[Amps] Tuned Input - IMD and efficiency

[Will Matney]

Tony,

My thoughts on using a tuned input is yes, always use one. One of the first rules in RF design is keeping components in a circuit mounted as close as possible. Without a tuned input, one is relying on the transceivers tank to tune to a varying load, and is seperated by a long length of coax. Though this may not have been documented per say by tests of actual amps and transceivers, in most handbooks this is said to be a no-no. The tuned input forms a load and a tuned match to the transceiver where the efficiency should be increased and the transform of power be greater to the tube(s). Here, the matching circuit should be mounted as close as possible to the cathode/grid of the tube(s). Heathkit did something in the SB-220 I didn't like. That was running long lengths of coax between the RF jacks, relay, and input / output tuned circuits. The relay should have been mounted in between the two RF jacks, and both jacks close to the relay. The input tuned circuit should have been mounted close to the cathodes of the tubes. That would have left one piece of coax to be used between the output tank and the relay. I always did think this was part of any problem with instability in the SB-220. IMD is another thing, and I'd venture to say that if tests were run, the IMD would be greater in one not using an input tuned circuit.

Best,

Will

*********** REPLY SEPARATOR  ***********

On 8/10/06 at 1:17 PM Tony King - W4ZT wrote:

>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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