How to change an amp's output impedance

[hasserl]

That's just not true. You should ALWAYS run into an equal or higher speaker load.

That is true if you are dealing with a solid state amp, but not with a tube amp. With a tube amp it is better to go with a lower impedance load than a higher impedance load, if you have to choose one or the other.

In this case we're only dealing with a 1:2 or a 2:1 ratio difference, neither one is perfect but they probably won't kill the system either. Just the same, a lower impedance load may run the tubes a bit harder, but a higher impedance could cause high flyback voltage which could cause damage in the way of arcing at the OT, the power tubes and/or sockets. The lower impedance is safer.

[Fred G.]

I'm going to dig in my heels and fall on my sword over this one -

What you are saying is the OPPOSITE of what needs to be done here. This is info that has been hashed over for years - it's old hat.

The general concensus is - use a speaker load equal to, or higher than, the output impedance of your amp.

I'm not looking for a fight here, but what your saying is just not true. Tell you what - run a seperate thread on it, and see what comes from it.

But you can read any of the tube amp books (Weber, Pittman, Mitchell, Torres (yeah, I KNOW)). You'll probably also get nearly 100% agreement with what I'm saying from folks posting here.

[hasserl]

I'm going to dig in my heels and fall on my sword over this one -

What you are saying is the OPPOSITE of what needs to be done here. This is info that has been hashed over for years - it's old hat.

The general concensus is - use a speaker load equal to, or higher than, the output impedance of your amp.

I'm not looking for a fight here, but what your saying is just not true. Tell you what - run a seperate thread on it, and see what comes from it.

But you can read any of the tube amp books (Weber, Pittman, Mitchell, Torres (yeah, I KNOW)). You'll probably also get nearly 100% agreement with what I'm saying from folks posting here.

You should be careful what you betting on, you'd loose this one.

First of all, remember that consensus does not make fact. Just because a lot of people agree to something that is wrong dioes not make it right, it just means that a lot of people are wrong. However, this particular subject has been discussed many times, and the "experts" do agree with my position. Hey, it's how I learned it. I used to think the same as you, because I read it a lot of times on internet bbs's. Finally I read the Q&A's on speakers on Ted Weber's site where he answers this question the correct way, then I read this here from RG Keen on Geofex:

Q:Will it hurt my amp/output transformer/tubes to use a mismatched speaker load?
Simple A: Within reason, no.
Say for example you have two eight ohm speakers, and you want to hook them up to an amp with 4, 8, and 16 ohm taps. How do you hook them up?

For most power out, put them in series and tie them to the 16 ohm tap, or parallel them and tie the pair to the 4 ohm load.

For tone? Try it several different ways and see which you like best. "Tone" is not a single valued quantity, either, and in fact depends hugely on the person listening. That variation in impedance versus frequency and the variation in output power versus impedance and the variation in impedance with loading conspire to make the audio response curves a broad hump with ragged, humped ends, and those humps and dips are what makes for the "tone" you hear and interpret. Will you hurt the transformer if you parallel them to four ohms and hook them to the 8 ohm tap? Almost certainly not. If you parallel them and hook them to the 16 ohm tap? Extremely unlikely. In fact, you probably won't hurt the transformer if you short the outputs. If you series them and hook them to the 8 ohm or 4 ohm tap? Unlikely - however... the thing you CAN do to hurt a tube output transformer is to put too high an ohmage load on it. If you open the outputs, the energy that gets stored in the magnetic core has nowhere to go if there is a sudden discontinuity in the drive, and acts like a discharging inductor. This can generate voltage spikes that can punch through the insulation inside the transformer and short the windings. I would not go above double the rated load on any tap. And NEVER open circuit the output of a tube amp - it can fry the transformer in a couple of ways.

Extended A: It's almost never low impedance that kills an OT, it's too high an impedance.

The power tubes simply refuse to put out all that much more current with a lower-impedance load, so death by overheating with a too-low load is all but impossible - not totally out of the question but extremely unlikely. The power tubes simply get into a loading range where their output power goes down from the mismatched load. At 2:1 lower-than-matched load is not unreasonable at all.

If you do too high a load, the power tubes still limit what they put out, but a second order effect becomes important.

There is magnetic leakage from primary to secondary and between both half-primaries to each other. When the current in the primary is driven to be discontinuous, you get inductive kickback from the leakage inductances in the form of a voltage spike.

This voltage spike can punch through insulation or flash over sockets, and the spike is sitting on top of B+, so it's got a head start for a flashover to ground. If the punchthrough was one time, it wouldn't be a problem, but the burning residues inside the transformer make punchthrough easier at the same point on the next cycle, and eventually erode the insulation to make a conductive path between layers. The sound goes south, and with an intermittent short you can get a permanent short, or the wire can burn though to give you an open there, and now you have a dead transformer.

RG has a lot more very good information on output transformers there on his site. Just go to geofex.com, then click on the link to guitar tube amps in the upper left corner and scroll you way down thru the information.

Since that time I've seen this discussed many times on other bbs's, and the "experts" on those other bbs's agree. The danger is too high of an impedance, not too low.

Peace bro,
Hasse

[hasserl]

Fred (or anyone that wants to read more about this), check out this thread and the answer from John Phillips:

http://www.thegearpage.net/board/sho...d.php?t=222589

The confusion seems to come from the difference between tube amps and solid state amps. As John points out, with a SS amp you can go ahead and run it into a higher impedance load all you want, there is no harm in doing so. But when dealing with a tube amp that ain't so. Somehow the truism about SS amps has been circulated out there as being applicable to tube amps, so we've got this confusion out there that refuses to die, and keeps coming up every so often.

