Biasing exercise using OT primaries

[xtian]

This method of calculating plate dissipation comes from Randall Aiken's page <http://www.aikenamps.com/Biasing.html> and sounds simple and elegant:

The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side.

So I'm checking out my 18 watt Lite IIb. Measuring from the OT, I get plate current (27.7 and 24.3 ma) and voltage (237vdc) giving me 6.6 watts on one side, and 5.8 on the other, which sounds like the numbers are reflecting what my ears tell me, that this amp is working and sounding good. Neat trick! I didn't have to remove the chassis from the head!

NOW, however, I'm looking at the EL84 datasheet, with it's fancy, Mr. EE professional graphs, and I'm not able to spot my numbers on any of these graphs. Can anyone shed some light? <http://www.r-type.org/pdfs/el84.pdf>

[martin manning]

I like this way of doing it too. Under limiting values on the data sheet see Pa max =12W. It's written right at the top of the first page too, and a 12W line is plotted on the plate curves. Your hotter side is only about 55%, so kind of cool. This is a cathode biased amp, so you are using plate to cathode voltage too, right?

[xtian]

OK! Now I can see, on page C2 of the datasheet, that I am running at about 50% of the Pa = 12W limit on dissipation.

This is a cathode biased amp, so you are using plate to cathode voltage too, right?

Not sure what you mean by "plate to cathode voltage," Martin. I can measure voltage at the plate (237vdc) and I can measure voltage drop across the cathode resistor.

[martin manning]

Yes, measure the plate voltage to ground and subtract the cathode voltage to ground from it. That will give the plate to cathode voltage, which is what counts for plate dissipation. The power lost by the voltage drop from ground to the cathode is dissipated by the cathode resistor.

[Gaz]

Yeah, the OT shunt method is great, especially when you don't have a fancy (or DIY) bias meter on hand. It also leaves out the screen current, which is a 'flaw' in the cathode current method. This method, however, is often referred to as "dangerous," but if you've been poking around in amps long enough you realize it's one of the safer things you've probably done

[martin manning]

This is not the shunt method, which measures current directly by shunting the OT with an ammeter. In this method you measure the voltage drop across the OT (safer than measuring amperage) and calculate the current using the measured DC resistance of the primary.

[Gaz]

Oh, my mistake, no wonder I never thought it was that dangerous! measuring current directly is not that dangerous either, but it does require taking the voltage out of the amp. Is there a name for this method?

[martin manning]

I think I've seen this method called the OT primary resistance method. The thing about measuring current is that there is essentially a direct short from one meter probe to the other, so if you slip and touch ground with one probe while the other is on the OT, you short the HT to ground (or whatever else you might touch) through the meter. When measuring voltages that can't happen.

[Reeltarded]

I bet it CAN! Not here though! lol

(knocks on skull)

[Structo]

I think I've seen this method called the OT primary resistance method. The thing about measuring current is that there is essentially a direct short from one meter probe to the other, so if you slip and touch ground with one probe while the other is on the OT, you short the HT to ground (or whatever else you might touch) through the meter. When measuring voltages that can't happen.

Except when you are measuring plate voltage on pin 3 of a 6L6 and slip and short against the heater pin 2........

Not that I have ever done such a thing.......

[katopan]

No, I've never slipped and shorted out the B+ either.

Putting in a B+ fuse saved my PT.