[HBR] Re: W1KLK receiver

[Bill Higgs]

Walt and the gang,

I'll begin by correcting myself on describing the input circuit: Not a Cohn, but two double-tuned circuits with bottom coupling. Doug claimed about a 25KC bandwidth, sounds about right with the number of poles used with decent inductors. Some loss, hence the 7044 grounded grid stage even on 75 meters.

Walt, you're right on several counts: Noise levels on 75 are so high that additional sensitivity simply means more noise off the antenna. If a signal has to be 3 microvolt to be copied above the noise, sensitivity is not what you're needing. Doug's purpose with the elaborate RF stage is to eliminate the image; with a 455KC IF, you generally need all the front end image rejection you can get.

As far as the 455KC IF is concerned, Doug used what he had. The image rejection he was seeking IS possible on 75 with a 455KC IF, but you have to jump through a lot of filter hoops to do it - which is what he did. Anachronistic? Possibly; HF crystal filters had been around a few years when he designed the receiver. Still, the better Collins filters had excellent skirt selectivity and good ultimate rejection. The major drawback is insertion loss; some of the better filters also had 10-12 dB insertion losses. Hence, often an extra IF stage was needed (Doug's design had three, plus the Q-Multiplier).

I apologize for missing the first part of the project, and didn't realize you were using such a high injection freq. You're right, at that frequency the tube capacitances tend to shunt the outputs to each other; unless that particular reactance is tuned out (impractical with a VFO), output can drop significantly. 'Twas just a random thought . . . I might experiment with it a lower frequency.

Regarding the 6JH8 mixer: Thanks for the biasing info. While I have seen circuits using the tube as a balanced modulator without bias, it seems to me that at least some DC bias (either way) would allow for a pot to balance the tube. -7 volt shouldn't be too hard . . . are you using a negative supply or developing the voltage via a cathode resistor?

Again, thanks for the hints!

Bill Higgs, NT4C