Pre-Phase Inverter Master Volume mods

Share your home builds, knock offs and ground up customs.
Post Reply
mspitzer
New Member
Posts: 4
Joined: Tue Jul 10, 2018 8:44 am
Just the numbers in order: 13492

Pre-Phase Inverter Master Volume mods

Post by mspitzer » Mon Aug 20, 2018 1:31 pm

I am working on a Hoffman 6V6 Plexi, and I am debating between whether to use a pre-phase inverter master volume, or go for a PPIMV. I have been reading George's posts about the advantages of a pre-phase inverter master in that it does not mess with the negative feedback. I have also read that the master on his Metro-plex is outstanding.

The Hoffman 6v6 pre-phase inverter master has a 1 meg pot from the treble pot. Are there any mods that can be made to preserve tone at low volume levels? It would certainly be easier to do a pre-phase inverter master volume than a PPIMV, but I need to know what mods would help to preserve tone. Thanks!

romberg
New Member
Posts: 15
Joined: Thu Nov 01, 2007 12:46 am

Re: Pre-Phase Inverter Master Volume mods

Post by romberg » Fri Aug 31, 2018 4:55 pm

If you have space on the chassis to mount two pots, I very much recommend that you install both a pre and post PI master volume. The pre is simple as you describe and just a 1m pot after the treble pot. This controls the amount of signal going into the phase inverter.

For a post-PI master I like the trainwreck type II or LarMar style master which replaces the grid leak resistors on the power tubes with a dual ganged pot. This type of master is totally out of the circuit when cranked full up, So, when this master is fully cranked the negative feedback is not affected.

The real magic of this setup (in my opinion) happens at lower volumes. One can either keep the post master cranked and control the overall volume with the pre-master. Thus all the clipping will be from the pre-amp alone. On a plexi, there will be some but not alot.

However you can crank the pre master all the way up and control the overall volume with the post PI master. Now the PI itself will clip. This sounds closer (but not identical) to the power tubes clipping. For lower volume "high gain" tones this is a pretty good compromise in my view.

Of course, there are all kinds of possibilities in between. The two masters (and your ability to dial in various degrees of PI clipping) can be very versatile.

Mike

Post Reply