[Harp-L] Bassman Reissue Tube Swaps

Apologies if this has been discussed before - I looked through the archives and couldn't find the answers I was looking for.

I have a Bassman reissue (the earlier one - not the LTD) that was "voiced for harp" by an amp tech.  It's an awesome amp, but I cannot get the volume up above about 2-3 before it starts to feed.  The tube layout (looking from the back of the amp, left to right) is:

5u4               6L6 (pair)      12AX7   12AX7   12AY7
Rectifier         Power Tubes        Pre-Amp Tubes   -

It's apparent that much has already been done (replacing the original SS rectifier with the 5U4, and replacing one of the original 12Ax7's with an Ay7).  The power tubes and the 12AX7s are all Sovteks.  The 12AY7 is a Marconi.

What kind of improvement can I expect if I replace the Sovtek's with American made NOS tubes?  Are there any other options (less expensive ones) for the power tubes?  Groove Tubes?  Any others?  Also, can somebody explain (with a minimum of electronics jargon please) why there are three 12A?7s, and what each of them do?  I found a post in the archives that says that the first two (on the right?) are gain stages.  The third one (on the left) is a "phase inverter".  I understand that a gain stage takes the signal from the guitar (or harp-mic in my case) and amplifies it enough for the power tubes.  I also understand that a Green Bullet with a CM element puts out a lot more power than a guitar pickup, and this is why replacing the 12AX7s with lower gain 12AY7s, 12aU7s, or 12aT7s will make the amp much more harp-friendly.  If I buy a couple of 12AU7 or 12AT7 tubes, do I want to experiment with them in the two right hand sockets?  Also, what does a "phase inverter" do (and why?)  Do I want to put a lower gain tube here too?

Many thanks to a great web-community...


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