[Boatanchors] UNSTABLE 6K8 OSCILLATOR

[Richard Knoppow]

On 1/28/2015 3:07 PM, n2lxm at juno.com wrote:
> Good Evening Bill,
>
>          The 6K8 can give you trouble if the lead lengths are to long. Had
> a homebrew radio that did what your describing. Rebuilt the oscillator
> and shorten up the leads. Worked O.K. after that. But the 6K8 would not
> be my first choice for a Converter. A 6AS7 would be a better choice.
>
>
>                                                      Jeff N2LXM
>                                                        American
>                                                          Patriot
>                                      Proud Father of Two U.S. Marines
>
>
     I think you mean 6SA7, thhis is about equivalent to the miniature 
6BE6 and is a decent mixer.  An even better choice would be the 6SB7Y or 
its minuature equivalent the 6BA7, these two are about the best of the 
pentagrid mixer tubes.  There are better nuxer types than the pentagrid 
such as a "Pullen" mixer, which is a dual triode with common cathode and 
oscillator injection on the cathode. It has far lower noise than the 
multiple grid type and still has good oscillator isolation.
      The 6K8 was a popular combination mixer and LO tube until the 6SA7 
came out in the late 1930s.
      The drift problem sounds like its coming from something other than 
the mixer itself.  The fact that it does not drift in a constant 
direction suggests its not the coil, perhaps a defective resistor.


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL