12AT7 in Pro Jr. Squeals

[rjs6700]

Over the weekend I tried a new EH 12AT7 in a Pro Jr. and as soon as the amp warms up it squeals. The JJ 12AX7 I had in there works with no problems as does the new EH 12AX7.

Did I just get a bad tube? I can sub a 12AT7 for a 12AX7 right?

[phsyconoodler]

You may have a bad tube,however using a 12AT7 instead of a 12AX7 can cause issues because they are biased differently.
I would suggest a 5751 or 12AY7 instead.

[lefty kwan]

It's probably a bad tube firstly. I read somewhere that 12at7 should not be used in gain positions like the V1,something about the tube design is better suited as a phase inverter tube. I've tried a 12at7 in the V2(PI) in my Pro Jr but thought it sounded a bit wimpy. I now use a GT5751m in the V2, which makes the amp slightly cleaner and a lot less compressed sounding.

[rjs6700]

Thanks for the replies guys. Tried it again tonight in V2 and its as quiet as a mouse. I guess it doesn't work in V1. Now I know!