Cabnet impedance related to drive.

[fil]

My head is a Sentura II, and my set-up right now sounds great.  I have a 4x12 sterio cabnet with Celestion 30s (16ohm).  Since I only have one head, I'm only using 2 of the speakers.  So, I was going to re-wire the cab so it isn't sterio.  Here's where the impedance thing comes in.

When I looked inside the cabnet, in addition to the speakers, there are 2 coils, one for each speaker pair.  My guess is that the 2-16 ohm speakers are in parallel, and 8 ohms of coil in series to give a 16 ohm load (resistance measures a little over 13 ohms).

I can wire the cab, with only speaker load, to 4 or 16 ohms.  Which will overdrive the power tubes more?

I'm loving the tone with the set-up I have now, Its just kinda big and heavy to be using as a 2x12.

thanks for any input

fil

[mastercaster]

I'm not real familiar with that amp, but a quick glance at the schematic would lead me to believe that you should wire it for 4 ohms and use the ext. speaker out.  The 16 ohm tap is not available to you, looks to be used for the feedback loop.  Sunn amps in general tend to be by design fairly linear animals, if you're looking for output tube distortion, prepare to bleed...   You could play with biasing them a little on the hot side, but avoid the death glow of the plates!  I'd recommend putting some sort of overdrive pedal in front of your amp, yeah it's different than output tube distortion but try it.    Good luck

[Isaac]

I also use a Sentura II. My outputs are 16 ohms and 8 ohms. There is also a 4 ohm tap on the transformer, but it isn't used on my amp. If yours is the same, I think you'd be best served by wiring the cabinet for 16 ohms. I get plenty of distortion, especially when the midboost is on, but it's also really loud, and I'm only using 4 tens, not twelves.

I don't know what the coils are for in your cabinet. Perhaps to smooth out the treble response?

[mastercaster]

That's interesting Isaac, does your amp look stock that way? Does the questioner's amp look the same way?  The schematic lead me to think otherwise, hmm...  One more point, in general it's usually better to use the highest output impendance available (assuming you can provide the matching speaker load) since that uses the larger or largest amount of coil on the transformer.

[fil]

My amp definitly has 8 and 4 ohm outputs.  They very much look original.

It makes sence to use the higher load, due to using more of the transformer.

What I was getting at, is I am using >8 ohms load in the 8 ohm output.  This gives a nice distortion at a reasonable volume (OK, its loud, but not "bleeding" loud).  I supliment that tone with a tubescreamer TS9 reissue.

If I wire my cab up at 16 ohms, and run it through the 8 ohm output, would you expect it to distort at a lower volume?

thanks for the feedback.

fil

[JoeArthur]

"One more point, in general it's usually better to use the highest output impendance available (assuming you can provide the matching speaker load) since that uses the larger or largest amount of coil on the transformer."

Where does folklore like this come from?  Matching the output impedance to the speaker load will indeed provide the greatest amount of power transfer... but nothing is being lost by not using the largest amount of coil on the transformer - as this seems to imply.

This belief has no factual basis... it is a groundless rumor to get you to buy speakers.

[mastercaster]

Fil,

I'll ignore Joe's jab as best I can, just trying to actually be helpful and of course my comments are strickly my opinion....  anyway, I understand your question fil and honestly I couldn't tell you if the 16 ohm load on the 8 ohm tap would provide quicker (less volume required) distortion, it just might but, I've always stuck with matching the load.  Especially (caution more of my opinion coming)   avoiding a higher impedence load on a lower output tap due to possible damage to the amp.  Will it kill it?  Not necessarily, is it recommended practice for distortion purposes or anything else, NO.  My advice is to keep things matched, 4 ohm cab into 4 ohm tap since that's the only match you have available and look elsewhere for distortion tweaks....pedals, biasing, tube types, master volume mod and the like.

[JoeArthur]

Aw Mastercaster... that wasn't a jab.  Now if I had said "rubbing eye of newt on the output transformer will give you that elusive tone"... that would have been a jab.

I just happen to know what maker of speakers put forth that "need to use the most windings".  

I did agree with you on matching impedance to maximize the power transfer!!!

Mismatching impedances... is something that can be done if you have pretty healthy and heavier than needed output transformers.  The mismatch is reflected back into the power amp and can change the tone - it is all personal preference if the tone change is better or worse.  

Personally I avoid driving a lower impedance speaker load from a higher output impedance at the amp... driving a 4 ohm speaker load using an 8 ohm transformer tap.  This to me would be just asking for trouble... as the 8 ohm transformer tap would be attempting to put about 41 percent more voltage than the 4 ohm tap would... assuming the same output wattage - and the 4 ohm load would be trying to pull more current.  Heavy transformer iron can take it... but something operating right on the edge might go belly up.  

I have done the reverse without any adverse effects (not actually on purpose but as an emergency fix)... drive an 8 ohm speaker load from a 4 ohm tap - there is a tone change and less power output due to the lower 4 ohm voltage.

[mastercaster]

That's cool Joe, and fil I hope some of this helps. I don't sell speakers but I do know who you're talking about. Perhaps the best myths are those wrapped in plausibility... I'll have to 'cypher' on that one some more Uncle Jed.