@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.
****************
@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.
