Thursday 5 December 2013

Some Work required................

Some time ago I borrowed an old KW1000 linear amplifier from a local radio amateur friend. It had been here on long term loan for some time. My friend used it mainly on modes like DRM (digital SSTV) on 80 and 20 metres. Driving the linear with 10 watts it would happily run these high duty cycle modes giving around 100 watts.

Now this is a vintage piece of gear, at least 40 years old and surprisingly there is not that much information about it on the net although I do have a manual and circuit diagram for it.

I used the linear mainly on SSB and noted that whilst the linear would produce 350 watts or so on 40 metres with around 30 watts of drive power was down significantly once you got above 20 metres with a maximum output of 150 watts or so on 28mhz.Not too much of a problem at first as I only used the linear on the odd occasion and then mainly on 80 or 40 .

Now I had been warned that the output valves (or "tubes") were probably "soft" and this was likely to be the reason for the low output. indeed the linear should have been capable of a higher output on its current best band of 40 metres!

To cut a long story short the owner of the KW had bought another old linear (A Yeasu FL2100) and offered me the KW at a knock down price, too good to resist. So now owning the linear I thought I would treat it to a new set of 572B valves. I sourced a matched pair of Chinese made 572bs and ordered them up. They were quite expensive, in fact costing more than I paid for the linear itself!

When the new valves arrived I set about removing the covers of the linear, not an easy task on the KW1000 I think the case is truly a horrible design for easy access!

PA Compartment open

It took me almost an hour to get to this stage!

The 572Bs were then removed. Far from being a matched pair the existing valves were not even a pair. One was a Chinese made valve, the other a U.S.A branded "Cetron". They didnt look physically damaged in anyway unlike the 6146bs I removed from my TS830

 U.S 572B
Chinese 572

One of the new 572's on left with the two old ones. The glass envelope looks a lot cleaner on the new one.

Anyway I fitted the new valves and well............errr very little difference- everything works but output power still down. So what next? Well on 80 metres the load control is fully closed yet It looks as if I need more capacitance. with some help I think we have identified a capacitor that it switched in on 80 in series with the load control- this may need replacing plus there are some other issues that may need work- watch this space! Given the voltages inside however I am VERY wary and extremely careful when working on this unit.

I am hoping to get this linear working a little better. It does seem more robust and easier to use than the "Loudenboomer" I still have here in the shack.


  1. Well keep at it and see what needs to be done to get the unit up and running!

  2. Hi,I also have the same problem, at 80m the load is fully clockwise. Replaced the bias caps. Notices my HT voltage at idles is 1.8kv, I understand it should be at 2.4KV. Next stages to change the smoothing caps and resistors, and may also update the tripler with new diodes and caps.
    Noticed there are some variable inductors with cores, that will probably need re-alignment. Trying to get hold of service manual to no avail. CCT diagram i have, again any help here please?
    Brilliant unit to match the rest of my Kw2000, (Next project)
    Any comments would be gratefully received.
    Craig M0OYH