# Clarification needed on impedance matching



## Spellcaster (Jan 7, 2008)

Having worked in the audio field with solid state, I hve a good grasp of what happens when impedance loads change with a transistorized amp. I'm a bit foggy about tube amps though...I've heard that running a higher than nominal impedance load with a tube amp can be just as detrimental as too low an impedance load is with solid state. I'd like to run my Fender SuperTwin (recommended minimum 4 ohm load) with a cabinet that's 8 ohms as a static load), Will this make the amp run hot, or possibly damage the output transformer? The combination would be used at reasonable volume levels, never balls-out loud. Thanks for your patience in answering this...I see a lot of impedance/wattage/matching questions in the forum, but a search hasn't turned up anything that addresses this specifically.


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## Gunny (Feb 21, 2006)

With your disclaimer that you're not going to run it flat out, I'd say that the typical statement about tube amps applies - you can have a 100% mismatch particularly from 4 to 8 Ohms or 8 to 16 Ohms and vice-versa. The only danger is going down to 2 Ohms or less when the amp should be matched to 4 Ohms.


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## Spellcaster (Jan 7, 2008)

Thanks Gunny. Does that mean the same applies as with solid state? Is too low an impedance the thing to avoid with tubes as well?


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## Gunny (Feb 21, 2006)

Spellcaster - yes, for the simple reason that 2 Ohms and lower is quickly approaching a short circuit - and all that that implies...a hazard for ss or tube amps.


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## Wild Bill (May 3, 2006)

Spellcaster said:


> Thanks Gunny. Does that mean the same applies as with solid state? Is too low an impedance the thing to avoid with tubes as well?


Here's the scoop, SC! Since you say you are familiar with solid state electronics you'll know that with the usual transistor amplifier the lower the speaker load the more current/power is delivered. That's why you have a limit for the minimum load so the transistors don't cook themselves but you don't care about how high is the speaker load. If you plugged in a 16 ohm cab into a SS amp that had a rating of 100 watts @ 4 ohms all that would happen is that you would see a lot less power.

Tube amps are different in that tubes want to see an optimum load in a specific range. If you look at tube data sheets you will often find curves of plate load vs. output power, and also distortion. Rarely do best power and distortion happen at exactly the same load. So a hifi design might sacrifice power for a better distortion figure. Besides, the difference is like 5-10 watts and with a 50 watt amp who can hear the difference anyway!

The range for a pair of 6L6's might be 4000-8000 ohms. Most Fenders designed for 6600 ohms. EL34's in Marshalls typically run at 3400 ohms.

Anyhow, since the tubes must see a high fixed impedance we use an output transformer to convert that 6600 ohms or whatever down to the speaker load. Since it's a transformer we get a fixed ratio. We can have 6600 ohms on the plate side and 8 ohms on the speaker jack side. If we plug in a 4 ohm cab we will get only 3300 ohms on the tube plate side. The ratio is fixed.

To give us different speaker impedances from the OT we tap the secondary winding to give us a new ratio, just like we might tap a voltage transformer to give us 6, 12 and 24 vac. So that Marshall can be switched for a 4, 8 or 16 ohm load and still deliver 3400 ohms to the EL34's.

The reason tubes are different from transistors in that they absolutely hate trying to deliver power into too high a load is that with such loads we get very high voltage swings in the transformer windings that can actually break down the insulation between the wires in the windings and short out the transformer. Consider, if you plugged a 16 ohm cab into a 4ohm output on an amp. Because the OT is a fixed ratio that would mean that since 16 ohms is 4 x the proper load the load given to the tube plates would also be 4 x higher. Those 6L6's would see 4 x 6600 ohms, or 26,400 ohms! This is a huge mismatch far outside the proper range.

Not plugging in ANY speaker load means the tubes would see an infinitely high load! Scary high voltage swings would happen. 

Hope this is useful!

Wild Bill

:food-smiley-004:


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## Spellcaster (Jan 7, 2008)

Wow.....I just learned more than I've been able absorb in a long time....and I've spent a lot of time online trying to get an understanding of this. Now I can see why.....it's such a different scene with tubes than solid state, and I've been trying compare them in the same frame of reference. It's finally starting to make sense. You guys on the forum are great - I get more good info here than anywhere else and I thank you!


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## Wild Bill (May 3, 2006)

Spellcaster said:


> Wow.....I just learned more than I've been able absorb in a long time....and I've spent a lot of time online trying to get an understanding of this. Now I can see why.....it's such a different scene with tubes than solid state, and I've been trying compare them in the same frame of reference. It's finally starting to make sense. You guys on the forum are great - I get more good info here than anywhere else and I thank you!


Glad it's useful, SC! I'm not a teacher but I have taught a few training classes and ham radio courses over the years. Whenever I'm called to explain something I try to put myself in the beginner's shoes. It seems to make things come through more clearly.

If you didn't already know, there's a fabulous site called Ampage:

http://www.ampage.org/

I spent a lot of time there and there are some very knowledgeable guys who are great at helping "newbies". Then I discovered this site! The fact that it was Canadian tickled my patriotic bones but being a relatively new board there didn't seem to be much help for techie questions happening. So I jumped in. Very soon the sysop gave us our own forum and other technical guys from across the country showed up. We've got a great crew now and the word keeps spreading!

Stick around! We have some fun!

:food-smiley-004:


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