845/211 tube amplifiers with kef reference 5?

@thegreenman  Unless you are in a small room there is simply now way any SET is going to do that speaker justice and the other way 'round.


Don't do it! Your speakers need a bit of power, being rated at 90dB. KEF rates the speaker at 8 ohms, but the fact that it employs dual woofers in parallel and a look at the impedance curve (published by Stereophile) shows that in the bass region, its really a 4 ohm load. So in that region, its actual efficiency is more like 87dB rather than 90 (Sensitivity is 2.83volts at one meter; converting to efficiency, which is a more useful spec for tube amps, you get 2 watts into 4 ohms, subtract 3dB to get back to 1 watt, you get 87dB)! So a tube amp with no feedback (like most SETs for example) will be incompatible with this speaker; even if the amp made enough power (which no practical SET does). On this speaker, it would be bass shy.


The speaker expects the amp to behave as a voltage source (putting out constant voltage regardless of load) but SETs being tube amps with no feedback behave as a power source (they try to put out constant power regardless of load). If you want to know more about this see
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
If you want to put a tube amp on this speaker, get one with some power; I would recommend 60 watts as a minimum. Run it on the 4 ohm tap. Keep the speaker cables short (monoblocks are helpful in this regard).

Bel Canto SET80 use 845R power triodes and output 70Watts per channel.

https://www.stereophile.com/tubepoweramps/598bel/index.html

Bel Canto SET80 use 845R power triodes and output 70Watts per channel.

This is why this amp won't work with the KEF:
http://www.atma-sphere.com/en/resources-paradigms-in-amplifier-design.html
The Bel Canto is a Power Paradigm device; the speaker is a Voltage Paradigm device. Anytime the two technologies are mixed, you run the risk of a tonal anomaly. In this case, it will be bass shyness.

op

listen to ralph (atmasphere) and scott (verdant)

211/845s won’t go well with big kefs...

you want low watt sweet single ended amp sound to play at real volumes, you need to hunt HARD for the right speakers... i still have a set of gordon rankin’s lovely 300b wavelength cardinal xs’s from 20 years ago... still can’t find a speaker i like enough and can live with to use them as the main amps

thegreenman OP
845/211 tube amplifiers with kef reference 5?

You should be fine with P/P 845 or 211’s, or even SE ones with the speakers being 90db, and if the amps have a 4ohm tap, and you don’t want to go too loud with the SE’s.
The 8ohm tap will give more watts but "could" a problem in the bass.
https://www.stereophile.com/images/1017KEF5fig01.jpg
The impedance peak at 2.5khz will be bought back down by the increase in -phase angle around there also, so a fairly flat 4’ish ohm impedance will be seen by the amp

This is what Stereophile had to say

Fig.1 shows how the impedance and electrical phase vary with frequency. While the impedance lies above 8 ohms in the low treble, it remains between 4 and 5 ohms throughout the midrange and bass and in the top octaves. The minimum value was 3.3 ohms between 90 and 100Hz, but as the phase angle is generally benign, the Reference 5 should work well with tube amplifiers from their 4 ohm output transformer taps.

Cheers George