[ken]

I own a pair of Tad tube monoblocks driving a pair of Morel Preludes with a stated impedence of 8 ohms and sensitivity of 91db.  From what I understand tubes seem to prefer a steady reactive load as opposed to  constant impedence swings.  I contacted the manufacturer and he stated that the average impedence throughout most of the audible frequency spectrum averages approx 5.6 ohms and said it's pretty flat throughout.  I've been listening to the 8 ohm taps and was just wondering if 4 ohms might be better. Any and all suggestions welcome 

[FullRangeMan]

For long tube life 16 ohms speakers is better, second place is 8 ohms.  Looks good listen the same music in both taps and choose the best according your ears.

This is a great TL speakers, Morel-USA site looks dormant, this maker is out of bussines yet??

[SET Man]

Hey!

   I wouldn't sweat it. Yup, just try both taps and see which sound better

   Actually 5.8 ohm average for 8 ohm rated speaker is pretty good. I think 8 ohm tap might do.

Take care,
Buddy

[ken]

Thanks for the responses thus far.  I did do some listening with the 4 ohm taps for about an hour or so and think it may sound a little better.  I started thinking after reading an article about what makes a speaker tube friendly, (a very intreresting and informative article incidentally)http://www.symphonysound.com/articles/tubefriendly.htmland there are several factors that come into play, impedence, phase angles, emf etc... with speaker sensitivity mattering the least, and after finding out that my speakers measured closer to 4 ohms (5. then 8 through most of the freq spectrum  I figured they may sound a little better with the 4 ohm taps.  I'll have to do some further listening but I think the imaging may be a little more focused and tighter.  8 ohms didn't sound bad by any stretch but 4 may be sounding a little better though :thumb

  I'm very happy with the Preludes, they do an awful lot right and very little wrong, and yes, Renaissance is still in business

[ken]

Sorry, I didn't realize the link I provided wasn't valid .  Should  be ok now.

[Quiet Earth]

Forget the theory and the name brands for a moment. Set the volume control to your normal listening level and leave it there for the test. Listen to music on the eight ohm tap. Pause the music like you're playing musical chairs (leave the volume control alone). Switch to the four ohm tap and listen again.

The tap that sounds the loudest and/or has the loudest bass is the one to use. If you hear no difference then leave it at eight. It's that easy.

Now you can ponder the theory. Have fun.

[BobRex]

Forget the theory and the name brands for a moment. Set the volume control to your normal listening level and leave it there for the test. Listen to music on the eight ohm tap. Pause the music like you're playing musical chairs (leave the volume control alone). Switch to the four ohm tap and listen again.

The tap that sounds the loudest and/or has the loudest bass is the one to use. If you hear no difference then leave it at eight. It's that easy.

Now you can ponder the theory. Have fun.

One caution - when you swap the 8 ohm lead leave the common connection in place.  If by chance you have to move the common too, then turn the amp off to do the swap.  Output transformers don't like to be unterminated.  While it usually won't matter for the 5 or 10 seconds needed to swap the leads, why take the chance.

[Quiet Earth]

Interesting point Bob. 

I never heard that rule about output transformers before, but since there are a so many amplifier designs out there I suppose we should cover all of the bases. I know for sure that when you press "pause" on your CD player there will be no audio signal flowing through the amplifier and therefore, no chance of damaging your output tube(s) due to power reflection. In other words, the amplifier should be quite capable of idling unloaded with no audio signal. Mine suffer no damage under this condition.

I would not turn the amplifier off to perform this simple A/B test because you would have to wait too long for the amp to warm back up to the same playing condition. Be careful not to touch the hot speaker lead to anything other than the other positive tap when you switch taps and everything will be fine.

Also, Ken, remember that an eight ohm tap is not exactly eight ohms just like an eight ohm speaker is not exactly eight ohms. For example, I think my amplifier taps are closer to 6 and 3 ohms even though they say 8 and 4.

The louder, more lively and energetic tap is the one that delivers the power most efficiently to your speaker, and that's probably all that really matters. 

[ken]

Thanks for all the responses.
Quite Earth, I think I'll do that A-B comparison like you suggested, I know it's usually better to go with the higher impence taps if you don't hear much of a difference. 

[BobRex]

Interesting point Bob. 

