[Elecraft] Amplifier Input Impedance

Tue Jan 27 11:24:08 EST 2009

Steve,

The KPA100 will tolerate a 2:1 SWR with no problem.  That is not 
uncommon with solid state amps.
If you do have a HI SWR problem with the input circuit, it would likely 
indicate a defective component in the input circuit (most likely a 
capacitor).

My GLA1000 is not up and running either so I can't give you any 'tried 
and true' data.

73,
Don W3FPR

Stephen W. Kercel wrote:
> Thanks to all who have replied.
>
> The amplifier that I have in mind is an old Dentron GLA-1000B that I am 
> restoring. It is a grounded grid sweep tube amplifier and uses fixed pi 
> networks (a different network for each band, selected by the band 
> switch) at the input. It is supposed to have a better than 2:1 SWR at 
> input. Back in the day, I used to run it with a Kenwood T-599D 
> transmitter whose output tank circuit was a wide range pi network, and 
> there never was a problem coupling the transmitter energy into the 
> amplifier.
>
> However, my K2 is much touchier about the range of loads it will tolerate.
>
> As the amp is not yet restored, I have not yet tried running it with the 
> K2. Thus, maybe I'll have a problem and maybe I won't. I did notice that 
> at least one poster to the list has had a problem the SWR seen by a K2 
> looking into a cheap tube type amplifier. Thus, I'm thinking what I 
> might do if I run into the problem.
>
> If I do have an SWR problem, I see three possible solutions:
>
> 1) (If even possible) Change the values of the mica capacitors in the 
> amplifier input circuits to obtain a better match in the relatively 
> narrow CW segments in which I operate.
>
> 2) Insert an attenuator between the K2 and the amp. A 2 dB attenuator 
> would knock down a 2:1 SWR to 1.53:1. The down side is that I'd probably 
> need to construct the attenuator from thin film resistors. The other 
> down side is that I'd be contributing about 35 Watts to the Heat Death 
> of the Universe. The 63 watts (or so) coming out of the generator is 
> adequate drive for the amp to give me 400 Watts out. The up side is that 
> there is no tuning.
>
> 3) Insert a tuner (which do have on hand) between the K2 and the amp. 
> This is the cheapest solution, but it makes band changing take many steps.
>
> Regarding AD6XY's precautions:
>
> 1) This is a valve amplifier and not solid state.
>
> 2) I've never had a problem with amplifier oscillation in the past. It 
> is a grounded grid configuration and relatively low gain. It is not 
> impossible that it will oscillate, but it is less vulnerable than other 
> designs.
>
> 3) I have a very high quality KW dummy load for sush purposes as 
> adjusting amplifiers.
>
> Thanks agasin & 73,
>
> Steve
> AA4AK
>
>
>
>
> AD6XY wrote:
>   
>> I would say something slightly different. Firstly I am assuming the PA is on.
>>
>> It might help to measure the input, especially if it is a valve amplifier.
>> If it is solid state the input match is more likely to be RF power
>> dependent, but if the match is really bad at low power it is not likely to
>> get better at high power and such an amplifier would not be linear. A better
>> way would be to measure at the power you intend to use but that requires a
>> directional coupler. It is not ideal but an SWR meter connected with a very
>> short lead would probably do.
>>
>> If the amplifier oscillates then there is a very high chance of damaging the
>> analyzer, but if that were the case, then at least it did not destroy the
>> rig. Make sure the amplifier output has a wideband matched load - definitely
>> NOT an antenna. 
>>
>> I don't think a blocking capacitor would be any use unless there is DC on
>> the amplifier input line. If there is, there is probably a problem.
>>
>>
>> Note: You can use an antenna analyser to initially measure matching circuits
>> of a high power valve amplifier, but only when it is off. You need to load
>> the anode with an appropriate impedance equal to the operational anode
>> output load and tune and load for a good match at the output. It will not be
>> spot on but it should be a good starting point. It is very hard to optimise
>> the tank circuit in a valve PA any other way because of the high voltages
>> both DC and RF. It does not tend to work so well with transistors as the
>> impedances are so low and often dominated by the device capacitance.
>>
>> Mike
>>
>>
>>
>> WILLIS COOKE wrote:
>>   
>>     
>>> I would say no to all three questions.  The input impedance to the amp
>>> will not be the same when it is not powered and driven.  The SWR
>>> indication on the driver will give you some idea of the input impedance. 
>>> It should not hurt the antenna analyzer if the amplifier is not powered,
>>> but I don't think it will tell you anything useful.  With power on the
>>> amp, it might.
>>>
>>> Willis 'Cookie' Cooke 
>>> K5EWJ
>>>
>>>
>>>     
>>>       
>>   
>>     
>
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