Autoformer vs Speaker impedance Curve

@almarg
No. If that were the only factor transistor amps would be more sensitive than tube amps in this regard, and I don't see that happening (although transistor amps don't have a filament circuit to cool off). I suspect Kirchoff's law might be in play there. But we use triodes and they are usually more susceptible to the actual B+ level due to reduced plate resistance and lower mu.

bdp24  

Thanks for citing those replies. I hope I have covered all if it but new questions are welcome. 

Several posters think a autoformer is evidence of a bad amplifier design. I don't think that at all though we have to be aware how a particular amplifier responds to a particular load. OTL amplifiers with reasonable number of tubes want to see 16 ohm and above speakers. So my recommendation is either get a high impedance speaker or get an autoformer to make your speaker high impedance. 

Several European radio manufacturers built OTL amps into common table radios. In their wisdom they used a 600 ohm speaker. Thats really cool.

McIntosh's use of an autoformer recognized that transistors like low impedances. If someone came to me and said "what impedance would you want to design an ideal SS amp"? I would say 1-2 ohms. That is exactly what McIntosh did.

So far I see this discussion has entirely ignored the resistance of the wire back to Hoover Dam. You can very easily measure this by plugging in a 1000-1500 watt space heater and noting the voltage drop. I do this when I look at buying a new house. Its so easy, the agents are amused, and you find out the actual resistance of your wall socket .... back to Hoover Dam. Sadly the lake is almost dry so I have to find another source to cite. I just like the sound of HOOVER DAM. Nice clean water powered electricty. 

The best I have found is a drop of about 3 volts at 1200 watts so that is 10 amps and the resistance is therefore 3/10 or 0.3 ohms. A poor house might have 10 volts, really bad wiring, which would be one ohm back to the Grid. It is pretty amazing to consider how you can get 0.3 ohms over all that distance, but transformers and 500,000 V transmission lines do the trick.

Now everyone knows their voltage fluctuates and frankly a good amplifier doesn't much care. The power loss with voltage in any well designed amplifier is related to V squared. If we loose 2 % voltage we loose 4 % power not 40%. Something is not right here. Good tubes operate with at plus or minus 5% filament range with no problem at all. No loss of power, none. Weak tubes are another story, but not good fresh tubes. I do this test on every batch of RAM tubes running the heater from 5.7 to 6.9 volts. The change in characteristcs is vanishingly small, within experimental error. 

However, now and then I do find a bad batch. I test in batches of 100 or more and one time I found some KT88s from Russia that went all over the place with filament variation. The provider of these tubes said they had sold thousands of that lot to... well im not gonna embarass anyone, but you know, the big guys, and nobody complained. 

So back to nuts and bolts. I don't believe this post of 40% and if it is true you had better go buy some kind of regulatiing power conditioner. The big one from Monster with the motor driven variac is quite good and I have used one at locations where I have large and frequent line variations. Otherwise I'm turning the variac all the time. My daytime voltage averaged 117 and 125 at night.

The resistance of a 5 ft, 16 ga power cord is 0.04 ohms not 0.4. Keep in mind that the primary of the power tansrformer in a typical large amplifier is wound with 50-100 feet of 16 ga magnet wire and and then another equal amount in the secondary. That's 10 to 20 times the resistance of a $5 power cord. The total equivalent resistance of a 500 watt transformer, referred to the primary is often 1-2 ohms.

Do what you will with power cords, but what are you going to do about the wire all the way back to Hoover Dam.

Bascom King (just one b please) is fine friend from Santa Barbara. I have visited his home and lab many times as he has visited mine. His lab is in an out building and I doubt with high current lines. We should all use a variac to measure amplifier power to make it a level playing field. In addition to that I like to run the line up and down at least 5% to see what happens to the bias and distortion. Often bias is very unstable which explains why high line can be a problem. Some lines these days run 125V which may be a problem for on the edge amplifiers. I have seen amplifiers (not mine) where a 5% rise in line causes a 10-20 % rise in bias current which added to the voltage rise causes a 15-30% or more rise in dissipation and possible thermal runaway. 

Theres a lot more to say about this but lets at least get real about the numbers. I don't see how Kirchoff applies though Thevenin is how one analyzes these situations. Kirchoff is for circuits with lots of nodes and branches. This is simple series resistance. 

 

almarg8,371 posts09-14-2018 1:08pmRalph, would I be correct in thinking that the low B+ voltages that must be used for the 6AS7G power tubes in your amps, compared to the much higher B+ that is supplied to the tube types used in most non-OTL amps, would tend to increase the degree to which output power capability is sensitive to AC supply voltage variations?

This is actually a good thought and I agree with you. Because the saturation (minimum voltage at some current) of the 6AS7 is going to be a large (say 50) percent of the B+. Much larger than in typical amps where is is 20-30%.

The RM-10 uses 720 volts on the EL-84 plates and the saturation is around 100 V which is only 14%. This is why it can produce 40 watts per channel. (50 at the plates).

Roger, I recommend you try some measurements on this bit yourself. Its obvious to me that you have not. Its simple enough to do. Bascom is a nice guy and I've always had respect for him, but he screwed this one up. People are human. 

Its a common question as to what about the wiring in the wall and such? Part of that is what substations are for; to make sure that power at a local level is the correct voltage. In tandem with that are the power transformers mounted on power poles that serve local nodes. The one on the pole in my back yard serves 5 houses. The AC power sent to that transformer is at a higher voltage; this is done to minimize current loss thru the power lines. As you might know, a different waveform is employed on high voltage transmission lines (like the ones which come from the Hoover Dam; that is why you can hear them on the AM radio if you get close to them) for this same reason. This of course is in a nutshell.

Once in the house, voltage drops can indeed occur, but the Romex found in most homes is very high performance. The problem is how to get the power out of the wall, since Romex as you know is solid core and very stiff; thus being illegal for use in a power cord. IOW the real challenge is how to build a power cord that does the job properly and still be flexible and legal. Its doesn't have to be expensive- it just has to be right. That is why its worth it to replace a power cord, even though there is wiring in the wall going all the way back (in effect) to the Hoover Dam.

We employ a simple bias mechanism that allows our amps to autobias and so respond to changes in wall voltage. It won't do for example to have the AC voltage to be very high and possibly damage power tubes! So the amp reduces the bias level when the AC line voltage is high and increases it as the line voltage drops.