I never heard that rule about output transformers before, but since there are a so many amplifier designs out there I suppose we should cover all of the bases. I know for sure that when you press "pause" on your CD player there will be no audio signal flowing through the amplifier and therefore, no chance of damaging your output tube(s) due to power reflection. In other words, the amplifier should be quite capable of idling unloaded with no audio signal. Mine suffer no damage under this condition.

I would not turn the amplifier off to perform this simple A/B test because you would have to wait too long for the amp to warm back up to the same playing condition. Be careful not to touch the hot speaker lead to anything other than the other positive tap when you switch taps and everything will be fine.

The problem is that even with no signal you have an energized primary and an unloaded secondary.  This has the potential to set up an oscillation in the output tranny and can burn out the tranny.  The output tubes aren't affected, but the tranny wiring is.  Most tube amp manufacturers recommend NOT letting the amp idle without a load for this reason.

As far as warm up is concerned, the amp will be off for what - 30 seconds?  I wouldn't worry about warm up after a 30 second power cycle, the caps haven't begun to discharge and the tubes will still be hot.  The amp will be operating at peak temps as soon as you power it back on.  If you kept it off for an hour or more, I'd agree, but 30 seconds (or more realistically under 10 seconds) will not be a problem.

[Quiet Earth]

The problem is that even with no signal you have an energized primary and an unloaded secondary.  This has the potential to set up an oscillation in the output tranny and can burn out the tranny.  The output tubes aren't affected, but the tranny wiring is.  Most tube amp manufacturers recommend NOT letting the amp idle without a load for this reason.

I can't recall ever seeing that warning before Bob, but thanks for heads up. I'll keep that in mind if I ever disconnect for more than a few minutes.

I probably should have mentioned that I use banana plugs on my speaker cables. Changing xformer taps is so easy that it takes more time to get my butt up from the listening chair than to switch the taps. 

[SET Man]

I can't recall ever seeing that warning before Bob, but thanks for heads up. I'll keep that in mind if I ever disconnect for more than a few minutes.

I probably should have mentioned that I use banana plugs on my speaker cables. Changing xformer taps is so easy that it takes more time to get my butt up from the listening chair than to switch the taps. 

Hey!

    This is real. Some tube amps will blow if there is no load. But so far I've heard only one case of this happen. It is a good thing that BobRex bought this up.

     My SET amps were build from kit and in the assembly manual there is a warning about this also.

    Than again components should be turn off before changing any thing anyway.

Take care,
Buddy

[Steve]

Interesting point Bob. 

I never heard that rule about output transformers before, but since there are a so many amplifier designs out there I suppose we should cover all of the bases. I know for sure that when you press "pause" on your CD player there will be no audio signal flowing through the amplifier and therefore, no chance of damaging your output tube(s) due to power reflection. In other words, the amplifier should be quite capable of idling unloaded with no audio signal. Mine suffer no damage under this condition.

I would not turn the amplifier off to perform this simple A/B test because you would have to wait too long for the amp to warm back up to the same playing condition. Be careful not to touch the hot speaker lead to anything other than the other positive tap when you switch taps and everything will be fine.

Also, Ken, remember that an eight ohm tap is not exactly eight ohms just like an eight ohm speaker is not exactly eight ohms. For example, I think my amplifier taps are closer to 6 and 3 ohms even though they say 8 and 4.

The louder, more lively and energetic tap is the one that delivers the power most efficiently to your speaker, and that's probably all that really matters. 

A couple of points to add to the nice post.

1) The overall distortion will lower and the harmonic spectrum of the distortion will change using the 4 ohm tap under the conditions specified.

2) Also the overall frequency response will be slightly flatter with 4 ohm, as well as increased damping factor.

Cheers.

[rollo]

Forget the theory and the name brands for a moment. Set the volume control to your normal listening level and leave it there for the test. Listen to music on the eight ohm tap. Pause the music like you're playing musical chairs (leave the volume control alone). Switch to the four ohm tap and listen again.

The tap that sounds the loudest and/or has the loudest bass is the one to use. If you hear no difference then leave it at eight. It's that easy.

Now you can ponder the theory. Have fun.

 Thats it in a nutshell. Have fun trying. 


charles

[ken]

I've been listening with the 8 ohm taps the last couple of weeks and I'm now going to giveto the 4 ohm ones a try for a few weeks.  I shouldn't have switched back to the 8 ohm taps so soon

[silver_strings]

I think maybe a 4ohm may sound a little better since it has less resistance so more sound will get through, this is just speculation though