S/E OT "Karma"

[matndona]

Seems alot of time is spent finding the perfect Tube or Tone Cap. Here's another angle:

I've been working with some SE OT's for a P1X amp, trying to find that balance between audio range & roll-off. Working strictly to design rules, in general, if the Inductance of the XFMR rolls off 20% from OCL reading with x milliamps applied, you'd call that the current rating.

If I butt stack the laminations on the core on a "125DSE" sized XFMR I can carry 100mA plus by this measure, but if I stack the lams 4x4 it rolls off more than 20% but sounds 'better'....at least if what you crave is somewhere in the Joe Walsh to Black Hole Sun range & you love rich harmonics. The inductance is higher, which in theory can get me down to 80Hz. The tone's great with a 6L6, 6L6GC and 6V6GT, but it doesn't quite seem to quite pull off an EL34 with the same grace. I suspect the big bottle needs too much juice.

On paper & to my ears it seems like the 4x4 stack set-up is rolling off right around or above 50-60mA and handling 10-11W. If I look at Duncan's CLASS A S/E tube data specs for the 6L6/6V6 family I could be at the balancing point where all the electrons are scrambling to be heard before falling off a cliff. When I get back home this weekend, I'm going to try and measure current/power to check these assumptions.

There seem to be a lot of smart folks here. Anybody know what the tube should be doing from a output & distortion standpoint right around it's max current rating? Does it want to live there? I'm thinking this is where power amp section distortion comes from.

Does it make any sense that the XFMR should 'matched' to that tube operating point?

Or are there valid reasons to keep some spare milli-amps around?

[Steve Conner]

If you call "Maximum Current" of the tube, the idle current that it can sit at in a SE Class-A stage without overheating, then it can deliver plenty more than that when needed on audio peaks. AFAIK, 6L6s or EL34s or whatever can crank out 300-500mA if you mash the grid as far positive as it can go. But they can't take a standing current much more than 60mA or whatever without red plating.

So I think rather than the tube running out of steam, you're more likely to experience the OT saturating. Since it's sitting near the saturation point of the iron, the positive half-cycles will push it further into saturation, so they'll get squashed up compared to the negative ones. This will add even harmonic distortion to loud low-pitched notes.

Now that might sound good, or it might suck, so I recommend testing it cranked up as loud as you'll ever want and playing the lowest notes you'll ever use.

[matndona]

You could be right. With a 6L6 or 6V6GT and the amp 'dimed' it sounds pretty sweet now though. I might just be lucky. 1st pass I had it stacked 1x1 like a Push-Pull transformer & it did suck!

Right now I'm using the transformer unvarnished, so I'll be able to restack it with a gap and see what happens. I've also been winding the 8ohm and 4ohm windings parallel so I can set-up the EL34 and get pretty much the same leakage inductance (losses) with different impedence ratios.

Still a couple more things to try. Living and learning!