Parallel SET with sufficient power


Hello all:  I'm looking to move back into SET land after 15 years. I have a pair of KEF Reference 3's at 88 dB sensitivity which I'd like to keep, so I'm considering a parallel Single Ended design with at least 30 WPC. I do like the '300B sound' so friends have recommended the 6C33c tube in parallel to deliver 30-40 WPC. I've read some very positive posts on Audio Mirror and Wall Audio of Germany was briefly mentioned. Smaller boutique like companies give me some concern as most of these fine designers or engineers build amplifiers in their spare time, and quality control may be compromised.  I'm not a huge fan of 211 or 845 sound but these amps are another option.  Thanks for allowing me to think out loud and I do appreciate all opinions!

normie57
I'd be cautious in extrapolating from experiences others may report with various SET or PSET amps, unless they were using the same or similar KEF speakers.  As shown in Graph 4 of  this review the impedance of your speakers varies from around 3 ohms in parts of the bass region to as much as 18 ohms in the upper mid-range/lower treble.  The interaction of that impedance variation with the relatively high output impedance of most SET and PSET designs will in many cases result in weak bass and an over-emphasized upper mid-range/lower treble.  To a degree that will vary considerably among different SET and PSET designs, due to their differing output impedances.

An audition with whatever amplifier you may end up considering, using your speakers, would therefore seem particularly in order. 

Good luck.  Regards,
-- Al
 
I know Vlad Bazelkov with Audio Mirror personally, and can attest to his knowledge and helpfulness.  Great guy, and his gear both sounds great and is very well built. He does some unique things with his designs, and he can answer any questions and concerns you might have about speaker match-ups.  contact@audiomirror.com.  Tell him Keith sent you :). Good luck!
@ normie57,

You might want to read this; it will help you understand the issues of driving a speaker with various kinds of amps.

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

If you are planning to use an SET, the speakers you have in mind do not seem appropriate. They are intended as Voltage Paradigm devices where the amps you are contemplating are Power Paradigm devices. The combination often results in a tonal coloration (which Al mentions above). If you are planning an SET, more efficiency is recommended to really hear what the SET can do. As a general rule of thumb the speaker in the room should be efficient enough that the amp never exceeds about 20% of full power.

Good Luck!
2 for 2.
That's all the advice I would need.
I appreciate your candid and very helpful comments. I will rethink my decision, for once before the purchase!  Thank you all, especially Al and Ralph.

Parallel SET with sufficient power
Parallel GM40's monoblocks
More power better sounding tube. And has 4ohm taps.
The negative phase angle of the Kefs is not severe, so the amps should drive them well.

http://www.tubeguru.eu/poweramplifiers-/gm-70-pse-triode-parallel/

Cheers george
If you wanted to stay with SET, amps based on the 845 usually put out about 30 watts a side. And then there's the ever-classic Cary 805 at 50 watts each: https://www.audiogon.com/listings/tube-cary-audio-design-805c-single-ended-triode-monoblocks-50-watt...
I have a pair of Lampizator 211 mono's that run a pair of 211 per side in parallel, and I can confirm that Al and Ralph are tipping you off correctly.  My 211's boast 85 watts of a tube power, but when I tried them on a speaker that dipped down to 4 ohms in the lower mid - bass range, it resulted in an anemic balance.  It isn't so much about watts - - it's about proper impedance matching to achieve proper frequency response and tonal balance.
 My 211's boast 85 watts of a tube power, but when I tried them on a speaker that dipped down to 4 ohms in the lower mid - bass range, it resulted in an anemic balance.
This could very well have been the output transformer, being 8ohm. If it had a 4ohm tap it could have been a different story, also Lampi may use no or very little negative feedback this will also have a bearing if it can drive a lower impedance as well. (having feedback lowers the output impedance) 
The amp I linked to from reading on the web has user adjustable negative feedback (which lowers the output impedance) as well as 4ohm taps.

Cheers George 
@georgehifi , there’s more to it than that... generally speaking, SETs behave as power sources rather than voltage sources (which might be your typical solid state amp). The reason to go this route is to avoid the brightness and harshness often associated with loop negative feedback (harshness and brightness is after all a coloration and not one that is particularly pleasant).

Because no feedback is used, you can’t rely on the Voltage Paradigm for flat frequency response, and the 4 ohm tap may or may not help (likely won’t, as the speaker may be looking for +3db worth of power in the woofer section, and merely moving to the 4 ohm tap won’t get you that).

There are speakers that are designed to work with amps of higher output impedance (and no feedback): Merlins, Coincident, Audiokinesis, Quad 57 and Quad63, Classic Audio Loudspeakers, Rogers LS35A, Lowthers, PHY, Festerex, most horn speakers and many more.
Sorry Ralph I was talking about driving the OP’s speakers with two low 3ohm impedances around 50hz and 150hz, my experiences and measurements on the same amp a 4 ohm tap output tap has a better chance of driving a speaker that dips to 3ohms in the bass as the OP’s do at 50hz and 150hz, than an 8ohm tap on that same amp.
Also if on that same amp the global feedback is increased, it will drive it even better again as it lowers the output impedance even more. But may not sound as "euphonic".
Sure there are spl limits with a 45w set/ or push pull just as there are with a 45w solid state

Cheers George
George, are you sure that TubeGuru amp actually has a 4 ohm tap?  I couldn't find a rear panel photo, but I would interpret this photo, which is one of those shown for that amp at the TubeGuru site, as suggesting that it may have only a single output tap.  Also, the website description states "Output Impedance: 4-16 OHM," which could very conceivably mean that there is only a single output tap, with those numbers being the limits of the range of speaker impedances that tap is recommended for use with.

Regards,
-- Al
    
George, are you sure that TubeGuru amp actually has a 4 ohm tap?
Hi Al, yes when I Googled those amps one of the forum/reviews? said to use the 4ohm tap for hard to drive speakers. And the feedback is adjustable from 0 to some 30db That’s what led me to think so.

I did the same on my 805 sets and zero fb had soft ill defined bass, nice mids/and not much sparkle on the highs of the Monolithiths. Max 35db fb tightened the bass considerably and gave more (still not enough)sparkle in the highs (1ohm load) ouch!!.
In the end I went big bi-amp solid state, as 50w set was not enough for loud level transients, but then the Monoliths are only around 86db where the OP’s Kefs are a touch higher and not as bad loading, as the Monoliths have far worse negative phase angles at 3 points with low impedance’s.

BTW: Edgar Kramer (reviewer) of Australian HiFi/ 6Moons/ and Audio Esoterica had a pair of (Elson Silva) Cymer Southern Star SE35, 35w single 13E1, set monoblocks and his drove his Wilson Watt 8’s comfortably, but in the end weren’t enough when the crew came around and wanted it turned up more than he does.
http://www.6moons.com/audioreviews/cymer/3.html

Cheers George
Correction, they were Wilson W/P Sasha, even harder to drive than the WP 8's.

Cheers George
Wise advice from almarg and atmosphere. They are the comments I'd be taking notice of. Good luck.