[Collins] 32 s 1 self oscilation

[Dr. Gerald N. Johnson]

On 4/21/2012 7:21 PM, Carl wrote:
>
> ----- Original Message ----- From: "Dr. Gerald N. Johnson"
> <geraldj at weather.net>
> To: <collins at mailman.qth.net>
> Sent: Saturday, April 21, 2012 7:11 PM
> Subject: Re: [Collins] 32 s 1 self oscilation
>
>
>>
>>
>> On 4/21/2012 11:54 AM, Carl wrote:
>>> long with the other suggestions Ive tamed an unruly S1 and others
>>> with a 22
>>> Ohm 1/2W carbon resistor right at the grid pin of V-7 and a 100 Ohm 1/2W
>>> right at at Pin 8&3 with C58 at the tube side and C59 at the other
>>> and both
>>> with very short leads.
>>
>> Those can be most effective at preventing VHF parasitic oscillations at
>> some possible cost of gain on 10 meters.
>
> That is only one of the benefits. It can also tame a stubrorn low
> frequency oscillation or a not perfect neutralization. Very common in
> many HF circuits. Unless a circuit is already drive starved it shouldnt
> affect 10M at all.
>
>
>
>
>>>
>>> For the finals be sure R119/120 and R43/48 havent drifted high and as
>>> long
>>> as you are at it check the cathode resistors.Next install other 100
>>> Ohm 1/2W
>>> carbon resistors at pin 3 of both tubes with added .01 discs at the
>>> far end
>>> as with V7.
>>
>> On my 32S-1 schematics, R43-48 ARE the cathode resistors of one of the
>> 6146 with R49-54 the cathode resistors of the other 6146. I can't find R
>> 119-120 on the schematic or parts list. I find them in the 32S-3
>> schematic, part of added grid parasitic suppressors not shown in my
>> 32S-1 manual or the 1959 edition of the yellow book.
>
>
> I used the 32S1 schematic on collins.org

that's collinsradio.org and the 32S-1 manual there is a couple years 
later than my 1959 edition yellow book, and my earlly 32S-1 manuals.

I see in the 1961 schematic that they added a plate parasitic suppressor 
to the 6CL6 driver and to the PA grids. Must have found some squirrely 
tubes!
>
>
>>>
>>> Why Collins didnt use some of those basic stabilazation components
>>> across
>>> all S Lines is beyond me.

They were suggested in the tube data sheets sometimes.
>>
>> You'll have to ask Warren Amfahr W0WL about that.
>>>
>>> Also dont overlook a gassy 6AH6 or 6CL6 which would likely be running
>>> hotter
>>> than normal.
>>
>> Also tubes with grid emission will counter act the fixed or ALC
>> generated grid bias and draw more current and in pentodes the gain goes
>> up as the plate current goes up.
>
>
> No different in a receiver where it overides the AGC voltage and in
> extreme cases drives the grid positive with severe signal overload and
> sometimes oscillation the result.
> I see this in receivers, transmitters and transceivers on a regular basis.
>
> Carl
> KM1H
>
There were those in the TV service industry that felt the big tube 
tester checking emission a transconductance were not good for servicing 
that a grid emission tester, which is much simpler, was a better money 
maker because it found more bad tubes that solve weird circuit 
performance quickly. The S-line receivers run the 6BA6 hard and so the 
cathode coating sputters to the grid and the grid runs hot enough to 
emit electrons in a while so 6BA6 with grid emssion seem to be a 
standard problem. Its a toss up whether the grid emission makes the 
controlled IF stage distort or just puts too much drive on the last IF 
stage and the product dector by fighting the AVC voltage. The result is 
the same, clipping distortion. And like I said, the gain of a pentode is 
directly proportional to the plate current and when the grid is biased 
to zero or a little positive, the tube plate current is high so the gain 
is high making oscillation very possible and not necessarily at the 
tuned frequency.

73, Jerry, K0CQ Technical Adviser to the Collins Radio Association.