Electronics > Projects, Designs, and Technical Stuff
Guitar amp dummy load box
akis:
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?
SiliconWizard:
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.
gbaddeley:
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.
Teledog:
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 4/8/16R
Not 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 testing
https://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 wisely
G'luck! :-+
akis:
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?
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