speaker load question

[12AX7]

In the past i've at times had speaker loads that were not ideal for a given amp. For this example lets say you're  running a 4 or 16 ohm speaker off a amp with a 8 ohm OT. Lets just assume for the sake of argument the amp has only a 8 ohm tap. When i have tried it in the past i found when running a improper load, one way yields a brighter hard almost harsh tone, the other way a darker duller tone.

The question is this. I can't recall 100%, but am i correct to assume it's running a lower load (4 ohm speaker, 8 ohm OT) that gets brighter harder sounding and a larger load (16 ohm speaker, 8 ohm OT) sounds duller/darker?
Consider the tonal descriptions very general because i'm sure others hear it differently, but i think in general that describes it. But the  question is which load mismatch yields which tonal description closest.

[pompeiisneaks]

I found this article that covers a bit about how tone changes with different impedance mismatches... it seems it either raises or lowers the mid range depending:

http://blog.hughes-and-kettner.com/ohm-cooking-101-understanding-amps-speakers-and-impedance/

I don't know personally, I was using a 15 Ohm speaker on an 8 ohm output and it seemed 'darker' to me and that would fit the 'more mids' idea.

~Phil

[12AX7]

sounds about right, tho not sure if i'd say more mids with higher loads but less highs. At least thats what it sounds like to me, but i wasn't sure if it was the direction i though but that seems to confirm it. I added a 7 ohm 25w resistor in series with my 8 ohm speaker just now to simulate 16 ohms and it was far more subtle than i recall, tho maybe somehow a resistor and speaker in series doesn't create the same sound even tho the load looks more right to the amp. Right now i have a smaller load than the amp wants to see and i wanted to see if a 16 ohm speaker would sound less strident before i spend $ on one. So i tried the resistor in series. I think it did sound slightly smoother but very very little and even then i couldn't be sure.

[pompeiisneaks]

No a passive load (resistor) has quite a different impact on the tonality of the output.  It helps make sure the OT is seeing the right load, but it impacts the tone greatly (tone suck to most).  The speaker changes it's resistance based upon the frequency of the signal, but the resistor remains constant.  This means it soaks up some of the frequencies instead of passing them to the speaker.

[12AX7]

No a passive load (resistor) has quite a different impact on the tonality of the output.  It helps make sure the OT is seeing the right load, but it impacts the tone greatly (tone suck to most).  The speaker changes it's resistance based upon the frequency of the signal, but the resistor remains constant.  This means it soaks up some of the frequencies instead of passing them to the speaker.

Thanks, that helps make a decision as to whether i should go ahead and buy a speaker, the answer being yes. Since thats the only way i can know and because it should steer the tone in the direction i want. Without going into why, i can only use a 16 ohm speaker with this amp and retain the proper load. I've been using a 8 ohm because thats what i had but felt at stage volume it gets very bright and thin and i think using the proper load will alleviate that to some degree.

[drgonzonm]

I know you are talking tonal effects, so the easiest way to test your theory would be to use the same model speaker, each with a different resistance.  As the gurus, on this forum have pointed out, the reflected impedance changes. 

I would expect some volume differences too. 

12ax7, have you tried series'ing a another speaker to your amp? 

[HotBluePlates]

... am i correct to assume it's running a lower load (4 ohm speaker, 8 ohm OT) that gets brighter harder sounding and a larger load (16 ohm speaker, 8 ohm OT) sounds duller/darker? ...

The problem with generalities is that often the specifics of a situation add new complications.

That said, a pentode/beam power tube data sheet which shows a graph of distortion and power output vs. load impedance shows a tendency for more 3rd harmonic distortion as load impedance increases.  Below a certain critical point where 2nd harmonic is at a minimum, lower load impedance results in more 2nd harmonic distortion; above that critical point, rising load impedance results in more 2nd harmonic distortion.

Taken together, you would expect that distortion will rise if load impedance goes up, and that the 3rd harmonic will contribute to "edge" or "sizzle" in the sound.  But while overall distortion may increase with lower load impedance, reduced 3rd harmonic could sound "duller" by comparison, and the extra 2nd harmonic contributes as a "fatter" sound.

[drgonzonm]

PRR,
You hit the harmonic right on the node,
When you are describing load resistance, am I correct that you are discussing the reflected load resistance on the tube?  (which then a function of the mismatch/match of nominal resistances)

Others, one of the reasons I suggested the same model speaker, with different nominal resistances, is to minimize the effects different speaker models/brands have on tonal qualities.   

[12AX7]

Ok, i'm trying to wrap my head around this but i just can't to any usable degree. Let me ask you what you think about my situation. I have a 4k 30 watt heyboer OT running 2x6V6 with a marshall PI and have only 8 and 16 ohm taps. (custom OT made for classic 30) The speaker is 8 ohms and running it from the 8 ohm tap. Do you think i'd likely get a better tone in either smoothness or complexity by running a 16 ohm version of the same speaker off the 8 ohm tap considering the OT's primary impedance? Right now it gets a bit too thin and hard at stage volume to easily dial it out. 

