Fender Bassman & Super Reverb

[emrr]

For a more in-depth explanation search for "flyback" on this page: Weber's "Let's Talk Speakers"

Even if the 'flyback' voltage is doubled by the impedance mismatch, you have to consider that the output tubes are also putting out a higher level due to the extra freedom of a light load, and that this will counter some of the interaction.

Air is a pretty good insulator, and my gut is telling me the tightly spaced winding insulation in the output transformer is more likely to breakdown and arc before the pins external to the tube are.

The larger the amp becomes, the more critical this will be. I personally like making correct matches, and I keep autoformers and matching transformers around for matching different amps to different speakers.

The flyback would be stepped up by a factor of 30 or so, according to what the article in the link above says. I'm not clear on the math of all this, but I do know that the Bassman head I rebuilt had a fried poweramp section, open resistors, fried caps, etc. from hours of being on top of a 8 ohm cabinet. I also saw this happen to a Bandmaster and Bassman Blonde. I haven't seen what happens when a 4 ohm amp drives a 2 ohm load for very long, as it sounds so bad it isn't really worth it anyway.

Regardless whether anyone believes the speaker manufacturer or not, we can agree the gist of this conversation is "don't mismatch."

True on the last part.

It's always possible that a mis-match is enough to drive an old amp with near-failing parts over the edge, and an amp with perfectly good parts may not suffer any abuse. Hard to say what the exact mechanism is. The step-up and step-down ratio is the same, and the one with higher current (step-down driving the speaker) is going to cancel a large part of the lower current step-up signal, assuming they are related signals. Which they have to be. The speaker coming in is negative polarity signal, the speaker going out is positive polarity signal. Anything else mixed in due to the speaker is going to be lower in level than the actual drive signal. The voltage going backwards into the amp from random speaker motion can't possibly be larger than the voltage that already exists at the tube plates, given an identical turns ratio and a lack of some other external drive source. To presume that the speaker could drive the amp in any meaningful way suggests an impossibly poor damping factor from the amp AND a larger random motion signal from the speaker than the original signal! We need Weber to comment further!

[emrr]

And previously ignoring the obvious, sure you are likely to have tube arcing when there's no load at all, since tube output is gonna follow path of least resistance.

But arcing when there's a signal path that's at least close to correct in impedance?

Flyback would have to be large enough and unrelated enough to overcome that. To me, flyback being unrelated enough to cause disturbance seems highly theoretical: you'd have to have a signal with no natural decay envelope for a speaker to generate a truly separate voltage back into the amp, like spaced tone bursts with no decay. An instrument signal will almost always have a decay factor that will continue to drive the speaker with something larger than anyting the speaker might attempt to generate in reverse.

[A-Barr]

As I understand it, the circuit is designed so that when the signal swings one way, there is extremely high current and zero voltage and at the other end of the swing, voltage is at its max. and current is null.

So running into too high or too low a load offsets the operating point of the tube(s) and results in either very high current or very high voltages in the output transformer (forget which results with which). Depending on the transformer, it may take it like a man, or it may be prone to fail under high voltages or more prone to failure under high current, you won't know which until it blows.

FWIW, I blew up an OT running cranked into a -50% load, but it was a $12 transformer so that's what I get!

[emrr]

Voltage rises with lack of current, but it's not going to rise any higher than the power supply can generate, and it's not going to rise much with a 100% mismatch. It will rise a good bit with no load whatsoever, and that will change the bias point on the output tubes.

Lower than rated load impedance will result in higher current, which is more likely to cook an output transformer.

Higher than rated load will increase voltages a bit (depends on the kind of amp we're taking about here as to how much), but if a bit of voltage increase can cook a transformer, well, it's a faulty transformer design in the first place. Think about line voltage variations from the wall; those will cause more fluctuation than a 100% mismatch up in load impedance.

With a Super Reverb 720 VAC power transformer, seeing a 110 to 125 VAC swing from the wall will translate to a swing of 689 to 782 VAC on the rectifier tube. This translates to a similar percentage swing on the DC hitting the output tubes, and the output current is going to go up and down with it. That's before we get into any output load mismatches.

The simple version is still that too heavy a load (4 ohms driving 2 ohms, etc) looks more like a short; 2 year old sticking a fork in the wall socket. All depends on how much current (powe output) we're talking about.

[A-Barr]

The simple version is still that too heavy a load (4 ohms driving 2 ohms, etc) looks more like a short; 2 year old sticking a fork in the wall socket. All depends on how much current (powe output) we're talking about.

But a short is much friendlier than an open circuit to a tube amp, or so I have read, hence the shorting output jacks.

Not trying to be argumentative, just playing devil's advocate....

[emrr]

neither condition is gonna make the amp last very long; we need to stop talking in absolutes here and consider only the conditions in question. 50% down or 100% up.

I'll admit I don't understand the shorting jack preference, and need to do some research there.

[emrr]

bit on speaker jack wiring here reads as the exact opposite; flyback being more of an issue with no load whatsoever(?):

http://www.rru.com/~meo/Guitar/Amps/Kal ... eaker.html

[Randy]

bit on speaker jack wiring here reads as the exact opposite; flyback being more of an issue with no load whatsoever(?):

http://www.rru.com/~meo/Guitar/Amps/Kal ... eaker.html

No load whatsoever is also an open circuit, the opposite of a short. You can think of it as infinity ohms. If there's nowhere for the current to go, it will rebound or flyback as if it were bouncing off a brick wall.

[emrr]

I think we covered that already

[infiniteposse]

Buy one of these: https://taweber.powweb.com/store/zmatch.htm and never have to worry about cabs not matching amps again. One of the most useful tools in my space:)

Lee

[Jeff White]

Buy one of these: https://taweber.powweb.com/store/zmatch.htm and never have to worry about cabs not matching amps again. One of the most useful tools in my space:)

Lee

Nice!

[riantide]

I have a Super that I absolutely love to death, but I would say for recording you might be better suited by a bassman head with a couple different cabs, maybe an open-back 4x10 and a closed 1x12. That would give you some nice options. And the bassmans are pretty easy to mod to blackface specs.

Plus I'm really not wild about a lot of the silverface Supers I've heard. If it was blackface, that would be a no-brainer for the Super.

Anyway, my two cents.

[themagicmanmdt]

not to be the butt of the conversation here...

if an amp can't handle the load (as in, 4 ohm amp, 8 ohm load), science ignored, sounds like a worse off scenario than an amp supplying more to the speakers than it's matched to.

[???????]

not to be the butt of the conversation here...

if an amp can't handle the load (as in, 4 ohm amp, 8 ohm load), science ignored, sounds like a worse off scenario than an amp supplying more to the speakers than it's matched to.

a little knowledge is a dangeous thing.