[Collins] 51J4 Alignment Question

[Dr. Gerald N. Johnson]

On 8/21/2013 3:26 PM, Grant Youngman wrote:
>
>> Most likely to compensate for tube degradation and minimize time in
>> a service shop.
>>
>
> Probably so.  I haven't compared the with vs. without schematics in
> detail.  Unless something else changed, the control would only reduce
> gain.  I don't see any reason to mess with it at this point.  Perhaps
> I have a fortuitous weak tube(s) somewhere :)

That control doesn't see IF energy, only DC. As you lower the resistance 
you increase the tube plate current and in the 6BA6 (and most other 
pentodes) greater plate current means greater gain. But running it balls 
to the wall, means enough cathode current to cause the cathode material 
to splatter on the grid, then you get grid emission, if you don't 
overheat the tube and cause it to get gassy. Though for a 10K pot, that 
region of excess current is only going to be in the last few degrees of 
rotation before reaching the shorted position on the pot. Resistance 
probably under 250 ohms for high plate current. With the two tubes in 
the mechanical filter module that stage doesn't need much gain, compared 
to a J3 without the filter module but other wise the same circuit.
>
> FWIW, a 51j4 with the Lankford product detector and AGC mods (and 4:1
> tuning with a weighted knob) works exceptionally well.  I did have to
> make a couple of minor changes to the Lankford circuit values for
> best performance in this particular radio, but I am really pleased
> with the results.  And Sherwood modified my SE-3 to work at 500 khz
> also -- best of both worlds.

My J4 has the BFO converted to a product detector and a considerably 
modified fast attack slow decay AVC circuit and it works decently but I 
like other radios better.

73, Jerry, K0CQ, technical adviser to the Collins Radio Association
>
> Grant NQ5T
>