the importance matching impedance with tube amps

[Steve Conner]

Gross! I once tried to destroy a small radio speaker by hooking it up to a 150w solid-state amp and playing bass through it. The cone tore into shreds, but the spider and voice coil kept on flapping...

I'm not sure about the 12% thing. I've measured both my homebuilt amps with one step mismatch in various directions.

The 50 watt "Ninja Toaster" (originally a trash-picked Sansui Q-55) only has a 16 ohm output. With 6L6s or EL34s, it used to give 50 watts into 16 ohms and about 25 into 8 ohms. This power drop was accompanied by red plates. With 6550s fitted, it gave 65W into either 16 or 8 ohms. This is good as it saves me buying another O.T. I'm convinced that the Sansui O.T. has ninja powers.

The 40 watt "Ninja Deluxe" (made from the guts of a late model Treble'N'Bass 50) gave 20W when fitted with 6V6s and mismatched one step upwards. (16 ohm load on the 8 ohm tap.) With 16 ohms on the 16 ohm tap, it gave 13W.

None of these is a 12% difference...

[Old Tele man]

I'm not sure about the 12% thing. I've measured both my homebuilt amps with one step mismatch in various directions.

...some further reading over on the Pentode Press website.

...the referenced Seeley book explains it quite clearly.

[Paul P]

I can't hear any difference when I switch the speaker from the 16 ohm to the
8 ohm tap on my amp's OT (SE parallel 6v6s, running an 8 ohm speaker off
the 16 ohm tap). On the scope the only difference to a test sine wave at
significant volume is a very slight distortion to one side of the wave. There's
no reduction in intensity.

Paul P

[6267]

I think sometimes that both a few different loads may work well, within reason, because each frequency has a different response to the impedance of the primary. I have an 8-ohm amp that sounds much better through a 4-ohm speaker for harp, but it tends to roll of the highs and emphasize the lows now too much for guitar. If anything, it's louder, but I wonder if the speaker is just more efficient.

[black_labb]

i believe the matching has alot to do with the output tranny and the speaker. the rated impedance is an average. some frequencies will have a higher or lower impedance. a different impedance will emphasise the frequencies that would otherwise be higher/lower than the nominal impedance used (depending on wether you use a higher or lower than rated impedance). the frequencies that are the other way will not be as loud. it also depends on how the output tranny is wound, as they will not usually be right on. even different tubes could change how accurate the tranny winding is.

[Steve Conner]

Old Tele Man, thanks for the link, but I'm not saying that I don't understand the theory behind it.

I'm saying that my actual measured results from real tube amps disagree with the theory, and so I think the theory is wrong. The actual power drop from a mismatch will not always be 12%, rather it will depend on a whole bunch of factors.

(Pentodes and beam tetrodes don't have a well-defined Rp, but the theory assumes it to be constant, so there's one major problem with the theory for a start.)

[tubeswell]

My understanding is that 'matching' is related to the load resistance seen by the output tubes. The OT primary and secondary an dspeaker could theoretically be configured different ways, as long as when you hooked up a speaker to the OT, the Tube sees the right load resistance to prevent the plates burning up.

[kg]

Old Tele Man, thanks for the link, but I'm not saying that I don't understand the theory behind it.

I'm saying that my actual measured results from real tube amps disagree with the theory, and so I think the theory is wrong. The actual power drop from a mismatch will not always be 12%, rather it will depend on a whole bunch of factors.

(Pentodes and beam tetrodes don't have a well-defined Rp, but the theory assumes it to be constant, so there's one major problem with the theory for a start.)

i don't think the theory is wrong so much as incomplete.

in the end i could care less about impedance matching in MI amps.

[Old Tele man]

Pentodes and beam tetrodes don't have a well-defined Rp, but the theory assumes it to be constant, so there's one major problem with the theory for a start.

...actually, their Rp is no better/worse than triodes--for variance over changing loads--however, the REAL problem is that the "theory" number assumes a non-reactive RESISTIVE load, while "real-world" numbers have to contend with reflected IMPEDANCE that's changing with both frequency-content and power level.

...never-the-less, when you're MEASURING an amps' power output across a fixed, non-reactive RESISTOR--like it's supposed to be done--then my statement still stands...you typically won't more than about 10-15% change in the readings between the Z/2-reading, the exact Z-reading or the 2*Z-reading.

[kg]

...actually, their Rp is no better/worse than triodes--for variance over changing loads--however, the REAL problem is that the "theory" number assumes a non-reactive RESISTIVE load, while "real-world" numbers have to contend with reflected IMPEDANCE that's changing with both frequency-content and power level.

