Why is impedence mismatch necesary in RKII?

[sleepingtiger]

When using a dual cab setup? Couldn't they have engineered an auto-switch for impedence, when using two cabs. You always have a mismatch when using individual cabs in order to prevent a dangerous mismatch when using both.
This was my ONLY disappointment with my RKII. I figured you techies could explain it .
Thanks-in-advance
Tony

 

[Rocky]

Rule of thumb: as long as your speaker is a higher or equal ohm rating you are fine.

 

[sleepingtiger]

Rule of thumb: as long as your speaker is a higher or equal ohm rating you are fine.

I realize that it's safe that way, but you can't use one cab at a time & be "right". With 2-8 ohm cabs, singley they are mismatched, but correctly matched with both running, when using the 2-4 ohm jacks. In order to get a correct match when using them singley using the 8 ohm jacks, you can NOT use both at once AT ALL. I just don't understand why proper impedence matching wasn'tengineered into the switching system.
Thanks,
Tony

 

[trem]

Just to make sure, are you talking about channel assignable cab switching?

 

[Boltino]

Rule of thumb: as long as your speaker is a higher or equal ohm rating you are fine.

This rule applies to solid state amps. The tube amp rules are more complicated and depend on the specific amp and transformer. Running my 64 Tremolux (4 ohms) with a 16 ohm cab is not ok.

 

[Rocky]

Rule of thumb: as long as your speaker is a higher or equal ohm rating you are fine.

This rule applies to solid state amps. The tube amp rules are more complicated and depend on the specific amp and transformer. Running my 64 Tremolux (4 ohms) with a 16 ohm cab is not ok.

Boogies manuals go with the rule of thumb that I stated, so do Soldano, THD. Marshall, Hiwatt, VHT and many other manuals. As long as your speaker is a higher rating you are fine. Your Fender will be fine running a 16 ohm cab off a 4 ohm source. Ohm's law and the general science b/w source load and destination load doesnt change for one company....unless Fender built something into the amp that prevents it from working. And I doubt that.

Depending on if you are running series or parallel will, of course, make exceptions but the 'Speaker > source = OK' nearly ALWAYS applies.

Tube amps can actually withstand a mismatch better than SS amplifiers.

 

[Boltino]

Rule of thumb: as long as your speaker is a higher or equal ohm rating you are fine.

This rule applies to solid state amps. The tube amp rules are more complicated and depend on the specific amp and transformer. Running my 64 Tremolux (4 ohms) with a 16 ohm cab is not ok.

Boogies manuals go with the rule of thumb that I stated, so does the advice from Soldano, THD and VHT.

Tube amps can actually withstand a mismatch better than SS amplifiers.

As I said, certain amp companies use transformers beefy enough to handle the mismatch. Certain amp companies do not. Blanket statements about this can get readers into trouble. It would be a shame for someone to damage their amp due to reading something that doesn't apply to all amps. IMHO, mismatches need to be evaluated on a per amp basis.

I absolutely disagree that tube amps can withstand a mismatch better than solid state amps.

 

[Rocky]

Most important thing for a person to do is check with their manufacturer to see what they recommend and go with that. A company like Fender not allowing any variation or engineering their products not to take a mismatch is not surprising.

A 16 ohm speaker or cab will actually task the transformers/output section LESS than a 4 or 2 ohm speaker/cab. A beefier transformer would be needed to match to a lower ohm rating, not a higher one.

 

[ja22y]

Here we go again. We should make impedance topic a sticky.

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

 

[nomad100hd]

Rocky dude your way off, SS amps are completely different than tube amps. Tube amps actually prefer a short to an open, a SS amp prefer and open to a short. The higher the load impedance above what it was designed for taxes the transformer and output tubes more. A general rule of thumb with a tube amp is you can plug a 4, 8, or 16 ohm speaker into an 8 ohm jack. However as Boltino said not all amps are made the same, some amp companies actually have a mismatch load on the tubes intentionally so you don't want to mismatch it even more. Depending on how many watt the transformer is rated for. Something to do with not having a load makes the amp dissipate all the power in the tube and OT.

 

[edward]

Lots of technical nuances here (that I will leave to the truly knowledgeable), but the rule of thumb is one step higher or lower is "generally" ok. So a tube amp's 8 ohm tap "should" be fine with a 4 or a 16 ohm cab. BUT BUT BUT, this all assumes that the tranny is stout enough to handle the load differential. Going over taxes the secondary windings; going under the primary windings. From what I've read/heard over many many years, this is not at all an issue with Boog trannies (appluase to Randall, if you will). But there are many amps out there you do NOT want to apply this rule of thumb to. Ask an owner of a vintage Marshall! Nor does Vox allow it for some of theirs. So there's the "usefulness" of such a generality; be smart and check for your particular amp.

Also consider that as said load changes, so will the tone. Some folks intentionally mismatch up or down because they prefer the tone. In other amps, it is a necessity if they swap to a diff tube (read the manual, eh ).
...and that was the "short" answer

Edward

 

[sleepingtiger]

Just to make sure, are you talking about channel assignable cab switching?

Yup, that's what I'm talking about. The Boogie rep's recommendation are stated in my last post.

Tony

 

[trem]

Just to make sure, are you talking about channel assignable cab switching?

Yup, that's what I'm talking about. The Boogie rep's recommendation are stated in my last post.

Tony

I get what you mean. I don't think there's any way around that. Semantic wise, it's not required, just a necessity for that kind of configuration. I truly believe their OT is made to withstand that kind of abuse though. Do you notice any loss of power when you switch to 1 cab only?

Automatic impedance switching is a good idea. You should tell them about it.

 

[sleepingtiger]

Automatic impedance switching is a good idea. You should tell them about it.

I did & got a bunch of double speak. "It's not that simple", "That would be too much to cram in the head", "It's really not necessary", etc. I don't think they want to admit that they overlooked something so fundamental.
Let's face it...if it wasn't an issue, they wouldn't have 4, 8 & 16 ohm jack in the first place. I guess I'll just hook it uo using the 4 ohm jacks & decide if I even want to use both cabs at once. If not, I'll use the 8 ohm just, being VERY careful not to put the switch in the back to "A+B"!

Tony

 

[trem]

You have a point, but there are some merits in their point of view. There are always different ways in design to tackle one problem, each with its own consequences. From their perspective, it's much easier to build a tough OT than to add that kind of switching.

 

[nomad100hd]

if you have a 4Ω and 8Ω cab you can run them both if you plug the 8Ω into the 16Ω jack and the 4Ω in the 8Ω jack.

 

[sleepingtiger]

if you have a 4Ω and 8Ω cab you can run them both if you plug the 8Ω into the 16Ω jack and the 4Ω in the 8Ω jack.

I have 2-8 ohm cabs. a closed 4x12 & an open 4x10. THAT'S not the problem. I won't be redundant, but my issue is stated in my original post. I just wish I could avoid a mismatch altogether. Unfortunately, it's a case of deciding on the lesserof 2 evils.....

Tony