Dentron used nothing but quality Centralab and HEC ceramic caps for
padders in the MLA-2500 and all their other KW+ amps..
The same brands used by Alpha.
Carl
KM1H
----- Original Message -----
From: "Joe Subich, W4TV" <w4tv@subich.com>
To: "'Adrian Rees'" <rees.a@btconnect.com>; <amps@contesting.com>
Sent: Friday, August 01, 2008 6:26 PM
Subject: Re: [Amps] Dentron MLA2500 SWR Question
>
>> I will investigate the bandswitch tomorrow, as that sort of
>> makes sense.
>
> While you're in the area of the bandswitch also look at the
> padding capacitors ("doorknobs") since they are notorious for
> thermal drift with high current. A bad padder would also be
> consistent with high drift on one or two bands depending on
> the particular switching arrangement and how much of the
> total capacitance was being provided by the padder.
>
> Dentron was not known for the highest quality components
> and might well have substituted capacitors with marginal
> current capacity if they could get new ones at a lower
> cost.
>
> 73,
>
> ... Joe, W4TV
>
>
>
>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Adrian Rees
>> Sent: Friday, August 01, 2008 5:49 PM
>> Cc: 'amps@contesting.com'
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>>
>> There is a 200 Ohm Swamping Resistor in place.
>>
>> I replaced the original (which had been removed by the
>> previous owner) with a 200 Ohm non inductive resistor made up
>> of a few resistors in Series / Parallel arrangement to
>> achieve the 200 Ohms at 50 Watts.The resistor was also
>> replaced to lengthen the life of the 8875's as well as to be
>> clean on air.
>>
>> I will investigate the bandswitch tomorrow, as that sort of
>> makes sense.
>>
>> 73 for now
>>
>> Adrian
>>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Alex Eban
>> Sent: 01 August 2008 18:42
>> To: 'Carl'; n8de@thepoint.net
>> Cc: 'amps@contesting.com'
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>>
>> Hi Adrian:
>> It's not hysteresis!
>> What happens is more akin with motor boating: the power
>> cutback isn't instantaneous, so that it takes a little time
>> for it to activate and then to release when it detects the
>> lowering of the SWR. You need the input resistor in order to
>> equalize the input impedance of the tube over its operating
>> cycle, don't go without it: maybe a little more power but
>> also a lot more distortion.
>> Alex 4Z5KS
>>
>> -----Original Message-----
>> From: amps-bounces@contesting.com
>> [mailto:amps-bounces@contesting.com] On Behalf Of Carl
>> Sent: Friday, August 01, 2008 7:15 PM
>> To: n8de@thepoint.net
>> Cc: amps@contesting.com
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>> Another option is to build a broadband input assembly patterned after
>> the 2 hole 8874 Alphas.
>>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message -----
>> From: <n8de@thepoint.net>
>> To: "Carl" <km1h@jeremy.mv.com>
>> Cc: "Adrian Rees" <rees.a@btconnect.com>; "amps@contesting.com"
>> <Amps@contesting.com>
>> Sent: Friday, August 01, 2008 12:22 PM
>> Subject: Re: [Amps] Dentron MLA2500 SWR Question
>>
>>
>> Carl beat me to it.
>> The swamping resistor is probably kaput, disconnected, or
>> missing. Don N8DE
>>
>> Quoting Carl <km1h@jeremy.mv.com>:
>>
>> > Is the input swamping resistor connected? Many clip it out
>> so they can
>> > get another 200 Watts and fry the tubes faster.
>> >
>> > Also check that the bandswitch doesnt have a fried contact.
>> >
>> > I have a MLA-2500 here as backup and the TS-940 drives it with no
>> > problem on any band.
>> >
>> > Carl
>> > KM1H
>> > ----- Original Message -----
>> > From: "Adrian Rees" <rees.a@btconnect.com>
>> > To: "amps@contesting.com" <Amps@contesting.com>
>> > Sent: Friday, August 01, 2008 4:56 AM
>> > Subject: [Amps] Dentron MLA2500 SWR Question
>> >
>> >
>> >> Hi all
>> >>
>> >> I have a Dentron MLA2500 linear, which mostly works fine, and I'm
>> >> getting lots of RF out.
>> >>
>> >> However, on 3.5MHz (no other band) it is difficult to tune and
>> >> was
>> >> wondering if someone could explain to me what is going on,
>> and how I
>> >> may cure the the problem ? So here goes, remember this is just on
>> >> 3.5MHz (80 M)
>> >>
>> >> Prime mover is IC740
>> >>
>> >> I tune the Dentron with about 10w from the IC740 into a Dummy
>> >> load.
>> >> When I have peaked the linear, the SWR seen by the IC740 is around
>> >> 1.8:1, as I increase the drive, the SWR seen by the IC740
>> goes up to
>> >> as much as 5:1 or more, thereby reducing drive power. This slow
>> >> cycling between lots of drive, but high SWR, then the
>> IC740 reducing
>> >> drive (to protect the finals) reduces the SWR, the power
>> drops, and
>> >> the whole thing starts again.
>> >>
>> >> I have got a small ATU installed between the Linear and the Radio,
>> >> but this is not really curing the problem. My other HF radio
>> >> (756ProII) with its Auto ATU is a little better, but again
>> it cycles.
>> >>
>> >> Both Prime movers get to about 25 - 30 watts of drive and
>> the cycling
>> >> starts. (Which means I only get about 250 - 300 watts out).
>> >>
>> >> I know the Dentron doesn't have a tuned input, hence the small ATU
>> >> between it and the Prime mover.
>> >>
>> >> I guess the amp is going into some form of hysteresis ?
>> Perhaps I am
>> >> wrong, but some insight into whats happening would be
>> good, and any
>> >> suggestions as to how to fix this annoying action would be great.
>> >>
>> >> Thanks
>> >>
>> >>
>> >> Adrian MW1LCR
>> >> Wales
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>>
>>
>>
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