The THD Hot Plate power attenuator is a device placed between the speaker output and the speaker cabinet.
It acts like a huge master volume control and lets you turn up your amp most of or all the way while absorbing most of the power generated by the amplifier and turning this power into heat. It passes a small part of the power to the speaker. The THD Hot Plate is designed for use strictly with vacuum tube guitar amplifiers, and each version is optimized for a specific impedance (4,8 or 16 ohms.) These inductive loads are frequency compensated, which means it will give you the best possible sound for that impedance speaker and amplifier combination.
How does a Hot Plate work?
A THD Hot Plate is a tuned network of capacitors, resistors and inductors which adjusts the overall EQ as the volume is turned down to compensate for the human ear's frequency response. Your ear perceives sounds differently at different volumes: the louder the sound, the more sensitive your ears are to highs and lows. As the volume drops, your ear becomes more sensitive to the mid-range, and less so for highs and lows. The Hot Plate compensates for this, working like the Loudness switch on a hi-fi. The THD Hot Plate is the first, and for now, the only attenuator that is frequency compensated.
Why do I want one?
Guitarists use attenuators because their amps sound good at high volumes, especially with full distortion, but in rehearsal, small clubs, or at home, they can't turn the amp up. They are also used in recording studios for better separation between the instruments, and for use when the speaker sounds best at low levels, or if it can't handle the power output. The Hot Plate also makes a perfect dummy load.
THD needed a good attenuator for in-house testing of their amplifiers. The amps have to be tested at all volumes, clean and pure to wild distortion, and they were tired of their ears hurting. They tried all the attenuators on the market, but they muffled the sound and made even THD amps sound dull. Now THD's design engineers don't have to worry as much about hearing loss, and neither do you.
How do I use it?
Connect the speaker output of the amp into the input of the Hot Plate, then connect the speaker cabinet to one of the two speaker outputs of the Hot Plate (doesn't matter which one). The amp thinks the Hot Plate is a speaker, so the sound stays true even when you turn the volume down.
What else does it do?
Built-in Noise Reduction
Adjustable Line Out
The THD Hot Plate is the only attenuator on the market with built-in noise reduction to reduce the hiss and hum between notes. THD has designed a passive, single-ended noise reduction system which provides approximately 10 dB of broad band noise reduction without gating or pumping, and without affecting the tone of the amp. A by-product of the noise reduction circuit is that it generates light as you play. The harder you play, the brighter it glows.
The Hot Plate is also the only attenuator offering a Bright switch and a Deep switch for tailoring your sound. The Bright switch gives you two different high frequency levels so you can compensate for an overly bright, or dull speaker cabinet. The Deep switch offers two distinct bass settings to help you fill out the bottom end, or reduce the bass in a cabinet with too much low end.
THD Hot Plates also feature a Line Out, which is adjustable by its own volume control giving it a wide range of applications. At higher settings, it can provide enough signal to drive the input of a separate power amp for slaving. The middle range of settings is useful for most rack mount effects. And turned most of the way down, the Hot Plate's Line Out will drive the instrument input of another guitar amplifier for extra power and volume.
Will it hurt my amp?
The THD Hot Plate will not damage your amplifier. When you play continuously at full output, you cause your tubes to age more quickly than they would at lower volumes. Using a Hot Plate will maintain the life of your tubes at exactly the same rate, no more or less, as when you play straight through the speaker.
Using a Hot Plate will also not hurt your transformer any more than playing through a speaker, as long as the impedances are matched (i.e. 8 ohm setting on the amp, 8 ohm speakers and an 8 ohm attenuator). If you are using a well-made amp, then the transformer should last indefinitely, regardless of whether you are driving a speaker or attenuator. If you are using an inferior amp and the transformer blows, it would have done this whether you play through a speaker or an attenuator. The Hot Plate puts the same load on the transformer as a speaker (which is why it makes such a good dummy load).