[Amps] Do 8875's die fast after weakening?
garyschafer at largeriver.net
Tue Mar 17 20:17:16 EDT 2015
If you have a shorted tube try taking it out and taping on the top with a
hammer. This sometimes loosens the gold balls and lets them drop to the
bottom. Do not turn the tube over after you do this or the balls may again
cause a short. (per Richard Measures)
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Kim Elmore
> Sent: Tuesday, March 17, 2015 10:10 AM
> To: qrv at kd4e.com
> Cc: amps at contesting.com
> Subject: Re: [Amps] Do 8875's die fast after weakening?
> 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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