If this is a 376 (not a "367"), the 12AU7 is the correct tube. Are you SURE that you inserted the new tube correctly?
Why were you replacing the original tube in the first place? Preamp tubes like that are much less likely to "go" compared to power tubes. If the original tube was going bad, there may be a power supply issue.

It definitely is the 376 (the mic pre). I put in a 12AU7A (as opposed to a 12AU7), would that make any difference? The output with the original tube had become distorted, thin - I assumed it was the tube so I replaced it with the RCA? I understand now that paying more for the RCA tube was probably a waste of money since this such a low voltage plate, but it should at least work. I'll re-check the install (it seemed fine - the pins can only line up one way and they all settled in right to the base of the tube).

Thanks,

Don't discount that the RCA tube may be bad...after all it is "new old stock" at the very least. That extra "a" designation won't determine the tube's gain, shouldn't make a difference. And what you described of the original certainly does sound like it may be a bad tube.

The RCA tube came from Tube Depot and apparently was tested before it was sent out. But I don't think its the power supply because all the other lights on the front of the unit work. Hmmm. I guess the only thing left is to check the install. Thanks again.