845/211 tube amplifiers with kef reference 5?

Am curious, how about push-pull 845 amplifiers in monoblock configuration? Eg 50w class A, each channel driven by 2 845 or 211 tubes? Would those be sufficient to drive the kef reference 5?

If the amp uses feedback you'd probably be alright. The feedback is to allow the amp to behave as a voltage source.

it won't be a disaster like he's making out, he's got a bee in his bonnet, over something.

'Something' is misinformation. In case there is any question, I am a manufacturer of tube amplifiers that employ zero feedback. I know all too well that such amps don't work on speakers of this type. You're not seeing me recommend our amps here. That should tell you something. I just don't like to see people flush $$$$$ down the loo for nothing.

If the speaker is bi-amplified, then an SET might work on the top. But you'll have to try it with a loaner- you might find that even with bi-amplification (and by that I mean also using an electronic crossover) that the SET still doesn't have enough power even if it can handle the load.

Again, to get the most of an SET, it should not be used past 20-25% of full power.

That’s just what sets do, look at ANY SET into the Stereophile simulated speaker load frequency response graph, most sets act like tone controls.

Sheesh! The Stereophile simulated speaker load ignores the fact that SETs do not function a voltage source (Voltage Paradigm) devices. As a result they will 'act like a tone control' (more accurately, will exhibit a tonal anomaly)  on speakers that expect the amp to behave as a voltage source (which is most speakers).

But! If the SET is running on a speaker **designed** for this sort of thing (IOW the speaker is a Power Paradigm device too, which is *all* speakers made before 1955) then there will be no tonal issues and the SET can be neutral other than the coloration brought on by the lower ordered harmonics, which audiophiles call 'warmth' or 'bloom'. Speakers that fall into this category will often have controls on the rear to adjust the speaker to the voltage response of the amp (midrange and tweeter level controls). But some speakers, like Coincident loudspeakers, are simply designed to expect a high output impedance of the amp and don't need the controls.


Put another way George is spreading misinformation, apparently out of a misunderstanding of how tube amplifiers behave.

Yes as I also said the bass is the problem unless there’s a 4ohm tap, then ultimate loudness is reduced

This statement is false. Loudness or sound pressure is a function of power output. If the amplifier is making the same power into 8 ohms as it does into 4 (which is what the taps are for) then the sound pressure will be the same since its power will be the same if properly loaded. Now if the speaker is 4 ohms and you use the 8 ohm tap to drive it, you may find that there is less sound pressure on that tap because the power tube is now loaded at an impedance well below optimal. This will cause it to run hotter since some of the power it makes will be absorbed by the tube itself, and because that is the case there will be less output power available to the speaker (and it will be higher distortion). 

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.

What George is saying here is bad advice. The Stereophile comments apply only to tube amplifiers capable of acting as a voltage source (IOW, employing feedback). Since 99% of all SETs are zero feedback, they will tend to behave as a power source rather than a voltage source. Since the design of the speaker clearly is expecting a voltage source (and Stereophile clearly shows this in their impedance curve measurement) you'll encounter a tonal issue (weak bass or brightness, depending on how you look at it) with such an amp.

One other thing to keep in mind with all SETs! You can't run them up to full power like you can with most push-pull amps; to really get the most out of any SET the speaker should be efficient enough that the amp never needs to make more than about 20-25% of full power. If this rule is broken, the SET will be making quite a lot of distortion at higher power. Almost any SET has about 10% THD at full power. If you keep the amplifier power under 20% its going to be a lot less- typically under 0.5%. When you push the SET harder, the additional distortion is perceived by the ear as 'dynamics' since the higher orders are showing up, initially on transients. The ear uses the higher orders to sense sound pressure- hence 'dynamics' but its really just distortion.


Its just not a good match. If working with a tube amp of that power, (60-70 watts) a push-pull amp employing feedback is the way to go.

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.

@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).