[Amps] QRO Heat management

Mon Nov 10 21:36:06 EST 2008

Carl wrote:
> Ahhhh yerself.
>
> I take it then that you have never bothered to read the SB-220 front 
> panel or the tuneup procedure in the manual.
>
> Or the NCL-2000, SR-2000, Hunter Bandit 2000's, and many others.
>
> The meter time constant was put in the rules at the request of 
> National, Hallicrafters and others as well as getting an OK from the 
> FCC that advertising a 2KW PEP amp was OK by them with the above 
> caveat. I was at National when they formally asked the FCC for an 
> interpetation.
But prior to the clarification the reg was only 1 KW average input. Even 
with the 1/4 second time constant I don't remember the reg stating any 
PEP limit.
>
> It didnt require rocket science to realize that if the proper voltages 
> were selected that going from CW to SSB positions  would double the 
> power and maintain the same plate load impedance. In order to save a 
> few bucks the xfmr iron was the limiting factor anyway for most 
> products and the manuals stressed using the Tune or CW position for 
> tune up.
>
Right.
> The MLA-2500 is a fine amp once owners realized the 8875's were only 
> rated at 300W dissipation and not 400 as with the 8874. If the input 
> resistor was not cut out and the amp run per instructions the tubes 
> last a long time. I use one as the amp for the 2nd station. Its fine 
> at 1000W in the button out position (labeled differently between 
> versions) and the regulation is as good as any other comparable amp.
I ran mine at "tilt" and never had a moments problem with either of them.
OTOH, although it had nothing to do with the power level, I had a low 
voltage cap (bout 5/8" diameter and maybe an inch long) in the DTR-2000 
blow and I do mean blow.  It sounded like a 12 gage had been fired right 
next to me.  The metal can hit the inside cover so hard it looked like a 
38 wadd cutter that had hit a steel plate. Mushroomed out quite nicely.
>
> Its mainly incompetent users that gave that amp a bad name while 
> others praised it. It was a popular DXpedition amp, mine had spent 
> years traveling around the world on its original set of tubes.
>
When I finally moved on mine were still putting out the rated output.
Sure was different picking up that MLA-2500 and then lifting an Alpha 76 
<:-)) or an HT-33B

73

Roger (K8RI)
> Carl
> KM1H
>
>
> ----- Original Message ----- From: "Roger (K8RI)" 
> <sub1 at rogerhalstead.com>
> To: "Carl" <km1h at jeremy.mv.com>
> Cc: "Rob Atkinson" <ranchorobbo at gmail.com>; <amps at contesting.com>
> Sent: Monday, November 10, 2008 6:33 PM
> Subject: Re: [Amps] QRO Heat management
>
>
>> Carl wrote:
>>> What many also forget or are too young to realize is that many of 
>>> the older amps were built when the power was FCC regulated as 1000W 
>>> CW and 2000W PEP SSB INPUT. Those full legal power amps had a CW/SSB 
>>> switch for
>>
>> Ahhh... I don't remember any PEP limit on input. Just 1KW average 
>> based on a plate current meter with a 1/4 second time constant.
>> My voice characteristics are such that I used to get over 2KW PEP 
>> output with the old 1KW average input which included the drive power 
>> in a GG amp.
>>> that reason and I guess a few hams actually used them (;
>>>
>> I always used them. Tune in the tune position and go full bore for 
>> operation.  The current and voltage ratios were such that the amp was 
>> properly tuned when switching to the high power position. Many hams 
>> couldn't get past the idea of tunning up at the power they planned on 
>> running to get maximum output but many old amps had such poor voltage 
>> regulation (such as the MLA2500) that you had several hundred watts 
>> more PEP out when running full bore after tuning in the tun position 
>> than you did when tuning up in the full bore position. I mentioned 
>> the 2500 as I had several and they all put out at least 200 watts 
>> more when tuned in the tune position and then switching to the full 
>> power position.  That means they were also tuned properly and running 
>> more efficiently at the full power position.
>>
>> 73
>>
>> Roger (K8RI)
>>> When run at full bore of 1200-1800W OUTPUT on CW the duty cycle 
>>> warms things up a bit more than at the old limit which is roughly 
>>> 600-700W output.
>>>
>>> Wassamatter with AM? Properly adjusted 1500W output linears work 
>>> fine at 375W carrier which is 1500W PEP. Thats easier on them than 
>>> RTTY and if using controlled carrier AM its even lighter duty. A 
>>> DX-60 with some audio mods sounds great with a SB-220.
>>>
>>> Carl
>>> KM1H
>>>
>>>
>>>
>>>
>>> ----- Original Message ----- From: "Rob Atkinson" 
>>> <ranchorobbo at gmail.com>
>>> To: <amps at contesting.com>
>>> Sent: Monday, November 10, 2008 12:36 PM
>>> Subject: [Amps] QRO Heat management
>>>
>>>
>>>
>>>> Heat can be an issue with some ham amps built over the past 30 or 40
>>>> years (can't say that about all of them since I have not owned all of
>>>> them).  There are several reasons for this.   When SSB became the
>>>> popular voice mode, designers realized they could come up with amps
>>>> that would work okay for most users operating ssb and cw (duty cycles
>>>> of no more than 50%) because most transmissions were no longer than a
>>>> few minutes.   They could build amps with lighter power supplies, make
>>>> them compact and relatively light weight to sit on a table top, and
>>>> pass the lower costs on to the customers.
>>>>
>>>> These amps were cooled adequately, but some hams complained about
>>>> noisy fans.   Now we have some ham amps with whisper quiet fans to
>>>> make these users happy.  I've seen amps advertised and reviewed
>>>> positively for being quiet.   They work okay for the above described
>>>> operation such as a typical ssb ragchew where no transmission is
>>>> longer than 5 minutes but if you run RTTY or heaven forbid AM, with
>>>> one it will darn near melt down pretty quickly.   Another practice
>>>> that I think Dentron among others got into, was to try to cram as much
>>>> as possible into the smallest cabinet possible, which never made sense
>>>> to me, but maybe some users thought a compact amp was a good thing.
>>>>
>>>> So anyway, there's nothing wrong with augmenting the cooling and in
>>>> fact you may have to if you have one of these desktop "quiet" amps and
>>>> you want to run slow scan or make long transmissions on cw.  In my
>>>> opinion the trick is to not have the added fans somehow obstruct
>>>> airflow rather than increase it.  The goal should always be moving air
>>>> in and out of the cabinet.   If the chamber to be cooled is sealed,
>>>> you have to be sure the amount of air at any instant being pushed in,
>>>> is the same amount as that being sucked out, otherwise a fan is going
>>>> to be worked against or pushed.    This is usually an easy problem to
>>>> fix by adding some vent holes, or having all fans blowing in or
>>>> drawing out.  The best amps have big cabinets with lots of space
>>>> around all the components.   I think these days, if you want an amp
>>>> like that you have to build it yourself.
>>>>
>>>> 73
>>>>
>>>> rob / k5uj
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>>>
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>>
>
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