Biasing 1968 Pro Reverb. Help needed.

[Lydian]

Does anyone have a precise article they know of for AA165 style biasing procedures? I've read so much that it's starting to all melt together. Thanks for any links.

-Dalton

[g1]

Procedures would be specific to the particular "method", of which there are several. The method will depend on the type of equipment you have at your disposal.
Bias probe type method is great for doing several different models of amp.
If you are just sticking with the AA165, it would probably be easiest to install 1ohm cathode resistors and use them to check idle current.
Some info on a couple methods here:
What Is Biasing?

[Lydian]

Thanks G1, I'll have to chew on that one a bit.

[xtian]

Aiken is a good resource, but you want this page: The Last Word On Biasing

Skip to:

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. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

[g1]

Sorry, that wasn't even the article I wanted to link , this tells more about the measurement methods:
The Last Word On Biasing

Edit: xtian beat me to it. Also, a paragraph below the one xtian quoted, he explains about how to install the cathode resistors. That is probably the most common method used.

[xtian]

install the cathode resistors. That is probably the most common method used.

True that. Cheap, easy, and even safer than measuring voltage on the OT primary.

[Lydian]

I'll read it tonight. Thanks guys.

[robrob]

Aiken is a good resource, but you want this page: The Last Word On Biasing

I'm a recent convert to Randall Aiken's bias method of measuring the output transformer's voltage drop and resistance. I used my Fluke 87 for his method and the "transformer shunt method" where you actually measure the plate current. The two methods came out within 1 millamp. Aiken's method is safer and probably more accurate since milliamp measurement is a multimeter's least accurate measurement.