[Heath] audio tube sb101

[Kenneth G. Gordon]

On 28 Feb 2014 at 8:53, RAY FRIESS wrote:

> Anyone have a spare audio output tube for an sb 101?

That is a 6GW8. I buy them off eBay. By careful searching, I can get them for 
around $5.00 each. Otherwise, they are around $30 each. AES has them for $32, 
which is too much IMHO.

>  My rig howls when I turn
> up the volume more than one quarter.

Check C-928, a .05 mfd cap which is UNDER the PC board. It is supposed to 
connect one lead of the audio output transformer to the cathode, pin 7, to 
provide negative feedback. It MAY be connected to pin 6, the grid, instead, 
which would cause positive feedback and oscillation. Or the cap may be bad.

In point fact, C-928 isn't needed in the SB-101. I routinely remove that. Doing 
so provides much greater audio output at the expense of "fidelity" but I never 
noticed any difference in audio quality.

You might try disconnecting that to see if your oscillation goes away.

>  I removed product detector to isolate the
> audio stage and it still occurs.  Removing the audio tube eliminates all sound. 
> Think the tube is bad.

Not necessarily. Those tubes don't usually go bad in that way.

Ken W7EKB