Tubes and recommended load resistance

tardishead · Sep 10, 2009

I am a bit confused about load resistance. As I have read the recommended load resistance for push pull 6v6 6f6 tubes is 10000 plate to plate. However I have seen on some schematics output stages with transformers as low as 5000 ohms primary for example the Collins 26w limiter.
As I understand it a lesser load resistance results in more current drawn - a steaper load line so clips more easily.
I do have a big PP 5000:600 OT - can I run it for PP 6v6 6f6 for most purposes?
If I bias the cathodes colder is this the way out to create more headroom and less current?

PRR · Sep 10, 2009

> recommended load resistance for push pull 6v6 6f6 tubes is 10000 plate to plate.

Close enough, for Vg2 similar to Vp, for maximum output at tolerable distortion.

> as low as 5000 ohms primary for example the Collins 26w limiter.

Push-pull 6F6 can do 10 Watts! The limiter does not have to be a Power Amp. 1 Watt, +30dBm, into the line is very ample. It was customary to use an amp significantly bigger than the required power, so it would work down in its low-THD zone. 4% at 11 Watts might be 1% at 1 Watt or 0.5% at +28dBm. Also it would not go SPLATT if somehow +31dBm was passed through it. Also 6F6 was VERY widely used, in-stock everywhere, and as-cheap-as any smaller power tube.

So for the limiter, MAX Power Output is not a concern, some mis-loading is very acceptable.

Electrons do NOT like to jump off solid matter and go through empty space. Tubes are high impedance. Low-Z tubes cost more, need more drive, more heater, etc.

Transformers choke and leak. Hi-Z windings have much more bandwidth trouble than low-Z windings. A low-Z winding is easier to design for wide bandwidth. It also uses fatter copper, less cost to hammer-down thin, less turns, less breakage, so faster winding.

And Standard Parts often trump custom parts. Unless using teeny-triodes like 6SN7, 5KCT was a very standard big-line-output part.

> for most purposes? .... more headroom and less current?

What are you building? A line-amp which works easy and "clean"? Or a LOUD amp which you will beat to the MAX? In easy audio working, current is essentially idle current. Set this high enough so current hardly changes at the highest level you really need. P-P 6F6 with 5K load will make over 5 Watts or +37dBm. In modern studios you never want over +20dBm, 1/10th Watt. You could actually work near 35V and 10mA, squeeze almost +20dBm out. Working at 200V and 60mA, it will do way-way over +20dBm while "idling".

If you gonna BEAT the beast, as in a guitar amp with a drummer, the current demand MUST be estimated from loadline. If the tube were ample (6F6 aint), then for P-P Pentode take the p-p load impedance, divide by 2, and assume the amp will suck B+ like a resistor that size. 5K/2= 2.5K, so with big 6L6 etc at 250V it could suck 250V/2K5= 100mA at MAX output.

pyjaman · Sep 10, 2009

hi,
PRR reply faster than I did. Lucky you.
anyway, if you want to, you can have a look at this simple article showing how you can basically determinate your operating point for PP operation.
http://www.freewebs.com/valvewizard1/pp.html

Laurent.

CJ · Sep 10, 2009

practically speaking, changing output transformers is not as easy as changing resistors, for instance.

you may not have a 5k, 6k, 7k, 8k, 10k set of outputs laying around the garage.

so you look one up.
you are asking "what do i look for?"

all the different makers will have what they call their "6v6 p-p, or 6l6 p-p, kt66, kt 88, 6550, 5881, etc " choices, and they all work for their respective tubes, some even cross over, but the specs will be all different between makers..

the cheaper one will have a bigger gap because it is more stable, spec wise off the assy line, this will mean more bass distortion than the .001 gap, but it will handle more dc imbalance, because of the wider gap, so you have all these tradeoffs, that is why prr hinted at the off the shelf 6v6 from triad or whatever, with the end bells missing, might be as good as the Bartolucci C core special.

emrr · Sep 11, 2009

RCA used 5K PP primary with both 6L6 and 6V6 frequently; the same iron. The old rule in the 1930's lit is that a heavier load (down to matching condition) on a power tube (looking for current rather than voltage) will result in less distortion. That does not make any sense to me from a modern POV. Any output stage feedback will play into the picture.