• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tube Amp with No Spk Load???

I have a Tubes4HiFi ST120 Tube amplifier (Dynaco Copy) I built a couple years ago. I have a number sets of speakers that I like to audition from time to time. It is my understanding that you are not to disconnect speakers from a tube amplifier while it is running or for some time after you have disconnected power or damage will result?
Can I build a switch box of some type that would put 8ohm resistors on the outputs with the amplifier still on, while I switched speaker leads from one set of speakers to the other.
If "yes".....What type of switch or circuit would I build for such a device?

Thanks
 
Everybody will tell you it will damage your amp.
I would have to simulate a design to see what happens.
I have seen protection diodes in guitar amps, I think they are from the plates to ground but would have to check.
Most guitar amps short out the OPT secondary if no speaker is connected.
If the NFB becomes unstable you could add a zobel network across the output to prevent this.
You could also use a make before break switch.
 
OK simulation shows that with no load and large input signal the plates of output valves can reach very large positive and negative voltages which will damage them. If you place fast recovery high voltage diodes from each plate to ground in reverse direction you solve this issue. You need them to be at least 2xHT volts and handle and few hundred ma.
 
The simplest way to switch speakers on tube amp is a make before break (MBB) switch and no need to add 8Ω resistors.
A MBB switch with current rating of 5A should be sufficient for your ST120 tube amp. However, it's best to turn the volume down to zero while switching.
 
Thought that was a tail. But actually, 100R across the output offers considerable protection according to simulation. Problems only occur if you turn up the volume to clipping levels. I think that maybe people don't hear anything from the speakers and so just keep turning the volume up. Thats when the damage occurs.
 
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Here's some handy 'Tech Talk', Rbertalotto! 🤓

Shorting or make-before-break switches ensure the next position is connected before the previous position is disconnected.

The moving contact is wider than in a non-shorting or break-before-make switch.

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1668966817058.png
 
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