>
>> 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? ... ...
/\ Shallow-dip means heavy loading - which makes for minimal
grid-current. My guess is that the grid-current meter is not accurate.
A quick comparison with a DMM will tell what's what. .
cheers, Tom
- R. L. Measures, 805.386.3734, www.vcnet.com/measures.
end
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