Tubes load resistance and output trans impedance

[Jeffy74]

Hi!

I would like to understand something. Is the primary impedance of output transformer is really important? Because for a type of tube, load impedance will change depending of voltage, circuit, push-pull, SE, trioded mode... It's really difficult to choose the right output tranny. Let say that I would like to use SE 6L6 in standard mode and triode mode (with a switch). What type of transformer (primary impedance) will I need to choose? If the primary impedance is to high or to low, what will append? Is there a tolerance (like +- 2000 ohms) reasonable?

Thanks for your help.
Jean-Francois

[ashok]

Hi Jeffy74,
The transformer primary /secondary ratio will determine its impedance ,and depends on your load. The load is not a uniform figure and I guess tube amps will work best with speakers that have fairly uniform impedance over the bandwidth.
The impedance seen at the primary will be turns ratio squared multiplied by the speaker impedance ( I see 5 and 6 ohms often ).
When the primary ( reflected) impedance is itself a few kilo ohms, an error of 2K ohms is probably not acceptable.

Matching the impedance is basically determined by power transfer plus some other factors. Maximum power transfer requires the load impedance to be equal to the source impedance.But I think in practice distortion also comes into the picture. You will see that some tubes 'sound better' with loads that don't quite meet this criteria. That was determined by building and listening I guess.

Hope I have this figured out correctly !
Cheers,
Ashok.

[ray_moth]

I agree with you that the issue of setting the right load impedance for output tubes is quite confusing. It can vary so much for the same tube, depending on the conditions under which you are running it (e.g. plate voltage, plate quiescent current, screen voltage, class of amplification, mode of connection, fixed or cathode bias, etc.). In theory, you should be able to sort out all of this by using the published plate current/voltage curves for the tube but that's too hard for me! I always end up referring to the manufacturer's recommendations.

The load impedance is quite critical for optimimum performance in tetrode/pentode mode. It is far less critical in ultra-linear mode and somewhat less so in triode mode. I understand that it's better to err slightly on the low side for tetrode/pentodes and on the high side for triodes, in the interest of minimizing distortion.

[G]

The easiest route for me has been to listen to the experts and not waste my time second guessing them. Not much anyway.

[misok]

uh, every time with this topic its just mysterious like this.

and i'd like to finally order my OPTs for 6S19P SE amp..

anode resistance = 400 Ohms
desired load line is (340v B+) / (100mA @ 0V) = 3.4 k

so it makes 3.8k -> 5k OPT primary is suitable ?

or i'm completely wrong .. please help. thanks