David McGown wrote:
But it is so easy to have test points with permanently installed resistors on the cathode on each tube. You don't have to shutdown the amp, remove the power tubes and plug them into the test sockets. If you put a test point next to the tube socket on the top of the amp, connect the cathode pin to the test point, and install a 10 ohm (or 1 ohm) resistor to ground at the test point, then you can get individual bias readings with the amp in operation, to allow touching it up after it warms up, checking for bias while in operation. You don't even need a switch. You can use a switch if you have an installed meter on the amp, otherwise connect the (-) probe to the speaker ground terminal, and measure at each test point. Easy Peasy.
This all assumes one is designing and building an amp from scratch, and not trying to implement bias measurement on an existing amp and disturb the cosmetics.
Should like to point out that switching out the cathode connection for installation of a meter in-line does have risks, not the least of which is contact reliability and forgetting to switch back to normal connection.
David
The bias meter is temporary -- you set it up -- let the amp heat up for about a half-hour -- then go about setting your bias. What makes the bias meter a valuable addition to the tool box is that with all four tubes plugged in at once, you get to see how setting the bias of one tube affects the others. You can "diddle" with the settings until all four tubes are reading the same quiescent cathode current. You can even ear-test the results with the meter installed. That way you can find the "sweet spot" for the bias as I did. Once the bias is set this way you're good to go for the long term.