue to follow with the initial calculation as the limit as it is better to err on the side of safety than to follow arbitrary and often incorrect web advice.

The "hot" range resistor in the bias-set network is there to allow different tube types to be used and to accommodate low-gain samples of tubes.

ALWAYS SET THE BIAS POTS FOR MAXIMUM NEGATIVE VOLTAGE AT THE TUBE SOCKET PRIOR TO INSTALLING THE POWER TUBES

The bias sweep should extend to 15% of the absolute value of Vs for proper control over all tube samples. Ideally, you should be able to turn the tube completely off (zero current).

Pre-connect the meter so that you can see what the idle current is doing as the tubes warm up. If the meter reading approaches the maximum calculated value, there is something wrong with the bias-set values. If things are normal, the tube may indicate a low number of mA.

Adjust the first tube by meter to a safe current . Err on the low side as you can always increase it later.

Adjust the second tube by ear for minimum hum with all MVs, level at zero.

There is absolutely nothing magic or correct about 70% of full dissipation, nor 50%, nor any other value you see on the web. The most widely parroted advice is usually incorrect.

[makinrose]

Hi Guys

Note that this discussion is with respect to fixed biased amps.

It is safest to use the unloaded supply voltage for the calculation of maximum idle current for the tube type you want to use.

It is easiest to calculate the maximum current and then the related voltage that will appear across the cathode current-sense resistor. If we assume these are 1-ohm each, then numerically the milliamp goal is the same as the millivolt reading.

Why are we calculating maximum values for the tube instead of arbitrary 50% or 70% values? The maximum dissipation of the tube sets the limit to the maximum current IN THIS SPECIFIC AMPLIFIER and that is what we need to know.

As I stated in TUTs and elsewhere on this forum, the idle current for the tube can be set to any value between zero and the maximum calculated for the specific voltage environment of the amp in question.

Yes, as you dial the tube current up from zero, the plate voltage will drop, since PT regulation in most guitar amps is about 20%. Still, continue to follow with the initial calculation as the limit as it is better to err on the side of safety than to follow arbitrary and often incorrect web advice.

The "hot" range resistor in the bias-set network is there to allow different tube types to be used and to accommodate low-gain samples of tubes.

ALWAYS SET THE BIAS POTS FOR MAXIMUM NEGATIVE VOLTAGE AT THE TUBE SOCKET PRIOR TO INSTALLING THE POWER TUBES

The bias sweep should extend to 15% of the absolute value of Vs for proper control over all tube samples. Ideally, you should be able to turn the tube completely off (zero current).

Pre-connect the meter so that you can see what the idle current is doing as the tubes warm up. If the meter reading approaches the maximum calculated value, there is something wrong with the bias-set values. If things are normal, the tube may indicate a low number of mA.

Adjust the first tube by meter to a safe current . Err on the low side as you can always increase it later.

Adjust the second tube by ear for minimum hum with all MVs, level at zero.

There is absolutely nothing magic or correct about 70% of full dissipation, nor 50%, nor any other value you see on the web. The most widely parroted advice is usually incorrect.

Good post! I usually have measured the Va with the bias current set on the low side but now that I think about it using the unloaded bias current make a lot of sense from a safety perspective.  One thing I run into is that my customers sometimes take the one the web 70% to heart and will measure the bias current and be dismayed it doesn't meet the number they expect. I find it hard to explain to them.... there is a ton of misinformation out there.