So I bought a quartet of Tung Sol 12ax7's to replace the tubes in my recently acquired Rock Master pre-amp. When I did the replacement, the sound was barely audible. So I replaced the old tubes and started swapping them one by one to find the dud. It turns out it's not the tubes but the last position that seems to be acting up. If I put three Tung Sols in and leave the 4th spot for one of the originals, it's fine. As soon as I drop the 4th Tung Sol in...back to nearly inaudible sound (doesn't matter which of the new Tung Sols I use either).
I looked at the tubes and they appear to be slightly different looking...is it possible the Rock Master houses 12aT7 and nto 12aX7 tubes??
Any thoughts on what might be going on here??
I 'm pretty good with this stuff usually but I'm truly stumped here.
:x
That is a stumper.
The 12AT7 and 12AX7 have the same pinout, and the 12AX7 has more gain than a 12AX7, so that shouldn't explain your situation.
Most plausible explanation I can come up with is that one socket contact in the fourth position is slightly misaligned, and the original tube has a pin that's just bent enough to make contact with it, while the replacement tubes don't. Take a close look. Also possibly there's a bit of corrosion or dirt in there that for some reason will still allow the original tube to make contact but the replacements aren't doing it. Try some contact cleaner like De-Oxit and see if it helps. Sooner or later that original tube will have to be replaced.
"A cheerful heart is good medicine."
Wow...excellent tips!! I will have a look at these scenarios.
Thanks!
