Preamp tube substitutions (ex 12AX7 to 5751)

[blackba]

I have done preamp tube swaps to lower gain preamp tubes in the past, but I just wanted to clear up some questions I am unsure about. I am aware of the preamp tube gain chart and how a 12AX7 has more gain than a 5751 and both have more gain than a 12AU7.

So for example say if you go from a 12AX7 in V1 to a 5751:

1. Will the headroom of the amp increase or decrease?
2. Will the amp breakup sooner or later? (very similar question to number 1)
3. Will the amp be louder or softer?
4. Will you get more or less power amp breakup as you turn up the amp?

Here are what I think the answers are:
1. The headroom will increase, it will take more for the amp stages to distort the next one?
2. The amp will breakup later as the V1 is not driving the V2 tube as hard
3. The max amp volume will be softer
4. Not sure about this one. Thinking less power amp breakup.

 

[D-Rock]

I think most of those questions depend on what preamp spot the lower gain tube gets put in.

 

[jerrydyer]

search Myles Rose and 12ax7. He wrote all kinds of cool stuff of preamp tubes. the 5751 has almost identical characteristics of a 12ax7 but about %30 less gain.

 

[blackba]

I think most of those questions depend on what preamp spot the lower gain tube gets put in.

I mentioned V1 (ie the first preamp tube gain stage). Just looking for a general guide.

 

[jerrydyer]

all YOUR answers are correct except 4
it follows suite.