> There are places on the web that covers a gold sputtering
> problem with the 2500 and the 8875 tubes due to VHF
> parasitics and other high Mu tubes for that matter.
The issue that kills the tubes in that amp is over drive and
overheating the grid, not "parasitics." Due to the transverse
cooling the 8875 is rated for 300 watts plate dissipation
(not 400 like the 8874). Although it is capable of more, the
MLA2500 should be kept to 1200 watts or so of output.
Install a grid trip circuit and replace the inductive swamping
resistor with proper non-inductive resistor directly at the
cathode. You could even consider increasing the amount of
swamping and/or converting the swamping network to a properly
matched attenuator. In an ideal world you should add a tuned
input but there is not enough space below the chassis close to
the tube cathodes.
73,
... Joe, W4TV
> -----Original Message-----
> From: amps-bounces@contesting.com
> [mailto:amps-bounces@contesting.com] On Behalf Of n4zed@comcast.net
> Sent: Friday, January 25, 2008 11:01 AM
> To: amps@contesting.com
> Subject: Re: [Amps] MLA-2500
>
>
> I suspect something is wrong also but I think it is in the
> design and not in any particular component failure. I've done
> the usual Google search and found the loading problem is
> common with this amp. During the search I have found no
> fixes to the problem.
>
> This amp had never been powered up when I got it about 3
> months ago.. The power cable had never been connected to a
> plug and the tubes were still in the original boxes from
> Eimac. The relay contacts as far as I could tell had never
> been energized and showed no sighs of any electrical contact
> at the contacts(that sounded weird), none of the connectors
> on the rear had any sigh of a connector being screwed on or
> pushed on.
>
> There are places on the web that covers a gold sputtering
> problem with the 2500 and the 8875 tubes due to VHF
> parasitics and other high Mu tubes for that matter.
>
> There are others that say they have ran their 2500 for 20
> years and had no problems.
>
> The amp works fine, good output, good reports. So why fix
> something thats not broken right? I just want to keep it that
> way, the tubes are way too expensive to replace. One good
> thing about buying something that has been out in the field a
> long time is that you get to see what kinds of problems there
> were with the design and hopefully fix it before it hits you
> in the wallet or something very close to the wallet.
>
> Ken
>
> **************************************************************
> ***********
>
>
>
> Somethings definitely wrong then.
>
> Id start by closely inspecting the bandswitch and plate choke.
>
> BTW, the cover has to be in place when tuning, the steel that
> close to the
> coil has a huge effect.
>
> Carl
> KM1H
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>
>
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