Do you need 2 digital multimeters to bias an amp?

[BobbyZ]

When using the shunt method, both meter probes are hot with plate voltage and you've got to control both of them carefully to prevent shorting the plate to ground or shocking the piss out of yourself.

Yeah that little really hard to change 1/2 amp fuse inside the meter will blow if you even think about touching the probe in the wrong spot.
But that little &^/#/&** won't blow if you're shocking the piss out of yourself !

Of course I still use the shunt method. Lol

 

[peteb]

http://www.aikenamps.com/index.php/the-last-word-on-biasing

• The plate current can also be measured using the "shunt" approach, where an ammeter is paralleled across each side of the output transformer. Since the internal shunt resistance of the ammeter is usually small in comparison to the resistance of the primary winding of the output transformer, most of the current is diverted through the ammeter, giving a fairly accurate reading of the actual plate current. This can also be dangerous because of the high voltages involved. One slip of the probe, and your expensive output transformer primary is shorted to ground through the low resistance of your multimeter. At best, you will blow the fuse in the meter. At worst, the output transformer primary winding will burn out in order to protect the multimeter fuse. This method is also inaccurate to varying degrees depending upon the make/model of the meter used. Some digital multimeters have fairly high internal shunt resistances (particularly on the lower current ranges), which will result in a reading that is lower than the actual plate current. This can result in setting the actual bias current too high, which can cause premature tube failure. Note that the shunt current measured on each side of the output transformer will be the total current drawn by all the tubes on that side, so if there are two tubes on each side, you must divide the measured shunt current by two.

 

[peteb]

Aiken says the shunt method will read low, how low depends on the resistance of your ammeter.


The danger he says, is not to you, but to the OT. A slip will place high voltage across the primary. Even with the Aiken method, to perform it expediently, requires connecting the probes to the same two high voltages, it’s just that with a slip, the Aiken method won’t harm the OT. Tryng to picture it, with the shunt method, if the probe on the center tap, or the red wire on the champ, slips and makes contact with the chassis, then the B+ is going nearly straight through the OT primary to ground



He says a low reading will lead to a higher setting than expected

 

[peteb]

i tried the shunt method and compared it to two other proven methods, Aiken’s method of using ohms law to measure voltage to calculate plate current and also measuring cathode current and subtracting screen current to get plate current. This on a champ with a 486 ohm cathode resistor, 978 ohm screen resistor and 270 DCR OT primary.


Aikens method:

Plate current = 12.1 VDC / 270 ohms = 44.8 mA




Cathode current method:


Cathode current = 24.25 VDC / 486 ohms = 50 mA

Screen current = 4.9 VDC / 978 ohms = 5 mA

Plate current = 50 mA - 5 mA = 45 mA



Shunt method:


First try 0.041 amps on the amps setting

Second try 42 mA on the mA setting







Interpretation.


The Aiken method’s plate current works with the calculated cathode and screen currents.


The shunt method’s plate current doesn’t jibe with the calculated cathode and screen current. The shunt method current came in low like Aiken said it would.

 

[Old Tele man]

Resistors -- especially carbon composition -- change their resistance as they heat up. Something to remember when trying to measure screen current as a voltage across a screen resistor...ie: always measure the screen resistor value immediately after you've measured the voltage across that resistor.

 

[peteb]

Next question, does the current increase or decrease when the amp warms up. To try to understand this I ran the amp for one hour and remeasured.



The shunt method gave the clearest answer. The plate current increased from 41 or 42 mA to 43 mA. That’s the best part of the shunt method, you just see a number like 42 or 43.





The other methods gave uncertain results, it was hard to gauge how much the resistance had changed in the cathode and screen resistors, and even more in the OT.




Judging by the change in the voltage drops across the cathode and screen resistors, there was a slight increase of current through the tube and the plate, consistent with the change measured with the shunt method.




the Aiken method when the amp heats up is different. The resistance of the OT did appear to measure 10% more resistance or 25% more or even 40% more but I have thrown out these numbers. Because the cathode and screen currents do not change very much when the amp heats up, the plate current cannot change much either, and neither can the resistance of the OT.

 

[peteb]

Thanks Old Tele Man,


I know enough now that I don’t plan to accurately measure the resistance of warm or hot resistors. I plan to stick with the Aiken method on a cool amp and consider that the current will heat up a bit as the amp heats up.

 

[Old Tele man]

Both copper wire (in OT) and carbon composition resistors change resistance with heat...which is why one should MEASURE resistance and NOT assume the color bands are 100% all the time.