THD hot plate serious question. DID IT KILL MY AMPS?

[Fantomaz]

Hi guys,

I still have two venerable thd hot plate that have served me well as attenuators over the years.

I sometimes use them as load boxes/di with an IR loader in the computer for recording.

I always use them on the "load" position when no speaker is connected as per the manual.

I recently had an unruly client that changed the settings during a multi amp recording session and set the hot plate on the "-8db" position on an EXPENSIVE 100w amp and "-12db" on another EXPENSIVE 50w boutique amp without me noticing.

We did a full 4 hours recording session, I went to shut the amps down and went BALLISTIC at the client when I saw the dangerous settings on the hot plates, the amps are mine.

Then I listened to the tracks, I know these amps very well and have recorded them a thousand times with the hot plates as dummy loads. I listened again and again, and compared to previous tracks recorded with the same amps, and I HAD to conclude that the amps sound BETTER when the hot plate is on -8db or -12db.

By now you must have realised that while I have a really good set of ears and I have been recording a long time, I am no electrical engineer.

So here is my question: what happens when you run a dimed 100w tube amp in a THD hot plate that is set to lower setting than "load" without a speaker connected?

Both amps have survived the experience and sound just as good as before, but I wonder, did I come close to transformer meltdown, or is it ok to try it again? Is there a risk for the amps? Am I cooking my output tubes faster than normal?

Thanks for your help!

[mhuss]

By putting an attenuator on a setting that expects a load, and not hooking up a load, you're presenting a higher impedance to the amp, which puts a bit of strain on the output transformer and output tubes. As you've observed, doing this isn't an instant death sentence, but it's not good for the amp. The closer you get to "-0db" or direct, the more stressful it is for the amp. You might try connecting a dummy load where the speaker is supposed to go, and see if the sound is still the same. If you replace a 16 ohm speaker with a 16 ohm dummy load, the amp will be happy on any attenuator setting.

[Fantomaz]

thanks for your reply!

Is this the kind of stuff we are talking about?

https://www.tube-town.net/ttstore/high- ... anguage=en

I will try that in the speaker out of the hot plates.

I have both the 16ohms and the 8ohms model, I have modern stuff too (fryette, suhr, two notes, tone king, ox etc) but I still like these hotplates better on some amps.

Thanks a lot!

I will let you know if the lower settings with the dummy load improves the sound, it is never too late to learn a new trick!

[Reeltarded]

That isn't how it works. There is an impedance mismatch when not in LOAD selection. The farther you get is lower than expected load.. -12.. is less.. -8.. even less.

Adding a dummy will act like a speaker. The load won't change, just the attenuation. The sound will only change because of resistance vs speaker inductive shift.

Hotplates are not great for 100w amps. If mismatched low it is hammering Hotplate beyond it's capacity of dissapation.

[Fantomaz]

Thanks for your reply,

So, just to make sure, you say that it is ok to set the hotplate 16ohms at a lower than "load" setting AS LONG as I have a 16ohms dummy load in the the speaker out and it won't damage the amp? Is that correct?

I have 7 tube amps in the studio connected permanently to different attenuators/reactive loads, the idea is for my clients to chose quickly the tone they like and then we decide if we use speakers + mics or go direct + Irs. (these days the Irs get used more and more)

So I guess I should leave a 16ohms dummy load connected permanently in the speaker out of the 16ohms hot plate and the same thing with a 8ohms load in the speaker out of the 8ohms hot plate to avoid future mishaps.

You say the hot plate is not good for 100w amps if not set on load, even with the dummy load in the speaker out, is that correct?
The amp I use it with is a "Bruce Zinky" Fender custom shop Tone Master 100w head that has a sweet spot with the master around 8, should I use another, more modern reactive load with that amp?

Tonally, the Tone Master works great with the 16ohms hot plate when the amp is set it on 16ohms; but it does get pretty hot and the lights on the hot plate do light quite bright.

Thanks in advance,

[Reeltarded]

Read the manual. On attenuation settings the device can eat a 100w amp, that is provided a compatible load (speaker or otherwise) is plugged in.

As a dead load without speakers or a resistive external device, the capacity of the unit is around half of peak.

RMS is a hard number to actually know without measuring each amplifier, but a 100w gain amp can peak far above RMS. My 50w amps knock on 120-130w meters, frequently.

I am hyper paranoid. I ate two of those boxes operating inside stated paramaters in the same week. They changed the way they were manufactured but I never trusted one again with 100w amp. or a dimed 50.

My point was really to say that your mismatch sounded better to you but is only available by running the unit is an unprescribed way that is dangerous to the Hotplate AND the amplifier. If you plug in a dead load and set attenuate, the sound will not be like your dangerous mismatches.

For fun, plug a speaker cable into the speaker in on Hotplate and meter the impedance through the attenuation settings. You will see load changes on every click.

[wpaulvogel]

The Tone Master is a solid state amp, isn’t it?
Solid state amps can be operated with a high impedance load and won’t cause any damage. It’s when they are operated under their rated load capacity that causes problems. Example: 100 watt @ 4 ohms rating. Don’t attempt to operate it below 4 ohms like 3 or 2 ohms, 8 ohm, 16 etc is fine and just creates less output power. Solid state drives are current drivers that operate on a specific voltage rail without the use of an impedance matching transformer. The greater impedance only limits current output and that lowers the power output. On tube output sections, using an impedance matching output transformer, the increased impedance causes the output voltage to rise significantly because of the inductance of the output transformer. This high voltage generation is what damages the transformer because it rises past the insulation limit of the winding separation and the cross conduction creates paths for electrical leakage within the transformer. Once this occurs, the transformer is almost always “leaking” internally and therefore creating a short circuit.

[mhuss]

Yes, a solid state amp is more forgiving on the high impedance side. With the rare exception, you can even run a SS amp open-circuit with no damage. Modern high quality SS amps have built-in protection that will cut the output when a very low output impedance is detected.

Tube amps are more forgiving on the low ohm side. A short circuit (0 ohms) is easier on a tube output transformer than an open circuit is (but still not recommended!)

Many output attenuators (including Hot Plates) have coils and caps inside to more accurately model the impedance of an actual speaker, and some explicitly recommend against use with a solid state amp. (From the THD Hot Plate manual: "The THD Hot Plate is designed for use strictly with vacuum tube guitar
amplifiers...")