[Amps] 8877 Max Grid Current

[Richard]

>Rich,
>
>My data sheet shows 25 watts which works out to be a grid current of around
>380mA.  This seems too high.  I fear that it may not be that simple (mixing
>AC and DC characteristics) although my tube theory is fluffy at the best of
>times.
>
>Popular opinion from all who have responded to me (and thanks to you all) is
>a maximum of between 80 and 100mA.

The 8877 grid is widely viewed as extremely delicate.  My guess is that 
this is due to the tendency of gold to evaporate from the grid and cause 
sometimes-fatal grid/cathode arcing.  Roughly half of the 8877s that I've 
autopsied became kaput from this phenomenon.  In Feb., 1986, a tube specs 
guy at Eimac (W. Foote) told me that gold evaporation was the result of 
vhf or uhf oscillation.  He described the autopsy process used at Eimac.  
I decided to have a go at it.  I found that the gold had evaporated from 
the grid, that the cathode was coated with numerous gold melt-balls.  
There were also gold melt-balls on the anode insulator - which expained 
the leakage current indication for the tube prior to autopsy.  The gold 
evaporation pattern from the grid indicated that there had been more 
current at the grounded end of the grid than at ungrounded end -- much 
like a quarter-wave vertical antenna.  I concluded that the frequency of 
the current that evaporated the gold was likely in the uhf region.  At 
Figure 24 on my Web site, there is a photo of an autopsied 8877 that died 
from gold evaporation.  [note -- gold evaporation does not occur until it 
melts and boils.  At one-atmosphere of pressure, gold boils at 2966ºC].
-  One computerized amp design program/app. predicts that the 8877 has 
enough internal feedback-C plus enough high-frequency gain, to become 
marginally stable above 100MHz.  One way to address this problem is to 
artificially reduce high-frequency gain through the use of a vhf/uhf 
parasitic suppressor. 
>
cheers, David

Today's trivia questions:  How much AC grid current flows through the 
grounded 8877 grid at it's max ratings of 4000Vdc and 250MHz?  How much 
at 144MHz/4000Vdc?



>VK3HZ
>
>----- Original Message -----
>From: "Richard" <2@mail.vcnet.com>
>To: "David Smith" <vk3hz@wia.org.au>; " AMPS" <amps@contesting.com>
>Sent: Tuesday, June 18, 2002 11:57 AM
>Subject: Re: [Amps] 8877 Max Grid Current
>
>
>>
>>
>> >I'm currently building a 2m 8877 linear amp, based on the K1AGB design.
>> >
>> >For my metering/protection cicuit, I need to know the maximum safe grid
>> >current that an 8877 can handle.  It's not specified in the Eimac data
>> >sheet.  Can anyone help me with this?
>>
>> There were two, published ratings for 8877 max grid dissipation, 20w and
>> 25w.  Since the typical grid-cathode driving V is c. 65Vrms, the
>> corresponding grid current can be calculated from ohm's law.
>> >
>> cheers, David.
>>
>> -  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
>> www.vcnet.com/measures.
>> end
>>
>
>


-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
end