Help! Weird problem on PR build

[56goldtop]

Hi guys!

I recently built a Marsh Princeton Reverb kit. It sounds great clean, and the reverb is wonderful. But when I turn it up past about 4, it kind of "buzzes" or "farts" when I play (only on the notes, it's silent when I don't play anything).

Even when the reverb is turned off it does this. However, when I disconnect the reverb tank and/or remove the reverb driver tube (V2 - 12AT7), it sounds good when turned up. Of course, then I don't have any reverb, and I want reverb on this amp .

Just for the heck of it, I subbed the JJ 12AT7 with an EH 12AX7 I had laying around, and now the reverb works without the weird buzz. I guess the 12AT7 was the problem. Unfortunately the amp doesn't sound as good with the 12AX7 in V2, and I also hear a very quiet bright noise along with the normal sound from the amp.

I popped the 12AT7 (which I guessed was broken) in a single ended EL84 amp, to make sure that the tube was the problem. But in this amp the tube sounds fine! But everytime I put it back in V2, the same buzz problem occurs.

Does this make any sense to any of you? Why would a seemingly fine 12AT7 not work as it should, while an EH 12AX7 does? Could it be that the 12AT7 is faulty, but still works as a normal V1 tube? I don't have another one of those laying around, so I can't try another 12AT7 at the moment. I would like to be sure before I buy a new one.

Also, I tried two other 12AX7's in V2, both were JJ's. They are working just fine in the metro JTM45 I took them from. One of them worked in the PR, and the other one had the same buzz problem.

So:
JJ 12AT7 in V2 - buzz problem
JJ 12AX7 in V2 - buzz problem
JJ 12AX7 in V2 - no buzz
EH 12AX7 in V2 - no buzz

Any suggestions on where the problem is?

Thanks guys !

[elronhoover]

Does this make any sense to any of you? Why would a seemingly fine 12AT7 not work as it should, while an EH 12AX7 does? Could it be that the 12AT7 is faulty, but still works as a normal V1 tube? I don't have another one of those laying around, so I can't try another 12AT7 at the moment. I would like to be sure before I buy a new one.

The 12at7 as used as a reverb driver in a fender is run pretty hard, compared to say a normal preamp gain stage. It could be that in this usage the tube is just not up to the task. As a general recommendation, it's better to use a quality NOS 12at7 for this spot if possible, they're still pretty cheap and available. You definitely want to use a 12at7 in this spot too..

Not sure otherwise why you would get buzz/no buzz with your tube swaps as mentioned..

Dave

[56goldtop]

Thanks man, I'll buy a NOS 12AT7 and see what happens

I've heard some pretty bad things about the current production 12AT7's, so I'm hoping this will take care of it.