[HBR] RFamp circuit of HBR-11+

[N2EY at aol.com]

In a message dated 12/11/06 5:24:13 PM Eastern Standard Time, 
Hopperdhh at aol.com writes:


> 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, but I haven't seen any errata that addresses it.  I'm hoping to start 
> 
> one of these receivers soon, so I haven't gotten to that point yet. That is, 
> if 
> I could I would try it both ways.  Just wonder what the experts out there 
> think.
> 
> 

Well, I'm no expert, but here's what I think it's about.

First off, it's not an error AFAIK.

Second, it's negative feedback so as to reduce the chances of 
self-oscillation at the signal frequency.

Third, and probably most important, it helps reduce Miller effect. Miller 
effect is when the effective input capacitance of an amplifier changes because of 
changes in the operating conditions, such as changes in gain. The tuned 
circuits of the HBRs are high Q and relatively low-C, so Miller effect could cause 
significant detuning on the higher bands.

VHF/UHF parasitics in receivers are dealt with the same way as in 
transmitters: parasitic suppressors in the grid and/or plate leads. The good news is that 
they can be very small in receivers. 

73 de Jim, N2EY