VTA ST-120 power output question

So the VTA ST-120 puts out 2x60W into 8 ohms - how much would it put out into 4 ohms? I  would expect around 90W but  that is my solid state experience speaking.. and my rudimentary understanding of the 'valve way' is giving me an itchy feeling that is not so ^^


My current speakers are Kef reference model three~two  4 ohm ( dips to 3.5 lowest ) and an efficiency of 91db..

Roy D wrote:So the VTA ST-120 puts out 2x60W into 8 ohms - how much would it put out into 4 ohms? I  would expect around 90W but  that is my solid state experience speaking.. and my rudimentary understanding of the 'valve way' is giving me an itchy feeling that is not so ^^


My current speakers are Kef reference model three~two  4 ohm ( dips to 3.5 lowest ) and an efficiency of 91db..

Tube amps don't quite work that way, as you suspect. If you look at how output transformers work, the various taps are at various points on the same winding, with the highest impedance tap incorporating the entire winding.

http://www.dynakitparts.com/image/data/Dyna-ST70.pdf  

Close matching of the nominal impedances of the speakers and the tap will give the most power to the speaker - all at/around the nominal amp rating. But, in practical terms, that will be a vanishingly few watts. Where there are (barely) measurable changes is when low(er) impedance speakers are connected to higher impedance taps. And 91dB Kefs will do very, very well on any tap.  

Now, cutting to the chase with solid-state amps - the increase/decrease based on speaker impedance is a matter of design and amplifier stability. I have a couple of SS amps that are fully stable into loads as low as 1 ohm, and some that would go *POOF* at anything presently that low a load for any more than a few seconds. The double/double is not universal. My brute-force Citation 16 is rated at 120/16; 150/8; 250/4, and if strapped 500/8. So the results are not linear. My ST120 (also solid-state) comes out at 30/60/80 - a bit more linear.

^^ yay starting to fathom things a bit then..
Yes my main amp is also a bit odd in that respect - a Dutch made class A 100w in 8 and when it sees a 4 ohm load it will deliver 150.
It too should do 1 ohm loads - was actually designed to drive electrostats.. Think its damping factor is about a thousand or so. Thats going to be different on valve isnt it

' Uncle Doug ' on youtube ' Output Transformers: A Discussion of Basic Principles ' some very relevant info round 5m36s .. always connect an 8ohm speaker to the 8 ohm tap, 4 to 4, etc..

So.. My wife's resistance to the build is all but worn to a nub lol.. Now my own practical considorations are coming into play ^^ Still hoping to hook up with someone relatively close to home to go and have a listen to and ST-70 or ST-120 build to get a better idea what I am in for before I take the plunge. My EE neigbhour a few doors down is good for practical advise and a helping hand when the time comes though he just shook his head and looked at me compasionately over this endevour with a valve amp when he has seen what we got now powering things which is a marvel of transistor engineering in class A throwing enough power at the speakers to make the earth move.
My first ever bit of kit was this huge Philips tabletop tube radio set when I was maybe 12 and it sounded magical =) Shudder now to think how I cleaned up the insides with no precautions whatsover cept having it unplugged from the mains. It weighed a tonne, the wooden case got pretty hot and the glow from the tubes was enough to read by lol....

A good friend of mine in Portugal is tickled pink though with the idea - tube fan, open baffle speaker nut, and he's like go go go do it!

Roy D wrote:' Uncle Doug ' on youtube ' Output Transformers: A Discussion of Basic Principles ' some very relevant info round 5m36s .. always connect an 8ohm speaker to the 8 ohm tap, 4 to 4, etc..

Well.... not always....

It is OK to go *up* a tap, from say... 4 to 8 or 16, but not *down* a tap from 8 to 4. As one goes up a tap, distortion and power drops (slightly), and damping increases (slightly). Going down puts a strain on the amp and increases distortion.

A couple of examples: I keep a pair of AR3a and a pair of MGIIIa speakers.

Now, back in the day (and, yes, I was 'there' back in the day), the OEM 70 was often described as "tubby" or "loose" into the 3a. It is a nominal 4-ohm speaker.

Impedance Curve here: http://www.classicspeakerpages.net/IP.Board/uploads/post-100690-1136075975.jpg

And as you can see, it spends much if its time at/near/below 2 ohms.

