845/211 tube amplifiers with kef reference 5?

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

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.

What George is saying here is bad advice.

That’s wrong

you’ll encounter a tonal issue

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.

thegreenman OP
As I had 805 sets drive just that same speaker off the 4ohm taps, like I said the max loudness wasn’t quite there, but it sounded great, for tube set. The Kef looks like a benign 4ohm load to an amp when impedance and combined -phase angle is taken into account.

Cheers George

Peaks in SPL are usually at lowest frequencies.  I have ne problem driving less efficient speakers with a 45w 845 push-pull to reference levels because I never pushed the limit in terms of SPL because the speakers rolled off at 60hz.  And to Ralph's point, my amps were Push Pull and not SETs.  Pushing a SET to hard doesn't sound as good.  I have done a direct comparison of the Art Audio Carissa vs. Quartets.  With efficient speakers, Carissa kills the Push-Pulls but as efficiency drops, the Push Pulls 

That will ne be the case with the Reference 5s.  You will have peak output of about 100dBs before the amp clips.  This is going to mean VERY flabby bass.  

Seriously, wouldn't it make more sense to bi-amp.  Get a great 845 for mids and treble and power the bass with a nice, SS amp.  Get a used Krell or Chord power amp, even Musical Fidelity to pair with it and the net result will be brilliant.