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

[Carl]

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 at sbcglobal.net>
To: <qrv at kd4e.com>
Cc: <amps at 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 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
>>
>> Safe & Secure Search Engine: duckduckgo.com
>>
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>>
>> Restored to design-spec at Heaven's gate 1Cor15:22
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