[Amps] Do 8875's die fast after weakening?

Kim Elmore cw_de_n5op at sbcglobal.net
Tue Mar 17 11:09:52 EDT 2015

I have experience with the MLA-2500 through my Dad (W5JHJ, SK 2007). It's a good amp and makes about 1200 W out on it's best bands, which were 80-20 m if I recall correctly. 

The 8875s will last forever BUT they have very low grid dissipation limits (5 W comes to mind) and no protection. The amp was poorly metered for grid current from the factory -- my dad modified his so the grid current was 0.1 x the meter reading so the meter actually defected. 

The grids are gold plated, so If grid current is exceeded the gold vaporizes and is deposited on the indirectly-heated cathode, poisoning the cathode and reducing emission. This almost always is what happens to the tubes in an MLA-2500.

If 600 W is all you need, you should also consider an AL-80A or B. The 3-500Z is as close to bullet-proof as they come and replaceable.  

Kim N5OP

"People that make music together cannot be enemies, at least as long as the music lasts." -- Paul Hindemith

> On Mar 16, 2015, at 10:12, "qrv at kd4e.com" <qrv at kd4e.com> wrote:
> I like the Dentron MLA-2500 and am seeing lots of them
> for sale at Hamfests and online.
> I understand that the "B" model had a tap for higher
> voltage for higher SSB power, but otherwise was the
> same as the plain 2500, though there have been some
> reports of occasional models with a tuned input.
> Some claim 100% output tubes, others 50-75% output,
> and some are without tubes.
> Prices range from $300. with no tubes to $800. with
> 100% tubes and fresh caps and bleeders.
> I'm not up to doing conversions, looked at that and
> decided not to go there.
> I'm OK with soft tubes - a reliable 600w output meets
> the need - but there's the question.
> Do 8875's die fairly quickly once they start to fade?
> e.g. If they are only showing 75% in an amp where
> another pair show 100% how long might they last in
> moderate SSB use?
> I know that there are other amp alternatives but
> right now I'm studying the MLA-2500.
> Thanks - David KD4E
> -- 
> *David* KD4E
> ARES-EC Bulloch County, Nevils, Georgia USA
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