[Collins] help needed

Tue Jan 31 06:50:30 EST 2012

Gentlemen, what wonderful treatise that you presented.  I may use this 
material in the next Collins Journal if you permit.

I would only add that when modifying the conventional power supply from tube 
rectification to solid-state rectification defeats the low noise floor of 
the Collins S-Line receiver and also makes these receiver susceptible to 
spurious oscillations.  Unless, of course, one takes appropriate measure as 
follows:  I would only add that the line voltage be no more than 115 VAC and 
if one is inclined to go to solid-state rectification in the power supply, 
that he reduce the output voltage using a dropping resistor.  It's 
phenonomel what a difference there is when a 75S-3 or KWM-2, etc. is 
operating on 110 VAC - for example -  using tube rectification.

Using a dropping resistor in the center tap of the secondary or even in the 
B+ line may seem good "engineering practice" but completely ignores the rise 
in filament voltage when the transformer "sees" 120+ volts on the primary 
winding.

As everyone knows by now, there has been this rush to go out and buy these 
solid-state kits from vendors which I will not name.  However, when I am 
asked whether to replace the tube rectifiers in the Collins 516F-2 power 
supply, I am rather emphatic - NO.  My response is to recommend reducing the 
line voltage using a thermistor, variac or bucking transformer 
configuration.

IMHO

David Knepper, W3CRA/W3ST
Collins Radio Association - Join Today
www.collinsra.com
Editor of the Collins Journal
Secretary of the Collins Radio Association
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----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: "Carl" <km1h at jeremy.mv.com>
Cc: <collins at mailman.qth.net>
Sent: Monday, January 30, 2012 3:41 PM
Subject: Re: [Collins] help needed

>
>
> On 1/30/2012 8:41 AM, Carl wrote:
>> Good points Jerry. I fight parasitics often in vintage gear as many
>> circuits were marginally stable and the fixes of that era were to place
>> a paper and foil cap across the tube socket between input and output and
>> move wires around until it was stable.....the latter is actually in
>> Riders as part of the service instructions. When replacing with film or
>> disc caps it was back to step one and pushing wires around wasnt an 
>> option.
>
> The long leads between the socket and the active elements in the tube in
> all but loctal and miniature based tubes makes them hard to tame. When
> its a quarter wave of wire from the bypass to the screen, that tube
> socket bypass doesn't have much effect. That's where a trimmer cap for a
> screen bypass sometimes worked to series resonate the stray inductance
> in the screen lead to bring the screen to ground, at least for a narrow
> band HF or VHF amplfier (and in the pre loctal/miniature days, 20 MHz
> was the threshold of "VHF."
>>
>> What has always worked for me is a 10-27 Ohm carbon comp resistor in the
>> grid lead right at the socket, triode or pentode. For the screen a 100
>> Ohm carbon comp right at the socket and a .01 or .02 disc cap at both
>> sides to ground with minimum lead length. Carbon and metal films rarely
>> work in these cases. A few very squirrely early 30's big pin glass tubes
>> needed a few turns of wire around the grid resistor to form a complete
>> parasitic suppressor. Having a spectrum analyzer handy saved a lot of
>> waster time.
>
> Most I've seen like a 47 to 100 ohm on the grid and not putting any
> bypass next to the tube on the screen with that amount of series
> resistor. Film type resistors probably have a bit more capacitance
> between the end caps so a higher value may be needed.
>>
>> Since the S1 tone oscillator is at a 1.3kc audio frequency the screen
>> bypasses should be .1's. discs which weren't economical in that value in
>> 1961. Note that C-107 is a paper cap, guaranteed to be leaky, and should
>> be replaced along with all other paper caps.
>
> Not only is the paper cap leaky, its inductive. .01 to .05 sometimes
> were series resonant at about 455 kHz though the makers did offer some
> intentionally tuned to series resonance at 455 kHz. The better film like
> the orange drops are extended foil and don't have nearly the inductance
> of the standard paper.
>>
>>
>> While the S Line is a bit newer the tubes have more gain and the cures
>> are still the same. Reducing gain in the tone oscillator is another
>> point to look at; having 260V on the plate and 268V on the screen is way
>> too much IMO. The feedback cap C-111 is another item to look at. I see
>> no reason that someone with a bit of time cant make that stage 100%
>> stable with any 6EA8 or other similar substitute. This isnt rocket 
>> science.
>
> As they did in the 32S-3 with a resistor on the grid, and in the KWM-2
> with both the grid and screen resistors. The feedback capacitor and
> phase shift network is a high pass filter so encouraging oscillation and
> some high frequency where the phase shift is again 180 degrees. Since
> they immediately drop the AC voltage by a voltage divider down to 6.8%
> of the amplitude on the plate, lower voltage should be fine, IF the
> oscillator starts good at the lower voltage. At least in the S-1 and
> KWM-2 this tone is the carrier oscillator for CW. Its at 1.3 KHz so
> harmonics are outside the mechanical filter passband. In the 32S-3 its
> only sidetone and less critical.
>>
>> While at National in the 60's there were many differences between
>> manufacturers in very common tubes such as the 6BE6, 6BZ6, 6GH8, and
>> many others. RCA was the worst by the late 60's as the end was
>> approaching and cutting quality was common, performance of tubes in the
>> same bulk lot were all over the place. We found Sylvania and Telefunken
>> to still have integrity.
>
> How much tube manufacture in those numbers was manual and how much was
> automated? Was the poor quality a lack of rejection at inspection or
> multiple automated machines drifting out of adjustment missing the
> mechanical parameters? Or did they grab any prewound grid even though
> each tube had a nearly unique design?
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
>>
>> Carl
>> KM1H
>>
>>
>>
>> ----- Original Message ----- From: "Dr. Gerald N. Johnson"
>> <geraldj at weather.net>
>> To: <collins at mailman.qth.net>
>> Sent: Monday, January 30, 2012 12:16 AM
>> Subject: Re: [Collins] help needed
>>
>>
>>> There was a factory service bulletin about the tone oscillator having a
>>> VHF parasitic oscillation, cured with 47 or 100 ohms in series with the
>>> control grid, probably. The 6EA8 has a bit more gain so probably was the
>>> part of the problem. The latest 32S-3/3A manuals show 47 ohms there. I
>>> didn't find a SB for that, but it might keep the 6EA8 more calm in the
>>> 32S-1 6U8 socket.
>>>
>>> ASAB1004 added 100 pf control grid to ground for the 6U8A in the KWM-2
>>> to control parasitics. The latest Rockwell KWM-2 manual shows sometime a
>>> 47 ohm resistor was added between the tube screen pin and its bypass to
>>> suppress a parasitic. That was in KWM-2/2A SB-10.
>>>
>>> All the tone oscillator circuits are very similar so parasitic
>>> oscillation tendencies could be similar.
>>>
>>> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
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