Steve, It sounds like you have a good handle on all of the issues. I have the same problem with my old Swan 1200X sweep tube amp that I have used with my K2 and with my current K3. This amp has no input matching networks at all! I also have an old SB200 Heathkit amp that does have input L-network matching networks. I found that the SB200 worked fine with the K2 at reasonable (around 1.5 to 1) SWR although I only used it on 80 and 40 meters. I'm sure that I could have easily tweaked the input networks on the SB200 for improved SWR if I had needed to do so but I so seldom used it that I never felt it was necessary.
With my Swan, though, the solid state radios would not drive the amp at all. I used an antenna tuner in between the K2 and the amp for a while but eventually modified the Swan to add small input matching networks built up from small ferrite toroids and mica capacitors switched by a rotary switch on the front panel of the amp. It was a fairly simple matter to find the correct values by cut and try. With your amp, which already has switched input networks, it should be a fairly simple matter to tweak the existing networks to get a very low SWR. I don't think you will have a big problem getting it to work the way you like. Don K7FJ > 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 _______________________________________________ Elecraft mailing list Post to: [email protected] 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