UNTIL! the user connected the 3a to the 8 (good) or 16-ohm (better) taps. Then things tightened right up, albeit at lower power, a little. Damping with that big, fat 12" woofer is a significant factor.

The Maggies are nominal 4-ohm speakers and they are so across the entire bandwidth - no variance.

http://www.integracoustics.com/MUG/MUG/articles/speakers.html

And to the extent that they will play with so little power, do fine from any tap. Nor is damping a factor with Maggies. The room they are in is 5 m x 7 m x 3 m, so the Citation 16 is required.

Now, your KEF speakers are quite efficient, and so will do far better than either the Maggies (85 dB) or the ARs (86 dB). Not to worry.

Be careful of received wisdom. It is useful in keeping you out of trouble as you learn, but as you learn, you will start to understand its limitations.

As to your wife: My wife has long-since thrown up her hands at my several hobbies (vintage radios, clocks, audio et.al.) as she enjoys the music and I generally confine the mess to a single room.

But, you may also use the very basic excuse:

a) It doesn't eat.
b) It keeps me close to home.
c) It is cheaper than another woman.

a) neither will I if She wills it   b) There are several scenarios which has her thinking of it being best not to be    c) it would definately not be lol..

Yeah - according to the video   Up is   ok but  might not condusive to sound quality ( depending on the speaker )   Going down is never good for sure though..

Not too sure about the figures of my Kef's though Peter - the measurements given were without the relevant ISA or other standards - read some comments by people using speakers of a slightly older kef vintage the actual efficiency measurements are about 3-4db lower than given in reality..  These Kef reference three two's are four way, with a fairly complicated crossover - two large woofers in the cabinet, two mid range drivers up front and one tweeter in the center of one of the two midrange drivers..  
I reckon the impedance chart does its best to imitate a sinewave

Funnily enough though   when I got these Kef's home   to replace the B&W Matrix we had before, measuring quite a bit taller than,   I was the one who said  maybe these are just too much for us =S
but she heard em and said  hush   they are great..

Every now and again I will still suggest perhaps we should get a pair of smaller stand mounted speakers and a sub so the placement is not so much of a thing ( these kefs need space in every way )   but when we go out and listen to whats out there   again  she says  hush   these beat all I've heard so far ^^

On sub-sat systems:

I have had three and retain one at this time:

AR Athena - this system I have retained. It has a stone-ugly sub, but low enough to fit under/behind many things. The mid-tweet boxes are industrial-modern, and so acceptable. The sound is excellent for a smaller room, does very will fed from tubes, and as it is so compact will be kept. It, together with an AR tuner and integrated amp is on-reserve for the second grand when he decides he wants it (must be middle-school age or better).

Revox Piccolo System - now in the hands of my oldest grand together with my second Revox A720 and matching A722. I am told she uses it every single day, sometimes at volume. Her room is gabled room about 5 meters by 4 meters with perhaps a 2.5 meter ceiling in the center. Does VERY well.

Earlier AR passive system - sold that to a friend in Phoenix, AZ for a song (but 3 x what I paid for it) as her friend had the same system and she was wildly jealous. She sent one of her students who lived nearby (to me) to pick it up, avoiding shipping costs.

I have heard powered-sup systems.

Bottom line - the closest thing to a sub-sat system that could match even a moderately good "box" was the Piccolo system. The sub is massive, the mid-tweet boxes decent speakers in their own right, and they accepted lots of power. But, my Dynaco A35 and my AR 4x speakers blew them out of the water.

Keep your Kefs.

Revox A series  that was - is -   good stuff..

I had an A77  and two for donor parts - sold them to some folks who drove all the way down here from Berlin to get em.  

Yes,  I think you are right.  And I have grown  very accustomed to the coaxial tweeter 'thing', and what is out there from Kef and others with this design that could stand mount, a lot is lèss sensitive than the kefs we have now.
The much lauded Kef  LS50's are quite nice but  the midrange humps and treble humps sound harsch to my ears and my wifes.. Flattening those out there are mods to do to the crossovers, but if you add up the cost of a decent fast sub from SVS or REL, what we have now  really is  something to cherish.

With the little one running around during the day here though, their drivers face the back wall..About 45kg a piece, I got them on felt so turning them around into playing position is not so hard  but its wearing shiny marks into the wood floor..