Download Build a Power Attenuator for Your Low Wattage Tube

Build a Power Attenuator for Your
Low Wattage Tube Amp
You quit playing your 100 watt amp to avoid alienating those you love and now you play
a low wattage amp. The problem is: it is still too loud. You want to get a good tone and
can’t get it with the volume turned almost off. What you need is a little power
attenuation. A power attenuator is a device that can be placed between your amp’s output
and the speaker. This allows you to turn your amp up, but attenuate the power before it
gets to your speaker. It differs from the master volume concept because it allows you to
drive the output tubes hard. Output stage distortion is round and sweet; it is what I call
the “good” distortion. Here are the plans for making a power attenuator that allows you
to turn down your speaker by 6 db without turning down your amp. The parts to build this
attenuator will cost about $30. It features a toggle switch that will attenuate 6 db in one
position and go back to dead-nut stock in the other position. Your tone will not be
affected in the least – only the volume will be cut to about a quarter of stock power.
Theory of operation
The idea is to correct the load impedance while using a resistor matrix to cut the power
by 6 db. For now, let’s assume we are working with an 8 ohm speaker load. First, we
will put an 8 ohm resistor in parallel with the 8 ohm speaker load. This will cut the
current in half, because there are now two paths for the current to go; namely through the
speaker or through the resistor.
Ordinarily, this would change the load impedance from 8 ohms to 4 ohms. To correct the
impedance and cut the voltage in half, we will add a 4 ohm resistor in series with the
speaker/8 ohm resistor combination. This will bring the load impedance back to 8 ohms
and the voltage will be divided between the speaker/8 ohm resistor and the 4 ohm series
resistor.
When we cut the current in half ( -3db) and we cut the voltage in half (another –3db) that
the speaker “sees”, we end up with a –6 db attenuation while maintaining the proper load
impedance of 8 ohms.
If you were doing this with a 16 ohm load, you would use a 16 ohm resistor in parallel
(R2) and an 8 ohm resistor in series (R1). With a 4 ohm load, you would use a 4 ohm
resistor in parallel (R2) and a 2 ohm resistor in series (R1).
Parts List for 8 Ohm Power Attenuator
These plans are for use with a low wattage amp, perhaps 25 watts or less. For larger
wattage amps, I do not recommend making your own attenuator. You would need much
larger wattage resistors and 6 db probably will not be ample attenuation on a high
wattage amp.
R1 – 4 ohm 15 watt wirewound resistor. I recommend Mouser part # 284 – HS15 – 4
R2 – 8 ohm 10 watt wirewound resistor. I recommend Mouser part # 284 – HS10 – 8
SW1 – DPDT Toggle switch. I recommend Mouser part # 10DS238
J1 - Output jack. I recommend Mouser part# 502 – 11
P1 – 1/4” right angle phone plug. I recommend Mouser part# 171 – 1206
Chassis – metal enclosure. I recommend Mouser part# 537 – 136 - P
Heat sink compound – I recommend Mouser part # 524 – 8109 – S
Speaker wire – about 2 feet. Use 18 gauge.
Hook-up wire – about 2 feet or less. Use 18 gauge.
Mouser can be reached at 1 – 800 – 34 MOUSE (1- 800 - 346 – 6873)
Let’s Get Started Building
Look at all the parts and decide where they can go in the chassis and not interfere with
each other. Take a minute to plan. Look over the schematic in the diagram and spend a
few minutes planning – before you drill any holes. You’ve heard of the carpenter’s rule,
to measure twice and cut once. The ampbuilder’s rule is to plan it out twice and drill the
chassis holes once.
Although it is possible to build the attenuator inside the amp, I advise not to drill any
holes in a vintage tube amp. Use a separate chassis and maintain the integrity of the
vintage amp.
Notice that the wirewound resistors are heatsinkable. We will mount them inside the
chassis, leaving enough room for the output jack, switch and a hole for the input wire.
The heat sink compound is applied between the resistor and the chassis. It allows for
superior heat transfer from the resistor to the chassis. You must have adequate
heatsinking for the resistors to actually perform at the specified wattage rating!
Let’s Hook It Up and Play
The 1/4" right angle phone plug (P1) is attached to the short speaker wire and will act as
the input for this unit. The plug would connect to the amplifier’s output jack. The 1/4"
female jack (J1) will act as the output and will connect to your speaker.
When the toggle switch is in the position shown on the diagram, then attenuation occurs.
But when the toggle switch is switched to the other position, the circuit returns to stock.
I first designed this attenuator for one of my employees that played regularly in Austin,
Texas. There is a sound ordinance in Austin and my employee complained that even his
22 watt Kendrick 2112 was too loud. He didn’t need a large wattage attenuator so I
designed this for him. It has worked perfectly for him and it can work perfectly for you
too.