>At 05:23 AM 8/11/97 -0800, you wrote:
>>>At 05:55 PM 8/10/97 -0800, you wrote:
>>>
>>>>How much grid current equates to 8.2W of grid dissipation?
>>>
>>> 55 ma per tube.
>>...
>>55mA divided into 8.2 watts equals 149V rms. Since the grid/cathode
>>driving potential for an 8877 is around 50V rms, where does the extra 99V
>>rms come from, Phil?
>>Rich---
>
>The 55 ma is only the DC component of the total grid current. This allows
>the use of a simple DC amp meter to sample the TOTAL complex AC, DC, and RF
>currents flowing in the grid.
At 4000V and 250MHz, the normal AC current in the 8877's grid is 40A rms.
At 4000V and 1.9MHz/160m, the normal AC grid current is 0.3A. I don't
see how a simple DC current meter can tell the difference
>By following the Ip and Ig curves in Care
>and Feeding of Power grid Tubes, ...
My copy of *Care and Feeding....* does not appear to contain such curves.
IMO, there is no direct relationship between anode current and grid
current. According to the characteristic curves for the 8877, grid
current is primarily a function of minimum anode voltage during the
conduction half of the cycle. Significant amounts of grid current do not
flow until the anode voltage decreases to under 900v.
> Also from these curves, we can get the corresponding DC current
>values that will make up the DC component that we need to watch closely
>so as not to exceed maximum grid dissipation.
If this be the case, then why doesn't Eimac specify a maximum DC current
rating for the 8877's grid?
................
- Has anyone on this discussion group looked through a low power
microscope at the grid of a gold-sputter damaged 8877?
Rich---
R. L. Measures, 805-386-3734, AG6K
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