THD Hotplate Attenuator (8 Ohm)

the hotplate is an attenuator designed to allow you to crank your tube amp to stage levels and then attenuate the volume down to usable levels. the hotplate accomplishes this by using a series of load resistors and other components that are used to dissipate the power of the amp--part of this is done via a fan that is activated by the output of the amp, and part of this is done through a lamp which also acts as part of a noise-reduction filter.
operation is very simple. there are basically two knobs and two mini-switches on the front of the unit. one knob is an attenuation knob that increases attenuation in discrete steps of 4dB--there are settings for 0dB, -4dB, -8dB, -12dB, -16dB, and load. when the control is set to the -16dB setting, the second knob becomes active--this second knob is a continuously variable attenuation knob that allows you to dial in anywhere from -16dB of attenuation to full attenuation. with the left control set at "load," you can completely unplug the speakers from the amp and use the amp as a giant fuzztone into a power amp or into another amp. the two mini-switches activate "deep" and "bright" boosts, and are useful for adding back both low and high end when heavy attenuation is used. evidently, this compensates for the ear's tendency to hear less lows and highs when the volume is very low and works very similar to the loudness control on a home stereo system.
there are four jacks in the amp. one is for the input from the amp, while there are two speaker output jacks. the last jack is a line out, which is also accompanied by a knob for setting the line-out level--this allows you to slave your amp into another amp or power amp. the deep and bright switches are interactive with the line-out, however: engaging the deep or bright switch will cause a DECREASE in deepness and brightness in the line-out signal. be sure to look at the jacks carefully--you can do some very major damage if you mix up the jacks. also, be sure to use very high-quality SPEAKER cable when connecting the hotplate--using an instrument cable will fry the cable and will likely damage your amp, the hotplate, and perhaps your speakers, as well.
the manual is very well-written and comprehensive.
one more thing...the hotplates are designed to work with specific impedances. you CAN use them with different impedances, but ONLY if the speaker cabinet you're using is rated at a HIGHER impedance than the hotplate. in any event, the amp should be set at the hotplate's impedance level or LOWER--setting the amp any higher than the hotplate's impedance level will nuke the unit.

the THD gives me mixed feelings. every single attenuator colors the tone of the amp, and the hotplate is no different. no matter where the attenuator is set, there is some coloration, though the deep and bright switches help to offset this effect. however, i can honestly say that even at -4dB, there is enough coloration to make a difference--specifically, there is a loss of both high and low end, and i would liken the tonal effects as to having several sheets over the speaker. the bright boost adds a bit of scritchy top-end to the tone, and it's not a frequency that i like very much--the deep switch is more effective. THD has designed these switches well, however, in the sense that they INCREASE in effect as you INCREASE the attenuation. the hotplate works very effectively at what it's supposed to do, but there is a tonal compromise. in the future, i'll continue to bring the hotplate to gigs, but if i have the opportunity, i won't be attenuating. i hate to give this analogy, but using the hotplate is in many ways like using a condom. it's still the same in many ways, but you do lose something.
as far as tone and sound quality goes, i've heard many times that the maximum attenuation threshold on the THD is about -8dB before you really start to lose tone. that's about right--after -8dB, you do get some severe tonal coloration. bear in mind that these units were meant to tame extremely loud amps such as marshall plexis down to CLUB-approved volume levels. while you can actually tame a 100-watt tube head down to bedroom levels, there is a severe loss in tone when you do that, so keep that in mind. compared to the other units available out there, such as the scholz power soak and the marshall powerbrake, the THD is clearly superior, though it definitely does color the tone. i'd be interested in hearing the kendrick airbrake if i could get my hands on one.

look solid as a rock. the casing is very heavy-duty, and the components inside look top-notch. i wouldn't worry about gigging with it.

i've talked with THD's owner, andy marshall, once, and he was very helpful and knowledge. he was also very patient in answering my questions and gave me some guidelines for using the unit.

overall, i'd say the THD does what it does well. my opinion is that it tends to work better on non-master volume amps, as using it with master-volume amps tends to introduce yet another control valve for the amp. the attenuator with a master volume-equipped amp is in many ways like adding another attachment on your garden hose--you can't not expect that to color the tone. on the other hand, with a non-master volume amp, these units work great. you can use the hotplate on a marshall plexi or an old fender amp and get the wonderful power-tube distortion tones without losing your hearing or getting thrown out of the club.
if i had to buy it again, i'd do it again, but i'd limit the use to non-master volume amps.