Tube amps, power and speaker impedance

[Klimon]

What's the story on tube amps, power and speaker impedance? When looking at the formula for calculating output power [ Po = (Ve-Va)*(Ve-Va)/(8*loadimpedance) ] one would conclude that the amp has twice the power when coupled to a 4ohm speaker than compared to a 8ohm (using the same opt winding ratio). However I've also read that in the case of a tube amp, contrary to SS, the power increases but doesn't really double.

Or let's put it another way: I have a 2 watts single-ended amp and a diy line array, now wired as a 16 ohm speaker and connected to 8ohm output taps. If I would rewire the speaker to a 8 ohms; would it play twice as loud (thanks to the 3db gain in sensitivity) at the same volume pot setting?

Parker audio speakers apparently use this trick (2 ohms speakers) to get more spl out of the 1.5 watt decware zens (probably at the cost of additional amp distortion and reduced damping factor)

BTW I'm well aware of the fact that a speaker doesn't have a constant impedance

Cheers,

Simon

[Miles Prower]

Originally posted by Klimon
What's the story on tube amps, power and speaker impedance? When looking at the formula for calculating output power [ Po = (Ve-Va)*(Ve-Va)/(8*loadimpedance) ] one would conclude that the amp has twice the power when coupled to a 4ohm speaker than compared to a 8ohm (using the same opt winding ratio).

How a VT final operates is considerably more dependent on the load resistance than are transistors. The power formula you quote probably won't work since halving the load will, more likely than not, force the VT into either saturation or cutoff before the extremes of Vpk are reached. The best way to design a VT stage is to plot the load line against the plate characteristics.

Or let's put it another way: I have a 2 watts single-ended amp and a diy line array, now wired as a 16 ohm speaker and connected to 8ohm output taps. If I would rewire the speaker to a 8 ohms; would it play twice as loud (thanks to the 3db gain in sensitivity) at the same volume pot setting?

No. The relationship between Po and percieved "loudness" isn't linear. A 3db increase in power is just barely recognizable as an increase in loudness.

Parker audio speakers apparently use this trick (2 ohms speakers) to get more spl out of the 1.5 watt decware zens (probably at the cost of additional amp distortion and reduced damping factor)

Cheers,

Simon

They do that simply for braggin' rights. A 50W SS amp with Zl= 8R0 becomes a 200W amp at Zl= 2R0. A 2R0 speek is not a reasonable load for a VT amp, and isn't really all that desireable for a SS amp either. I would avoid those like the plague.

[Eli Duttman]

2 Ohm Parker speakers should be used ONLY with DECWARE SE84 amps. The SE84 is atypical for a tube amp. It has single floating taps on the O/P trafos and power production will increase as the load decreases, down to 2 Ohms. I would hope Parker speakers have a "flat' impedance curve.

FWIW, I run a pair of monoblock strapped SE84Bs into 8 Ohm nominal speakers made from 94 dB. efficient drivers. That setup works well. Approx. 8 WPC is a good match for 94 dB. speakers.

[Klimon]

Allright, I get the picture after little thinking. So the way is to half the (relative) primary impedance of the opt when drawing the loadline for a 4ohms load and then calculate the power for 8 ohms load?

Approx. 8 WPC is a good match for 94 dB. speakers.

Sounds luxurious to me; I'm getting much enjoyment from 2W amps + 90db/w speakers in a small room. Although dynamic range is restricted when directly compared to a 30W pp I still prefer the little amp.

Thanks!!

Simon