> If there is a problem with the metering circuit as you suggest,
> Tom, whereby the grid meter is partially indicating plate
> current, then shouldn't the amp be reading some grid current
> under conditions of zero drive due to ZSAC?
The problem can be a breakdown problem in a component, or
something in the auto-bias. That amp doesn't have zero signal
quiescent current because of the auto bias.
The amp might have multiple problems.
The only thing I know for sure is 200 mA of grid current, with very
high anode current and a shallow dip, points towards metering
errors. Any 8877 I've seen, with 200 mA of grid current, shows a
sharp dip in anode current and sharp peak in grid current at the
tank is tuned through resonance.
If the blocking cap is open, the anode current will be very low and
grid current almost off scale with only a little drive power.
If the RFC or tank input is "shorted" or on the wrong band, grid
current will be much lower than normal no matter how hard the tube
is driven.
If the input is on the wrong band, the grid and anode current will be
low.
I can't think of any RF load or drive condition where the grid current
on an 8877 would actually be 200 mA while the anode showed high
anode current with only a very shallow dip, can anyone? Grid
current always is high only when the anode is sharply dipped. Grid
current, when high, peaks sharply with the tuning. It also increases
all of a sudden at a certain drive while plate current fails to increase
much if the tube anode sees a high load impedance. If the anode
impedance is too low, the grid current will be low and anode current
high with a shallow dip.
If the tube is weak grid and anode currents are low. If the tube or
blocking cap is shorted the amp will not drive at all, or HV will flash
over.
The toughest problem to guess at or trouble shoot is one where the
readings make no sense at all. It sounds like the current
description is wrong, perhaps from metering problems.
73, Tom W8JI
W8JI@contesting.com
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