On 4/5/2011 7:08 PM, Roger (sub1) wrote:
> On 4/4/2011 10:44 PM, Bill, W6WRT wrote:
>> ORIGINAL MESSAGE:
>>
>> On Mon, 04 Apr 2011 15:17:23 -0400, "Roger (sub1)"
>> <sub1@rogerhalstead.com> wrote:
>>
>>> , a true 1500 watt
>>> amp needs to be capable of nearly twice that to hold together.
>> REPLY:
>>
>> That doesn't make sense. If it can't "hold together" at 1500 watts, it
>> is not a "true" 1500 watt amp.
How about we put some figures on this that are pretty much applicable to
the real world and I hope I can keep the numbers straight without making
a mistake. <:-)) That and there are others on here who are much better
than I at making these calculations for real amps.
SSB is figured at 20% duty cycle and CW at 40% for RF exposure. These
figures should be close enough for design work. This is neglecting any
off time. The typical on, off time is some times calculated at 10 minute
transmissions and receiving 10 minutes which in a 30 minute cycle would
be 66.7% on time. Of course in that scenario the other guy is only
running a 33% duty cycle.
The following is an over simplification as the transformers calculated
would likely be insufficient due to cool down time and heat retention.
it also makes the overly optimistic assumption that efficiency will be
60 % on all bands. These sizes should be considered absolute minimums.
In real life or IRL as the flight simmers say, I'd add at least another
20% to the transformer size just to be safe. Actually I'd add quite a
bit more for comfort.
So if we are running 1500 watts output at 60% efficient then the input
needs to be 1500/0.60 or 2500 watts pep input, but it's only 20% duty
cycle. or 500 watts. Neglecting the inefficencies in the power supply we
can build a legal limit amp with an HV transformer of only 1/2 KVA.
Switch to CW with 40% duty cycle and we suddenly need a 1 KVA
transformer. IOW one that weighs twice as much. But what if we want to
run digital or any other mode considered 100% duty cycle. Then we need
a transformer of at least 2.5 KVA, or 5 times the size of the one we
used for SSB only. An amp capable of 1500 out at 100% is *Heavy*!
So this comes out for a legal limit amp the HV transformer needs to be
capable of delivering in CCS:
SSB = 1/2 KVA
CW = 1 KVA
Digi= 2.5 KIVA
And at these ratings the transformer will get hot. The other PS
components need to be sized accordingly:
Still each of these amps could be advertised and sold as a legal limit
amp and it would do it.
The reality of it is, there are a number of amps out there who's
components are sized like this. There are a few where these are actually
generous. Why? For one thing most hams aren't going to be contesting or
running digital. They will never know there amp is incapable of running
100% key down at 1500 watts out. Older amps designed when the legal
limit was 1 KW DC input or 2 KW PEP will typically give 1200 PEP out
with a low duty cycle. Most of their components, including the band
switch are not sized to handle today's power levels.
However lets look at these legal limit amps differently. Lets say the
ham who purchased the first amp with a half KVA transformer for SSB
decided to run CW. He's only going to be able to run about 750 watts out
before overheating the PS. What about if he wanted to run digital? He
only has an HV transformer rated for 1/5 of what he needs so his shiny
legal limit amp is only going to be able to put out about 300 watts
continuous without over heating.
Add to this the amount of cooling air required will change substantially
from SSB to Ditital along with the associated noise.
A tube with 1 KW plate dissipation at 60% efficiency should be capable
of 1000/0.60 or 1666.7 watts out but with many tubes it's not quite that
simple. There you are limited by the cooling and cathode emission and
even how the manufacturer calculated the dissipation. The FU728F is
rated at 1200 Watts dissipation while the 4CX1500B is rated at 1500.
The FU728 will happily put out 2500 PEP and over 2KW nominal but it's
been pointed out on here that the 4CX1500B will not reach those levels.
Other tubes may have far more emission capabilities and can exceed their
plate dissipation ratings.
Typically the more air you put on them the more power you can get out...
Up to a point. Higher plate voltage (within the capabilities of the
tube) also gives the capability for more output power.
However, regardless of how we get that extra power the laws of physics
dictate the power transformer has to be capable of delivering the
required input power WITHOUT over heating or excessive voltage drop at
the required current. The diodes have to handle the current and the caps
have to be sufficient to give the desired regulation and ripple reduction.
There are also tubes that can put out considerably more power than for
what they are rated, BUT at the expense of reduced life. The 811A is a
prime example of this.
This takes me back to my original statement. If you want to run the full
legal limit 24 X 7 and have the amp stay cool it needs to have much more
capacity than being able to just run the legal limit.
Yes, you can select a tube based on plate voltage and drive available
that will nicely run the legal limit that can not be driven much beyond
that point. Even that does not negate the need for the robust power
supply and a temperature controlled cooling system for the tube. Or you
can go with all conservatively rated components and use computer control
which takes you back to the 100% rating and 2.5 KVA transformer in the
example with the computer keeping you legal with the lower duty cycle
modes. These amps are also available, but at substantially more than
any of the manual tune amps.
73
Roger (K8RI)
> And if that is the case there are a lot of amps being sold that either
> don't make sense, or are using false advertising.
> Do 4 811As running 800 watts make sense? How about 8877s with anemic
> power supplies that will hit 1500 pep in casual conversation, but won't
> last through a contest? How about a pair of 3-500s running 1500 watts
> from an amp you can carry under one arm? It'll run the legal limit, but
> not for long on digital or contesting. The 8877 and 3-500 examples are
> legal limit amps, but they won't do it all modes key down or even SSB in
> contesting.
>
> So I stick with the above statement. If you want it to hold up
> contesting, or any other mode using a high duty cycle then it needs to
> be able to do almost twice that ICAS.
>
> 73
>
> Roger (K8RI)
>
>> 73, Bill W6WRT
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|