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Tube Amp mismatched impedance

Ajak

Ajak

Guest
Mar 31, 2012
314
4
Bern, Switzerland
#1
Hello TB,

I played a gig last friday where the headlinder provided a bass cab and I could just bring my Peavey VB-2 head.
Their bass player had a Randall 410 which was 16 Ohms. The impedance switch on the VB-2 only offers 2, 4 or 8 Ohms.
What would happen if I used the amp with the switch on 8 Ohms? Would I blow the amp or would it just shorten tube life in long-term use?
BTW the other bass player was kind enough to let me use his head so I didn't risk anything, I would like to know for future situations though.

(In future I'll ask about the exact cab model... I have to say that I never expected a 16 Ohm cab though)

thanks
 
#2
It will probably be fine for short term use and just blow a power tube under long term use.

Markbass 610s are 6 ohm, I used to run an SVT-CL with mine. I kept the tap at 4 ohm. The head worked fine for 12-18 months and eventually blew a single power tube. Swapped it out and it was fine.
 
#4
Possibility of getting reflected voltage spikes back on the primary of the output transformer which can damage it and the output tubes but I've never seen or heard of it happening. Some amps have protection diodes to prevent this kind of damage from happening but I think those are mainly there in case the amp is run with no speaker load which is very very bad for a tube amp. Going the other way is not a problem like say you have only 8 ohm on your amp and you are plugging into a 4 ohm speaker cab. That should not damage your amp but you will not have full power and the output tubes may run a little hotter.
 
#6
Lots of threads about this sort of thing. It seems that the common different views are,
1) Always match OT and speaker impedance with tube heads .... just do it.
2) When mismatching, it is better to have a lower than appropriate cab impedance. A higher than ideal cab impedance is more of a problem.
3) While not ideal to mismatch, you can mismatch thisaway or thataway and get away with it.

I own three different tube heads, and I wouldn't willingly mismatch impedances with any of them.
 
#7
InsanityAmps said:
Possibility of getting reflected voltage spikes back on the primary of the output transformer which can damage it and the output tubes but I've never seen or heard of it happening. Some amps have protection diodes to prevent this kind of damage from happening but I think those are mainly there in case the amp is run with no speaker load which is very very bad for a tube amp. Going the other way is not a problem like say you have only 8 ohm on your amp and you are plugging into a 4 ohm speaker cab. That should not damage your amp but you will not have full power and the output tubes may run a little hotter.
Click to expand...

Complete agreement.
 
#8
A lot of how much/well mismatch is tolerated depends on the primary impedance of the amp in question's OT given the power tube compliment (as well as other factors like plate voltage and the OT's power bandwidth rating), which is why you will hear some manufacturers state mismatch is OK one way or another (higher/lower) - so it can and does vary from amp to amp. For example, if I'm running a pair of KT88s into a 4500 primary, I can probably go either way with impedance and be fine depending on how close to the voltage limits I'm running the tubes. If I'm running them into a 3200 primary, the amp may become unhappy running into lower mismatched impedances but be fine into higher loads. Regardless, mismatching skews the amp's clean bandwidth, and while guitards may like it, I think it sucks for bass (and for guitar for that matter). High power bass amps are likely less forgiving since they are running higher plate voltages and generally optimized for clean output while guitar amps are on the other end of the design spectrum.

As an aside, amp designs that go to half power by disabling pairs of power tubes are more of a gimmick than anything else, at least for bass anyway.
 
#12
My experience of mismatched impedances:

When I first bought a SVT-CL I was using if with an 8 ohm 410 while I was waiting for my 410HLF to come in for a couple months. I got the 4 ohm cab before I had any issues but had a power tube failure very shortly after on very low time tubes (this is about 2001 before issues with new ampegs seemed very common)

The other was with a used Pignose B100V, I plugged a 4ohm cab into the 8ohm output by mistake and it ran for about 5 minutes before I blew a tube.

coincidence maybe... but I don't mismatch impedances either way.
 
#13
Ajak said:
It was this one but with 16 Ohms:
Link Removed

At least the other guy said so, it had no sign on the back.
Click to expand...

Here's further proof that everyone's gig bag should include a small/inexpensive multi-meter.

You could have used it to discover what ohm that cab truly was, because even if it had an ohm-label, someone could have replaced (or re-wired) the speakers to a different ohm.
 
#14
As a general rule for (tube amps only) short term use you can go one click either way away form the correct impedance. So if your cab is 8 ohms and your amp choices could be 16 or 4, or your cab is 4 ohms and your amp can be 8 ohms. That being said I am an the anal retentive type and would never mismatch ohms if at all possible.

Of course there are other factors to consider. Impedance of a speaker cab fluctuates as you play different frequencies through the cab and the construction of the drivers will affect this situation as well.

For instance a 4 ohm cab with two 8 ohm 12" drivers with 2 inch voice coils as compared to say something crazy like a 4 ohm 2x18 sub cab with two 18" drivers with 4 inch voice coils. The 2x18 cab will be able produce more lows lower (design) thus allowing the impedance of the cab to dip way lower than say the 2x12 cab. You can take most any of the solid state micro amps available today and hook to the 2x18 4 ohm cab, crank the volume, and the bass control, and the amp will shut off/go into protect mode. It would not do this with your average 2x12 4 ohm cab.
 
#15
Red Planet said:
You can take most any of the solid state micro amps available today and hook to the 2x18 4 ohm cab, crank the volume, and the bass control, and the amp will shut off/go into protect mode. It would not do this with your average 2x12 4 ohm cab.
Click to expand...

perhaps, but I would never really feel good about having to crank the volume and bass EQ on *any* amp, not strictly class D micro amps. It's, generally speaking, asking for trouble in the long run.
 

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