12ax7a in "S" "D" 84-40

[roml3]

I was playing through my 84-40 amp, and the volume on ch. 1 was fluctuating. One of the 12ax7 was dim. another guy there had an old one I plugged it in the socket and all seemed fine...for a few seconds and it blew the fuse twice and I shut my amp off. I ordered some tubes and capacitors and all is well. However when I pulled the 12ax7s out to start repairs the tube my friend gave me was a 12ax7a could that have caused the fuse blowing? also when I turned the amp on with the 12ax7a one of the el84 tubes was glowing way more the other two before the fuse blew.

what is the difference between 12ax7 and 12ax7a?

 

[GilmourD]

Re: 12ax7a in "S" "D" 84-40

12AX7A is just a revision to the spec, mostly for noise. They're interchangeable, just like you could also pop a 12AT7, 12AU7, 12AY7, 12DW7, 7025, or SRV's favorite 5751 in there.

 

[GilmourD]

Re: 12ax7a in "S" "D" 84-40

If you blew a fuse your volume fluctuation was probably more likely due to something in the power section, since that draws a good deal more current. Were your power tubes redplating?

BTW, some 12AX7s don't have much of a glow at all, so that's not really an indicator of them not working.

 

[roml3]

Re: 12ax7a in "S" "D" 84-40

Yes one of the el84 was glowing bigtime

 

[GilmourD]

Re: 12ax7a in "S" "D" 84-40

Yes one of the el84 was glowing bigtime