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

To: "Kim Elmore" <>, <>
Subject: Re: [Amps] Do 8875's die fast after weakening?
From: "Carl" <>
Date: Tue, 17 Mar 2015 21:56:45 -0400
List-post: <">>
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.


----- Original Message ----- From: "Kim Elmore" <>
To: <>
Cc: <>
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, "" <> 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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