...never-the-less, when you're MEASURING an amps' power output across a fixed, non-reactive RESISTOR--like it's supposed to be done--then my statement still stands...you typically won't more than about 10-15% change in the readings between the Z/2-reading, the exact Z-reading or the 2*Z-reading.

OTM, perhaps you're having trouble understanding steven:

he's saying that WHEN HE TESTED IT, it didn't hold true.

i've known steve for close to 10 years now and he doesn't just make stuff up. i believe him.

so where does that leave us? we have a theory that does not jive with practice. that means we have to investigate the theory to find out WHY, because it is incomplete.

frankly i can think of one glaring reason why it would not hold true: is the original load impedance "perfect," or is it greater or lesser than "perfect?"

obviously if it is offset then you will find different changes in output power as the impedance is varied in one direction vs. the other.

the question becomes how many tube output stages have a "perfect" load impedance for max power output?

secondarily, we have the issue that since the effective impedance of a speaker is frequency dependent there is a variation on this power output change WRT frequency content.

[Old Tele man]

OTM, perhaps you're having trouble understanding steven: he's saying that WHEN HE TESTED IT, it didn't hold true.

...no, I think I did understand...what I didn't know was *how* those tests were performed, hence my added comment about using non-reactive RESISTOR...his posting didn't state what he used to perform his tests.

...and he doesn't just make stuff up.

...didn't say that he did...and, certainly did NOT wish to imply such either!

we have a theory that does not jive with practice.

...that "theory" has been around longer than you or I have been around.

that means we have to investigate the theory to find out WHY, because it is incomplete.

...when you find out what's making the theory 'incomplete,' I believe you'll find it's already been said.

i can think of one glaring reason why it would not hold true: is the original load impedance "perfect," or is it greater or lesser than "perfect?"

...agreed, on the amp HE was using...remember the "theory" is foundationed upon laboratory "tests" and "proofs," using components with "...as close to optimum..." components as possible.

the question [now] becomes how many tube output stages have a "perfect" load impedance for max power output?

...if you're asking me, I'd say pretty much 'none' of the present-day E-I transformers and only 'some' of the toroidal transformers being used.

...we have the issue that since the effective impedance of a speaker is frequency dependent there is a variation on this power output change WRT frequency content.

...let me quote myself: "...while "real-world" numbers have to contend with reflected IMPEDANCE that's changing with both frequency-content and power level." ...doesn't 'effective' impedance encompass 'reflected' impedance; and, likewise, doesn't "...impedance that's changing..." encompass '...impedance (of a speaker)...is frequency dependent..."?

[kg]

so, if by your own admission, NONE of the amps any of us use/design/construct are going to have optimally designed output stages operating into non-reactive loads, how useful is your original comment?

i could tell you but i think we all know the answer.

in theory solid state amps sound better than tube amps, and bees cannot fly.

[Old Tele man]

in theory solid state amps sound better than tube amps, and bees cannot fly.

...(A) ss amps can make both good/bad distortion; and, (B) the honey-bees can fly, it's the bumble-bee that raises peoples eyebrows!


P.S.--I've followed your BAGA from way back...do you recall my inquiry to you about "perveance" years ago?

[Leftymatson]

The original question, way back when, had to do with power rating and impedance matching with no signal applied. The OT's primary only sees the secondary when there's some kind of signal so if the amp's powered up with no or very little signal (hiss), an 8 Ohm 1 Watt resistor would make a perfectly acceptable load. Just watch for smoke if there's more that a Watt's worth of noise. Again, with guaranteed no signal the impedance and the power rating don't matter.

[Old Tele man]

We all know the importance of matching our tube amplifier's impedance to our speaker load's impedance. But does the wattage of the amp relative to the wattage of the cabinet play a role if you're just powering it up with a load attached and not playing it?

I'm about to service a 100 watt Laney head that has a switchable impedance selector (4/8/16) ... but i realized that the only cabinet i have here at home to connect to this head is an 80 watt/16ohm 2x10 extension cab. It doesn't seem safe to power the amp up connected to a cab with a maximum wattage that is smaller than the amp - even though the impedance can be matched at 16ohms.

...I read this as "...will dumping 100W into an 80W cabinet be OK?" to which I tried to illustrate that even by "...playing with the switchable impedance selector" that the 100W output would only be lessened/attenuated by about 10-15% (ie: not a very appreciable amount)...which would still exceed the 80W speaker cabinet rating.

...obviously, I didn't phrase things perfectly for some of the readers...to whom I appologize.