SET tube amp and blowout?

Hey guys,

I'm a noob to the world of DIY home audio, so please forgive any
ignorance on my part.

From what I understand the SET amps are popular because the distortion
adds to the pleasant sound, even though they are low wattage devices.

Knowing that most speaker blowout is caused from low wattage amps
distorting the signal, what is the likelihood of speaker blowout on SET
amps? And is there a way to determine a "red zone" for speakers when it
comes to harmonic distortion and wattage?

Thanks again
Paul Moore

 

Huh? No, speakers blow from exceeding ratings (note that most speakers
rated "RMS" are damned if they can handle even half that, in real watts!).

You could add a circuit to detect when the output tube's cathode current (or
grid voltage) starts drifting, coincident with drive signal peak coming near
0V (when the grid goes positive, it pulls current from the cathode and a DC
voltage appears on the grid leak resistor, shifting bias). Or a peak
detector when grid voltage peak approaches say, -5 to -1V with respect to
cathode.

But that's useless, because it'll sound like shit by the time you reach that
anyway. Consider music that has a dynamic range of 20dB: that means, from
an average low level of say 0.1W, you have peaks up to 10W. This might
barely be acceptable on a 5W amp, since the peaks are getting clipped only
3dB, for a dynamic range (at the output) of 17dB. If you want to play it
twice as loud, you just crunched your headroom to 7dB and it *will* sound
nasty (especially on bass, which needs a lot of power to register the same
intensity as midrange, and especially on peaks, which need a lot of power
for only a short duration).

Tim

 

Well, sometimes.

The "RMS" rating, which is wrongly named, usually refers to the correct
average heating power rating of the speaker. Most "Pro Audio"
speaker/cabinets, i.e. the ones used by live bands, from reputable
manufactures, will handle their "rms" rating with continuous white noise
and most music signals. If the amp does not clip, then the average
average power outputed by the amp will actually be less than its rated
output.

Kevin Aylward

http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

The speaker blowout you are referring to from low wattage amps
is where a clipped signal has so much harmonic content
that it overdrives the tweeters. This is (or used to be) a problem
in disco houses that tried to get away with using consumer speakers.
(Consumer grade speakers typically have a tweeter power rating
that is much lower than the woofer rating, since most music has
much less energy at the high frequencies.)

But the "coloration" distortion from a single-ended triode
(when not driven into clipping) is typically much milder,
no danger to tweeters under anything remotely like normal
listening.. Added to that, most of these amps are only capable
of a few watts output anyway. You'd have a hard time blowing
anything but the cheesiest speakers!

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

 

This is very good information, but as to the question of blowing
speakers, the amps the original poster is referring to are transformer
coupled. Therefore any DC on the grid resistor, plate circuit or any
other element of the output stage would be isolated, and not present at
the speakers.

 

Um? I see no reference to DC on the output, and obviously, bias current
changes (including bias itself) are not going to be present, since SET OTL
is well...yeah...

Tim