Electronics > Projects, Designs, and Technical Stuff
Guitar amp dummy load box
TheUnnamedNewbie:
--- Quote from: gbaddeley on November 13, 2019, 01:49:39 am ---A tube guitar amp must never be operated without a load, as it could damage the output tubes or transformer. I suggest the 16R load should be permanently connected, and switch another 16R in parallel to get 8R and switch another 8R in parallel with those to get 4R. This makes more efficient use of load resistors and is cheaper.
--- End quote ---
I know this is slightly off topic, but I have heard this wisdom said many times, but never actually heard a good reason for why. It is not really a RF amp, so I don't think any power is really 'reflected' - what mechanism destroys the amp here?
gbaddeley:
--- Quote from: TheUnnamedNewbie on November 18, 2019, 06:58:18 am ---
--- Quote from: gbaddeley on November 13, 2019, 01:49:39 am ---A tube guitar amp must never be operated without a load, as it could damage the output tubes or transformer. I suggest the 16R load should be permanently connected, and switch another 16R in parallel to get 8R and switch another 8R in parallel with those to get 4R. This makes more efficient use of load resistors and is cheaper.
--- End quote ---
I know this is slightly off topic, but I have heard this wisdom said many times, but never actually heard a good reason for why. It is not really a RF amp, so I don't think any power is really 'reflected' - what mechanism destroys the amp here?
--- End quote ---
With no load on the secondary, the output transformer no longer provides an impedance matched load for the output tubes. The tube anodes can then swing to a much higher voltage range and possibly oscillate. This can cause breakdown (arcing) in the tube, socket or transformer, and possibly permanent damage.
calexanian:
Having professionally designed a few of thees systems in the past I would highly suggest a few things.
First realize the sound from this direct out is not going to be anywhere near the actual sound of the amp due to the lack of dynamic speaker impedance. Much of the sound of the amplifier comes from that, particularly in tube amps due to their low damping factor. Look up load simulators for information on that. They re introduce the approximate response curve of a typical speaker impedance.
Secondly I highly recommend make before break switching, especially if it is to be used with tube amps. If not available as mentioned before you will need some kind of resistance across the input from the amplifier to reduce the excessive voltage spikes switching a powered speaker will cause. The voltage will ramp until it finds a path somewhere. You would prefer it not be the sockets of the tubes!! I used to use a 470 ohm 5 watt resistor and a 10 ohm 5 watt resistor with a 150 volt MOV in series with that in parallel to the 470 ohm to give two levels of protection from big spikes. I would put that arrangement right at the input from the amplifier.
Apart from that remember your thermal considerations. I liked to use 4X wattage in resistors. Generally a fan was not required. Also I would use screw terminal type resistors as PC mount ones tend to un solder themselves at times!
gbaddeley:
Agree with previous post. If you need the sound of the amp & cabinet combination driven to a certain volume, there is no easy way to simulate it at lower volume. Heck, at higher volumes, the room acoustic and feedback to the guitar become part of the overall sound. Maybe you should try a modelling amp that can produce “big” sounds at low volume.
darrellg:
The proper way to accomplish this is with an isolated speaker. Here is an (expensive) example of what I'm talking about: https://www.sweetwater.com/store/detail/SilentSisTan--rivera-silent-sister-75-watt-1x12-inch-isolation-cabinet-tan. It's a speaker in a box with microphone mounts. The box is insulated to contain the sound. You plug your amp output into the speaker in the box so you can turn your amp up to a level that allows it to distort as needed, and it gives you the frequency dependent impedance that only comes from a real speaker.
There are also cabinets that will hold the entire amp, but they tend to cause overheating problems. Some venues will have separate rooms for guitar amps. Both of there solutions make it difficult to adjust the amp during a performance.
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