Getting into Tube Equipment

- Isn't the lower impedance floor also a key consideration in amp matching? I'm using EL4s (4 ohm port) to drive 4 ohm speakers, that according to JA at Stereophile are" 5 ohms for the entire audioband other than the region between 100 and 300Hz, where it reaches a minimum value of 3.5 ohms at 165Hz". There are some 8 ohm speakers with minimums well below 3, isn't this a significant consideration?

Yes, and Yes.

If playing tube amps, I think I mentioned earlier that one has to beware of speakers with dual woofers- when you see that, make sure that the woofer array is not wired to be 4 ohms, or you simply won't get everything out of the amp that you intend.

In the old days, when tubes were the only game in town, speakers were more efficient and 16 ohms was a lot more common. This is for the simple reason that there is no sense in making the tube amp (or any amplifier for that matter) work hard for its keep. You want the amp to be effortless. It makes a difference!

Given you keep active on this site, it surprises me to hear that. It’s become the preponderant current criticism. Someone just made that statement here on Audiogon in the past week.

Just because other people say it doesn't make it true. I've heard a lot of older ARC amps and CJs that seemed a lot more syrupy than the ST-70.
But a lot depends on condition- keep in mind when the ST-70 was new, so were the parts in it. That is why many of them have to be replaced, as the older parts when out of spec result in distortion, bandwidth problems and so on. IOW many people are describing an amp that is out of spec. I didn't.

Folks historically dismissed the Dynaco with its slow, syrupy, pipe and slippers sound that represented the typical products of the time after hearing the reinterpretations; most of it came down to a case of comparing apples and oranges. The new amplifiers got built with modern parts, which provide a night and day difference in perceived speed, openness, clarity, and refinement. Build an original with metal film resistors (and good modern caps), and you’ll hear what I’m talking about.

I've never heard that sort of sound from any Dyna! Of course, the power supplies have to be up to snuff and if not that might account for that reputation. They are plenty lively stock.
But I agree that replacing parts helps. The carbon film originals can drift in value and the metal films won't. I also replace the coupling caps. Over the years we've modified many ST-70s; usually using a 12AU7 voltage amplifier/driver circuit, simpler, with less parts than the original.

I was thinking about building one of Latino’s amps.  Do you own one and have you compared it to an updated/modified ST70? I was also thinking of just buying an ST70 and updating it using modern day parts in key areas while keeping the output trannys.

@grannyring the big weakness in the ST-70 is the use of a single 5AR4 rectifier. Its marginal when the amp is at power and so it the most likely tube to fail in the amp- not the power tubes! There aren't good solutions for the older amps, solid state rectifiers run the B+ too high, the transformer can't support dual rectifiers (and there's no room); the solution is simply to not run the amp hard. Otherwise the original circuit is excellent- rivaling amps that cost a lot more (Marantz 8B) with similar bandwidth and less distortion. 

I'd go with Latino's versions- very nice.

Regarding references to the Tekton 4 ohm speakers, why would one pick a 4 ohm version of the same speaker instead of an 8 ohm? Any advantages? I don't think I've ever seen an amplifier that prefers a lower load.

There isn't one (tube, solid state or class D)!  That is my point. I'm sure people get excellent results with the 4 ohm models- there are plenty of posts on this site to that effect. Here's the deal:
****IF**** you want to get the most our of your tube amplifier****THEN**** that amplifier investment dollar is best served by a speaker that is 8 ohms or more.

Its that simple. If you have a 4 ohm speaker, you are leaving amplifier performance on the table. How this manifests will be less transparency and sounding not as smooth, likely less bass extension.

This forum is about high end audio. To that extent, the goal is simply to get closer to the music. Tube amps are really good at that; why on earth would you want to limit their abilities?

They probably do alright, but the 4 ohm versions can be problematic if you use an amplifier that does not employ loop negative feedback. This is pretty much all SETs as well as a number of push-pull amps. Essentially you may encounter problems getting the bass energy right. Its not worth messing with, since the amp’s ability to play 8 ohms is better and will result with more neutral presentation, one that is more transparent and likely also smoother.
If you’re on a budget, but are also committed to a tube amp, why not get the most out of it? I compare it to buying a Porsche but not setting it up with the right tires. It might be fun to drive, but it would probably be a lot more fun if you had the right tires on it.

