Phase matters more than I expected

[Phil_S]

It's been almost 2 years since I built my 'wreck deviant. Although hiss and brightness have been the main problems I was trying to address, I was also getting the telltale howl from the presence control that lets you know the phasing is wrong. I wrongly figured that turning down the presence control would let me work on the other problems first, and I really didn't have much luck.

The reason for stalling on dealing with the phasing was that I didn't follow a standard layout and swapping the OT leads involved calling Major Payne, who I didn't want to tangle with. Well, yesterday, someone suggested reversing the PI inputs instead of the OT leads. This was both simple and obvious, but just didn't occur to me. It was also fully accessible and the job took me about 5 minutes.

Not only is the howl gone -- that was expected -- but the ice pick treble is cured! It seems to me, there was something riding on the signal that made the hiss hard to take. I can't say there is less hiss -- and it was never THAT bad -- but it is much easier to hear it now.

A bit of hum cropped up. I had been tinkering with the ground scheme in an effort to quiet the hiss; didn't really have expectations here, but not much to lose. I will probably put it back to the way my instinct says it should be.

I was considering putting the amp on the auction block as it was just annoying the heck out of me, great tone and all. Now, I'm not so sure. Fixing the NFB phasing changed the character of the amp considerably. I'll need to get to know it all over again!

So, I post this to note that howling should be at the top of your list, not the bottom, when cleaning up in the "afterbuild" phase.

--Phil

[Jana]

[paulster]

Did you swap your 82K and 100K plate resistors at the same time? Your PI is going to be significantly unbalanced if you didn't.

I think I'd just have swapped the OT primaries...

[jjman]

Hopefully he flipped the PI outputs (post cap) and not the "input(s.)"

[Phil_S]

I reversed the wires on the power tube grid stoppers to simply reverse the phasing. The PI should still be balanced, eh?

The OT primary wires are very difficult to access in this particular build -- bad planning? Also one OT wire is too short, making that approach even more challenging.

Thanks for the comments.

Phil

[paulster]

You'll be fine then. It's an equally valid way of correcting the phase at the OT and achieves exactly the same result.