[Amps] Tuned Input - IMD and efficiency

Tony King - W4ZT amps080906 at w4zt.com
Thu Aug 10 13:17:20 EDT 2006


Tom,

Thanks for your comments on topic...

Tom W8JI wrote:
>> Ok, here we have finally gotten a documented number; 4 to 
>> 6 dB HIGHER
>> with an untuned cathode driven amp than a tuned input. 6 
>> dB is a
>> significant amount in my book.
> 
> I wouldn't take that number to the bank since the result 
> would have to depend heavily on the impedance seen at the 
> cathode, how the exciter behaves, and even cable lengths.

Well it is a "range" and I did ask the question had anyone done testing. 
Apparently none of the people on this list have ever done the tests and 
these figures are all that any of us have to go on unless someone comes 
up with something better.

I agree that there are many variables and hence we can't nail the number 
down but other than the data presented in the book, no one else can show 
their test results.

> 
>> Now it would be nice to see validation of these numbers on 
>> real amps
>> that we find in use today.
> 
> It might be something that never happens because there are 
> too many variables.

Right, it may not, but if someone took the time to do it on whatever 
amps they had available, there would be SOME data that could be used as 
a yard stick for everyone that is interested in the question of tuned 
input or not.  Don't get me wrong, I agree with your statement below. I 
believe that it is much better to use a tuned input than not.  The 
empirical knowledge indicates that one with a tuned input is easier to 
drive, has higher efficiency and is more neighbor friendly. BUT, there 
are plenty of old school Hams that still subscribe to the "you don't 
need a tuned input" theory and they are Elmers to a lot of people.

> 
>> It is also very interesting reading in the last paragraph 
>> regarding the
>> coax used to connect the exciter to the amp.
> 
> If it affects efficiency, and I know it does without a tuned 
> input or with an incorrect type of tuned input, it would 
> also have some effect on IM.

I really do believe that. I don't have the test equipment to prove it or 
quantify it, but I believe it.

> 
> Quite frankly I've never measured the IM change because not 
> having a tuned input causes too many other headaches.

I see
> 
> I highly doubt a simple shunt capacitor can be effective 
> from 1.8 to 30MHz.
> 
> 73 Tom 

Even with a tuned input, I subscribe to the theory that the output 
capacitor for the ten meter tuned input should be placed right at the 
tube cathode. It will be shunted by much more as you go lower in 
frequency so there aren't any negative effects by doing so.

That said, it would only be through trial and error that one might find 
that it helps with a particular external tuner scenario. Unfortunately, 
that tuner is usually removed from the cathode by some feet which takes 
us right back to the coax separation issue and associated problems.

By the way... thanks for bringing this thread back to the topic and out 
of the parasitic black hole.

73, Tony W4ZT


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