AT7 in V3 for phase inverter

[Boogie Chillen]

I'm going to get some tubes for my FM100 combo. I've read that some people are replacing the AX7 PI in V3 with an AT7. What would be the results of such a manoeuvre and has anyone tried it in their Fillmore?

 

[Boogie Chillen]

Anyway I've decided it's a bad idea. I don't want to change the gain structure of the amp or compromise it in any way. It was designed to have an AX7 in V3 so that's what I'm going to stick with.

 

[rarebitusa]

Just one mans opinion... don't have a Fillmore but have played with a Jan 12at7 in a few Mesas. With less overall gain the difference with the at7 is not huge but noticeable. In my IIC+ HR the at7 in the V5 PI cleaned up Ch 1 and provided better lower end clarity with lower Vol 1 settings, which is what I was looking for. In my V:90 V7 the lower gain was evident as the output volume was lower with the same settings, but there was no noticeable tone difference so have since reverted back to the 12ax7.

The choice really depends on what one is looking for, with the PI it can't hurt to try it out.. it may end up being meh or you could end up liking it 8)

 

[Boogie Chillen]

I don't know which phase inductor circuit is used in the Fillmore specifically which makes it a bit of a lucky dip as far as how changing the tube type is going to affect the amp, tone or gain structure. I read somewhere that there are three main types of PI circuits used in amps. I can't remember what they are but I reckon it would pay to find out before changing the manufacturing specs of the amp.

 

[Boogie Chillen]

Found a bit of info, although it's way over my head. The PI circuit is most likely a form of the "long tailed pair" phase inverter. Some Fenders and Orange use the "Cathodyne" phase inverter circuit. And then there is the one called the "Paraphase".
If the PI circuit is LTP then it might be worth trying an AT7 from what I've managed to find thus far. More clean headroom and smoother break-up.