Ultra low hiss = crosstalk

JJman

Member
Messages
994
12ax7-17.JPG



I picked up this 12ax7 tube recently and noticed it had no hiss at all in my homebrew Champ. I was really looking forward to using it in my high gain head. While running a signal generator into the Champ I could hear significant sound with the volume fully down. So I changed the volume pot but no improvement. I looked at the phantom wave at various points with my scope and could see it was not on the cathode or grid of the 2nd stage. But it was on the plate of the 2nd stage. So I figured I needed to move some wires around but no change. I noticed the wave would reduce, then flip in phase when I turned the volume upwards. Then I decided to try another tube and the problem disappeared.

So this holy grail zero hiss (and balanced BTW) tube is so quiet because the crosstalk between the plates lets the hiss in stage one cancel in stage 2, I guess.

Anyone ever see this? Is it more likely in long plate tubes like this one? Is this still a holy grail 12ax7 for a guitar amp?
 

pdf64

Member
Messages
8,917
I've got a mazda branded ECC83, looks pretty much like a mullard construction, that has the exact same cross talk issue, ie turn down the vol control between the 2 stages and signal (in the opposite polarity ie inverted!) gets through to the 2nd section plate. As the volume is turned up, the polarities null out and signal drops to zero, then the volume increases again in the expected polarity (non-inverted).
 

Timbre Wolf

>thermionic<
Platinum Supporting Member
Messages
12,023
Anyone ever see this? Is it more likely in long plate tubes like this one? Is this still a holy grail 12ax7 for a guitar amp?
I have to admit that I don't exactly understand the issue here. That tube is a Raytheon, and they do seem to be consistently very quiet. But not all the amps/positions I've tried them in actively use the second triode, so that wouldn't phase cancel out hiss (that is what you're suggesting, isn't it?).

- T
 

Pete Cage

Member
Messages
1,614
Don't assume that the two behaviors are related. You might just have a quiet tube that also happens to have some crosstalk.

A sample size of one does not indicate a trend.
 

pdf64

Member
Messages
8,917
I never really noticed the hiss level in that mazda, good or bad. However, the gain was on the low side for both sections.
 




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