Shot noise in 6AU6 tube?

[WB3JOK]

For a few months, my HW-101 has been exhibiting shot noise (random "crackle" spikes which deflect the S-meter) and slowly getting worse. About three years ago I replaced the 6HS6 (1st and 2nd IF) with new surplus 6AU6WC's as suggested in modification articles. Apparently the 6HS6 has more gain than needed, and insufficient plate dissipation in the 101 circuit.

Anyway I went looking for the noise today, but couldn't find anything loose or corroded, and tapping on the tubes, pressing the PC board, wiggling parts did not cause or change the random noise. On a hunch I replaced both 6AU6's with known good used ones. The noise was gone  and didn't return when I put the mil-surplus tube back in the 1st IF, but it came back immediately when I put it back in the 2nd IF socket.

The tube is not microphonic, and I can't see any odd glow or arcs from within it. I was just wondering if anyone else has experienced this (shot noise) with receiving tubes that otherwise test and function well. 

-Charles

[w5rkl]

The noise you discribed is quite similiar to what I am experiencing in my Drake R4C receiver. It appears you have found the offending tube causing the noise. I'll just have to replace the tubes in my R4C to get rid of the noise.
73's
Mike
W5RKL

[n3lrx]

I had a similar problem in my Mohawk.  Only it was loading on the output. I had the audio tapped into the diode and used the RF Gain as a volume control. If I had no speaker on the output and the volume got turned up for some reason if something hit with a strong signal, usually lightning crashes with sharp and rapid amplitude peaks it would arc over in the socket. And make a pop inside the tube and/or chassis. I just stuck a junk 8 ohm squaker on it so if it got turned up I could hear it and know the volume was up.

[WA4FUU]

I wonder if it is caused by grid emission? Some of the free barium from the cathode coating finds its way to the control and/or screen grids; when hot enought, the grid(s) can become another emitting surface.  Just a thought.  Some tube checkers test for this condition and if the condition exists recommend replacement of the tube.

73,
Richard
WA4FUU