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
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
Playing a signal through a valve amp when no load is connected is to be avoided.
I'm sure someone will come up with a circuit diagram, but a 'make before break' switch would be desirable.
I'm sure someone will come up with a circuit diagram, but a 'make before break' switch would be desirable.
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.
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.
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.
I was told that p[utting a 100 ohm resistor across the speaker output terminals would not degrade the sound and would protect the output transformers....??
It is very cheap insurance. 50 cent resistor or $100.00 transformer....................Doesn't require much thought.
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.
Thank You!....Very helpful....I think a simple 4PDT "make before break" toggle switch would do the same thing? But if I protect the output transformers with a couple 100 ohm resistors, then a "break before make / center off" switch would also work?
Try and use a good quality switch, it does not take much to affect the audio quality with the low impedance.
Toggle switches don't come with a 'make before break' option - that only applies to rotary and slide switches.
As long as the output transformers are now protected with resistors, I guess it doesn't matter any longer. A simple "break before make" will be fine.