Guitar amp dummy load box - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: November 12, 2019, 10:42:18 pm
content: Hello,I am thinking of making a simple dummy load box. It will have a simple resistive load, for example 8R. At this stage all we want is to (over)drive the output valves of the guitar amp, and we do not care about dynamic performance on a real cab and output transformer etc.Further, I would like to tap the signal on that load for further processing, eg a small 5W amp, or maybe headphones, line out etc.I have made something like the (simplified) below, but am not sure I am doing it right.Am I on the right track? In particular the way I tap the signal with different transformers for different selected cabinet loads, does it look OK?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 12, 2019, 11:16:33 pm
content: Just a couple first thoughts.- It's probably obvious, but don't forget to properly select the load resistors so they can safely dissipate the power they will have to dissipate.- If you're going to use a transformer for the output signal, why not take advantage of this to get a symmetrical output so you can connect it to an XLR connector. A clean way to further connect the signal to some other audio gear.

author: gbaddeley
author_rank: Regular Contributor
timestamp: November 13, 2019, 01:49:39 am
content: 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.

author: Teledog
author_rank: Regular Contributor
timestamp: November 13, 2019, 05:40:20 am
content: I agree with gbaddeley.You probably just need a couple of 8R 20W power resistors (or less ..with heat sinks)You shouldn't be maxing out the amps unnecessarily anyway, unless you like paying for expensive tubes/parts.Switch/wire them for 4/8/16RNot sure why anyone would use transformers for power amp testing..I've never seen that before, perhaps 'scoping the output / load, but that's about it.This guy just has a ~10W 4R resistor permanently soldered to a 1/4" jack for most of his testinghttps://www.youtube.com/channel/UCRs15W2eE8Ugg4kXxOz0hSQ{edit again} ..perhaps I'm wayy off base here..I'm assuming you want to hear/monitor the sound when the amp is overdriven, without the volume to annoy the neighbors?By using a fixed load, and tapping it off, you won't ever hear what it actually sounds like with a speaker.Speaker impedance varies slightly with frequency..and the speaker itself.Plug in a Marshall speaker, then try the same with a Celestion..way different sound..There's some L-pad/attenuators for amps..but haven't heard any rave reviews on them.Have a master vol mod installed on the Bassman 50 - I like it, but it's used wiselyG'luck!

author: akis
author_rank: Frequent Contributor
timestamp: November 13, 2019, 08:02:55 am
content: I will summarise hoping to answer the questions.We have a couple of very expensive valve guitar amps. A large part of their sound characteristic comes from their output stages, both at normal levels and overdriven.Under normal listening conditions indoors, we play very quietly, thus missing out on the true performance.I want to design a dummy load box to allow us to drive the output stages, while still using the same cabinet/speakers, but at comfortable listening levels.Aware of the limitations of this setup, we have run tests with hard wired resistors etc and are very satisfied with the result. Additionally, we have decided that 5W is the absolute maximum we ever want to send out to the cab/speakers.The problems:I cannot think of a way to send only some of the signal to the speakers, and with a volume control independent of the guitar amp, other than by tapping it off of the load resistor and then re-amplifying it with another amp. I have thought of using bulky rheostats but I can't come up with a decent, totally passive circuit.Since we only need 5W to fill a normal room and still annoy the neighbours, I only need a small little amp inside the box, like the LM1875 for example, it will be plenty.I use a small, step-down transformer, with a windings ratio according to the selected load, 4R, 8R or 16R (that is why there are three primaries on the schematic), so as to keep the tapped signal level to standard +4dBu (~ 1.25V RMS).As my first question I am just not sure I am doing the tapping properly, it simulates OK, but maybe there is a better way?

author: 001
author_rank: Super Contributor
timestamp: November 14, 2019, 10:51:47 am
content: Quote from: akis on November 13, 2019, 08:02:55 amWTF?Spend money to heat resistor? Resistor is not a loadspeaker anyway.U need some wisdom to buy small amp

author: gbaddeley
author_rank: Regular Contributor
timestamp: November 16, 2019, 10:21:48 am
content: You don’t need to reamplify. Across the high power resistor load, add the speaker cab in series with a high value resistor, say 47 or 100 ohms. Add Rheostat in series too, if you want some I dependent adjustment of volume.

author: akis
author_rank: Frequent Contributor
timestamp: November 16, 2019, 04:05:32 pm
content: Yes but as a speaker has wildly different impedance vx frequency, so adding a simple resistor in front of it will also alter the attenuation of different frequencies. For example my speakers (eminence tonker) go from 8 Ohm to over 300 Ohm, depending on frequency, and it's not even gradual or linear.

author: Audioguru again
author_rank: Regular Contributor
timestamp: November 16, 2019, 05:56:11 pm
content: A resistor in series with a speaker alters its frequency response and alters its damping. With the poor damping then it sounds like a bongo drum.The added output transistor also alters the sound.

author: gbaddeley
author_rank: Regular Contributor
timestamp: November 18, 2019, 05:47:07 am
content: Quote from: akis on November 16, 2019, 04:05:32 pmTrue, but your 300R is extreme. Most speaker cabinets would be fairly smooth rated R in range ~200Hz - 2KHz with gentle rise above that. The max R is at bass resonance and would usually be not much higher than 3-4 times the nominal R.The series R that I suggested actually gives a slight bass boost on normal cabs due to underdamping (look at speaker theory) but this may not be audible at lowish volume.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: November 18, 2019, 06:58:18 am
content: Quote from: gbaddeley on November 13, 2019, 01:49:39 amI 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?

author: gbaddeley
author_rank: Regular Contributor
timestamp: November 18, 2019, 12:14:59 pm
content: Quote from: TheUnnamedNewbie on November 18, 2019, 06:58:18 amWith 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.

author: calexanian
author_rank: Super Contributor
timestamp: November 18, 2019, 09:06:53 pm
content: 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!

author: gbaddeley
author_rank: Regular Contributor
timestamp: November 19, 2019, 12:25:54 pm
content: 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.

author: darrellg
author_rank: Unknown
timestamp: November 20, 2019, 06:22:06 pm
content: 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.

author: akis
author_rank: Frequent Contributor
timestamp: November 21, 2019, 07:59:10 pm
content: I have thought about muting the speaker. I thought that the enclosed air inside the sealed box will affect the cone travel and general feedback behaviour compared to open air? Also some cabs are quite large, you'd need a very large box to house them in and not very practical, compared to small a settop box, like the Mesa Cabclone (which I do not like BTW).I had thought to take a speaker, cut out most of the paper cone, leave a few bands/stripes to keep it suspended from the outer rim, it will also give us some form of silent speaker emulation and present a very similar load to the amp? We could tape over the remaining stripes to stiffen it and account for the difference in feedback to the coil from the whole cone in air. Of course it means destroying the speaker, but at £80-100 is a great deal cheaper than other solutions.Another very important factor is the actual speaker which gives a very characteristic sound, not overdriven but just by its nature, for example there is a world of difference between the Tonkers, the Stonehenge and the Jensens we have tried here. The Stonehenge in particular really stands out it is so different.

author: Teledog
author_rank: Regular Contributor
timestamp: November 27, 2019, 12:46:35 am
content: A few attenuators here (excuse the 3rd party links);https://www.thegearpage.net/board/index.php?threads/speaker-soak.1418300/