6SN7 front end

[dhuebert]

I would like to use all octals for the bass amp I am building now. Previously I used a 12AX7 for the front end, it needed a shield to keep buzzing down, without it the amp displayed all sorts of bad behavior. I would like to replace one stage of AX7 with two stages of 12SN7 adjusted to give the right gain. My question is what does you think of this idea and do I need some kind of sheild for the SN7?

One of the considerations is: if the amp goes on the road its much easier to come up with 12A*7 and 6*6 than Kt88 and 12S*7 but I like the all octal idea.

Don

[erichayes]

Hi All,

Don, have you considered using a couple of 6SF5s or 6J5s (or a combination of the two). They're both metal types; the gain of the 6SF5 is the same as a 12AX7 and a 6J5 is identical to half of a 6SN7.

FYI, another problem I've run into with high gain circuits using glass tubes is photosensitivity, particularly with fluorescent lights . . . hummin' and buzzin' and all sorts of hell raisin'. Put a soup can over the tube and it's quiet as that mouse and his cotton ball.

There are 12 volt versions of both. Another one to consider is the 6/12SQ7. That was part of the octal "All American Five" lineup, and is still fairly easy to find.

[mesherm]

Eric,
Regarding the fluorescent lights, have you tried putting a opaque non-metal container over the tubes to see whether its photosensitivity or just radiated EM. The fluorescent lights in our lab play hell with some of our sensitive millivolt measurements on strain gauges. I have to use a 5 hz low pass filter most of the time.

[erichayes]

Hi All,

Yeah, once we discovered the phenomenon, it was Science Fair Time. The guilty tube was a 6AQ7, which is ½ a 6SL7 and 3 diodes. We exposed it to regular fluorescent, incandescent, Mag Light (made a "whump" sound when the flashlight was swept over the tube) and covered it with everything from the aforementioned soup can to a Celestial Seasonings tea box to wrapping the thing in duct tape. We got poohpoohed and razzed by everyone we told. About a year later, some friendly competition were working on a condenser mike pre and had it happen to them. We served up large portions of crow shortly thereafter.

[mesherm]

Sounds like the three diodes were acting as a photomultiplier

[erichayes]

Sounds like the three diodes were acting as a photomultiplier

I misspoke. The 6AQ7 is a dual diode/triode with independent cathodes for the diode and triode sections.

Nope; the plates of the diodes were grounded, as was the cathode of the dual diodes. I had a wee bit of experience with PM tubes back in the early '70s when I was employed by a spectrophotometer manufacturer, and this ain't that.

[EWBrown]

On a related note, neon bulbs are also very photosensitive.

For a quick experiment, plug in a neon (NE-2 and 220K resistor) night light into a variac, and turn the voltage down to the point where the NE-2 lust barely lights up. Then shine a light (doesn't have to be bright, and even IR or UV works) onto it, and it will light up full intensity.

/ed B in NH