Where the old tube amps underpowered?

weegee

Active Member
I was looking at some old ads from the 1950's. The amps I saw were 10 to 30 watts with a few a little higher. Where the old tube amps underpowered?
 
My most powerfull tube amp is 35wpc. My lowest powered amp is around 6wpc. If your speakers are efficient enough lower powered amps can do very well.
 
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Under-powered is a relative term. If you want to run low efficiency speakers that were intended to run with an amp at 100+ wpc on something making 25 watts, yes. If you want to run 3 watts on speakers with efficiency numbers in the 100+ range, then no.

Tube amps that make 100 or more watts do exist, they just tend to get really stupidly priced as the power goes up.
 
Same reason hundred horsepower automobiles were a rarity in the 1920s, state of the technology of the time.
A high powered domestic Hi Fi tube amp of the late 1950s to mid 1960s period might be rated around 60 Watts per channel for stereo. Dynaco Mk-3 on the low priced end to McIntosh MC60 on the other end.
A good middle range amp may be rated at 30-35 Watts, Dyna Stereo 70 to Marantz 8/8B or McIntosh MC225/ MC240.
The common good home systems tended to be based on p/p 6BQ5/ EL84 at ca 12 - 17 Watts. Many examples in that class, the Dyna ST-35 possibly the best of the budget end to the Leak Stereo 20 at the other end.
Many consoles used the p/p 6BQ5 amp as the core of the system.
There were of course lower powered amps for basic/ entry level systems.
 
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The 50s were the beginnings of home audio and closed box speakers. This is also when high power beam pentodes were marketed. This allowed much higher wattage from 2 tubes up to 100 watts. Which allowed speaker designers to develop even larger multi element speakers. The demand for these products or the home audio evolution is tied into the rise of the 'middle class' which come from WW2 era workers having much more disposable income to spend on amenities or lifestyle improvements.
 
Measure how much power your speakers consume at your desired listening level. I would be surprised if it is more than 25 watts, and in reality is probably closer to 5 watts than it is 100. 35 watts per channel is plenty of power and even with the most inefficient speakers it will usually be sufficient.
 
I've had a Realistic APM wattmeter hooked to my stereo system for many years now. Most of the time, the needles rarely go beyond 1W RMS. Granted, most of my speakers have had efficiency ratings at or near 90dB, but even an absurdly inefficient speaker like the AR 2ax (78dB) can be driven to decent volume levels with a Dynaco Mark III, and perhaps even something smaller.
-Adam
 
I've had a Realistic APM wattmeter hooked to my stereo system for many years now. Most of the time, the needles rarely go beyond 1W RMS. Granted, most of my speakers have had efficiency ratings at or near 90dB, but even an absurdly inefficient speaker like the AR 2ax (78dB) can be driven to decent volume levels with a Dynaco Mark III, and perhaps even something smaller.
-Adam
Adam - I have the Radio Shack , LED Peak power meter. You have the old analog, ballistic Averaging meters, so you may not know if there's a 10 or 20w peak being produced. RMS power is one thing but for full dynamics on musical peaks, you need to have an amp. that can handle power surges on transients.
I also have AR2ax and mine measured 85 dB SPL, 1 meter,1watt, not 77.
 
Adam - I have the Radio Shack , LED Peak power meter. You have the old analog, ballistic Averaging meters, so you may not know if there's a 10 or 20w peak being produced. RMS power is one thing but for full dynamics on musical peaks, you need to have an amp. that can handle power surges on transients.
I also have AR2ax and mine measured 85 dB SPL, 1 meter,1watt, not 77.
APM-500?
 
I've had a Realistic APM wattmeter hooked to my stereo system for many years now. Most of the time, the needles rarely go beyond 1W RMS. Granted, most of my speakers have had efficiency ratings at or near 90dB, but even an absurdly inefficient speaker like the AR 2ax (78dB) can be driven to decent volume levels with a Dynaco Mark III, and perhaps even something smaller.
-Adam
APM-200?
 
The thing that somewhat "saves" low power tube amps, is the fact that due to the lower level of global feedback, they can be run closer to clipping (less crest factor) than solid state amps. In other words, tube amp clipping is much more benign than solid state amp clipping, in most cases.

Feedback is a double-edged sword- it's great at reducing distortion when the amp is below clipping- but once clipping occurs, it actually "doubles down" on the errors- by trying to force the amp to produce more gain (which is what happens when clipping "compresses" the output), it actually causes an even greater degree of distress within the amp (harder clipping) than if the amp had no feedback. This causes all sorts of issues- generation of high-order harmonics (well beyond the usual second, third, and fourth harmonics that usually dominate the distortion of a typical amp- clipping, you can have even significant distortion up into the 10th harmonic or higher), and even stuff like momentary latch-up in some designs of less stability (fortunately, not usually a problem in modern SS amps- but that was a real issue with the first 20 years or so of SS amp designs!)...

So, while you may need to run a 100w solid state amp on an 88dB sensitivity speaker to have enough dynamic headroom (usually, you want 10-20dB of headroom beyond the normal steady-state volume level- and a 20dB increase in level is 100 times the power)- a tube amp may do acceptably well with 30 to 50 watts on the same speaker. Yes, the tube amp clips momentarily, more often than the SS amp, at the same SPL levels- but the consequence of that clipping is much less detrimental with the tube amp...

This is one reason why many of the current modern SOTA tube amps, even higher powered ones, still use more moderate levels of feedback- it's more worth it to do the hard work and make the amp as linear as possible before feedback- which is much more possible with a tube amp than with a SS amp,due to tubes starting out more linear than transistors on an individual component level. For one example- IIRC, the VAC Statement amps only use something along the line of 12dB or so of feedback... they simply don't need a lot of it, to do everything they can do. Hence, a feeling of dynamic "effortlessness", even beyond that of other amps of the same power.,..

Regards,.
Gordon.
 
In my main system, yes. In my office system, I use an APM-100, which is passive and RMS-only. I do have an APM-500, as well as a homebrewed LED meter which is similar, but they're not in use at the moment.
Adam - I have the Radio Shack , LED Peak power meter. You have the old analog, ballistic Averaging meters, so you may not know if there's a 10 or 20w peak being produced. RMS power is one thing but for full dynamics on musical peaks, you need to have an amp. that can handle power surges on transients.
I also have AR2ax and mine measured 85 dB SPL, 1 meter,1watt, not 77.
The APM-200 is a hybrid design, using analog meters for RMS and LEDs for peak watts. Granted, there isn't a high amount of resolution on said peak LEDs (0.05/0.15/0.5/1/2W or 5/15/50/100/200W, depending on the range setting), but it gives you a pretty good idea of what's going on. The peak LEDs on the APM-200 seem to be in-line with what the meters on my Pioneer SPEC-4 show. As for the 2ax specs, you may very well be right. I know that their efficiency rating is very low, and once saw a figure of 78dB quoted somewhere for the 2ax. Further searching has been inconclusive, but a figure around 85-86dB is commonly mentioned for AR speakers from this era.
-Adam
 
Gordon, I am puzzled by youe answer: "So, while you may need to run a 100w solid state amp on an 88dB sensitivity speaker to have enough dynamic headroom (usually, you want 10-20dB of headroom beyond the normal steady-state volume level- and a 20dB increase in level is 100 times the power)- a tube amp may do acceptably well with 30 to 50 watts on the same speaker". A 20 dB increase in level implies voltage level. The gain formula for which is 20 log A or 10. Yet, "Level is 100 times the power." implies the power gain formula of 10 log A. Which are you referring to? Please clarify.
 
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