NFB oscillation on SE issue

[jojokeo]

I have a single 12ax7 to S.E. EL34 (w/ vvr as seen on another thread I'm tweaking to dial in). On V1b the cathode resistor is 2.7k un-bypassed. I applied a 100k pot as a series resistor (using Geezer's technique) to the 16ohm tap and the other alligator clip end to the cathode of the 2.7k of V1b. I immediately got oscillation squeal. I attached the pot to the 8ohm tap and got an increase in "presence" i.e. more brightness/clarity, and slight vol increase and still bordering on going into oscillation squeel again.
In a pp build I would switch the OT's primary leads but this is single ended. It appears that I'm experiencing positive feedback on a SE tranny? Can this happen w/ SE OT's? I want to ask before I switch any of the wiring.

I've got the OT currently hooked up (using the diagram) w/ the red 8k tap to B+ and the blue/common wire to the plate and using the 16ohm secondary wire w/ an 8ohm speaker to bring pri z down to 4K for the EL34. What do you think?

Thanks for your time and input in advance.

[tubeswell]

The amount of NFB that you supply is important in terms of avoiding oscillation. (Ooops I meant to say Too much!) NFB can make the amp unstable and more than enough can kill the tone.

(If you aren't too fussy, a 'shortcut' I have found that is useful to killing high freq oscillation is a chunky (50-10W) 220-470R wire-wound resistor across the OT secondary - which doubles as a 'safety' load if you accidentally forget to plug a speaker in)

[VMS]

Can this happen w/ SE OT's? I want to ask before I switch any of the wiring.

Yes it can. Switch those primary leads so that blue/com goes to B+ and red or brown to the plate.

[PRR]

> Can this happen w/ SE OT's?

Yes. Same reason.

There does not seem to be consistency in how they apply the red and blue wires. Swap them.

"Too little NFB" will not make a simple tube amp unstable.

Too much takes the flavor away. Way-too-much is unstable but you may not be able to get that far with a simple amp (anyway the gain will be way low before you have too-much NFB).

[Geezer]

I attached the pot to the 8ohm tap and got an increase in "presence" i.e. more brightness/clarity, and slight vol increase and still bordering on going into oscillation squeel again.

That's the clue.....when NFB is connected, there should be a decrease in volume.
You have PFB (Positive FeedBack)......as has been suggested, switch the primary wires around.

G

[jojokeo]

Thanks all. The only real reason I want to try adding is because I'm suffering from some very high top end frequency piercing - almost slight ringing on high notes w/ V1a cathode bypass switched in getting the most gain. At the same time, w/out a treble bleed cap, I'm not quite satisfied w/ the darker tone for the majority of the guitar signal especially when a les paul/humber guitar is being used. I've tried smoothing caps across a load resistor, sm cap feedback btwn grid and cathode, btwn plate and cathode and btwn plate & ground all w/out success for a clear / bright-ish tone but not piercing.
So I'm wondering if the nfb w/ a treble bleed cap will combine the best of both worlds here? I was also considering a conjuntive filter across the OT's primary to get the tone I'm looking for?
Any tips or suggestions is much appreciated as I've spent hours already trying to get this magical tone just right. 

[tubeswell]

You could try a 220pF in parallel with the grid leak for the EL34 - or that large wire wound resistor I talked about before - or you could decrease the amount of NFB you are feeding back (by increasing the upper leg of the NFB voltage divider)

[jojokeo]

I haven't used the cap on a grid leak resistor before on a power tube. My goal is only taking off some of the very highest signal frequencies w/out any loss of clarity. Almost like an oxymoron. When I tried a small cap across the plate-cathode or grid to cathode it sort of worked but to get rid of the ringing high end it also took away some of the clarity of the guitar's tone w/ the amount that was needed to get rid of the ringing. Somewhere between 500pf & a .001uF. Using the 16 or 8 ohm taps didn't make any noticeable difference either.
Another thing I'll do w/ a treble bleed approach on various builds when the cap alone is too bright but w/out anything the tone is too dark on lower settings is put a 47k or 100K resistor in series w/ a 100pf - 500pf cap across the pot's lugs which helps get it just right. But nothing at this point has quite hit the bullseye yet.

[tubeswell]

But have you tried the 220R-470R 5W wire-wound resistor across the Pt secondary yet?

[jojokeo]

Here's what I ended up doing. I put a few different pf caps across the grid leak and it helps. I have to remember that because it didn't take too much away from the high end. But I ended up removing all smoothing caps, treble bleed filter, grid leak cap, nfb, etc. and lowered the vol pot from a 1M to a 500k. I probably could even have used a 250k vol pot and it would even get a little more clearer/brighter but it's the way it should sound now - go figure. I also changed a couple of other values for Pdiss on EL34 and tone stack and it's really good now.
I can tell I've hit the sweet spot. I can play my les paul, strat, tele, hollow body w/ p90s, whatever and they all sound like they should and through a couple different speaker cabs too. The tone of the guitars and pick-up settings really shine through. I didn't want to use nfb or tone loss caps if I didn't have to. There's no more ringing or harsh high notes, great sustain, and nice musical feedback w/ the gain up and vvr set down to 4. I guess simplicity is best sometimes? For some reason this one was a bit of a pain getting right but it was really my own fault from getting in my own way thinking I needed to add stuff when I didn't. Another lesson learned. Thanks so much for the help & suggestions all!