A question of value in today's hi-end.
- Thread starter DaveyF
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What high current? Are you kidding? Tube amps make their watts with high voltage/low current. Transistor amps make their watts with low voltage/high current. Typical voltage rails on a SS amp would be around 50v. Typical B+ on a tube amp would be around 500v. Some tube amps that use transmitter tubes can have plate voltages over 1000v, but voltages in the 400v-525v are much more common.
Technically this is not quite true. Yes, this is the situation on the primary side of the output transformer, here it is all high voltage, low current. But the the real action is on the secondary side of the transformer, the bit actually connected to the speaker terminals, which is why the quality of the transformer is so important: if it can't deliver decent amps to the speaker then the sound may suffer ...
Frank
Happy to oblige, Jack! I've never fiddled with valve gear, but the principles are straightforward. As Mark stated, valves are great at dealing with high voltages, not so good at the current thing. So, unless you wish to go down the esoteric road of doing OTL circuits which I have never looked at, the way to get around this is to use a transformer, the big lump doing the output thing. If you didn't use one, you would blow the speaker and/or the valves because the impedance of the speaker would be too low!
Transformers, if good at their job, transmit power almost perfectly from the primary or input side to the secondary or output side, trading off voltage for current, or vice versa. Just like the mains or power transformer. A simple example: a 4 ohm speaker handling 16 watts means an 8 volts signal is being fed to it, 2 amps are flowing through it. So that the 16 watts power is being fed through the output transformer, in the shape at the output of 8 volts, 2 amps; 8 times 2 equals 16 watts. Assume the transformer is a 10:1 step down, which means that 8 volts on the output came from 80 volts on the input. But the power flowing through the input side of the transformer has to be the same (almost) as through the output. Simple arithmetic says that current on the input side is thus 16 watts divided by 80 volts, equals 0.2 amps -- 0.2 times 80 also equals 16 watts. As Mark said, a high voltage of 80 volts, a low current of 0.2 amps does the job in the valve area.
If you directly tried to feed that 80 volts into the 4 ohm speaker, that would mean 20 amps are flowing through the voice coil, and the speaker is handling 80 volts times 20 amps current, equals 1600 watts -- something's gotta give ...!!
The winding of wire in the output of the transformer has to be very low resistance, which means thick copper, otherwise it starts becoming a significant percentage of that 4 ohm speaker resistance, and power is lost in the transformer needlessly ...
Does that make sense, or am I explaining what you're after?
Frank
Transformers, if good at their job, transmit power almost perfectly from the primary or input side to the secondary or output side, trading off voltage for current, or vice versa. Just like the mains or power transformer. A simple example: a 4 ohm speaker handling 16 watts means an 8 volts signal is being fed to it, 2 amps are flowing through it. So that the 16 watts power is being fed through the output transformer, in the shape at the output of 8 volts, 2 amps; 8 times 2 equals 16 watts. Assume the transformer is a 10:1 step down, which means that 8 volts on the output came from 80 volts on the input. But the power flowing through the input side of the transformer has to be the same (almost) as through the output. Simple arithmetic says that current on the input side is thus 16 watts divided by 80 volts, equals 0.2 amps -- 0.2 times 80 also equals 16 watts. As Mark said, a high voltage of 80 volts, a low current of 0.2 amps does the job in the valve area.
If you directly tried to feed that 80 volts into the 4 ohm speaker, that would mean 20 amps are flowing through the voice coil, and the speaker is handling 80 volts times 20 amps current, equals 1600 watts -- something's gotta give ...!!
The winding of wire in the output of the transformer has to be very low resistance, which means thick copper, otherwise it starts becoming a significant percentage of that 4 ohm speaker resistance, and power is lost in the transformer needlessly ...
Does that make sense, or am I explaining what you're after?
Frank
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Thanks for that feedback, Frantz. What I am trying to point is the possible misconception leading from Mark's reply to Lee, where Lee stated that the high current was driving his 1.6s well, which Mark disputed. 16 watts is 16 watts, yes, but it may be derived from different combinations of voltage and current. Most speakers need current, lots of it, but tubes can't do this by themselves, unless in OTL. Output transformers do the "magic", if you want to be silly you could make an output transformer that pumps out 100 real amps from the tube output stage! It would do a terrible job of driving real speakers but it could be done! So the output transformer is the crucial bit that matches how the tubes naturally behave, and what the speakers want to see driving them, and it is all important how well they are made that makes this coupling work efficiently. Hence why many tube amps have different output impedance terminals ...
Frank
Thanks Frank. You made it easy to understand. Now how about a solid state example for contrast? 
