I have found a few threads that briefly address this issue, but they are not entirely clear. My output transformer has 3 different speaker taps for different impedance speakers. I notice that Decware sells amps with such a switch and further they recommend switching the impedance "on the fly" to see which sounds better with any set of speakers. Is it as simple as using a make before break switch? The threads I have seen appear to make the issue much more complicated, but they all seem to be using break before make switches. Can anyone give me assurance that a make before break switch on its own is sufficient? Or tell me what I need to do to make it safe to switch output taps "on the fly"?
Thanks for any suggestions.
Thanks for any suggestions.
"Is it as simple as using a make before break switch?"
In theory yes. Assuming you are speaking about a rotary switch, also known as a shorting switch. But please don't use a cheap Chicom rotary switch. Use a good American ceramic style rotary switch like a Centralab. These are available from places like eBay. And you can get them in multi pole configurations and double (or triple) up on the contacts to assure a good solid connection. You don't want your amplifier left unloaded because of a poor switch. Also be careful not to do something like putting a four ohm speaker on a sixteen ohm tap. Your output tubes will be quite unhappy.
In theory yes. Assuming you are speaking about a rotary switch, also known as a shorting switch. But please don't use a cheap Chicom rotary switch. Use a good American ceramic style rotary switch like a Centralab. These are available from places like eBay. And you can get them in multi pole configurations and double (or triple) up on the contacts to assure a good solid connection. You don't want your amplifier left unloaded because of a poor switch. Also be careful not to do something like putting a four ohm speaker on a sixteen ohm tap. Your output tubes will be quite unhappy.
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If your amplifier has triode output, or pentode with negative feedback, use non shortcircuiting switch.
If your amplifier has pentode output, without negative feedback, use a shortcircuiting switch.
This is because pentodes behave as current sources, and they strongly dislike to have its output open circuit. Any sudden transient may cause arcing in tubes, transformers and wiring.
ITOH triodes are close to voltage sources that will be hungry with its at output short circuit.
If your amplifier has pentode output, without negative feedback, use a shortcircuiting switch.
This is because pentodes behave as current sources, and they strongly dislike to have its output open circuit. Any sudden transient may cause arcing in tubes, transformers and wiring.
ITOH triodes are close to voltage sources that will be hungry with its at output short circuit.
Make before break does temporarily short the output of the amplifier.
I would not try this on some amplifiers, unless the signal to the amplifier is turned down.
That output transformer primary can go from 5k Ohms to DCR Ohms of the primary.
Most amplifiers will not care; but look at the design to analyze the plate current during the near dead short while the switch is shorting two secondary taps in the "Make" connection as it goes on its way to connecting only one secondary tap.
I would not try this on some amplifiers, unless the signal to the amplifier is turned down.
That output transformer primary can go from 5k Ohms to DCR Ohms of the primary.
Most amplifiers will not care; but look at the design to analyze the plate current during the near dead short while the switch is shorting two secondary taps in the "Make" connection as it goes on its way to connecting only one secondary tap.
Open circuit (un-loaded) output tap for some tube amplifiers makes them into an Oscillator.
Other amplifiers with no load, and the volume turned all the way up, may cause arcing of output tube(s), output transformer.
Your Tube Amplifier Mileage May Vary.
200 Ohm loads on an 8 Ohm tap . . .
Suppose the output stage damping factor is 3. When the 8 Ohm tap sees a 200 Ohm load, the gain will increase:
8 / 3 = 2.67
200 / 3 = 66.7 That is an extremely light load, it might as well not even be there.
The voltage might increase by almost as much as 3 x. That 3 x voltage increase was normally not planned for in most tube amplifier designs.
A Make before Break switch has to handle the Peak currents caused by the switch shorting out the secondary windings.
8 Watts rms into 8 Ohms = 11.3V Peak. 11.3V Peak / 8 Ohms = 1.4 Amps Peak.
A lot of "8 Ohm" loudspeakers have minimum impedance of 6 Ohms.
11.3V Peak/ 6 Ohms = 1.88 Amps Peak.
A 32 Watt amplifier will have 2.8 Amps Peak into 8 Ohms, and 3.77 Amps Peak into 6 Ohms.
Make sure the switch contacts will handle the shorted current condition, and the running current condition.
Other amplifiers with no load, and the volume turned all the way up, may cause arcing of output tube(s), output transformer.
Your Tube Amplifier Mileage May Vary.
200 Ohm loads on an 8 Ohm tap . . .
Suppose the output stage damping factor is 3. When the 8 Ohm tap sees a 200 Ohm load, the gain will increase:
8 / 3 = 2.67
200 / 3 = 66.7 That is an extremely light load, it might as well not even be there.
The voltage might increase by almost as much as 3 x. That 3 x voltage increase was normally not planned for in most tube amplifier designs.
A Make before Break switch has to handle the Peak currents caused by the switch shorting out the secondary windings.
8 Watts rms into 8 Ohms = 11.3V Peak. 11.3V Peak / 8 Ohms = 1.4 Amps Peak.
A lot of "8 Ohm" loudspeakers have minimum impedance of 6 Ohms.
11.3V Peak/ 6 Ohms = 1.88 Amps Peak.
A 32 Watt amplifier will have 2.8 Amps Peak into 8 Ohms, and 3.77 Amps Peak into 6 Ohms.
Make sure the switch contacts will handle the shorted current condition, and the running current condition.
Thanks everyone for your insights on this. I can see that I have to think about this a bit more. With respect to the shorting, it seems to me that the 4 and 8 ohm taps are already connected together on the secondary winding. By connecting them both to the speaker at the same time, aren’t we only temporarily reducing the length of the secondary coil between them? Wouldn’t the transformer still be subject to the speaker load? Am I completely out to lunch with respect to the functioning of the transformer? (I readily admit that could be the case).
It's probably quite safe to switch taps as long as there is no input signal. As far as I know, the bad stuff usually happens when the amp is pushed into clipping without a proper load on the transformer. I burned a guitar amp OPT once by turning the volume up without a speaker connected, while I have hifi amps that has survived open outputs.
I guess there must be some kind of overload situation involved when OPTs are destroyed by open outputs, otherwise we wouldn't be able to use plate chokes and interstage that operate more or less unloaded?
I guess there must be some kind of overload situation involved when OPTs are destroyed by open outputs, otherwise we wouldn't be able to use plate chokes and interstage that operate more or less unloaded?
AFAIK, shorting out just a part of one winding is enough to turn the rest of the transformer into an expensive resistor (with some leakage inductance, probably).
Personally, I could possibly see myself doing this on a small SE triode amp but not on something that runs on high voltages and produces real power. Or has expensive output transformers/tubes.
How about a standby switch that cuts the HT? Without plate voltage it should be perfectly safe to disconnect the speakers.
This is based on calculations by @6A3sUMMER. Not sure if 33Ω is too low, when switching from center to 4Ω tap before the contacts open, the speaker is connected to the 4Ω tap, while the 33Ω resistor is still connected to the 8Ω tap.
Hoping someone can give some advice.
Hoping someone can give some advice.
Since I don't change speakers much, I connect the speaker directly to the amp's output terminals. Problem with a switch is that at the low impedances involved, any contact resistance can have an effect on system performance.
No, When switching from center to 4Ω tap before the contacts open, the speaker is connected to the 4Ω tap, while the 33Ω resistor is still connected to the 8Ω tap.
Center position
Make-before-break
4Ω position