>
>
>>>>Eimac rates the 8877 at 20w max. grid dissipation. Normal grid/cathode
>>>>potential is 55Vrms. With 75mA of grid current, dissipation is approx.
>>>>4.1w.
>>>>
>>>>Why do you limit grid current to 75mA, Phil?
>>>>
>>>>- Rich
>What would you limit the Ig to on your amp, Rich?
? After having tested around a dozen kaput 8877s, I know that
gold-sputtering is responsible for roughly 2/3 of 8877 failures. . .
In order for gold to sputter, it must be boiled. This requires an
incredible temperature and expendature of energy for the 8877's c. 50g
grid. . However, by passing a current with pronounded skin-effect -
such as UHF - through only the top layer of gold atoms, it is apparently
possible to evaporate thin layers of gold. This scenario is borne out by
the fact that gold evaporation is consistenly highest at the base of the
grid, as if the grid were acting lsomewhatike a UHF vertical antenna.
- Thus, I would design the grid protection to handle far more current
than could be obtained by mistuning. Furthermore, I would put an energy
absorbing glitch resistor between the stored energy in the filter caps
and the anode. // To be specific, I would design the grid-trip circuit
to be capable of interrupting 40a -- i.e., the full emissive capability
of the cathode. The actual trip current should be roughly equal to the
rated dissipation. // see below.
>Apparently, you think 75 ma is too low, although the tube is maxed out
>at 60-65 ma.
? I=P/E = 20w/55V =c. 0.36A. // The Revised 5-15-77 8877 spec. sheet
rated grid dissipation at 25w maximum.
>Limiting the Ig to 75 ma allows the tube to operate at its
>maximum power output. Would you put a 30 amp circuit breaker on a
>circuit in your home that is only rated for 15a?
>
Surely, if I discovered that the "15a" circuit was wired with #10 copper.
>
>>> I am in the business of having
>>>to buy tubes. Prices are soaring, as I am sure you know.
>>>
>>>Why push your luck on a very fragile grid
>>
>> I have a dissassembled 8877 in front of me. The grid is composed over
>>of slightly over 100 gold-plated molybdeum bars Elevating such a grid
>>to a high enough temperature to boil gold would require considerable energy.
>>>When lightly loaded, by mis-tuning or wrong antenna, the 8877 Ig soars more
>>>rapidly than any other tube I have used. Alpha 77 series owners will
certainly
>>>agree with this!
>>>
>>Alpha 77s do not have a truthful grid-current meter.
>>
>I was not referring to meter accuracy, Rich. You are correct; the Alpha Ig
>meters are off by 100%. I was referring to the rate of change when the
>loading cap is moved about.
>
? I have heard reports that the 77's grid meter is off by more than
120%. If a certain rate of change is being observed on a meter that was
designed by Mr. Errorhorn (sp?) to give false readings, what then?
>The toughness of the grid on the 8877 is a relative term. Comparing it to
>the grid of a 4-1000A, which a lot of us cut our teeth on, it is fragile.
>
? The grid of an 8877 is designed to handle over 40A-rms at 250MHz. The
4-1000A grid handles relatively little RF at full freq. and V ratings.
The screen, however, must handle around 15A at full freq/V ratings.
- What does a damaged grid of an 8877 look like, Phil?
- Rich..., 805.386.3734, www.vcnet.com/measures.
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