[Collins] 51J4 Alignment Question

[Carl]

>
> 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


Its a decent AM radio for crowded bands and Collins didnt have a decent 
ham/SWL AGC until the 75S3B/C. The 75A series were horrible.

If you want better for AM use a HRO-60 and align all those IF cans to suit 
followed by a ceramic filter to give steep skirts.

Carl
KM1H