[HBR] RFamp circuit of HBR-11+

[Walt Hutchens]

> I noticed that there is an unbypassed 220 ohm resistor in the cathode of the 
> 6AZ8 RF amplifier in the HBR-11 and also in the HBR-13C.  This seems unusual 
> to me ...

It might be a little unusual but it makes perfect sense.

Excellent HF receivers can be built without any RF amp at all.  You can 
get plenty of gain in later stages and you don't need an RF amp to get 
over the noise of any of the better mixers.

When an RF amp is used, one of the main reasons is often to give a place 
to apply AGC ahead of the 1st mixer.  In other words, the RF stage will 
run at LESS than unity gain most of the time.

At most only modest gain will be needed.  So why bypass the cathode 
resistor?  Even without it being bypassed, there'll be boodles more gain 
than needed.  And too much RF stage gain causes a lot more problems than 
too little, in the form of crossmodulation and related effects in the 
mixer and subsequent stages.

Furthermore, an unbypassed cathode resistor helps to reduce phase shift 
in the stage.  That means reduced 'Miller effect' -- the effect in which 
  phase shift that varies with stage gain (as AGC voltage changes) 
appears as a varying capacitance, causing detuning of the stage.

As to VHF instability, that's not a function of the circuit (HBR or 
whatever) but rather of the tube type used.  'Good' tubes have closely 
spaced electrodes (short transit times) and high gain -- these are the 
requirements for high gain and low noise at the near VHF frequencies, 
10M and up.  But they are also exactly the requirements for VHF 
oscillation.

The defense is twofold: Leads in stages using such tubes -- usually all 
of them except the mixers until you get to the 2nd detector -- must be 
kept as short as practical, so the VHF resonances are at the highest 
possible frequencies.  And you put a 100 ohm resistor right on the grid 
of each tube.  Cut the resistor lead to 1/4" and hook the end, stick it 
through the pin on the socket, and solder.  Grid connections to the tube 
go to the other end of the resistor.

Typically the VHF oscillator circuit consists of the leads to the (HF) 
tuned circuit shorted by the capacitance of that tuned circuit.  The 
tube socket is a relatively high current point -- pulses of grid current 
flow on positive peaks.  The resistor destroys the gain of this 
oscillator circuit but because no grid current flows in the amplifier 
circuit you intended, it has no effect on HF operation.

Very rarely I've had to put the resistor in the plate circuit or at the 
other end of the grid lead.  If building a standard circuit using the 
specified parts using a resistor at the grid as above will do the trick.

Walt
KJ4KV