[Collins] 51J4 Alignment Question

[Dr. Gerald N. Johnson]

On 8/21/2013 12:04 PM, Grant Youngman wrote:
> Just to follow up on my question.  My receiver is an early one, and
> does not have the IF gain control.

It was probably added to account for variations in various makes of 
tubes and to get the AVC control voltage distributed best between the 
various tubes with AVC control. Its easy to mess up the AVC (and so the 
S-meter) by having a stage with more gain than designed for.
>
> Finished the alignment of the receiver, and measured the gain from
> antenna terminal to IF output.  3 uV RMS in at the antenna terminals
> yields 250 mV RMS at the IF output port.  The alignment spec for
> receivers with the IF gain control is 270mV out.  I figure that's
> close enough not to worry about.  I think someone also indicated that
> the IF output signal was loaded down by the detector and was badly
> distorted and not very sinusoidal (?).  I did not find that to be the
> case.

I think the voltages should be RMS, signal generators are always 
calibrated in RMS and all VTVM of that era were peak reading calibrated 
in RMS.
>
> Which still begs the question … why did Collins feel the need to add
> the IF gain control in the first place?  I can only suppose it had to
> do with the fact that some contracting officer put it in a
> specification somewhere along the line.

Well I think its to control the S-meter better. I know that changing the 
RF tube in a 75A4 even to the ECG brand 6DC6 will cut the S-meter 
reading serious because it controls faster than RCA 6DC6. I need to run 
some experiments on those tubes and see how far out of 6DC6 spec they are.

I'm slow to respond to your original post because I've been traveling 
preparing for 10G+ operations and just got home yesterday. Saw lots of 
museums, few with tubes. Made a bunch of 10G contacts over the weekend.

Setting the IF gain to max will probably work, but will shorten 6BA6 
lifetimes because higher gain happens only with greater plate current 
and they will eventually exhibit reverse grid current from cathode 
material being sputtered on to the control grid wires. Then the S-meter 
goes negative as the IF tubes warm up and that grid current opposes the 
AVC voltage. Happens regularly in S-line receivers even with the gain 
set normally.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association
>
> Grant NQ5T
>
>>
>>> I've been reading over the alignment instructions for the 51J4
>>> getting ready to touch mine up.  In manual section 5.3.17 (1962
>>> manual), it gives instructions for setting IF gain.  My question
>>> is whether the input (antenna terminal) and output (IF output)
>>> voltages indicated are peak or RMS.  I presume it doesn't
>>> actually matter as long as both measurements are taken the same
>>> way, but thought I would ask.  I think I've also seen a comment
>>> in a posting somewhere that it isn't worth worrying about and
>>> that the IF gain should just be set to maximum.  Any input on
>>> this?
>>>
>