technichian question regarding impedance

[Baltar]

i didn't find a post regarding this, sry if it's already discussed before ....

what is dangerous for a tube amp? a speaker with too low or too high
impedance?
for solidstate amps it's clear, that too low impedance is making them
sweat. is it the same for tube amps?

i am asking, because of in which way i should move the tolerance of
my self built attenuator.

thanks in advance!

 

[larry_emder]

Re: technichian question regarding impedance

I also need to learn about this stuff. Help is appreciated!

 

[Danglin' Fury]

Re: technichian question regarding impedance

I'm in some electronics classes and I really don't think any impedence is "dangerous" for any amp. But I do know for a fact that if you want the maximum output from your amp, you have to match the speaker load to the output impedence of your amp. If your amp has an impedence of 16 ohms, then so should your cabinet. If anything, too low an impedence may be a problem but I don't think so. Just match the speaker load with the out and you will get the max power out of your amp. If the impedences don't match, it will be less loud.:6:

 

[Baltar]

Re: technichian question regarding impedance

I'm in some electronics classes and I really don't think any impedence is "dangerous" for any amp. But I do know for a fact that if you want the maximum output from your amp, you have to match the speaker load to the output impedence of your amp. If your amp has an impedence of 16 ohms, then so should your cabinet. If anything, too low an impedence may be a problem but I don't think so. Just match the speaker load with the out and you will get the max power out of your amp. If the impedences don't match, it will be less loud.:6:

you mean it doesn't matter for tubeamps? :eek13:
i melt down a ss-amp in the past, due to low-impedance speakers at
high volumes. don't you think, that eg the tubamps output transformer could
get broken, if you connect speakers with wrong impedance?

i heard, that for tube amps it would be the other way around (so a high
impedance is a prob), can somebody verify that right or wrong and estimate the dangerous range ?

 

[ErikH]

Re: technichian question regarding impedance

Yes, impedence matters. Tube amps are much less tolerant of mismatches than SS amps. Of course it is always best to match the impedence but you can get away with using a speaker or speaker cab with a rating HIGHER than the amp. Just don't go lower than the amp.

 

[innerdreamrecords.co]

Re: technichian question regarding impedance

For example on my amp it's says "minimum 8 ohm" on the extension cab output.

 

[shredaholic]

Re: technichian question regarding impedance

In my experience, speakers with a lower impedance rating than the amp are pretty bad, while higher impedance speakers probably won't hurt but it gets to a point where it's likely to damage the OT.

For example, EVH used 8ohm on his amp into 20ohm load box. Having said that, if you can get a load that matches, don't bother pissing about with it and just get something that you know is safe - it's not worth the risk.

 

[That90'sGuy]

Re: technichian question regarding impedance

Yes, impedence matters. Tube amps are much less tolerant of mismatches than SS amps. Of course it is always best to match the impedence but you can get away with using a speaker or speaker cab with a rating HIGHER than the amp. Just don't go lower than the amp.

Actually, wouldn't SS amps be much less tolerant than tube amps? Diodes and transistors have very strict tolerances, no?

The idea with heads and cabinets is that you always want to match the impedance. If you need to get by in the meantime, make sure your speaker have a higher ohmed total load that the head output. Think about it this way, resistance is a safe thing. If you don't have enough resistance on your speaker load, you could blow a transformer (too much current essentially).

 

[ErikH]

Re: technichian question regarding impedance

Actually, wouldn't SS amps be much less tolerant than tube amps? Diodes and transistors have very strict tolerances, no?

SS amps can handle it more reliably than tubes. You'll blow a tube or output tranny before blowing a transistor. Those little power amp chips can handle quite a bit, some as low as 2 ohms. Manufacturers put 8 ohms on there as a nominal safe rating. The lower the impedence, the hotter the chip gets.

 

[JacksonMIA]

Re: technichian question regarding impedance

I'm with Erik on this one. Solid state is usually much more resilient when it comes to imperfect voltage, current, etc. You can still fry 'em, though.

 

[Marcel]

Re: technichian question regarding impedance

I'm not sure (!!) but tube amps can handle lower impendance better then higher impendance...
SS can "always" handle higher impendance but lower will hurt them ...

 

[Marcel]

Re: technichian question regarding impedance

i heard, that for tube amps it would be the other way around (so a high
impedance is a prob), can somebody verify that right or wrong and estimate the dangerous range ?

That's what I've read too IIRC

 

[ErikH]

Re: technichian question regarding impedance

I was just repeating what I had been taught about tube amps. However, I read the following from geofex.com and I trust what R.G. says so it seems what I had been taught is all out the window (and this was years ago I learned it too)....

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#matchspkr

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch

 

[muttonchopsrule]

Re: technichian question regarding impedance

i've learned from experience that tube amps don't like to be mismatched too low. the dude who i sold my single rec to set it up incorrectly the first time he had it in his own hands. he had a 4 ohm cabinet connected to the 8 ohm output, and after about 10 minutes the fuse went. foolishly he put a new one in, and sure enough that one blew too. upon putting a 3rd fuse it, it was realized that the power tubes were cooked, but luckily enough that's as far as the damage went.

mismatching impedences can, in some cases, try to yank too much voltage through the output transformer. and in the worst cases the voltage will arc across the transformer's windings and short it out. then there goes your next paycheck to get the thing fixed.

needless to say, you should always match up your impedence with what the amp wants to see. if this means rewiring your cabinet, or even buying different speakers, it's still better than possibly damaging your gear.

 

[Baltar]

Re: technichian question regarding impedance

http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#mismatch

thanks for the link erik.

here they say what i wanted to have verified ...

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

 

[missionamps]

Re: technichian question regarding impedance

Transistor amps frequently have a circuit in their PA (power amp) that senses high current though the output transistors and this circuit (sometimes called a crowbar) will then shut the amp down, saving it's life.
Most all tube amps don't have anything like that... just sometimes a little 500ma to 1000ma fuse in the high voltage supply
High power transistors are very low impedance devices and can draw lots of current and need to be loaded with a lower value speaker load so as to make some power.
However, too low of an impedance is sure to draw even more current through the solid state PA and subsequently blowing the output transistors.
Tube circuits rely on voltage more then just current.
I know it's really the same thing but a pair of 50 watt power tubes draw only a couple hundred miliamps of current while a 50 watt pair of transistor draws tens of amps.
In a power tube amp, it is generally more safe to be hooked to a lower impedance speaker load then it was designed for then a higher one while a transistor amp PA can be unloaded completely from it's speaker load (infinite impedance) and be 100% fine but it will blow up with it connected to a very low impedance load where huge amounts of current can be pulled through the transistor.

Bruce