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Re: [Amps] Best way to tune a Dentron ML2500 ?

To: "Joe Subich, W4TV" <k4ik@subich.com>
Subject: Re: [Amps] Best way to tune a Dentron ML2500 ?
From: R.Measures <r@somis.org>
Date: Mon, 1 Aug 2005 06:05:03 -0700
List-post: <mailto:amps@contesting.com>
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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