Fas
16 watts can of course be 16 x 1 or 8 x2 or even 4 x 4 .. That could be true but if the load was fixed...and resistive .. It only can be one combination of Voltage and current... so for your 4 Ohms speakers (assuming these can be approximated as a resistive load), it will be the same 8 volts and 2 amps across its terminal whether the amp is tubes with transofrmer, SS, OTL, Hybrid or whatever you want to think of ... Only One combination 8 Volts across terminal and 2 amps through ...
Now assuming a 4 ohms resistive load (most speakers are not truly resistive but I digress) 100 amps in such a load is 40,000 watts ... Care to let me know of a "real" speaker that can take that ? Not any that would come from the top of your head or any head ...don't you think and how many amplifiers SS or otherwise do you think can swing that much power? Home amplifiers or Pro for that matter... So you will see those nice amps figure for SS amplifiers, the reality is that for a SS to push that much current the impedance has to be very, very low say at 0.1 ohm for example it would be a substantial but attainable 1000 watts ...
The maximum current an amp can deliver is limited by its output impedance. (and rail current , etc I have chosen to ignore these for now ) . On this ground SS amps do better than tubes amps. SS amps have output impedance routinely in the tenths of One ohm or even less..
Back to the thread
16 watts can of course be 16 x 1 or 8 x2 or even 4 x 4 .. That could be true but if the load was fixed...and resistive .. It only can be one combination of Voltage and current... so for your 4 Ohms speakers (assuming these can be approximated as a resistive load), it will be the same 8 volts and 2 amps across its terminal whether the amp is tubes with transofrmer, SS, OTL, Hybrid or whatever you want to think of ... Only One combination 8 Volts across terminal and 2 amps through ...
Now assuming a 4 ohms resistive load (most speakers are not truly resistive but I digress) 100 amps in such a load is 40,000 watts ... Care to let me know of a "real" speaker that can take that ? Not any that would come from the top of your head or any head ...don't you think and how many amplifiers SS or otherwise do you think can swing that much power? Home amplifiers or Pro for that matter... So you will see those nice amps figure for SS amplifiers, the reality is that for a SS to push that much current the impedance has to be very, very low say at 0.1 ohm for example it would be a substantial but attainable 1000 watts ...
The maximum current an amp can deliver is limited by its output impedance. (and rail current , etc I have chosen to ignore these for now
) . On this ground SS amps do better than tubes amps. SS amps have output impedance routinely in the tenths of One ohm or even less..Back to the thread
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English Frantz, English!
In that conversation, I'm sure, even though I'm only reading half of it, Frantz is the one speaking English.
Tim
No worries, Jack. An SS amp is very straightforward in comparison because its output devices, transistors and mosfets, work at exactly the voltages and currents that speakers like, so they can effectively be tied directly to the speaker terminals. In essence all that the output stage of such an amp does is adjust what percentage, what level of the power supply voltage is passed through to the voice coils of the drivers, they work like a little man winding a water tap up and down, constantly adjusting the level of mains pressure being fed through a hose (how's that for an analogy, eh??).Thanks Frank. You made it easy to understand. Now how about a solid state example for contrast?
This is why the quality of the power supply is so important, if it's "wobbly" then the sound is "wobbly". Yes, there are myriads of ways to minimise these effects, such as using local and global feedback, but if you make sure everything is as clean as possible in the first place then everything becomes easier ...
As regards the 100 amp output, as Frantz points out, speaker drivers in themselves never need these sort of currents. Where things get nasty inside speaker cabinets is within those dastardedly crossovers, where there can be a split second need for a spike of current to make those components happy, and if the amp can't deliver that then the sound suffers. Here's where Krell, Mark Levinson, etc, win ...
And why active speaker systems, or electronic crossovers with bi- and tri-amping do so well ...
Frank
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Jack,
If you want another perspective, consider that a the transformer multiplies the impedance of the speaker by a factor that equates the square of the tuns ratio between primary (tube side) and secondary (speaker side). This would mean that in typical practical cases, the tubes see a speaker with an impedance between 1000 and 6000 ohms, that needs high voltage and low current.
Confused?
If you want another perspective, consider that a the transformer multiplies the impedance of the speaker by a factor that equates the square of the tuns ratio between primary (tube side) and secondary (speaker side). This would mean that in typical practical cases, the tubes see a speaker with an impedance between 1000 and 6000 ohms, that needs high voltage and low current.
Confused?
Yeah, I left the output transformer out of the current discussion. The output transformer in a tube amp does convert high voltage/low current to low voltage/high current (relatively speaking). However, I’m aware of no tube amp that can output the current that a well-built SS amp can deliver. If anyone knows of a tube amp that can deliver more than 30 amps of a current let alone 60 amps, please let me know what amp it is.
there are positives and negatives of either approach---to insinuate otherwise is incorrect imo.
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