[Heath] Improving the GR-81?

[Kenneth G. Gordon]

On 5 Jan 2014 at 9:20, Guy Giacopuzzi wrote:

> Is it just my unit, or has anyone else noted the lackluster
> performance of the GR-81 regenerative shortwave receiver?

Hmmm....I don't have a GR-81, but those regennies I have used were 
pretty hot for as simple as they were.

>  Looking at
> the schematic, does anyone have any ideas to improve it?

Yes. Reduce the value of C-5 from 75 pfd to 20 pfd: increase the value of 
R-1 from 750K (or 680K) to 10 Megohms. This increases the impedance 
across the input tuned circuits, increasing the "Q" by reducing the "load" on 
them,  and thereby, increasing the selectivity and sensitivity.

>  Mine "works"
> and get's reception, but it lacks sensitivity and audio gain.  I've
> checked the wiring, and it's perfect...will check tubes next.

Tubes are usually the last things I check, unless they don't pass the simple 
"look-and-touch" tests. (Are they warm, can you see the filament, are they 
broken?)

The first things I measure are voltages. 

Should be 110 VDC on the plate of the 50C5, pin 7 and 115 VDC on the 
screen, pin 6.

Then 40 VDC on the plate, pin 6 of the 12AT7 1st audio stage.

Does the detector go into oscillation at high levels of the regen control 
(plate voltage to the 1st section of the 12AT7) ?

Have you tried feeding an audio signal into the grid of the 2nd section of 
the 12AT7 (1st audio stage), pin 7?

Have you tried feeding an audio signal into the 50C5 grid, pin 5?

Audio output is supposed to be 1 watt, which would be strong enough at 
full volume to literally drive you out of the room.

(You should be using an isolation transformer between the GR-81 and the 
wall socket before you do any of this.) 

>  Any
> other ideas?  Regeneratives are suppose to be incredibly sensitive and
> otherwise, loud...mine is anything but....

Then there is a problem...or more than one.

Ken W7EKB