Noisy NFB/Presence

[boots]

I am just finishing up a 5FA/JTM50-ish build. This is my first attempt at EL34's.

I'm getting a lot of hum when I connect the NFB/presence. The amp sounds pretty good without the feedback loop connected, and I may end up leaving it without NFB. I've never been a fan of the whole NFB idea anyway, But I'd like to get it working right and really evaluate the NFB/presence thing with an open mind.

Where is all the noise coming from? I've double-checked the wiring. The only thing I can think of is a lead-dress issue (always a possibility with my builds). It's not normal to have an increase in noise with the NFB, is it?

(Yes, I have reversed the OT primary leads, and they are correct - I get a loud squeal with the leads reversed the other way).

[Malcolm Irving]

How are you connecting the ground on one side of the speaker circuit?
I think the best way is to take a wire from the speaker socket(s) back to the local ground for the LTP PI tube.
Keep that ground wire close to (or even twisted with) the NFB connection wire.
EDIT: I'm assuming that your speaker sockets are insulated from the chassis.

[JazzGuitarGimp]

+1 isoltaed speaker jacks.

[boots]

Come to think of it, I think the speaker ground is floating.... oops!

I do have isolated speaker jacks, and the rings of the jacks are connected to one end of the OT secondary. But I did not connect them to a ground.

Thanks. I'll look into this when I get home later today!

[boots]

Thanks Guys! That was the problem - the speaker ground was floating.

The amp is near dead-quiet now, and the presence control works fine. I am still undecided whether to hard wire the NFB, or switch it. I really like the untamed "wild" sound of the amp with no NFB, but I think I'm starting to get used to the closed loop sound. I think the amp seems happier with the NFB loop.

The EL34's sound wonderful. Reminiscent of Duane Allman's tone on the live album, very woody and sweet.

I'll post some pics in a new thread. I'm not particularly proud of how sloppy this build turned out, but I'll probably be keeping this one for myself anyway.

[Firestorm]

Grounds, bad grounds, multiple grounds, etc. are 95% of amp problems. Glad you got it sorted out. 34s are becoming my favorite tube. Even the new ones, but nothing touches the Blackburn Mullards.

[rp]

The amp is near dead-quiet now, and the presence control works fine. I am still undecided whether to hard wire the NFB, or switch it. I really like the untamed "wild" sound of the amp with no NFB, but I think I'm starting to get used to the closed loop sound. I think the amp seems happier with the NFB loop.

Amps born and risen to acclaim w/ NFB should stay that way. IME you'll get bored of the wild sound 50 watter pretty quick. So many ways to play with a 5F6A/JTM but I always put the NFB right back. The real fun is bypass (boost) cap on V2 cathode (essential at least w/ a push pull pot IMO) upping the filtering to 50 all around or at least 50/50/32/32 and trying a ss rectifier. Hard to choose the best btw all of those, no one can, which is why one must build a 5F6A/JTM45, JTM50 tube rectifier, JMP50 (the loud ones), 2204. Get to work!