4 ohm cab + 8 ohm cab -> Orange AD200 head

[Wasnex]

When a tube amp says 4 ohms next to the jack that doesn't mean it has to be a 4 ohm load, that means you should not run more than a 4 ohm load.

Sorry but you are overgeneralizing. This may be true for Fenders, but not all tube amps are set up the same. Mesa manuals clearly spec their tubes amps to be run in a mismatch where the amp sees twice the expected impedance but caution against loading them with 1/2 the expected impedance. For years, I thought this was wrong information, but @agedhorse has confirmed it's accurate and a result of the way Mesa loads their output tubes.

 

[Wasnex]

Sorry Rick but I disagree. The Orange is pushing it’s output tubes basically at their limits. Adding a severe impedance mismatch could well push them too far especially with a bass.

IMHO, of more concern, I believe the design pushes the output transformer to it's limit. Keep in mind the tubes in this amp have around 675V on the plates so the tubes should have plenty of voltage to make the rated 200 Watts unless the power transformer really sags under load. Bench tests commonly show the amp puts out about 165 watts (clean). I have read that factory bias is 18-22ma, which is under 50%, so the tubes are run super cold. The general belief seems to be the stock output transformer is the limiting factor. An undersized transformer with thin laminations running an impedance mismatch sounds like a good recipe for mischief.

Don't get me wrong, I think this is an incredible sounding amp, and I am not aware of a bunch of reported cases of output transformer failures. The transformer may be intentionally undersized but robust for all I know. I just don't think its a good idea to run an impedance mismatch without first consulting the amp's designer to find out what is safe.

 

[Old Garage-Bander]

I'm old enough to remember when we still used tube amps because that's all there were. SS was just starting to be a thing in instrument amps. And what screaming demons they were at all of 25 watts. I was an early adopter.

The thing that always stuck with me was that when the load impedance matched the tube amp's rated output impedance, (nominal in all cases), you get the maximum transfer of energy (power). If the impedance were higher or lower, then the output was not at max. This is unlike a SS amp, where it will just keep trying to source more current as you load it heavier (until it blows, or protection kicks in).

Say you have a tube amp taped for an 8 Ohm load, with an 8 Ohm cab attached. The 8 Ohms gets stepped up to several thousand Ohms, by the output transformer. The tubes are happy at their designed load impedance. If the same amp is tapped for a 4 Ohm load and has a 4 Ohm cab attached, the tubes see the same load of that several thousands of Ohms that it did when it was tapped for 8 and running 8. If you put 8 Ohms on a 4 Ohm tap, the tubes see a higher impedance then they like and you don't get the optimum transfer of energy. Likewise, put a 4 Ohm load on an 8 Ohm output and again, the tubes are not as happy as when properly loaded, and you don't get the max energy transfer.

For mismatched loads, tube rigs are more tolerant of a load that is too low than they are of one too high.
SS amps on the other hand don't like too low of a load, but will usually do OK if it's too high. With SS you're not going for a match, just avoiding going too low. SS amp output stages generally connect directly to the speakers, because they are low output devices that already can match the speaker loads without an output transformer. Tube output stage are much higher impedance by themselves and need the OT to match the speakers.

 

[Vismund_Cygnus]

Isn't possible to change cab's impedance from 4 to 8? (yass, my electronic side ignorance)

 

[Mushroo]

Isn't possible to change cab's impedance from 4 to 8? (yass, my electronic side ignorance)

You'd have to replace the speaker driver(s) to change the impedance from 4 to 8 ohms. But unless you were certain the replacement driver will sound just as good, it would be an expensive gamble.

Probably better in my opinion to sell both your cabs and then buy another cab (or cabs) that is the right tool for the job and you like the sound.

 

[Fuzzonaut]

Hello, I've got Orange AD200 tube head, Orange OBC115 8 ohm cab and Acoustic Control Corp 4 ohm cab. It's very important for me to find the way to correctly plug them both into this amp (I allow any technical modifications) because working together they reach my expectations. And I know nothing about electronics more than how to use tube amp, how to play it and generally what it is.
My head's outputs:
-4 ohm
-4 ohm
-8 ohm
-Slave out
Thanx in advance for any help

Bold suggestion: You could get another Orange OBC115 8 ohm cab and a Invalid Link Removed:
slave the two 115 together for 4 ohm, then put those two and the Acoustic into the cab merger and then run from there in series to the amp: it will see 8 Ohm.

 

[Vismund_Cygnus]

Bold suggestion: You could get another Orange OBC115 8 ohm cab and a Invalid Link Removed:
slave the two 115 together for 4 ohm, then put those two and the Acoustic into the cab merger and then run from there in series to the amp: it will see 8 Ohm.

You are a genius Buddy : )

 

[Fuzzonaut]

You are a genius Buddy : )