Planning another PR Build. phase inverter improvements?

[Opry Audio]

Hey yall. I'm gonna do another PR build in the next few weeks. I want to work off of another hoffman layout, but wondering if anyone has had luck rearranging the board in diy layout creator to fit jupiter caps.

In the last one I initially used JJ 6V6's but had an issue with filament rattling out of the box. Changed them to EH and the amp sounded a lot better. Settled on 18mA per tube on the high power setting and changed a bit about the tremolo oscillation. One thing I would like to improve on the next one is how the amp behaves when the volume is at 10. The amp breaks up really nicely, but I'm wondering if what I was hearing was the limitations of the cathodyne phase inverter. It sounded like it was being sent into cutoff conditions. Not a bad solder joint or build quality issue (I dont think). If anyone has some wisdom on how to get it to behave, I would like to know!

Lastly, I really like the PVC coated wire that doug sells here. But, does anyone have any suggestions for 22awg cloth covered wire? I've bought 20awg cloth wire for heater wiring purposes before, but the insolation is so thick and looking for something where there isn't a drastic difference in size. Similar to what's in 60's fenders. Thanks!

[thetragichero]

the "PaulC mod" typically done on princetons greatly improved my fender excelsior (which is largely based off a 6g2) when cranked

[Opry Audio]

I've heard of the Paul C mod? I found some stuff about it but no real technical info about what it's doing. Also mentioned that it overloads some PT's lots of conflicting info. But yeah.. It's creating a voltage divider to create a fixed bias on the grid of the PI triode? If someone can explain better I would be interested to see if it's going to improve my builds. Thanks!

[brewdude]

Might try a large (470k-1M) grid stopper on the cathodyne triode.

[jordan86]

I went down this rabbit hole building my PR.

Grid stopper is a good option. Easiest. Merlin has an article on cathodyne PI’s and discusses the nipple distortion they can cause.

Paul C mod does not strain the PT. That’s the Stokes you are thinking of. The Stokes mod runs the PT harder. Great for more clean headroom in a PR. That’s what I did in mine. It’s actually what they did on the Brwnface PR as well. I’ve had no problems but I have slightly improved PT.

If you’re interested in the Paul C do some more research. Allen does this mod in his Sweet Spot.  It’s an ampeg thing actually. They did it first. If you google ampeg Paul C mod you may find more info. It does slightly change the tone. Some guys love it. Others who generally want that classic Princeton sound dislike it.

[jordan86]

Paul C discusses it here. Changes the way the PI biases and tends to control the nasty stuff.
https://www.thegearpage.net/board/index.php?threads/paul-c-and-stokes-mod.353375/

[Opry Audio]

Cool. Thanks for the link! I’m gonna do some more research and see if this mod makes sense with the Hoffman layout. I have some extra $ to throw into builds at the moment so I ordered enough to build 2 at the same time. I had very good results using a Hammond 290AX, and it seems like the stokes mod may be more feasible with a PT like the Allen TP25.

Either way, the last one breaks up nicer than a lot of true 65 PR’s. At least the ones that have passed my way. I also ordered some of the cloth covered 22awg wire that Doug sells, looks way better than the AES cloth wire I’ve been using. Can’t wait to check it out!

I didn’t manage to find a good way to fit Jupiter’s into the Hoffman layout without basing it off of the eyelet board and changing to turrets, but grateful that Doug’s layout fits Mallory 150’s just fine! Nothing against the radials.. but I love my 150’s.