2266c Combo to 16ohm Box?

[realbole]

Setting the Amp's output to 8 ohm, unplugging the internal speakers and plugging in an external 16 ohm box.
I'm wondering if this could harm the tubes or the Amp?

As I know from my other Tube amps, this reduces the volume slightly, but it's not dangerous to the Amp.
If you are trying this out, your amp should ALWAYS(!) be set lower then the box is:
Amp 4 ohm > Box 8 ohm
Amp 8 ohm > Box16 ohm

The reason I wanted to try this is that I only own a 16ohm box and wanted to try the amp without that 'ole "Tube Rattle" :)
But, as I have no experience in doing this with KT66's valves, I wonder if that could cause problems? Anyone?

Thanks in advance :)

[realbole]

Couldn't wait, so I tried it :).. Works just fine, sounds beautiful with 2x12 closed box, no Tube Rattle... Guess it'll be OK

[MKB]

It'll be good to hear Steve D's view on this. Personally I do not like to use a tube amp with the wrong impedance load. I have personally seen a Super Reverb blow up by using a 16 ohm load with it (although the SR is rated at 2 ohms nominal, a much greater mismatch than you are using). If you plan on running the amp like this quite a bit in the future, there is a very simple mod where you can connect the existing 16 ohm tap on your output transformer (it's in there) to the ohms switch in place of an impedance setting you may not be using (like the 4 ohm) by just moving a connector. I'd suggest you eventually do that.

[realbole]

Thanks for your answer! Yeah, I knew there was an extra transformer out that is not used in 2266c's, I red that here somewhere. The thing I'd like to do is to be able to use my 16ohm 2x12 along with the internal speakers which are both 16 ohm, and I guess it's not gonna fly.
2 x 8 ohm (the external box) = ether 4 or 16 ohm
2 x 16 ohm (internal speakers) = ether 8 or 32 ohm

And combining these, won't give me 4 nor 8 nor 16 ohm. In fact that is an interesting question that Steve D may want to illuminate. Can this combination somehow be used with VM (or for that matter, any other amp)?

I'm pretty sure by now that Amp at 8 ohm to speaker at 16 ohm won't give me trouble, because I used to gig like that at full volume with other amps and nothing happened and VM did not glow the tubes running like this in 30mins. But... you never know. You're right, Steve would be the Man to decide whether or not to do it. :D

[SteveD]

There are many schools of thought on this subject and much debate. It would be unwise of me to recommend you do anything other than match the load to the amp but that's not to say that damage will definately occur if you don't. Some people have done it for years without a problem while some blow the OT after a few hours/minutes. Who's to say that wasn't a pure coincidence and would have happened any way.

The volume you play at, the type of sound you use, the quality of your speaker cable etc. etc. are some of the many factors that can exploit potential weaknesses or 'flashpoints' that may or may not exist in your equipment. If you try it and blow something you will 'logically' assume that it was the load mismatch that caused the problem and in reality it could have been, but equally it could have been something else adding fuel to the fire!

In some ways output impedance matching could be viewed as a form of bias for the output stage which is meant to be optimised for maximum power transfer at maximum bandwidth. Skewing this bias should technically only alter the transfer function which in turn would affect the two aforementioned parameters, but, when we are dealing with power the potential faults that can occur tend to be more catastrophic and costly.

If you are happy to take the potential 'risk' then it's ultimately up to you but personally I prefer to match the amp to the load as best as possible and see no good reason to do otherwise really.

[realbole]

Ok, thanks a lot again Steve!

Hmm... so if I can read between the lines, it's not like "It's OK to do it" but it doesn't mean "Never do it" ether :D
Well, I'll give it a try because I haven't had any bad experience with doing this so far.

Again, I wanted to warn anyone who is going to try this, I heard (just heard and never tried it) that it's dangerous plugging the higher ohmage Amp in to a lower ohmage Box (like Amp at 8 or 16 ohm to a 4 or 8 ohm box), so...

But, one thing is still unclear to me:
What about parallel-connecting two Boxes that are of two different ohmages?
In my case 16 and 8 Ohm, parallel connection, measured with a multimeter on the Amp-end of the cord, gives me 4,4 ohm value.
Does this mean that I could switch the amp to 4 ohm and use it safely with both boxes?
Seems logical, but I'm not a Pro so... hmm

Answers are much appreciated, thanks in advance! :)

[MKB]

The only issue you have with the mismatched boxes is that more power will go to the 8 ohm load, and less to the 16 ohm cab. With a 50W amp, and the loads in parallel (for a total load of 5.3 ohms), using strictly the math, you will get 16.7W into the 16 ohm cab, and 33.3W into the 8 ohm cab. This itself isn't a problem, but if the speakers in both cabs are identical the 8 ohm cab will be a bit louder. If your 16 ohm cab speakers are more acoustically efficient than the 8 ohm cab speakers (hopefully by 3dB), the greater speaker efficiency may make up for the power difference, and the two cabs may turn out to be close to each other in volume. If you have the stock G12C speakers in the VM combo, and perhaps V30's or G12H-30's in the 16 ohm cab, they might match very nicely.

I'd probably use the 4 ohm OT tap to run the two cabs in parallel.

Question for Steve: could you tell us what the primary impedances of the output transformers on the 50W and 100W Vintage Moderns are? I'm a bit curious as this may feed into the situation a bit, I've read that the original Radio Spares OT's in JTM45's were run at 8k, which is higher than normal for a 50W amp.

[SteveD]

Ra-a = 8Kohm on the 50W and 4Kohm on the 100W as per the KT66 datasheet specifications.

[realbole]

WOW!
MKB, thanks a buuuunch you've taken the time to write this! I'm so happy now! It's like discovering the wheel all over again! :D
So there are actually endless combinations that one could try, combining different speakers, if the ohmage matching is as simple as that. All you need is a multimeter...
This board rocks! Keep it up forever!
Cheers to Master Steve too!

[slowpokerhino]

I once hooked my Twins "ext. speaker" jack to a 16 ohm Marshall cab and ran it along with my internal speakers (4 ohm). It blew the fuse as soon as I hit the first chord. I'm damn lucky I didn't fry my vintage amp. I no longer "experiment" with that amp.

[realbole]

Yeah, you have to measure out the ohmage of your setup using the multimeter first.
I used the headphone splitter like this one: to plug both boxes together in parallel, then measured the ohmage on the tip and the wider ring of the splitter's jack.
That gave me the value of 4.4 ohm which is good enough for the 4ohm setting on my VM, I guess. I'm at the 2nd day of testing at 100% volume, and it works :D