[UK-CONTEST] Amplifier Advice ACOM 2000's

Wed Jan 25 07:35:45 PST 2012

Chris

Unfortunately you are incorrect. Duty cycle on amplifiers needs to be
averaged over time due to the thermal inertia in the components used. In a
contest calling CQ  and listening for replies result in 50% duty cycle on
RTTY.

CW is even lower typically in a contest about 30% duty cycle and for SSB
lower still even with heavy RF clipping around 15 to 20%.

The hardest I work my amplifiers is in RTTY contests when I am dual CQing on
2 bands the highest average duty cycle that can be achieved is 50%. One of
my amplifiers is OK the other amplifier cut out after  3 hours - the
transformer's  thermal cut out disconnect the primary. I now have it in a
box with fans blowing air over it and now copes with the whole of the BARTG
March contest and others like it.

More rubbish is talked about power ratings of amplifiers than any other
area. If the amplifier can survive transmitting at full power RTTY for
several  minutes without over heating then the problem is not the RF device
cooling. Then look at the transformer, which is a large weighty item of
copper and iron with a long thermal  time constant. This means the
transformer can be overrun for a long period of time without failure as long
as it does not exceed the temperature  rating of the insulation on the
copper wire. On a modern transformer this is about 100c. The outside of the
transformer will be cooler as  it is the winding in the middle that is the
limiting factor. If the inner winding is at 100c then touching the outside
would burn your fingers unless you are blowing air over the transformer.

Transformer VA ratings are specified by the manufacturer for still air.
Blowing a lot of air around the transformer will increase that VA rating by
50 to 100%.  A 1000VA transformer with forced air cooling could have an
average load of 2KVA. But the diode capacitor power supplies used in valve
amps have very high peak current to charge the capacitor QED the VA rating
needs to be higher than the RF devices DC power input.  Could probably
average about 1500 watts DC input for a forced cooled 1KVA transformer. Note
the average DC input power. Thus a 1000KVA still air rated transformer,
forced air cooled used in a RTTY contest should be able to reliably deliver
about 1500 watts RF output for the whole contest but it would be running the
transformer at its limit. But without forced air cooling average input would
be around 750 watts  or for RTTY contest 1500 watts in and about 800 watts
of RF.

If your amplifier does not lend itself to transformer forced air cooling
then use the transformer intelligently. Use the thermal inertia of the
transformer by mixing say 30 minutes of Run with 30 minutes of S&P and
better still monitor the transformer temperature and maximise  its
utilisation.  

73 David G3YYD

From: Christopher Plummer [mailto:plummerc42 at hotmail.com] 
Sent: 24 January 2012 23:03
To: g3yyd at btinternet.com; uk-contest at contesting.com
Subject: RE: [UK-CONTEST] Amplifier Advice ACOM 2000's

David,

Sorry to be pedantic, but RTTY is 100% Duty cycle, its CW thats around 50%.

So always derate your linear for Data modes.

Dont forget to try a toe in the water of HF RTTY this weekend in the BARTG
RTTY Contest, its sounds like bedlam, but its actually fairly gentlemanly
(at times at least).

Chris G8APB
BARTG Awards Manager

> From: g3yyd at btinternet.com
> To: UK-Contest at contesting.com
> Date: Tue, 24 Jan 2012 22:03:31 +0000
> Subject: Re: [UK-CONTEST] Amplifier Advice ACOM 2000's
> 
> The Acom 2000 was designed to run 1500 watts continuous carrier output for
> hours on end. So it can be run on RTTY contesting with its circa 50% duty
> cycle with a huge reserve. 
> 
> Most amplifiers being so called under run the only bit you are under
running
> is the transformer. Half power out will dissipate almost the same power in
> the PA devices as running full power. However if the transformer is
> deliberately fan cooled will make a huge difference to its power
capability
> compared to still air.
> 
> 73 David
> 
> -----Original Message-----
> From: uk-contest-bounces at contesting.com
> [mailto:uk-contest-bounces at contesting.com] On Behalf Of Jim Balls
> Sent: 24 January 2012 20:24
> To: uk-contest at contesting.com
> Subject: Re: [UK-CONTEST] Amplifier Advice ACOM 2000's
> 
> Most of the video's on you tube I've seen of contest operations have the
> amps running flat out!!
> 
> If you got it, flaunt it ;0)
> 
> -----Original Message-----
> From: Bob Harrison
> Sent: Tuesday, January 24, 2012 7:15 PM
> To: uk-contest at contesting.com
> Subject: Re: [UK-CONTEST] Amplifier Advice ACOM 2000's
> 
> My Merc is governed to 155MPH...... of course I take it to the limit
> everyday
> 
> 73 Bob G4UJS
> 
> > -----Original Message-----
> > From: uk-contest-bounces at contesting.com [mailto:uk-contest- 
> > bounces at contesting.com] On Behalf Of David Honey
> > Sent: 24 January 2012 18:53
> > To: uk-contest at contesting.com
> > Subject: Re: [UK-CONTEST] Amplifier Advice ACOM 2000's
> >
> > On 2012-01-24 16:51, Ken Eastty wrote:
> > > 5. ACOM 2000, friends and contest groups use these (under-run of 
> > > course in the UK)
> > >
> > > Under-run? Pull the other one.
> > >
> > > Contest entries from UK stations using Linears capable of running 
> > > more than 1kW (or much less on 4m) should be disallowed.
> > > I notice that some operators are very reticent when it comes to 
> > > telling what amplifier they use.
> > >
> > An amplifier that is rated at 1kw is rarely rated for high duty 
> > cycles, and for contest use you want to be sure the amplifier can run 
> > for an extended period without overheating. Using a 1kw amplifier at 
> > 400w achieves that. I don't buy the argument that just because some 
> > one has the equipment to do something illegal automatically makes them 
> > guilty of using it in that manner.
> > It reminds me of the joke of an accused being charged with a crime and 
> > then asking the judge to also charge him with rape. After all, he had 
> > the equipment. :-) _______________________________________________
> > UK-Contest mailing list
> > UK-Contest at contesting.com
> > http://lists.contesting.com/mailman/listinfo/uk-contest
> 
> 
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