... am i correct to assume it's running a lower load (4 ohm speaker, 8 ohm OT) that gets brighter harder sounding and a larger load (16 ohm speaker, 8 ohm OT) sounds duller/darker? ...

The problem with generalities is that often the specifics of a situation add new complications.

That said, a pentode/beam power tube data sheet which shows a graph of distortion and power output vs. load impedance shows a tendency for more 3rd harmonic distortion as load impedance increases.  Below a certain critical point where 2nd harmonic is at a minimum, lower load impedance results in more 2nd harmonic distortion; above that critical point, rising load impedance results in more 2nd harmonic distortion.

Taken together, you would expect that distortion will rise if load impedance goes up, and that the 3rd harmonic will contribute to "edge" or "sizzle" in the sound.  But while overall distortion may increase with lower load impedance, reduced 3rd harmonic could sound "duller" by comparison, and the extra 2nd harmonic contributes as a "fatter" sound.

[pompeiisneaks]

It could just be the speaker itself.  If it's matched for the impedance of the tap, it may just be that way tonally.  You can easily try the 16 ohm tap on the 8 ohm speaker and see if that helps or maybe try a different speaker entirely.  What is the exact speaker? Maybe the speaker guru's can tell you about that speaker a bit more?

~Phil

[12AX7]

I have tried the 16 tap but it's a mismatch while with 4k primary the 8 tap with a 16 speaker should be more proper i'm told. The speaker is a 8 ohm celestion G12T-100 and I have a shot at a G12K-85 in 16 ohm i'm considering.

[drgonzonm]

I have tried the 16 tap but it's a mismatch while with 4k primary the 8 tap with a 16 speaker should be more proper i'm told. The speaker is a 8 ohm celestion G12T-100 and I have a shot at a G12K-85 in 16 ohm i'm considering.

As side note, I couldn't find a G12K in 16 ohm or at 85 watt.
As an other note, the g12t suggested use, is as a bass speaker. 

Have you looked at Celestion's website.  The response shows a dip in the g12t at about 1500 Hz, the G12K has less of a dip. 

[12AX7]

They don't make the K85 anymore but when they did a lot of them were 16 because they came in various manufacturer's cabs. I'm talking used, but i told him i'm going to pass for now because i'm not sure. A lot of great reviews but the graphs don't tell me the K85 (I have a graph i found online for the K85 and it;s the same as the K100) is better suited to my needs than my current T100.
I have indeed looked at the graphs,  and I'm not sure where you heard they were for bass but the G12T100 is a guitar speaker that comes OEM in lots of guitar amps. In fact i have a seconds one thats in my fender modeling amp that came stock in it. 

[drgonzonm]

This is a quote from the celestion website for the g12t "With a massive 100W power rating, this speaker sits perfectly in high-power or hybrid amplifiers. It voices a modern sound with a big bass presence."      

[12AX7]

That just means it's got a ton of lows. Lots of guitar speakers that have a huge low end say that. It says nothing about being used for bass amps, not that it couldn't but it's made as a guitar speaker.In fact, if you go to thier site theres a section for bass speakers and the G12T is not in it. By the way, can you guess what the G in G12 stands for ?

[drgonzonm]

Some say guitar, others say it replaced the designation P, so public announcement? 

Being a bass player, (started on an upright, arco style), I look at speakers with a broad range. IMO, electric bass is more about the harmonics on the E-1 string than the fundamentals.  Capturing fundamentals on an upright is hard on amps.  If the git-fiddle guys can use bass amps, then Bass players can use guitar speakers. 

[HotBluePlates]

... When you are describing load resistance, am I correct that you are discussing the reflected load resistance on the tube?  (which then a function of the mismatch/match of nominal resistances) ...

The speaker's impedance, reflect back to the tube by the turns ratio of the output transformer, is the only load the tube has.  So yes.

... Others, one of the reasons I suggested the same model speaker, with different nominal resistances, is to minimize the effects different speaker models/brands have on tonal qualities.

That's splitting amoeba-hairs, because the change from 8Ω up to 16Ω or down to 4Ω is the overriding factor.  All speakers are varying in their impedance with frequency; see any Eminence data sheet for a plot of impedance with frequency.

... The speaker is 8 ohms and running it from the 8 ohm tap. Do you think i'd likely get a better tone in either smoothness or complexity by running a 16 ohm version of the same speaker off the 8 ohm tap considering the OT's primary impedance? Right now it gets a bit too thin and hard at stage volume to easily dial it out.

Who knows? Tone is in the ear of the beholder.  If the amp sounds thin & hard with the rated 8Ω speaker, you may want to alter the amp rather than chase minor colorations from mismatching speaker loads.

It could just be the speaker itself.  ...

Yes!  Speakers vary enormously in their overall response, and the sonic effect isn't always obvious from frequency plots, etc.  You need to hear different speakers with your amp to know which is the best fit.  And if you have a known-ideal speaker, you may have to adjust the amp's voicing to work well with the speaker.

This video may be a starting point (it helps if you use very good headphones).  The G12T-100 sounds at once meatier, but with a harsher sizzle, than one of the benchmark holy grail speakers, the silver alnico G12 at 3:39.

Or maybe this video.