THD hotplate

Anyone seen a schemo for something like this?
http://www.thdelectronics.com/products/hotplate.htm

Cheers, :sam:

Tony.

I think there was a thread about this some time ago. I don´t know if it was in the old forum. Anyway, someone (NYDave?) came with a schematic for something like this.

Isn't that basically what Tom Scholz's PowerSoak is?

I have got a marshall power brake which is basically the same thing but without the filter caps..
works well getting my 50watter down to bedroom level (even though on 1 it is still very loud)

This link shows all the types of soaks and their variants
http://www.amptone.com/guitaramptable.htm#poweratten

Here is the DIY for one..
http://www.amptone.com/g243.htm

the THD seems the only one giving "decent" results for studio purposes.
Never really liked the Marshall powerbreak. Maybe i should built one and experiment, although the filters in the THD are what it is all about i think.

Thanks for the links! :guinness:

I have SOMEWHERE a simple design which used a combination of wirewound resitors in the form of heating elements and projector lamps.
I will look for it,
Steve

Muchas Gracias Steve!

The crap on the left simulates loudspeaker impedance. This is critical when loading many tube guitar amps because of their high output impedance. It also obviously must dissipate all the power coming off the amp, 50-100Watts in the 8 ohm resistor.

Values are VERY approximate. The 50 ohms may be too low for a cloth-surround gitar speaker. The LC network should approximate the bass resonance of a gitar speaker, but probably not exactly match it to allow for acoustic effects. All of this needs a LOT of research and ear-tuning.

What seems to be special is the "loudness" function. If you play soft you can't hear bass. So you don't get the "loud" sound at low volume. Also if you are checking tone at "11" volume, there will be a lot of hiss from the amp. They have a very clever idea. An incandescent lamp makes a power-controlled variable resistor. Put a frequency-selective load on it, and it may be flat at low power level but EQed at high power. Also when you stop, the lamp goes cold and reduces the hiss. I need a LOT more coffee to think how the lamp and caps go (I may be totally ack-bassward here), and they have some switches too, and a proper Line Out. But this gives a very general idea.

It is possible they use the lamp AS the main dummy load. Stick a 120V 60W lamp on a 15Watt 16Ω tube amplifier and play: it gives a good 15Ω load at low level and a higher load (and glow) at high level. So it can't be that simple, but they may have found a cleverer plan than my hasty guess.

Yeah, you could DIY it. Yeah, you could spend hundreds of hours trying to get the effects to come out right. If you don't understand speaker impedance, you could burn-up a few amps along the way. The retail price isn't cheap, but I think it is cheaper than DIY.

Or you could borrow one and copy it, but that would be stealing. Not illegal, but certainly amoral. It's not a trivial or "obvious" design. Give the dudes some respect for their work.

Check this out:

http://www.webervst.com/mass.html

Here's some good info on designing reactive loads for guitar amps

http://www.aikenamps.com/spkrload.html

Give the dudes some respect for their work.

Click to expand...

I must agree. Knowing Andy Marshall (owner of THD) as I do, not only is he a smart guy but a great dinner guest and an oud (sic) player to boot.

His old babysitter was THE George Massenburg.

> good info on designing reactive loads for guitar amps http://www.aikenamps.com/spkrload.html

Yes, his models have a more accurate theory than mine. His values may be more typical of gitar speakers (but there will be wide variations, and you may have to decide what you want to emulate before you can pick exact values). There are two impedance maximums: I've just used 50Ω to set both, he has separate 47Ω and 68Ω resistors to set each max, and he lays-out the branches accordingly. His second plan accounts properly for DCR and approximates eddy current losses.

This just gets the electrical impedance (almost) the same as the terminals on a speaker. The voltage/sound response of a speaker is complicated and must be modeled to get the expected tone. The bass resonance has a huge effect on impedance but less effect on acoustic output: say 15dB impedance rise but 5dB acoustic rise. So we either need to tap (or damp) the bass resonator, or add a bass dip after this network. The cone breakup above 1KHz gives (in most gitar speakers) a general rise followed by a fall-off, punctuated by dozens of narrow resonances. If your final output is another speaker, some of these will happen there, but if you take a line-out to a good hi-fi speaker, you won't have guitar-y top end.

