[Collins] help needed

Dr. Gerald N. Johnson geraldj at weather.net
Mon Jan 30 00:16:18 EST 2012


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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