Recommendation for higher power tube amp

If the amp that drives the Theil no longer has to make power for the bass, there will be more power for everything else.
IME this is a really tricky way to go since active electronic crossover (which is what you need to see to it that your amp for the Theils does not have to deal with bass) are another block in the signal chain and so some immediacy is lost.

You have to check the input impedance of the sub in parallel with your amps and then see if the preamp is OK with that.

Efficient speakers MAY help, but an efficient speaker may introduce a difficult load for that particular amp. The crossovers or some other feature may confuse the amp.

IME this is an *extreme* rarity. Usually efficient speakers are a much easier load; in fact I've never seen an efficient speaker that was harder to drive. FWIW most speakers of higher efficiency don't have weird phase angles; that is more commonly seen in lower efficiency speakers. So the crossover comment seems right out.

Wouldn't using a pair of Zero Autoformers help with the low impedance issue?

No. If anything, the terrible inefficiency of these speakers is more of an issue than the impedance, although the impedance isn't helping.

OK- we can do the math again. 200 watts is about 4x the power of the amp in the OP. That's about 6dB.


So the speakers are ***84dB 1 watt/1 meter***! If a speaker that was only 90 dB (which is easy to find) the amp he has now would make the same sound pressure. The thing is, the bigger you make a tube amp, the harder it becomes to make an output transformer that has bandwidth. Because big tube amps are expected to make bass, often its the highs that suffer but often the bass suffers a bit too. The only exception to this rule is if an OTL is used; then its possible to scale the amp to any power without loss of bandwidth.


But a 200 watt OTL on this speaker is impractical- and that would be only to maintain the bandwidth he has already. So it makes more sense to change out the speaker for something more efficient than it does to change out the amp- the latter is nearly a Sisyphean task. Even with 200 watts this speaker isn't going to be able to make satisfying sound pressures in most rooms if you want to play anywhere near realistic volume levels.

But you don't have to lose any resolution (in fact many think that you gain resolution) with higher efficiency speakers. In my example of my first post on this thread, I showed that an amp of 1.7 watt could keep up with his BAT amplifier if that amp was on a set of ZU Audio loudspeakers. This means that a 7 watt amplifier could keep up with a 200 watt ARC in the same circumstances.


The bottom line here is its very obvious that the speaker needs to be replaced if tubes are going to be used. 

Different speakers - I have considered this as a next step, and was thinking about Zu, Devore 3XLs, and Joseph Audio RM-25si, all of which I think would get my amp closer to the loafing so enjoyed by wolf_garcia's Carys. Any others I should think about? Maybe start another thread...

Watch the impedance. If you want the best performance out of a tube amp, the impedance best not be under 8 ohms especially in the bass.

@erik_squires Keep in mind his budget...
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@thosb
A used RM-200 would be a good amp. BUT - tube power is expensive, which is why in the old days there were horns. Without going that extreme, I am wondering if it might be more practical to change out the speakers instead of changing the amps.

The VK-55 is about 55 watts. The RM-200 is thus about a 6dB increase in power. The Thiel has a sensitivity spec of 87dB, but being a 4 ohm load this makes its actual efficiency about 84dB - borderline criminal as with such low efficiency, its hard to find an amp that can make enough power and *sound like music* at the same time. The RM-200 would be on a *very* short list!


How this works is sensitivity is a **voltage** measurement- 2.83 volts on the speaker measured at one meter. If 8 ohms, that's 1 watt; at 4 ohms that's 2 watts- meaning that the speaker's efficiency is 3dB less than its sensitivity. IOW it takes 2 watts to make 2.83 volts on a 4 ohm speaker.


Now efficiency is a **far** more useful spec when dealing with tube amps- this is because tube amps don't double power as impedance is halved; instead cutting power in half as impedance is doubled (if running a lot of feedback). In a nutshell efficiency lets you know if your tube amp makes enough power to work with the speaker. Sensitivity can fool you.


Its far more practical to run a speaker of higher efficiency. Let's explore this:
A speaker that might easily keep up with the Thiel (from a sonic perspective- you don't have to give up sound quality with higher efficiency) might be one of the ZU Audio offerings which you could easily find used within the price range specified. Now ZUs are pretty easy to drive and about 98-99 dB with 1 watt. So let's do a **tiny little** math:

With a 50 watt amp the Thiel can make x volume at the listening chair. Now each doubling of power is only a 3dB increase. The ZU is about 15 dB more efficient. I don't want to lose anyone here by going too deep into the math of decibels, so let's go with a 6dB difference- to make the **same volume**, a speaker 6dB more efficient needs only 1/4th the power. So if we had a speaker that was 90dB, a 12.5 watt amp would play the same volume as a **50 watt** amp on the Thiels! But the ZU is about 9 dB more; if we break that up in 3dB chunks and start with 6dB more, to make the same volume level as a 50 watt amp on the Thiel we would need only about 3 watts. Cut that in half again since we have 3dB left and you see how impractical it is to throw power at a speaker that inefficient!

The actual math- 55 watts- a 15 dB increase in speaker efficiency- results in a 1.739 watt amp able to make the same sound pressure. Think about it.


With 55 watts you would have nearly unlimited power- which is what you want- at normal volumes the amp will be loafing (which is also what you want) and so your tubes will last a lot longer.