Topband: R4C RF stage modifications
sire at iinet.net.au
Fri Jul 11 18:47:43 EDT 2003
For the last eight months or so, I have been playing around with an Drake
R4C, which has been solid-stated up to the first mixer - the only two tubes
left now are the 6BA6 RF amplifier and the 6EJ7 first mixer.
I can take no credit for the design (or most of the workmanship), which
has been carried out by friend Phil VK6APH with some suggestions by Tom
W8JI and uses the classic Sherwood 600Hz 1st IF roofing filter on CW.
The results - even though the R4C currently lacks an AGC - are exceptional
and it outperforms my FT1000MP on 40m CW, where I have spent a lot of time
playing around with it.
The R4C's performance on 160m CW is also excellent, but it lacks the really
sharp edge the R4C has on 40m over the MP. Tbinking about why this might
be so, we have become increasingly of the belief that this shortcoming lies
in the permability tuning of the 6BA6 RF stage.
Our surmise is that as 160m is, in reality, an afterthought on the R4C and
as 1.5MHz is the bottom end of its capability, it has occured to Phil and I
that the "Q"/effectiveness of the R4C's permability-tuning RF system may be
pretty low on 1.8MHz. Replacing the permability tuning with bandpass
filtering will also get rid of having to re-tune the R4C front-end every
time the receiver is tuned 5 or so kHz up and down 160m.
The idea we have is to replace the permability tuning with a switched
three-section bandpass filtering system - specifically the one sold by
Dieter W8DIZ for his Multi-Pig transceiver kit. The 6BA6 will be replaced
by a solid state RF stage.
I would be VERY interested in the opinions of the receiver gurus on the
reflector as to what they think of the idea of swapping the R4C permability
tuning for a three-section bandpass filter.
More information about the Topband