12AX7 A/B

[jeff]

 I'm building a test preamp so I can switch in different values and hear the effects. I was thinking of a 12AX7 A/B switch to test between different brands of tubes. Can I just wire the heaters, plates, and grids of two sockets in parallel and switch the cathodes from one or the other to the cathode resistors?

Has anyone tried this?

 I know you can turn on/off the heaters to turn the tube on/off but I want to be able to switch without waiting for one to cool and the other to heat up if possible.

[stingray_65]

Hey Jeff,

Dont forget that there are 2 sides to the 12--7's

Put in as many sockets as you want.

wire the plates ,heaters etc as normal.

put coupling cap after each plate and attach it to the out of your pre amp.

on the screen side, attach a spdt switch.

the center lug goes to the tube, the top lug to ground and the bottom lug to your source.

when grounded the tube has no output and doesn't matter, it's virtually invisible audibly.

when the switch is flipped up, it gets signal and sings.

But dont stop there!

wire in an octal too use the pin out for a 6SJ7 .

If you need, I'll draw up a schematic.

Ray

You know PRR has been saying for some time that this sort of thing would be the BEST tube tester, tests tubes in a real world environment and give real results.

Come to think of it, if you REALLY wanted to go all out, you could tap different plate voltages.

Some bottles respond best at higher voltages and some better at a lower voltage.

[stingray_65]

OK Jeff,
 I thought about it a bit and have a few more bits of insight.

Let us not use a cathode bypass cap. they can color and filter the signal and might be difficult to match values closely.

Let us use a rather high value of coupling cap so as to pass more frequency than the guitars normal range, up the value from .022uf to say .047. if it passes a wider range than the guitar's frequency range, we wont have to worry so much about component matching.

NOW, we have built our comparator, what do we plug it into?

I'm thinking maybe a mosfet follower and then into a nice solid state monoblock amp. not a guitar amp, a HiFi amp and a speaker tower with full frequency range.

There are a lot of small power amps for sale on eBay real cheap like this

http://cgi.ebay.com/Hi-Fi-Mini-Stereo-Audio-Amp-Car-Computer-MP3-25WX2-/220755825644?pt=Car_Amplifiers&hash=item33661293ec#ht_5717wt_1042

For that matter we might scrounge one out of an old 4 head VCR.


Ray

[jeff]

 I see what you're saying and that's cool. I wasn't thinking about trying to compare tube to tube but see how each tube sounds in the circuit without having to remove the tube and put in the new one. I'm not sure this is the same thing. Even though the tubes have no input they're all drawing current and the plate is connected to the B+ by 3 cap/resistor in series. I don't know if doing it this way is the same as pulling out the tube and putting in a new one. Maybe I'm overthinking this again and should do it your way to find the tube I like best and then put that tube in my circuit.

   Thanks
      Jeff

[RicharD]

The Champ Checker has come up quite a few times.  I don't think anybody has bothered to draw it until now.  This isn't my idea.  Please review and comment.

Thanks!
-Richard

Edit:
oops!  Delete C3.

Double oops, connect B+ to diodes, not filaments.

[RicharD]

better

[jeff]

Cool that's what I was thinking of.

What's the M1 on S3?

[RicharD]

It's a DC volt meter which switches between the cathodes.  If Rk is a precision resistor, then one can accurately estimate the tube's current:
VRk/Rk=I
You might very well be surprised that a weaker tube that is flowing less current may actually sound better.

Switching through preamp tubes shouldn't be a problem although I'd expect a pretty big thump.  I don't think it'd be a good idea to switch the power tube so I didn't show that.  I could be completely wrong.