Folks ask me about how to make a 100 watt amp less loud. They want the amp “singing” but they need to have the output valves starting to clip and this means LOUD.
Usually they are amps with outputs north of 60 watts. Exceptions are Vox AC30’s which are 30 watts. The discussion here is focused on amps that four or more output valves.
First thing to keep in mind is that a 100 watt amp is only twice as loud as a 10 watt amp. You would think the 100 watt amp would be 10 times louder than the 10 watt amp.
So what are your options?
A power does what it says on the tin. You connect it up to your amp output and it turns a lot of the energy which was originally going directly to the speakers into heat.
So you can have those lovely tones of your amp as if it was turned up to full whack without blowing your head off.
There is a downside though. You are always running your amp at almost full power. The valves are being made to work really hard and the output transformer as running at its full rating. The penalty is that the output valves will wear out much faster. The output transformer life may be shortened too. If you attempt to run a power soak with a vintage 19070’s Marshall amp, for example , it is very likely the output transformer will fail.
This is not just only for Marshall amps. It is true for many amps.
So is there is a compromise?
Removing Two of the Four Output Valves
A typical amp could have four EL34 or 6L6GC output valves. Removing two of the output valves would reduce the power by half. Keep i mind what I said earlier that will not reduce the volume by half but will help.
One output valve is unplugged from each of the push pull stages. Normally you would unplug the middle two valves.
However, there are more steps to be done to ensure no damage to your amp
When two valves are removed this affects the biasing on the valves and also the impedances presented to the transformer.
The impedance issue is easy to solve. If your speaker cabinet is 8 ohms then run the speakers from the 4 ohm amp output. If the speakers are 16ohm run them from the 8 ohm output.
This is a bit more tricky and depends on the schematic of your amp. It is very important to set the bias up again because there are now only two output valves present.
It is very important this is done otherwise you will cook the output valves.
Some amps such as the Vox AC 30 has no adjustable bias because, the biaising is done using a single resistor connected to the cathode of the output valves. This cathode bias resistor must be replaced with a resistor of double the value of the original value to ensure correct biasing of the EL84’s.
I would recommend consulting with good competent amp tech to deal with the biasing if you are not experienced or comfortable sorting out the biasing issue.