Is a Tube Amp Capable of High Current?

Thanks Everyone!

It seems then, if I'm understanding all of this correctly, that high current is what's needed to push the power into highly resistive or low impedence loads? So if my speakers stay somewhere close to 6ohms w/o much in the way of wicked impedence curves, I should be fine with just some high wattage, correct?

The ARC amp that I'm in the process of buying weighs close to 100lbs so I'm presuming that it has the kind of HUGE output transformers that will be required.

Here's another question...If my speakers are 6 ohms, should I use the 4 ohm or 8 ohm taps? Should I be conservative and go with 4?

Thanks again for all of the insightful advice. I think I'm starting to get it.

The idea of a 'high current' amplifier is a bit mythological. If your speaker is a 6 ohm load (not particularly difficult for tubes, BTW), then it is easy to show, using Ohm's law, what is really happening.

Let's say that you've settled on a tube amp that produces 115 watts. Let's further say that you have a transistor amp that will produce the same power into 6 ohms. By using a little algebra (Ohm's law is Resistance = voltage/current), we can derive how much current will be flowing in each case.

Power = (current)squared X Resistance, so:

115 = (4.38) squared X 6.

This is the same regardless of whether it is tube or transistor. IOW, 'high current' has little or nothing to do with it.

You'll probably want to use the 4 ohm tap to keep the load on the output tubes at a good operating point for low distortion.

Since the amp specs are rated from its 8 ohm tap, it likely that you will have a little less power on the four ohm tap as the transformer is less efficient on the four ohm tap. The loss will be minor.

My guess is the real issue is whether or not 110-115 watts is enough power. Unless you are in a really small room (and depending on your listening habits) there's a good chance you'll be running the amp out of power if you try to play it loud.

I have a 6ohm speaker(87dB) and use a 60 watt per channel tube amp with out any issue, and from time to time I like to let it rip. btw I use the 4 ohm taps on the amp- to me it sounds better. ~Tim

As long as your nominally 6 ohm speakers don't have any dramatic impedance dips or peaks, I think the ARC's will do fine. Use the 4 ohm tap - it's better to mismatch the load on the high side of the amp output impedance rather than the low side. You'll effectively lose about 20% of potential output power.

Given the nominal nature of speaker impedance and output impedance, I doubt you'll notice any difference.

BTW Ralph - The equivalent circuit for the amp looks like am ideal voltage source in series with an impedance. This impedance represents the output impedance of the amp. It can limit current into low impedance loads since it is in series with the speaker.

Also, for those whose math is rusty, when dealing with alternating current (i.e. audio signals) the voltage, current, and impedance are complex quantities (i.e. each is in the form a+bi, where i = sqrt(-1) ). For impedances, the imaginary part is the reactive component, and the real part is the resistive component. Complex arithmetic models the energy storage behavior of capacitive and inductive elements in the circuit.

Yes, Ghostrider, the output impedance of course is a current limiting factor as more and more of the output voltage is dropped across the output section as opposed to the load as the load impedance is reduced.

However in the case of the ARC this is not a serious issue with a 6 ohm load. So the issues of a 'high current' amplifier are rendered moot, for the most part.

Were it a 3 ohm load, I think a different solution (amplifier) might be advised. There is also a guy on the ASOG (otlamp.com) named Paul Speltz who makes a torroidal autoformer (a variation of one we used to make) for this sort of application. It has taps for 4,3,2 and 1 ohm loads and sets the amplifier load at 16 ohms. Works quite nicely on those otherwise hard to drive loads that tubes don't normally like.

Allows one to use tubes with, say, Appogee Scintillas, if you could find a set these days. For the most part though 3-4 ohms seems to be the most challenging most speaker manufacturers are doing these days.