Fixed bias, and power output.

[MWJB]

Within reasonable perameters, "No".

Take the Blues Deluxe & Custom Vibrolux Reverb, similar B+, 40W CVR has a B+ of 437vdc,Primary Z of 5K (from schematic), 38W BD has a B+ of 427vdc, primary Z of 7.5K (from schematic), but max clean output is comperable =/-2W. (Fender schematics seem artificially high in terms of primary Z, but proportionally the differnce is the same). I very much doubt that you would see any significant change in output unless you dropped well under 3K, or less.

Above 2.5K and below 8K for 6L6, primary Z is predominantly a tone issue.

[diagrammatiks]

that's not entirely correct.

calculating from an ideal loadline

the cvr is 59 watts maximun clean and the blues deluxe is 37 watts.

a 2k shift in impedance with all other things being equal will decrease the max potential swing by something like 10-15 watts.

[MWJB]

Measuring the amp, as per Fender it IS entirely correct, the CVR (40W @ 5% THD) cannot make anything like 59W with a B+ that low. BD is 38W (@5% THD) the 10 less volts at the plates accounts for that well enough.

Don't rely on calculation alone, measure.

[Steve Conner]

Power output does depend on load impedance, to be sure. But:

The dependency is not just a simple ratio. It's a curve, and you can sometimes get the same power into two different impedances. For instance, I have an OPT that I believe is 6.6k to 16 ohms. When used with 6550s, I get about 60W into a 16 ohm load, but I still get 60W into an 8 ohm load on the 16 ohm winding.

And, given any load impedance, you can adjust the power output of tetrode and pentode tubes by playing with the screen voltage and screen resistors.

So summing up, it's really quite easy to make two amps that give the same power output with different load impedances. I did it in a single amp by accident!

[R.G.]

Here's a chart from a 6L6 data sheet:

Nice! Thanks for posting that. I've seen that before, but I don't have a handy address for it on line.

Power output for tubes has a broad, flattish maximum. Over a very large range, the power doesn't change all that much ; the distortion is a minimum for some load impedance - and these two loadings are different in general for a specific tube, as well as being different curves for different tubes.

This is the stuff that makes the idea that matching impedances to a razor-sharp perfection for speakers is a losing battle. Even if the speakers didn't vary in load impedance wildly with frequency, the tubes don't care all that much.

[pdf64]

The GE data sheets often have that page, eg http://www.mif.pg.gda.pl/homepages/f...93/6/6L6GC.pdf last page and http://www.mif.pg.gda.pl/homepages/f...093/6/6BQ5.pdf

[diagrammatiks]

right accounting for losses puts the cvr right around 40watts.

I just have a hard time believing that the BD puts out 38 watts with an impedance that high.

Of course that's assuming that screens and plates are within 10 percent of each other.

If I had one here I'd gladly measure it to confirm but 38 watts is a real high number with a plate impedance that high.

Also why wouldn't the CVR be able to make 60 watts with a b+ that low?

The design max for a 6l6gt are 500 plate/450 screens with a 5.6k nominal plate impedance.

437screens/plates with a 5k is almost an ideal line. There's not reason it shouldn't be able to put out close to 60 watts.

[leadfootdriver]

I like to sit back, and watch the big brains do battle...

[Chuck H]

Back to the OP... (and non jargoned descriptions)

The reality of any reasonable OT difference from one amp to another (with plate volts being similar) is only going to be a few watts at the most. A few watts for a fifty watt amp is, in most cases, an inaudible difference in volume.

If the OT is a really cheap turd it may be marginally rated and have a small stack of laminates. This usually equates to poor bottom end. So an amp with such an OT would still be basically as loud in the upper and mid frequencies but would lack bottom end.

[J M Fahey]

Well, itīs true there are different "pairs" of parameters which can give roughly the same power output.

Easy to see by superimposing different load lines on a Power tube graph.
Thereīs a "maximum" one, the one I chose in the example "thinking as an Engineer" which goes from a low idle current, full +B, to maximum current compatible with the lowest saturation voltage.
Why?
Because saturation voltage and idle current both mean lost power.
That "maximized" load line ends in the middle of the curve corner, bend or knee, you choose the name you like.
I refer to the knee at 135mA and estimated 60V .... always "in this particular curve".
Now, if you choose other load lines, at a certain angle from this "maximized" one, one side or another from it, you will have pairs of impedance values , one higher, one lower, which will yield the same power, and not much less than the maximized one.
Thatīs what many refer to as "the broad area where you can play with load impedance and not lose much"
*Now* you start to modify things and choose with your ears, beyond the basic "Engineering" approach, although I think it still pays to start by doing things "by the book" , listening to it and considering it a valid reference point, and only then start experimenting bending things.
jm2c