Member's recommendations for tube amplification

I don't post much here anymore but sometimes follow a thread or two. This one piqued my interest because it mentioned a couple of my favorite amps. It also piqued my interest because I think there are a few points by other posters that should be reinforced.

First, I have experience with SET amps having worked with someone for several years well known for manufacturing transformers and designing several SET amp circuits for our own manufacturing and sales. I can say from his perspective, the designs were best put to use on speakers whose efficiency was 98 dB or higher. However, we found one interesting exception to that rule. Pairing a 12 watt 300B with a pair of Fritz Carbon 7 speakers (88 dB, 6 ohm) at a very popular audio show. That matching came about by accident, as a 50 watt PP amp failed on us and while it was being repaired we played around with the 300B. We didn't go back. Anyone who heard that combination was amazed it could work. We played it as loud as we could and it continued to perform flawlessly. Sometimes you get lucky, but as Ralph pointed out, the norm will have different results.

The M-60 OTLs will drive your speakers quite well. I have used them for several years now with a variety of speakers ranging from 88 dB to 94 dB, with both smooth and somewhat bumpy impedance curves. In fact I used them with AudioKinesis Jazz Modules for a majority of that time and now with Quad ESL 57, using autoformers to better match the impedance. The autoformers add nothing to the sound in my opinion.

Bill's (grannyring) suggestion of the Music Reference RM-10 is spot on. This amp has balls and puts out 35 watts per channel (double what most EL-84 designs can muster). I have owned mine for 12 years now and used it on many speakers. It just recently powered speakers I am playing around with that go down to a 2 ohm load in the 15 - 20k range. No problem whatsoever. It works great on the ESL-57s as well.

One thing I've learned over the years is there are other factors that can determine how an amp matches up with speakers. I believe Ralph mentioned the size of the listening room and how loud you like to listen.  Another is damping factor, or the amps ability to control the woofer. Lately a question I've been asking speaker manufacturers is what amps did you use to test your speakers. Generally that will give you a good idea of the damping requirements. Some speakers were designed with solid state in mind, others tube amps. You can learn a lot talking to the speaker designer.

I suppose you should take what I say with a grain of salt:

1. Jack Elliano is my business partner.

2. I am an Atma-Sphere dealer.

3. I work with Roger Modjeski

I used to own the original Micro ZOTL and used it on high efficiency speakers and it did quite well, were talking 98 - 104 dB. Again, I think a lot has to do with how large your room is and how loud you listen.

The ESL panels I am using now have adjustable sensitivity, from 70 - 80 dB. The impedance is pretty acceptable from 100 Hz to about 10K Hz whereupon it begins to drop down to 2 ohm pretty quickly (note the speakers do not go below 100 Hz and I bi-amp them).

Understanding the designers goal was to make a panel with a very small footprint that can provide extensive detail at low volume, he had to make significant comprises. So he said the heck with the rest of it and made a set of panels that are better than my ESL-57s, but not as flexible with amplification. While the recommendation was to use a 100 watt amp (ss or tube) I was able to use a tube amp that is 35 watts per channel to drive them cleanly getting SPL levels up to 90 dB, which is a good 10 dB more than my preferred listening level. The level of detail and clarity in the music is fantastic.

In addition to room size and listening levels, if an amps damping factor is synergistic with a speaker, a lot can be done with a little power. There is too much emphasis placed on headroom in 2-channel. although in home theater I think it's important. Less is more in many cases. Mind you I'm talking about the ability of an amp to drive a speaker cleanly to reasonable listening levels. Just because it can be done doesn't mean the listener will like how it sounds.

I'm not disputing Ralph's observation. I'm just saying there's too much emphasis placed on headroom in a 2-channel system. I have used low power SET amps with a variety of speakers and with the ability to measure output while music is playing could clearly see no more than 2 watts being used to achieve reasonable listening levels, without compromising sound. This using 8 - 12 watt SET amps, zero feedback.

Again, size of room, SPL level you listen to, damping factor all play a part in how much power you need to effectively drive a speaker. As per my previous example, 12 watts of SET power, zero feedback, and 6 ohm 88 dB speakers on paper would make anyone think "huge mismatch." Not the case from mine and others perspectives, include a reviewer who wrote up the system, and another reviewer who took the amps home on the spot to do a review.

Feel free to disagree. Better yet get the equipment, learn how to use it, and measure it for yourself.

I should have stayed in hibernation. Enjoy.

I'm back for a moment as Ralph contacted me about the thread.

For Bill, first of all I was not upset with your comments. I think Charles summed it up nicely. I left here a while ago because of the misinformation I kept seeing over and over again and how audiophiles were being influenced by it. Again, I come back every once in while because of something interesting I see. Your mentioning the Music Reference RM-10 was just that, as you rarely see that amp mentioned at all, let alone a discussion like this. A true injustice to the amp in my opinion and I owned one long before I started working with Roger Modjeski, the amps designer.

As I read over my last post it dawned on me that I could add a bit more to it to make it clearer. Again, Ralph's comment on SET power is not being disputed. It's true and something I have seen occur on test equipment in Jack Elliano's (Electra-Print) shop. It's not easy to get around the cold hard facts of how SET amps typically behave. Quite frankly some people like the distortion, it gives them the warm and fuzzies, and that's great. Until you know what to listen for, and when those nasty harmonics start to rear their ugly heads, as they will eventually as the power rises, then you should not find things so pleasant.

Jack Elliano developed a set of output transformers that allows two series triodes (in this case an EL-45) to provide about 12 watts of power. He also designed a summing loop that tapered off the rise in the distortion (IMD), which in no NFB SET amps happens pretty quickly after the first watt or two (when power to handle peaks in music is generally needed). So for this circuit IMD is less than 1.5% up to 3 watts (25% of full power). It will rise to 10% at about 7 watts (still extremely clean at greater than 50% of full power). The distortion finally causes the amp to clip at 12 watts (full power). That's why such a design (which we used in 45 and 300B designs as well) can push a 6 ohm speaker like Fritz's Carbon 7's to bliss, or as Charles noted those 4 ohm Hornings.

In a typical circuit without such a summing loop IMD distortion will rise to nearly 30% at 7 watts, and at this level you anyone should be able hear it (although the rise up to that point could be challenging). The first few watts will be fine for the most part (the 20 - 25% of full power Ralph alludes to), but the amp will turn out the nasties quickly after that. However, that's not to say such an amp can't be enjoyable in a given room with a given set of speakers at low listening levels, as I've heard it happen. On the other hand in the A/B testing of these two circuits I could hear the distortion as we cranked it up past 80 dB (on 94 dB 8 ohm speakers). You learn where your limits are pretty quickly.

There are a lot of nice folks here still, but I really would be doing myself a disservice if I hang around. That could change but for now life beckons and I'm knee deep enjoying life in my new surroundings in the Bay Area while still fiddling around with my system when time permits. I hope I answered your question Bill, take care everyone.