[Amps] Vacuum relays in Dentron MLA-2500

[David A. Pruett]

Don,

Not ordinarily one to contradict by good friend W8AV, but two vacuum relays 
are not the whole story.

I own a MLA-2500 and have often thought about switching to vacuum relays to 
get away from the noise.

There are two relays in the MLA-2500:

- A DPDT relay which switches the input and output of the amp in 
line.  This is the relay which W8AV refers to.

- A 4PDT relay which accomplishes one or two things, depending on your 
particular model of MLA-2500.  In all of them, one set of contacts switches 
the cutoff bias so there is little or no idling current on standby.  In 
some early models, another contact switches the blower from low speed on 
receive to high speed on transmit.

It's been my experience that the majority of the noise comes from the DPDT 
relay.  With my amp, I replaced the 4PDT with a quiet DIP relay by mounting 
it to a base from another 4PDT relay and it made little 
difference.  However, if you put vacuum relays in, I'll bet the 4PDT will 
become the next level of annoyance.

Most vacuum relays used by hams are 26 VDC.  All of these relays have 12 
VDC coils, so the relay supply in the MLA-2500 is 12 VDC.  My friend N8CC 
ran an HC1 vacuum relay in his MLA-2500 by changing the relay supply 
rectifier circuit to a voltage doubler.

Vacuum relays are to be preferred from the speed and voltage handling 
standpoint, but many amps get by with open frame T/R relays.  The SB-220, 
for example, while not totally quiet is a lot less annoying than the 
MLA-2500.  It uses essentially the same relay, but mounts it to the amp 
chassis through a rubber grommet.  This might be a viable approach to get 
quieter without requiring extensive re-design.  Just remove the relay, 
drill out the mounting hole and put in a grommet which the original 
mounting screw can pass thru.

73,

Dave/K8CC


At 12:46 PM 12/26/03 -0500, Don Knight wrote:
>Ok but most of the vacuum relays I've seen are 24 volt. Anyone make a 12
>volt vacuum realy? I assume I just hook the two relays up the way it's
>hooked on the single realy now and just split the tr/rx sections?
>
>                                             Thanks,Don
>                                                 n4dek
>----- Original Message -----
>From: <W8AV at aol.com>
>To: <n4dek at triad.rr.com>; <amps at contesting.com>
>Sent: Friday, December 26, 2003 12:35 PM
>Subject: Re: [Amps] Vacuum relays in Dentron MLA-2500
>
>
> > In a message dated 12/26/2003 11:53:08 AM Eastern Standard Time,
>n4dek at triad.rr.com writes:
> >
> > > Wondering if maybe someone that has access to the schematic for the
>MLA-2500 could take a look at it and let me know what I would need to change
>this amp to use vacuum relays. I've been told that I would need two relays
>but really don't know for sure. I've found several vacuum relays on ebay but
>need to be sure of what I need to get. One or two? Voltage? ect.   I can't
>deal with the relay noise in this amp any longer.
> > > Please help!
> >
> > You need two if you are using SPDT vacuum relays, one to switch the
>incoming RF from the transceiver to the input of the tubes and one to switch
>the output from the amp to the antenna.  The MLA-2500 uses an open frame
>DPDT relay to accomplish both of these functions.
> >
> > 73........de Goose, W8AV
> >
>
>
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