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

[Kim Elmore]

Carl is absolutely correct. My 1200 W output is optimistic: the best I ever saw (I wrote it down) was just shy of 1100 W PEP with everything at max. 

Kim N5OP

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

> On Mar 17, 2015, at 20:56, "Carl" <km1h at jeremy.qozzy.com> wrote:
> 
> 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
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