>
>
>>>When a good 8877 is properly tuned (slightly heavy loading)
>>>it will put around 2300 watts into the antenna with 55-65 ma
>>>of TRUE grid current. I have had several Alpha 77DX/77SX's
>>>and all of them read HIGH on the grid meter. I think Dick had
>>>in mind that it would help to save tubes. NEVER let the grid current
>>>on an 8877 exceed 75 ma under any circumstances. I set all my
>>>grid-trip circuits to this value.
>>>
>>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
>
>Rich,
>Eimac is in the business of selling tubes. 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.
. Under max. ratings of 4kV / 250MHz, an 8877 grid handles over 40a of
RF current, or roughly 0.4a per bar. It is my opinion that an 8877 grid
is not fragile. Sure, 8877s are known to fail from gold sputtering, but
my guess is that excessive HF grid current is not the reason.
>when all you need is 50-65 ma Ig
>for full output on a good tube? Also, a grid trip-circuit has a lot of slack
in it,
>(not as bad a a fuse, but slower than a QSK circuit.) The only time I ever get
>a grid-trip is when I select a wrong antenna in the heat of battle.
>Setting it higher than necessary is not a very good idea unless you have a
lot of cheap
>spares on your shelf. I want the amp off line anytime the Ig even approaches
15-18
>watts on peak. I think this approach is the reason for my success with the
>8877. I have a pair with date codes of 1979 that are solid black in color now,
>but put out within 97% of a new tube (1999) with the same Ig.
>
Blackness suggests that you have a natural gas pilot light in the same
building that houses the tubes. The hydrogen sulphide oderant present in
natural gas causes black silver-sulphide to form. Silver-sulphide is
conductive. It is of no consequence.
>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.
- cheers, Phil
- Rich..., 805.386.3734, www.vcnet.com/measures.
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