Confused About Impedance Mismatching?

[jcruppe]

This is impedance mismatching as I thought I understood it concerning tube amps. If the impedance is lower than the amp expects to see, the tubes work harder run hotter and do not last as long, e.g. 8 ohm amp into 4 ohm speaker cabinet. If the impedance is higher than the amp expects, the output transformer will work harder run hotter and could possibly overheat, e.g. 8 ohm amp into 16 ohm speaker cabinet.

I have also seen info in the Airbrake manual, from a DR Z rep stating you can use a Brakelight or an Airbrake with 4,8,or 16 ohm load but Running an 8 ohm speaker out with a 16 ohm cab will allow the amp to operate cooler.


I asked Dr. Z. this question myself ...the info on the Air Brake says something like run an vintage Vox or Marshall on 8 ohms, with a 16 ohm cab? Is this true with the Brakelight? This doesn't make much sense to me.

This is the answer I got.
John,
The Brake lite can be used with a 4, 8, or 16 ohm load. Running a vintage amp mismatched will put less stress on the output transformer. That is the reason behind mismatching a vintage Vox.
Thanks,

Brent Ferguson
Sales Manager,
Dr. Z Amplification

Am I missing something? It sure seems contradictory to me.

[jaysg]

Mismatching up...8 ohm setting & 16 ohm load, causes higher flyback voltages inside the tubes, especially with real speakers. Some tubes can take it and others cannot. This is why you'll often find a specific warning either about Marshalls or EL-34s. It's the EL-34 that gets internal arcs easier than most others. Apparently, the Dr Z guy isn't worried about el-84s.

[I'm no tube expert, but I know the math on this.]

[Merlinb]

Mismatching up...8 ohm setting & 16 ohm load, causes higher flyback voltages inside the tubes, especially with real speakers. Some tubes can take it and others cannot.

+1.
Using a load which is higher than specified will reduce the anode current swings in the valve but increase the voltage swings, which is potentially fatal to some OTs. It also increase the screen-current pulses, potentially leading to screen failure. Altogether, this is the more dangerous mismatch.

Using a lower load than specified does exactly the opposite. It is safer for the screens and causes lower voltage swings.
Its often suggested that because the average anode current will usually increase, it is harder on the valves. Depending on how high the screen voltage is to begin with, this could lead to red plating, but rarely does in practice. It has also been suggested that the increased current can cause the OT to fuse and fail, but I'm not convinced. It'd have to be a pretty puny transformer that couldn't take a couple hundred milliamps continuous.

In most cases a 2:1 mismatch won't lead to failure. But anything more is certainly pushing your luck. Dr. Z appears to either be talking rubbish or omitting some pertinent information...

[jcruppe]

It sounds like the devil is in the details. Amp mfg., model, tubes, tranny, etc... I would conclude that it would be best to check with the manufacturer before setting up a mismatch just to be on the safe side.

Thanks