rf gain pot in the cathode circuit and a tube choice question.

[W0EP]

I'm still looking at this homebrew receiver I picked up at a recent hamfest.
I had another topic asking about the tube complement and general layout
entitled:    1330 kHz and 8340 kHz

Now I am looking at the RF Gain control which is pretty flaky.

Rf amp socket has these connections:
pin 1:  33 ohm resistor coming in from bandswitch and small cap coming in from 100kc osc.
pin 2:  link to pin 7; 3k ohm to ground, parallel with 220 ohm (with bypass cap to gnd) then to point A
pin 3: gnd
pin 4: filament supply
pin 5: 15k ohm to B+
pin 6: 23k ohm to B+

I think those connections make sense with a 6au6.

Between point A and ground I have the 0-30k rf-gain pot.
Also at point A I have a 470 resistor to pins 2/7 of the 2nd IF amp tube (6ak5)
and a 470 resistor to pin 2/7 of the 3rd IF amp tube (6cb6).
So the pot is in the cathode ground line of the rf amp and the
two gain stages of the 2nd IF.

Is it "usual" to have the rf gain control as a variable resistance
in the cathode circuit?

The control itself
does not do much for 3/4 of it's motion, the sound is dead.
Then on the last 1/4, the sound starts to come up.  In the
last 1/8th there is a distinct gain jump and it seems like
other signals come in that may not be on this band, almost
like a regeneration effect.

Also,  I'm starting to wonder if that 1st IF amp should really be
another 6cb6 instead of a 6ak5  (the tying of 2 and 7 seems
strange with a 6ak5).

[K1ZJH]

That resistance seems a bit high... the pot provides gain control by making the cathode more positive than the
control grid, the same as applying AGC voltage to G1 to cut back gain.

A sharp-cutoff tube like a 6AU6 would be my last choice the RF stage.  A 6dc6 or 6GM6 makes a lot more sense,
especially if the stage is under AGC control. Take a look at some of the "HBR" receiver designs that were published
in QST back in the late 1950s and early 1960s. That will give you some good insight into Ted Crosby's
circuits, which are still popular to this day.

Pete

[W0EP]

Thanks!

The HBR series has been mentioned to me before, and I was too thick
to figure out the connection.  Now I have done some more googling and
I'm starting to see some of what I was missing.

[G3RZP]

Although Heath (AC5UP, please ignore!) used 6AU6s with AGC, I would go for a 6BA6 in the IF and a 6BZ6 in the RF sockets. There is an argument that if use a straight (sharp cut off) pentode with a screen dropper, as you increase bias to reduce gain, the screen voltage rises, giving the effect of a long grid base. However, at the cut off point, the change in gm is still sharp, so the distortion and cross modulation properties aren't improved that much.  Which is why 'variable mu' or 'remote cut off' tubes were introduced.

The use of a variable cathode resistor for RF and/or IF gain control is quite common. A number of receivers that didn't do that returned the negative end of the power supply through a resistor to ground and used the negative volatge developed across that for a negative line for biasing the output tube (example: US BC348, RAF R1155). People who connect the B- for a BC348 to chassis usually burn up the output transformer.

Where a variable resistor in the cathode line is used, it's useful to put something like a 25k ohm variable in series with the control with one side grounded. For normal operation, short the extra resistor: on transmit, adjust the variable for a convenient level of sidetone.

[N2EY]

Is it "usual" to have the rf gain control as a variable resistance
in the cathode circuit?

Yes. All my homebrew superhets do it that way. It works well if done right.

I usually have separate RF and IF gain controls, too.

Be aware that you may have a bad control. The symptoms sure sound like it.

73 de Jim, N2EY