SF Vibro Champ fuzz

[Dave Curtis, dB AudioTech]

no, just as long as it's not a lower impedance speaker. try it with the 8 ohm.

That information is correct for a solid state amp; it's the opposite for a tube amp (lower mismatch is better than higher. That said, running into an 8 ohm load won't bother it for the short time it takes to rule out the speaker.

[hollisdevillo]

That information is correct for a solid state amp; it's the opposite for a tube amp (lower mismatch is better than higher. That said, running into an 8 ohm load won't bother it for the short time it takes to rule out the speaker.

Really? That's interesting. Why is that, Dave?

[Dave Curtis, dB AudioTech]

Back EMF, aka "Flyback Voltage" from a tube amp's output transformer trying to drive too high an impedance.


From: http://www.geofex.com/tubeampfaq/taffram.htm

"Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load?
Simple A: Within reason, no.
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up?

For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load.

For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer."

[slidincharlie (Carlo P)]

(...)Extended A: It's almost never low impedance that kills an OT, it's too high an impedance. (...)

?!?! Gerald Weber in his books tells exactly the opposite thing (at least with reference to tubes life): he sais that a lower load than expected wears the output tubes much faster...

[rokplo]

it was the speaker ;-))

i took the speaker out of the cab again, bowed air inside it to eliminate particles that could create the buzz, and mounted it back in the cab, being careful to not screw it too tight .... and the fuzz effect disapeared !

thanx for the expertise ....

[Albert Kreuzer]

?!?! Gerald Weber in his books tells exactly the opposite thing (at least with reference to tubes life): he sais that a lower load than expected wears the output tubes much faster...

Not the opposite. Lower load is harder on the tubes, higher load may endanger the OT. Most tube amps will handle both "within reason", as R.G. states.

Cheers,
Albert

[rokplo]

finally, i took the speaker out of the cab again, blow some air inside it, between the cone and the spider, and put it back again in the cab, but i didn't screw it too tight ... and the fuzz disapeared !

thanx everybody !