And that is without considering directional response, which is crazy in the top of any speaker, and quite odd in the bass of an open-back big twin.

thanks for the links everyone. :thumb:

PRR, thanks for making me understand how these things work (or should work :grin: )

I'll dig deeper in that in a few days, have a session going that needs attention.

Didn't know George was babysitting technicians,
thought he only accepted Linda Ronstadt invitation to sit :grin:

Lucky him. Andy that is :green:

Finally, i didn't want to steel anything at all, i'm lucky still capable of buying a hotplate if i need one. Wasn't all that much into the powersoaking (compared a load of em and really didn't find what i looked for unfortunately) until somebody pointed me to the THD (what i very briefly tried a while ago, but need more time and other amps to get convinced...) I am known to be pretty anal about gtrssounds, sorry
Would make a nice box to build, inspired or not by the THD.

If it is so good as what a friend of mine thinks about it, then I WILL rush to the musicstore to get me one... untill then i remain a little sceptical towards ALL those gizmo's.
Hopefully time (and THD) will prove me wrong as i need something what does what "they" say about powersoaks if i don't want to end deaf as a tree.
Just a pitty that they come in different versions depending on the impedance you want to use them, making that in case they're fine i need at least 2 or 3 for our studio...

Peace!

Tony

[quote author="PRR"]
[/quote]
I love little expansion. But this looks like compressor.
Lamp to the ground or bridge performs better.
Lamp may be not so lazy, possible many of small lamps in paralel.
xvlk

thats essential right PRR its a blaanced reisitive network 10 watt custome wirewound resistors a 4pole 6 throw greyhill with inductive frequency compensation on the output all of that stuck into an aluminum extrudtion where the wirewound 's are incapsulated liquid ceramic into slots on the extrusion . and they has some of the funnest sayings on the bottem of the ckt boards Wil

Wilebee

actually the lamp is for compression with the newer hot plates they are switchable they tended not to sound as good with lower wattage amps ie 15-20 watt

Wil WIlebee

Tony,

If you can, check out the Webber Mass as well. I own one and can't hear any significant degredation when the cabinet is recorded loud or attenuated. I used to have a Scholz Powersoak that I would cringe at having to use live if the soundman demanded lower stage volume. The cool thing about the Webber is that it actually has a little voice-coil and magnet inside. It behaves like a speaker, because it contains a speaker. It just doesn't have a cone on it to push any air, instead it has a perforated former to provide the tension/mass reactance.

My only gripe is that the unit was rated for 100W, and I blew it out with a 100W Marshall. It was replaced with a new unit by Webber though.

I can and do use the Webber for recording, which I would never use the Powersoak for.

-Chris

amendment to my first post yes its an LC network I worked for Andy Marshall for 2.5 years as his design assistance and have built a couple hundred hotplates and Yellowjackets and yes he is a damn fine ude player hi whole family is quite talented including his brother Steven St. Crouix and his father who was a Nurologist who is some article George Massenberg quoted as say Dr Curtis Marshall was probably one of his greatest influences ( sorry if I'm mis Quoteing this GM)

Wil

Wilebee

[quote author="wilebee"]actually the lamp is for compression
[/quote]
I have correct this mistake. (Independendly to you, but thanks )
xvlk

Speaking of dinner Tim...seeing your name makes me crave some Vindaloo!!!

A few things about Hot Plates...
First off they're quite different from Power Brakes (build around a transformer) or Airbrakes (build around a L-pad).

The lightbulb is part of the noise reduction, and works an expander, not a compressor. Sorry Wil, I gotta call you on that. The rest is on though.
In old versions before we made it switchable it could be turned off by pulling th bulbs.
And I actually like it with small amps as well.

As for reliability, and how long it'll be around, the Hot Plate outsells the most popular competitor. And with 20000+ in the field we have yet to come across one that hurt any amp that was well maintained. But I sure wouldn't use one with leaky caps.