On Jul 31, 2005, at 7:34 PM, Joe Subich, W4TV wrote:
>
>
> R. Measures wrote:
>
>> Eimac specs the 8875's grid diss. at 5W max., typical grid-cathode
>> driving potential is c. 42v-pk, or c. 30V-rms. I = P / E:
>> 5W / 30V = > 0.1666A, so two 8875s should be able to withstand
>> 333mA of grid-I without exceeding the grid-diss rating.
>
> You've got to be kidding - 333ma is nearly the rated PLATE current!
- I am not kidding, Joe, and neither is either Ohm's Law or the Eimac
spec sheet. The rated plate / anode current for a pair of 8875s for
SSB is 1000mA. Is 333mA "nearly the rated PLATE current" ?
Cheers
>
> Eimac recommended a grid current trip at less than 100 ma for the 8877
> which has a significantly larger grid than the 8873/8874/8875.
Eimac does not mention 100mA on their 8877 spec sheet. They rate the
grid at 25w-max dissipation. Since the grid-cathode driving potential
is c. 65V-pk, or 46V-rms, I = E / P. 25W / 46V = 540mA
> I would
> never want to see more than about 25 mA of grid current per tube in
> an amplifier using the 8873/8874/8875 tubes. Unfortunately, Dentron's
> grid metering shunt in the MLA-2500 resulted in 1 A full scale on the
> grid meter. It made it very difficult to see when the amplifier was
> being over driven.
Grid-I indication can not verify overdrive, anode-I does. Overdriving
typically causes the cathode's emissive coating to flake off and short
against the grid.
>
> Abuse killed more 8875s than phantom parasites ...
Eimac autopsies of the two 8875s I sent them from a MLA-2500 showed
they died from internal leakage caused by gold sputtering off from the
grid. Are you saying that gold can be boiled off of the grid by HF
energy?
>
> 73,
>
> ... Joe, W4TV (ex K4IK)
>
>
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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