[Collins] help needed

[Carl]

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.

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.

Since the S1 tone oscillator is at a 1.3kc audio frequency the screen 
bypasses should be .1's. discs which werent 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.


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.

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.

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.
>
> On 1/29/2012 7:18 PM, Glen Zook wrote:
>> No problems with the 6U8A.  It is the 6EA8 that causes problems in that 
>> particular circuit and I don't really have a clue as to why.  Of course, 
>> I haven't spent any time learning why since I have plenty of 6U8 tubes!
>>
>> Glen, K9STH Website:  http://k9sth.com
>>
>>
>> ________________________________
>>   From: Bob and Sue Jefferis<ljranch2010-ham at yahoo.com>
>> To: Glen Zook<gzook at yahoo.com>
>> Cc: "drleborgne at aol.com"<drleborgne at aol.com>; 
>> "collins at mailman.qth.net"<collins at mailman.qth.net>
>> Sent: Sunday, January 29, 2012 6:42 PM
>> Subject: Re: [Collins] help needed
>>
>> Glen,
>>
>> Is a 6U8A close enough? I am aware of the fact that 6EA8 may not work 
>> reliably in the oscillator slot, but I believe the 6U8A is a little more 
>> robust than the 6U8 (definitely a Thanksgiving tube - Gobble, Gobble).
>>
>> Bob, KF6BC
>> On Jan 29, 2012, at 12:39 PM, Glen Zook wrote:
>>
>>> Check the tone oscillator, a 6U8.  There has to be a tone present before 
>>> you can tune the transmitter or to send CW.
>>>
>>> Replace the tube, if bad, only with a 6U8.  Although the 6EA8 can be 
>>> used to replace the other 6U8 tubes, the tone oscillator often will not 
>>> work properly if a 6EA8 is substituted.
>>>
>>> Glen, K9STH Website:  http://k9sth.com
>>>
>>>
>>>
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