I don't know very much about tubes, except that SET is a bit like Class A in SS. I have  pair of Image Revelations which are 90dB/1 watt and 4ohm. My reading suggests thats pretty marginal, even hard to drive from a SET tube amp. Thoughts and advice on choosing a SET amp, please. It would be driven without a preamp from a Mytek Brooklyn (or alternatively from a B1 buffer "pre".) Thanks.

Hi paulsaints - I always start with this SPL calculator - myhometheater.homestead.com/splcalculator.html

You can see that you'll need about 1 watt for 85dB average SPL (loud) at 12 feet (3.6m) distance.
It's nice to have say 10x that ie. 10 wpc, for plenty of dynamic headroom.

So, you can see that a 300B SE amp (8wpc typically) could do it.
Or other amp with similar wpc.
Note that SE amps do not have as good LF control as PP valve amps, lower Damping Factor => 'looser' LFs...which may or may not suit your loudspkr's in-room sound.
Ideally, trial a SE amp if poss.

However...
- you may be listening at not so loud levels (good) or v loud levels (more power needed).
- you may sit closer (less watts needed) or further away (more watts) than 3.6m.
- your room may be small (less watts typically) or quite large (more watts for good LFs)
- your spkrs may be placed well away from walls (more power needed) or fairly close (less power, some wall reinforcement).
- your music preferences may demand less power (jazz, etc) or more power (classical, heavy bass material).

90dB/W is a quite sensitive loudspkr.

4 ohm impedance - choose a valve amp that has a 4 ohm impedance Output Transformer connection option.

That might give you some food for thought.

DarkLantern blog - darklanternforowen.wordpress.com

Certainly try before you buy if at all possible. I have been down a similar track with a SET 300B with 91dB speakers which was flat and dull.

Were all different though and your room may be more suitable. 

Hi paulsaints, in addition to Owen's and Gryffles' excellent advice, I suspect some clarification of circuit topology would be useful here. SET means single ended triode. In which there is usually one triode output valve forced to run in class A. Class A means that the amp's output stage runs at full power all the time, even without any audio signal running through it. Class A operation achieves the lowest distortion and highest damping factor out of a triode. SS amps can also be made to run in class A where you can also have single ended topology with one output transistor. It's more common for a SS amp to be a push pull design in which any operation from the efficient class B, to the inbetween class AB, to full class A can be chosen.
Valve triode amps can also be push pull with two or multiples of two output valves. The main difference between SET and push pull triode amps is in the output transformer required. In push pull the current from the two output valves cancels magnetically within the transformer core so that the the whole transformer can be made smaller and with good frequency response. In SET amps the current from the valve has to go fully through the transformer and isn't cancelled magnetically. That means there needs to be a small gap incorporated into the transformer core so that it doesn't saturate magnetically. So the SET amp transformer has to be made very large for the power output and consequently with poorer performance than with a push pull design. As Owen says above that can lead to poorer bass control. However the midrange magic of a SET amp should be there as the tube is running in class A with low distortion and where the output transformer is most efficient, leading to great midrange.
Your Image speakers can run with 8 watts and it could be ok but for a speaker like this I'd use upwards of 20w, but more importantly one with good bass control. As Gryffles says, try before you buy!

AMR-iFi R&D

That's all very helpful and sensible advice. I have read around and understand the need for watts a lot better now - all to do with dynamic range, as much as absolute power needs. I do listen to quite a bit of classic music and hadn't previously realised that classic music generally has a higher dynamic range and thus needs a lot more watts to produce a full range sound. I am also understanding that small increases in dB seems to need an almost logarithmic increase in power. Given that the Image Revelations are designed to do base well it would seem that a low power SET is probably not the way to go, and even a high power one would be marginal. If the opportunity to try something out comes up, I'll give it a go, otherwise I might be better looking at something like a Prima Luna HP amp.

or maybe stick with the current Parasound HCA-1000's which in mono block are giving me 400 watts per side - maybe I've been spoiled for power without realising it ...

the images do like a bit of power.

do you have the original revelations or the 2 ?

single digit set's match better with a more efficient speaker.

Jul 22, 2017 10:57:00 GMT 12 paulsaints said:

I am also understanding that small increases in dB seems to need an almost logarithmic increase in power.

Yes, eg. you need 2x the amount of watts to produce a 3dB (1.4x) increase on SPL.

DarkLantern blog - darklanternforowen.wordpress.com

Personally think you are on the right track now paulsaints

Push-Pull El34 or KT88 would work well 40+ watts

Are the Image Revelations able to be bi-wired? In that case you could use a bigger amp for the bass and a SET for the midrange and treble.

AMR-iFi R&D

you mean bi-amp ?

doesn't always work

Yes, although it would need to be set up properly for it to work ok. Just a thought....

AMR-iFi R&D

the image rev can be bi-wired but the internal cross-over can not be bypassed.

That's good. Do you know if the upper terminals on the speakers runs midrange as well as the tweeter?
To set up a bi-amped system the bass would need to be rolled off to the upper frequency amp, as bass takes the most power to reproduce. The bigger amp used to drive the woofers can run full range. The speaker's own crossover is still used. You'd need to set the levels right between the two amps.

AMR-iFi R&D

no idea.

personally i'm not a fan of passive bi-amping.

do it properly, go active, bypass the internal x-over and use an external x-over

Rev 2's

Active would be great but most difficult and costly to do. It requires modifying the speakers to remove the crossovers and adding an extra set of terminals for the 3-way setup that they are. Not to mention 3 amplifiers, chosen for each driver, and a dedicated 3-way active crossover. Very expensive way to go, and you'd need to set it up properly which would need a bit of measuring. Also the sound of an active crossover in the signal path to consider, with its multitude of opamps and/or digital circuitry.

AMR-iFi R&D

agree, costly to convert a passive speaker.

much easier on a speaker with external crossovers like the magnepan mg3 series, avalons, living voice et al.

iirc there was a uk speaker with a plug-in crossover that coudl be removed for active use.

arcam ?

Naim SBL?

AMR-iFi R&D

not the one i was thinking of.

maybe arcam two?
x-over was in a small plugin box.
remove box and you got direct access to drivers.

Not sure...
Paulsaints, you could possibly get away with a 211 or 845 (or other tube setup) SET amp with 20+ watts. As long as it has a 4 ohm speaker terminal.

AMR-iFi R&D

Of course, with SE amps (incl SETs), it is more than simply power - the previously mentioned highish output impedance / low damping factor / loosish bass characteristic of SE amps, means that traditionally they match well with spkrs that enjoy some LF 'fullness' added, eg 'full range' drivers with limited LF extension.

My first 300BSE amp went quite well with my 87dB/W Spendor SP1s.

DarkLantern blog - darklanternforowen.wordpress.com

Some SET amps have an adjustable negative feedback control as well, that alters the damping factor by reducing the output impedance, and modifying distortion characteristics. They can be set to the user's sonic taste.

AMR-iFi R&D

Nice little SET amp - could suit a nice computer desktop speaker set

www.ebay.com/itm/PSVANE-T-845-SUPER-TUBE-POWER-AMPLIFIER-PAIR-All-New-/162597067970?hash=item25db8a5cc2:g:ENsAAOSwCkZZR0Wm

I'm worried no-one realised that was a joke ...

If you don't mind doing some soldering it is possible to build a tube amp running a single ended KT88 for about 12-15W.

I've even seen the odd kitset for such an amp.