The 8873, 8874, and 8875 are all identical internally and just the external
anode determines the plate dissipation.
The MLA-2500 cooling is adequate for the 1000W CW INPUT the amp was designed
for since that was the FCC rule at the time. Tune up for 700-800W key down
output and the tubes last forever; PEP SSB will be around 1000W.
There are 3 flavors of the 2500, the original has a single HV and the
Continuous Duty switch changes the fan speed.
The 2500A (not marketed as an A) has a dual voltage HV and the B version is
the same minus 10M/12M.
As with most Dentrons there are often minor differences even during a
production run.
The 100 Ohm 25W input swamping resistor must be in place for 70-100W
exciters, many took them out when tubes were cheap so they could get over
1500W out......real stupid IMO.
Carl
KM1H
----- Original Message -----
From: "Kim Elmore" <cw_de_n5op@sbcglobal.net>
To: <qrv@kd4e.com>
Cc: <amps@contesting.com>
Sent: Tuesday, March 17, 2015 11:09 AM
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@kd4e.com" <qrv@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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