One other tip- in this case, you already know you want tubes, so as a result if the question of which you get first, the amp or the speakers, now you have a better idea. You get a speaker that works easily with tubes and by all means you avoid a speaker that is considered 'hard to drive'.
Actually I don't see any point in 'hard to drive' speakers since all amps, solid state, class D and tubes make more distortion when asked to work hard. High end audio is about getting *rid* of distortion - getting closer to the music.
Tube amplifier power has always been more expensive than solid state, in fact when the industry went from tubes to transistors decades ago it was mostly driven by the higher profit margin that was possible with transistors. The point here is that keeping the speaker easy to drive will really help you with that amplifier dollar.

@captainblackleg, an earlier post recommended Revel speakers. If you want to keep this on a budget, I would pass on that recommendation, that is if you plan to use a tube amp with only 35 watts per channel!
The interface between the amp(s) and speakers is really important! You’ve not mentioned the size of your room or how lively it is and that plays a role too. For example if in a smaller room you might be able to use a speaker with less efficiency. In a larger room, higher efficiency speakers can really help!

So let’s assume that a 60 watt amp is all you can afford? In that case I would be hesitant to go with any speaker that is less than about 92 db. Now here’s the tricky bit- when dealing with tubes, speaker *efficiency* is more important than speaker *sensitivity*.
The reason is that the former is rated 1 watt at 1 meter, the latter is 2.83 volts at one meter. Before your eyes glaze over, the reason this is important is because if you have the bad luck to wind up with a 4 ohm speaker (which is usually bad for tubes unless the speaker is really really efficient, and by that I mean over 99 db), the 4 ohm speaker will be harder to drive than the specs seem to suggest!
There’s a little math involved, at 8 ohms the two specs are the same. But change the impedance to 4 ohms and the sensitivity spec will be 3 db higher than the actual efficiency of the speaker! So if you have a speaker that is 4 ohms and the sensitivity is rated at 92db, the actual efficiency is only 89db. That 3 db may not seem like a lot, but requires that you double the amplifier power to play at the same level than if the speaker were 8 ohms with the same sensitivity.
So if your amplifier dollar is on a budget, the higher sensitivities of 4 ohm speakers might be a siren song, but if your wits are not about you, it can lead to money flushed down the loo.

Also, all tube amps sound better on 8 ohm loads than they do on 4 ohms, because the output transformer is more efficient, so the amp can make a slight bit more power, have up to an octave lower bandwidth, and lower distortion. This gets even better with 16 ohms BTW and on 16 ohms, speaker cables get a lot less critical...
There is no reason you have to suffer less resolution because the speaker is higher efficiency. In this regard, there really isn’t a tradeoff until you get into speakers with a horn loaded bass array, which trade efficiency off for bass response (unless the horn is enormous).

With many tube amplifier companies, the smaller amps are often the ones that sound the best because they have more bandwidth. This is particularly true of Single Ended Triode (SET) amplifiers. The exception is OTL amplifiers which have no output transformer; this being because the output transformer in nearly all tube amps is the bandwidth limitation.

Some speaker recommendations: DeVore Fidelity, ZU Audio, Audiokinesis, Coincident Technology, Merlin (used only), Pure Audio Project, some of the Tekton lineup (stay away from the 4 ohm models); there are many more.

Here’s a tip: pay attention to the bass array in the speaker. If rated at 8 ohms but you see dual woofers, it may well be that the speaker is actually 4 ohms in the bass while ’nominally’ rated at 8 ohms! The B&W 802 is an example of this. A speaker like that might have you thinking that tube amps can’t play bass, and they totally can. So check with the manufacturer to make sure that the woofer array (if more than one driver) has an impedance of at least 8 ohms (16 ohms is better- the amplifier distortion goes down and the output transformer gets even more efficient) and overall the speaker efficiency is at least 92 db and you won’t go wrong unless your room is really big. If so, you’ll have to consider a more efficient speaker like a Klipsch (some of which are quite good with tubes).

One more thing- if you’re considering an SET (which can be really musical), obey this rule of thumb: match the amp to a speaker whose efficiency is such that the amp isn’t really going to make more than about 20% of full power. This is important as SETs are nearly 10% distortion at full power and the distortion starts to take off right at about 20% of full power. At that point the amp starts to sound really ’dynamic’ because distortion product (composed of harmonics that coincidentally the ear uses to sense sound pressure) starts to show up on the leading edges of the musical transients. IOW, the ’dynamic’ quality is just distortion interacting with physiology. Once you know that is the case, its not as interesting to hear that type of ’dynamics’. OTOH, SETs have a property of extremely **low** distortion if the amplifier power is diminished towards zero, and for this reason if you really want to hear what they do, you need a speaker than is more efficient than you will find others recommending, even on this site and possibly later on this thread! But don’t be fooled, and your amplifier dollar will be a lot better served.
Good Luck!