Tube DIY Asylum

RE: Bias adjustment? Scott 208

24.1.201.224

This Post Has Been Edited by the Author

In Reply to: RE: Bias adjustment? Scott 208 posted by walkstoslow on October 20, 2024 at 07:12:02

I'm sorry, but your belief is mistaken. Take a look at what Tre posted below:
"The problem is, knowing what the bias voltage is doesn't tell you what the idle current through the tubes is."
Tre is ABSOLUTELY SPOT ON!
Here is a section of my post in the FAQ section "Tube Problems That Aren't Tube Problems":
"Keep one last thing in mind - often you see a voltage "spec" on a schematic at the control grid of a tube. DO NOT TRY TO SET THE VOLTAGE AT THE GRID TO THAT SPEC! The negative voltage at the tube grid should be whatever value is required to set the tube's cathode current to spec. If the tube is passing the correct amount of current and the voltage at the grid is -20 volts then the hell with the "spec" of -22.5 volts or whatever it is. Making the grid voltage more negative with respect to the tube's cathode is the equivalent of stepping down harder on a garden hose to restrict the flow of water. And if you let up the pressure on the hose and more water flows - that's the same thing that happens if the tube grid is less negative!!"
The bias voltage is not the only variable that determines the current through the tube. Typical power tubes like the 7591s can have as much as 75-100% variance in cathode current from tube to tube with the same negative bias voltage. Variations in plate and screen voltages can change the cathode current. Variance in heater voltage can have some effect also.
And BTW - having the Variac set at 100 volts doesn't mean the AC supply to the amp will always be 110 volts. Variances in the AC voltage at the wall will raise or lower that voltage above or below your 110 volt setting.
So while it is nice that the Scott hac AC balance adjustment, that adjustment can only equalize the current between the two 7591s in each individual channel. You need to be able to adjust the bias voltage to raise or lower the cathode current to the desired level.
Please take a minute and read the FAQ post I linked to below. I am sure it will be of help!

Tube Problems That Aren't Tube Problems (Open in New Window)

Edits: 10/24/24

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