[Amps] Dentron MLA2500 SWR Question

Fri Aug 1 18:26:33 EDT 2008

> 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 at contesting.com 
> [mailto:amps-bounces at contesting.com] On Behalf Of Adrian Rees
> Sent: Friday, August 01, 2008 5:49 PM
> Cc: 'amps at 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 at contesting.com 
> [mailto:amps-bounces at contesting.com] On Behalf Of Alex Eban
> Sent: 01 August 2008 18:42
> To: 'Carl'; n8de at thepoint.net
> Cc: 'amps at 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 at contesting.com 
> [mailto:amps-bounces at contesting.com] On Behalf Of Carl
> Sent: Friday, August 01, 2008 7:15 PM
> To: n8de at thepoint.net
> Cc: amps at 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 at thepoint.net>
> To: "Carl" <km1h at jeremy.mv.com>
> Cc: "Adrian Rees" <rees.a at btconnect.com>; "amps at contesting.com" 
> <Amps at 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 at 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 at btconnect.com>
> > To: "amps at contesting.com" <Amps at 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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> 
> 
> 
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