[HBR] Need help with noise

[Ian Wilson]

The two 6EH7s in the IF strip are AGC controlled.

The first mixer uses a 6DJ8 with the second triode as cathode follower
(a Pullen mixer, with the different gms of the two stages arranged via
different plate voltages on the two triodes).

The first mixer scheme uses crystals 100kHz apart (well, in the 75S3, it
did. My scheme will use a Si570 but at the moment a crystal oscillator
lets me cover 7.0 - 7.15MHz (actually more since the first IF bandpass
isn't very steep).

Moving the antenna to the 1st mixer input lets 40m BC through. So I guess
the front end selectivity is doing its job, wonder why I didn't think of
doing that.

I'll see if I can test whether the RF stage is behaving linearly but it
will be
a day or two before I have time to try this.

Thank you for the things to check, etc.

73, ian K3IMW



On Thu, Jan 16, 2014 at 7:01 PM, Walt Hutchens <waltah at earthlink.net> wrote:

> Ian said:
>
> > 6EJ7 RF amplifier. 6DJ8 first mixer. 3.075MHz bandpass filter
> > (+-75kHz BW). 6EA8 second mixer. 455kHz mechanical filter (2.5kHz BW).
> > 2x6EH7 IF amplifier. Beam deflection product detector. AF stage
> > borrowed from the 75S3. Uses a PTO (actually Dubrow but very
> > similar to Collins) for main tuning.
>
> How are you doing the AGC?  The 6EJ7 is sharp cutoff, the 6EH7 is
> semi-remote, so that's going to be a challenge.  Is it just the 6EJ7
> RF/2 6EH7 that are on the AGC line?
>
> It's A LOT easier to do AGC if all the tubes on the line are the same
> or similar types. OR if most are the same type and one has a greater
> cutoff point than the others and so contributes only a little to AGC
> action but cannot be pushed into cutoff.
>
> What's the 1st mixer circuit? With such a hot RF stage you need more
> ability to handle large signals than gain; low noise is basically
> pointless.
>
> I assume the RF stage is fixed-tuned so the 2nd mixer sees 150 kcs of
> the band.
>
> What happens when you hook an antenna to the 1st mixer input? Or
> (equivalent) pull the RF stage and hook a 'gimmick' from grid to
> plate.
>
> I'm thinking some stage is overloaded and/or the RF stage is running
> at cutoff.
>
> All receiver projects go through this phase:  The stages all work but the
> receiver doesn't!
>
> Walt Hutchens
> KJ4KV
>
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