Amen to this Tube Amp post-we need more engineers on here...HI hemigreg,
Your have brought up some good points here and I hope you take my expansion on them as a compliment. Many of the readers of this thread have been pulled into impedance and power thinking by some new paradigms that I do not particularly agree with. So here goes.
We do need more engineers on here, for one thing to keep the known laws of electricity correct in our discussions. Some manufacturers like to make up stories to defend their equipment's performance, or lack there of. I am always entertained when John Atkinson, a very strait shooter, measures an amplifier and finds horrible results. It is unwise to send JA and amplifier that measures poorly and then have to defend it in the Manufacturers Comments.... which are even more amusing.
QUOTE :;"’6-18-2018 2:05amThis is an old topic...literally. In the times when tubes were dominant amplification devises, the output transformer was a necessity (one might say a necessary evil. The tubes are amplifying voltage, so that they work with high impedance loads. The loudspeakers normally have quite low impedance. Thus a transformer is required to match the output of tube amplifier and the loudspeaker. When first transistors have emerged, the schematic design did not evolve immediately. Thus, early transistor amplifiers were very similar to the matured at that time tube amplifiers.I think its safe to say that when engineers first got transistors they didn't know what to do with them and the evolution of design topologies took awhile to develop into what we have now. I was learning transistors at the same time and had the same early RCA and Motorola books. I was 14 years old and it really wasn't all that difficult to get something that worked. We were all making the circuits out of those manuals which were the most advanced at the time which was 1965. However those schematics did not look like tube schematics at all.
However, over time it was realized that transistors work better and amplifiers of current as opposite for tubes which work better as amplifiers of voltage. Thus, transistor amplifiers can and do work well with low impedance load such as loadspeaker directly.
While we consider that transistors are indeed current amplifiers they do not respond linearly to voltage inputs as tubes do. Pentodes act like transdconductance devices which means their current is proportional to their input votage which is a very handy thing to drive a speaker since we start with voltage (from the preamp) and want current for the speaker. Speakers are current driven devices by nature of their physics.
What makes tubes need a transformer and transistors not is the fact that high current transistors come easily but high current tubes do not The most popular transistor in the world is the 2N3055, which Dynaco, NAD and many others used. For $1.50 it can do 15 amps of current. The best Horizontal Output tubes (mistakenly called Video Tubes because they were in TV sets) can only do one amp of current but they can do this at a very high voltage, thus the need for the transformer. We are really not matching impedances here, we are using the transformer to exchange voltage for current, as all transformers do. Indeed output transformers are difficult to make, more on that if people want to know, and expensive because they have to get the best workers in the factory to make them. I had one guy I trusted and he made all the RM-9 outputs with exceedingly consistant results.
Now, still transformer (autoformer) can assist with the loudspearkers with various impedance...i.e. 4 Ohm vs. 6 Ohm vs. 8 Ohm providing more stable load to a SS amplifier. However, transformer alone is not a perfect transfer devise and making quality transformer for audio output is difficult and costs a lot of money. The typical issues with output transformers are: reduced damping factor and difficulties with driving complex loads, slew rate reduction, additional distortions, etc.This is the biggest myth being promoted in this threat. An Autoformer or any transformer cannot fix a difficult load and only affects stability in a poorly designed, on the edge amplifier. A good transformer does not reduce damping factor appreciably, or slew rate or change distortion except at the frequency extremes. M6 iron is extremely linear, low hystersis, and low eddy current loss. Those fancy amophous irons don't make transformers appreciably better.
What Transformers do is present to the amplifier a particular voltage and current that the output devices like. The taps on tube amps are there for that purpose. You might as well call them A, B and C and choose the one you like best. At least you have a choice. What an Audoformer does is give you that choice with ampifiers that do not have taps.
The RM-10 produces an unbelievable 40 watts with one pair of EL84/6BQ5 tubes (typically 17 watts) at less than 1% distortion with an 8 ohm load on the 8 ohm tap. Many people do not need 40 watts and they can get 25 by putting their 8 ohm speaker on the 4 ohm tap resulting in twice the damping and 1/10 the distortion. Most people do this, I encourage them in the manual to try it and it works on all well designed tube amps. Try it!
Sorry this is long, but there is a lot of work to be done on this topic.