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 >> >> >> > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