[Richard]

Well, the old Fenders have an extension speaker jack that puts the speakers in parallel and they don't have selectable impedance. Mesa actually suggests mismatching with a lower impedance load for a different tone. I haven't wrecked any amps with a mismatched load so I'll have to rely on other's experience.

[Steve Conner]

What I've always heard is that higher than rated impedances are ok for SS amps, but for tube amps it's the other way round: a lower than rated impedance is safer. The reason being that a too-low impedance just wears out the tubes quicker, but a too-high impedance could blow out the OT from overvoltage.

Exactly what you can get away with depends on the particular amp. The old Marshalls with 550V B+ ran the screen grids in EL34s to the edge of destruction even at the rated load impedance. A higher than rated impedance would probably explode them. Fender's high powered bass amps like the 300PS and 400PS are some other designs I wouldn't care to mismatch in either direction, as they tried to squeeze as much power as possible out of their 6550s.

As if that wasn't bad enough, the impedance of a speaker is nominal. An 8 ohm speaker can go up to something like 50 ohms at its resonant frequency.

PS: You should be able to hook a good quality solid-state amp up to anything. If it can't drive the load safely, it should shut down or limit to protect itself.

[Ray Ivers]

Steve,

+1

Ray

[Bruce / Mission Amps]

What is with this silly "+1" junk I've seen lately.... is it too hard to type or is the response not worth typing?

[Steve Conner]

On other message board software that has functions for rating people's posts, you can click a button to give an extra point to a post that you like. The most famous example is Slashdot with its karma points. If the board software doesn't support that, you just have to type "+1" instead. So I guess it's shorthand for "I agree with the content of the above post and if the board had a rating system I would mark it up". Thanks for the +1 karma Ray

BTW, I have no idea if my theories on impedance matching are actually right. I ought to go and try to nuke some old worn-out EL34s. I don't have a spare output transformer to destroy, though.

[R.G.]

I may be able to save you some time and tubes, Steve.

Modern solid state amplifiers are almost universally stable into anything from their rated load to an open circuit. What they are not usually good at is running too low a load. The basic amplifier circuit will - WILL - go up in smoke if you run it with too low a load or short circuit the output. This is because the output devices can provide massive currents but into too low a load this cannot change the output voltage, so the output devices have many volts and many amps through them - and thermal failure follows quickly.

Elaborate protection circuits are now standard equipment in SS amps. These circuits sense output conditions which are dangerous to the amp's continued health and prevent the amp from killing itself. Protection circuits are universal except on certain audiophile (... some would say tweako idiot) amps where the circuits have been removed because the seller or owner thinks they somehow ruin the music, or cover up that last 0.0001% of clarity and goodness the music might have.

But I agree with you - a good SS amp should not be damaged with loads from 0 to infinite ohms.

Tube amps are a different beast entirely . Triode and pentode output tubes are entirely capable of killing themselves with overdissipation, but the combination of limited currents and AC coupling into the output tubes and output through a transformer make overcurrent death just about impossible with a shorted load. The setup just can't provide a high enough DC current for long enough to kill the tube. This is NOT true with other circumstances like losing bias, for instance. That will kill your tubes dead.

In a tube amp, the output tubes feed the output transfomer, including the leakage inductance of the OT and the primary inductance of the transformer. When the secondary of the transformer is open circuited, the tubes are just driving the primary and leakage inductances. This has a couple of effects.

One is that the load impedance is an almost completely undamped reactive load. The second is that it's an inductor.

For feedback amps, those that take output feedback and cram it back into an earlier stage in the power amp, open loads runs the open loop gain of the amp through the roof at high frequencies because of the inductive loading, causes severe phase shifting at high frequencies, and the combination often starts the thing oscillating. It will oscillate itself to death in many cases.

Then there's that inductive thing. Any transients which are not damped are turned into massive voltage spikes by inductive kickback. This can flash over the insides of the tubes, causing tube failure directly, or can flash over the sockets, not only killing tubes but leaving a conductive trail on the socket to help the next tube fail.

So it's the high resistance loads that make tube amps fail. Not all tube amps will fail at the same point, same load, or in the same manner, because they don't all have the same combination of loop gain, feedback, and damping. Some tube amps will even live through protracted open circuits on the output, but it's still a bad idea, because that just means you have hit the fatal combination of conditions yet.

And I have killed both SS amps and tube amps in the noted ways, so there's amp blood on my hands... 8-)

[Steve Conner]

+1

BTW, what's your opinion on those high voltage diodes that some amps have connected from the plate of each power tube to ground? I never was too sure what they do, but lately I think they do much the same as the diodes that are put across the output transistors in a SS amp with BJTs. That is, clamp the kickbacks from an inductive load and so help the amp survive a bad load. The diode will stop the plate going below ground, so if the OT had perfect coupling (another story) the other plate could not go above twice the B+ voltage.

I know my Toaster amp is stable with an open-circuit speaker load, and will drive one happily as long as the power stage isn't cranked into clipping, at which point screens start to glow white hot and other nastiness. (Yes, I did once try cranking it into an open circuit to see what happened.) But I never tried my other 50 watt amp. The Toaster has no diodes, but it has spark gaps across the OT primary halves as a desperate last-ditch measure. They have never fired.