[HBR] Need help with noise

[Walt Hutchens]

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