Single Ended Odd Distortion - To NFB or not to NFB?

[chinrest]

I'm fiddling with a single-ended 6v6 circuit (ala 5F2). I have one of those problems that may have gone unnoticed (rather than a new problem). When playing higher notes (550hz) with humbuckers, I hear a little unpleasant distortion - a kind of hard-clipped, ghost note. Hitting chords produces no ill effect (or masks it). Lower output pickups don't produce this effect.

I've ruled out speaker and tubes. Preamp and power amp voltages look normal. It doesn't sound like the crackle of a cold solder joint. I'm left looking at this circuit's lack of NFB and the cathode bypass on the second stage. Both cathodes of this 12ax7 are bypassed with 25uf/1k5. I like to be more informed before making blind modifications, so I ask if this something to explore. This amp is a little gain monster, but I wonder if it's a little over the top.

Again, this subtle issue only occurs with cranked volume, humbuckers, and single high notes. Any input is appreciated, as always.

[pdf64]

Have you tried it with a different speaker?

[chinrest]

Have you tried it with a different speaker?

Yep. It's more or less noticeable on different speakers.

[Jazz P Bass]

You do not indicate at what power level you are playing the amp.
Are you playing it 'clean'?
SE amps will exhibit odd tones when overdriven.
You may be running the amp just at the onset of distortion & that particular note drives it off the edge.

[chinrest]

You do not indicate at what power level you are playing the amp.
Are you playing it 'clean'?
SE amps will exhibit odd tones when overdriven.
You may be running the amp just at the onset of distortion & that particular note drives it off the edge.

I noticed it with the volume cranked, and hitting a hard high note with humbuckers. The NFB might be a red herring. I was hoping to find out a little more about how it effects this type of circuit, and if a lack of NFB might cause strange distortion issues. Again, this is a 5F2 with no NFB, and a 25uf bypass cap on the second triode.

"Speak before you tweak" is my motto.

[EFK]

I'd be interested to know what level of drive you have going when this happens. Example, if you play softly, is it clean, and will begin to break up when you hit it hard? The reason I ask is, speaking for myself, I have yet to hear a single ended amp that sounds good to me at the "in between" stage. They sound great clean, and when driven to hard clipping all the time ("dimed," in other words, but it probably is circuit dependent) they also sound great. However, the sound of a single ended amp at the edge of breakup, or where certain notes/chords will start to clip when hit harder while the amp is still somewhat clean - this is a horrible sound to my ear. Very abrupt and harsh. I don;t know if it is a characteristic of all single ended amps, or maybe just the ones I've played. So I wonder if you are hearing this. If the amp is already being heavily driven when you notice this, then disregard all of the above and carry on!

Clear, bright speakers with little 1" voice coils can emphasize this effect.

[Jazz P Bass]

A lot of tube amps are that way.
Clean is clean.
Dimed dirty is nice.
Inbetween that, blech.
In a single ended output stage the bottom half of the sine wave will change before the top half (scope wise)
So you have to get to where both halves are 'distorted'.

[pdf64]

'A lot of tube amps are that way.
Clean is clean.
Dimed dirty is nice.
Inbetween that, blech.'

My experience is that often, global NFB on the power amp is a contributory factor to this, removing / reducing it can improve how smoothly the amp comes in and out of overdrive.
However, the JTM45 has about the highest ratio of NFB for guitar amps, and that tends to do the inbetween thing smoothly.
Pete

[chinrest]

I'd be interested to know what level of drive you have going when this happens. Example, if you play softly, is it clean, and will begin to break up when you hit it hard? The reason I ask is, speaking for myself, I have yet to hear a single ended amp that sounds good to me at the "in between" stage. They sound great clean, and when driven to hard clipping all the time ("dimed," in other words, but it probably is circuit dependent) they also sound great. However, the sound of a single ended amp at the edge of breakup, or where certain notes/chords will start to clip when hit harder while the amp is still somewhat clean - this is a horrible sound to my ear. Very abrupt and harsh. I don;t know if it is a characteristic of all single ended amps, or maybe just the ones I've played. So I wonder if you are hearing this. If the amp is already being heavily driven when you notice this, then disregard all of the above and carry on!

Clear, bright speakers with little 1" voice coils can emphasize this effect.

Play softly, magic. Hit it hard, more magic. The problem really is, as it's been described, at the very onset of clipping. It drives me a little batty that I only hear this glitchy sound with single high notes and humbuckers. Try as I might, I can reproduce the glitch with single coils. The Tele can do no wrong with this amp.

I'm gathering that adding NFB might enhance the issue in a negative way.

[EFK]

A lot of tube amps are that way.
Clean is clean.
Dimed dirty is nice.
Inbetween that, blech.
In a single ended output stage the bottom half of the sine wave will change before the top half (scope wise)
So you have to get to where both halves are 'distorted'.

Exactly the reason to learn a lesson from the little Supro amps and use a LINEAR VOLUME POT!!!!!! Works like a charm.

[kleuck]

Did you try to reduce the triode bypass cap ? 25µf is way too huge to play with overdrive imo, in similar circuits i use between 1 and 4,7 µf ! A grid stopper can help too, particularly on last triode, which has no grid-stopper usually. Last, non-polarized cap for the cathode bypass can clean-up things too.