>
>I finally got my YC-156 on line, sort of. As noted in my earlier
>transformer post, I am using about 3400Vdc B+, which effects my design RL
>of 2400 ohms which the amp was designed for.
>
>The following problem doesnt even require application of B+, and is
>something I haven't seen before.
>
>The amp design looks like any run of the mill 3-500Z cicuit, but is
>physically 3X bigger.
>
>When the filaments come up, the grid current meter pegs below zero.
>
How much v-drop do you measure across the grid current meter shunt with a
DMM? The resistance thereof? - thanks.
>When I short the bias cut off resistor the grid current goes way positive.
>
A positive indication is neg. current. This is normal.
>This is without excitation, or B+.
>
ok
>I am using a variable shunt regulator (two terminal device) in place of a
>bias zener, and this is located directly below the filament transformer on
>the other side of the chassis.
>I can vary the bias from .6 to 30V, the grid current goes from 25mA to "oh
>my god"
>
>I have checked the filament circuit bypass caps (.01 disc ceramics) with an
>ohm meter, and nother is shorted.
Is this a heater type cathode or a thoriated-tungsten filament type
cathode?.
>
>Nothing is shorted in the tube (all open).
>
One needs a high pot tester to find out what's going on in a tube. Ohm
meters are of limited use.
>I tore the covers off and checked my wiring of the metering circuits, all
>appears OK.
>
>The grid current indication is the same with the B+ attached or detached;
>as is with the dummy load and input circuit.
>
>What gives?
>
hot cathodes emit electrons whether or not the anode supply is connected?
- later, Jay
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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