Tube Amp Peak Ratings?

[Psycho Bass Guy]

I design my amps so that when they ship, they will do max rated power at about 10% THD. That doesn't mean they won't do better, but when you have to sell something at a reasonable price, and have it be reliable, sometimes you have to bias it a little on the cold side.

I meant no insult to your designs. I can personally attest that they can do their rated power at 1%THD or less, and I also follow your reasoning as to why you bias them for higher distortion. Since I get to 'keep an eye on' most of the amps I've worked on, and I'm absolutely anal about function, I run tigher tolerances. I was simply pointing out that the figure of 10% THD is NOT a necessary function of a tube amp. That's important because seamonkey always erroneously insists that tubes are inherently more prone to distortion that transistors.

Another problem I've run into is that if you design a tube amp to do rated power at 1% THD, when a customer tries to overdrive the power amp, the power tubes will be running out of spec.

Absolutely. That's why it's so important to have good tubes.

Even with feedback the cabinet will have a dramatic effect on the output response. Add a ton of feedback, and compare it to an amp with a reasonable amount and see which one people will pick in listening tests.

Yes, and that same cabinet will also have a drastic impact on power/frequency in a s/s power amp as wel. I'm not a big fan of lots of feedback myself. Again, I was pointing out to seamonkey that for his 'discovery' of yet 'another weakness' of tubes, there are design principles which compensate.

 

[seamonkey]

BbbyBld,
You must have access to some good equipment and analyzers - anything you can share?
I sure would like to see some real amp measurements.

 

[fdeck]

The equipment is pretty simple. A PC with free analysis software, and an attenuator so the output of the amp doesn't smoke your PC. A voltmeter is also needed. The most expensive part of the test is laying your hands on a tube amp.

 

[BbbyBld]

BbbyBld,
You must have access to some good equipment and analyzers - anything you can share?
I sure would like to see some real amp measurements.

What specifically would you like to see?

I won't make any promises about what I can show you, though.

 

[fdeck]

I would not mind seeing what that 10% THD waveform looks like on a typical tube bass amp going into a simple dummy load. I pretty much know what to expect on a solid-state amp, but not a tube amp.

 

[seamonkey]

You are very cool for even offering.
Let's see .. (rubs hands together)
Whatever Fdeck wants and except maybe a plot of the input and output into a speaker.
And say
Frequency response at 1 watt
Frequency response at 1%THD
Frequency response at 10% THD
All into a speaker load.
Wattage (sine wave) at 1% THD, and at 10% THD

 

[BbbyBld]

I would not mind seeing what that 10% THD waveform looks like on a typical tube bass amp going into a simple dummy load. I pretty much know what to expect on a solid-state amp, but not a tube amp.

Ok...that's pretty easy.

Frequency response at 1 watt
Frequency response at 1%THD
Frequency response at 10% THD
All into a speaker load.
Wattage (sine wave) at 1% THD, and at 10% THD

Response curves of this type are proprietary...sorry.

Your bottom request is reasonable.

 

[Mika]

You are very cool for even offering.
Wattage (sine wave) at 1% THD, and at 10% THD

When you request tests, the test conditions must be unambiguously specified. Here you have failed to provide an essential specification: the frequency of the test signal.

Also, the correct term for the quantity you're requesting to be measured is power, not wattage.

 

[anderbass]

Theoretically, since the peak and RMS of a square wave are equal, if you measured the power at "hard clipping" you could get "up to" a number of double the power at onset of clipping.

For various reasons you won't quite get double...

Thanks Jerrold...

 

[seamonkey]

I ran across a Peavey Patent:
Link Removed
by Sondermeyer, Jack C

Part of the description notes:
" ... Thus, the output tubes 16 and 18 become over biased beyond class-B and at severe output clipping significant crossover distortion is generated as well. Consequently, at overload, the output signal of tube amplifier 10 will be clipped at the peaks. However, it will not be as "dirty" as a typical solid state power amplifier operating under the same conditions, because a large portion of the overloaded output waveform is forced or compressed into the severe crossover distortion region. To a musician, such a waveform is much more musical in nature and "cleaner" (i.e., less harsh) than a solid state amplifier at overload. Due to the compression (i.e., distortion near the zero crossover), the actual peak output clipping is reduced and is far more tolerable than that of the solid state amplifier. This phenomenon is thus, tube power amplifier compression."


I found spice models closely match the patent information.
http://www.duncanamps.com/technical/ltspice.html

A schematic for amp that closely matches the tube amp in the patent:
http://www.duncanamps.com/zips/el34_push_pull.zip

When you load this into LTSpice and Right click on v1 and select: Amplitube[V] and set it to 2v.
Click the running man and you'll see pretty much the same as diagram 3f on the patent.
There's more crossover distortion as the voltage sag kicks in.

Since the spice models seems accurate I can get measurements from them.

 

[Hoss]

I hate to always quote the Ampeg book, but on page 182 it says:

"The power and output transformers were some of the biggest ever fitted to a musical instrument amplifier. The SVT was good for 300 Watts RMS and 700 Watts peak power."

I dunno how accurate that info is, but as trusting my ears I would believe that. What a loud amp that SVT is...

 

[seamonkey]

BTW - the spice models do exactly as the patent states. So do (some) modelers. Peavey Revalver lets you see this without playing with spice.