Gentlemen,
Veterans of this list may recall that I posted a plea for assistance/advice
some time ago regarding the article published in Dubus many moons ago,
regarding the DJ5RE GS35b 144MHz amplifier.
I'm delighted to say that the amplifier construction is now complete using
G3SEK's Triode board - I'd recommend this board without hesitation.
My problem, is that I cannot seem to get more than an extremely limited
amount of output from the RF deck.
I've initially set the anode HV to 2800v to try and mimic the DL4MEA DC
conditions so that I at least had some form of comparison, as Gunter has
issued a table showing the performance of his amplifier. At 4Kv of anode
voltage the gain of the GS35b does increase but the RF output still only
rises to 450w.
The initial DC conditions are as follows:
2.6Kv Anode Voltage (on load) and 2800V off load
400mA Anode Current
300mA Grid Current
80w input RF
300w output RF
150mA Anode Standing Current (giving 80mA of Grid Current with no drive)
I have tested the input circuitry and output line with my MFJ and I can get
good resonance at 144.400MHz on each of them. However, I cannot get any more
than 300 watts of output. I can adjust the input tuning and anode flapper
to 'peak' the readings. The output stub also can be made to 'peak', with a
corresponding 'peak' of 300mA of Grid Current.
300 watts seems a very low output for 80 watts of drive - but I cannot see
what is wrong. Anode input power is about 1200 watts, so I expected 600watts
of output - not 300 watts. Even this level of output would be a bit low for
this level of drive, I'd have thought. I am using a Bird 43 power meter with
a 2500W probe into a 5Kw dummy load.
There is no sign of arching of burning and the anode feed is as the author
suggested with a separate box for RFC5 via two feed-through capacitors.
There is quite a bit of heat from the anode air exhaust, so I think that the
output tuning may not be correct (even though it 'peaks'). Is the 'Q' of the
output stub likely to be higher when the stub is closest to the tuned line,
or higher when the stub is furthest from the tuned line? My thoughts are
that the 'Q' of the output tuning is low - hence the lower level of output.
Also, that the anode tuning flapper may need to be altered (i.e. made larger
or smaller) to get the anode back to resonance with the output stub in a
higher 'Q' position - or is my thinking wrong?
My question (for those that have ventured this far down the page!) is: Am I
doing something wrong? All of the dimensions of the metalwork check out OK
with the article.
I've tried emailing Thomas directly but to no avail (maybe I've got his
email address wrong: Thomas.Hoeppe@ASAMNET.DE).
If you've built one of these decks or have any suggestions as to how to get
the beast trotting along at full output, I'd be delighted to hear from you!!
Thanks,
Phil Smeaton
G0HSS
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