Speaker impedence aka ohms

[hackworth1]

How critical is the speaker impedence when using tube amplifiers? There are Carvin 10 inch speakers on ebay right now that someone is selling as single speakers. They are 32 ohm speakers. Seller says they are designed for tube amps. Would one or two of these be suitable for a 10 to 30 watt tube guitar amplifier?

What would happen to a 4 or 8 ohm speaker driven by an amplifier that is rated for much higher impedence speakers? Would such speakers blow up? Would they cause any damage to the amplifier?

[Fred.T]

Try one of the many sites discussing this

http://www.horrorseek.com/home/hallo...eMatching.html

"...According to legend and conventional wisdom, solid state (transistor) amplifiers can drive higher impedances than their rating demands. So a transistor amp with a label saying "4-ohm" will drive 4, 8, and 16 ohms with no troubles. You can go up but not down. Don't try 2 ohms.

Older legends say that tube amplifiers can drive lower impedances than their rating demands. So a tube amp with a label saying "8-ohm" may drive 4 and 8 ohms. You can go down but not up. Don't try 16 ohms. And never run a tube amplifier without speakers attached. Other legends of the tube age say that any attempt to use a tube amp with speakers of different impedance may cause damage to the amplifier.
...
Honestly, you are better off with a proper match. Just because your tube amp with 8-ohm output survives driving a 4-ohm speaker doesn't mean that it likes driving that load. You are probably reducing the life time of the amp.
..."

[hackworth1]

I bought this amp on ebay. The seller has used two 8 ohm speakers with it (series or parallel wiring unknown). I don't think the guy knows what the correct impedence is. Probably an old console radio amp. Can anyone here identify it?

http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT

[muchxs]

Originally Posted by hackworth1

I bought this amp on ebay. The seller has used two 8 ohm speakers with it (series or parallel wiring unknown). I don't think the guy knows what the correct impedence is. Probably an old console radio amp. Can anyone here identify it?

http://cgi.ebay.com/ws/eBayISAPI.dll...=STRK:MEWNX:IT

It's late '50s through very early '60s Magnavox. It's a mono phonograph amp very roughly equivalent to a tweed Deluxe. It needs an additional preamp stage for guitar use. It's designed to drive an 8 ohm load. It will drive 2x 32 ohm speakers in paralell for a 16 ohm speaker load. I wouldn't go out of my way to use 32 ohm speakers, a single 8 ohm 12" speaker would likely be a better choice.

Approx. 12 watts output means you can use just about anything for speakers, 8 ohm load is close to ideal. It's too bad you didn't get the original Magnavox alnico 15" speaker that comes in the console with this thing.

Post the transformer date codes, those will indicate the approx. date of manufacture.

[BuddyLee]

I would just buy the correct impedance speaker for your amp. On my MM '65 Reverb it has settings for 4 or 8 ohms. From my understanding you can run a speaker safely at 2x your channel setting, but you do not want to run speakers that have a lower ohm rating unless you set them up as a pair in series.

Example. If you had a 2 ohm pair of speakers you could wire them in series and get an ohm rating of 4. This would match the 4 ohm setting on my amp and be ideal. But in parallel the ohm rating would drop to 1 ohm because there is less resistance.

You also have to consider this when using an extension cabinet with a combo amp.

Those 32 ohm speakers are probably meant to be used in a 4 speaker cab wired in parallel. That would make an 8 ohm cabinet if I am doing the math right.

[muchxs]

Originally Posted by BuddyLee

From my understanding... ohm rating...

Please... call it either "impedance" or "speaker load". "Ohm rating" is an internet term made up by someone who had little idea what they were talking about.

"Impedance" is dynamic and frequency dependent. A speaker has a different impedance at 100 hz than it does at 1khz. When we say "an 8 ohm speaker" that means that the speaker's impedance falls in the general range appropriate for an "8 ohm" speaker.

Impedance differs from DCR, "DCR" stands for "DC resistance". That's what the speaker will read on a multimeter or a DVM. An "8 ohm" speaker generally shows less than 8 ohms DCR, it can be as low as 6 ohms, approx. 7 ohms, etc. So there's no such thing as checking a speaker and seeing a perfect 8 ohm DCR. Maybe it's out there somewhere but I haven't seen it.


Here's what you're doing in a tube amp (the old Magnavox for example):

The tubes want to "see" around an 8000 ohm load plate to plate in that little Magnavox amp. The output transformer primary may be set up for somewhere between 6600 ohms and 10,000 ohms in a more or less correctly designed amp. So: No problem! Hook an an "8 ohm" speaker (which connects to the OT secondary windings), let's say the tubes "see" 8000 ohms (8k) plate to plate to plate across teh OT primary. All good so far...

Let's connect a 4 ohm speaker to the same transformer. The tubes now "see" 4000 ohms (4k) plate to plate. That's not exactly ideal but we'll live it with it. Your tubes will live with it, too.

Let's say the amp had a transformer set up for a 6600 ohm (6.6k) load plate to plate with an 8 ohm speaker. O.k., 6.6k is a little on the low side for 6V6s. Swap to a 4 ohm speaker and your tubes "see" 3.3k, that's very much on the low side. Substantially less than ideal.

Let's go the other way. OT wound for 10k plate to plate with an 8 ohm load. Roughly 5k plate to plate with a 4 ohm speaker, that's not all that bad.


NEVER operate a tube amp without speakers or a dummy load connected. That generally results in a melt-down...


Solid state amps: One of the things manufacturers found attractive about good ol' analog solid state amps is that the speaker(s) can be connected directly to the power amp without expensive transformers. Manufacturers would prefer not to pay for transformers if they don't have to. That's where we get solid state power transfer like 200 watts @ 8 ohms, 400 watts @ 4 ohms and 800 watts @ 2 ohms. It doesn't work like that for an amp with an output transformer. The vast majority of tube amps have output transformers.

Let's look at our solid state example again. Jeez, 2 ohms? That's awfully close to a short circuit. A short is 0 (zero) ohms. Any idea how much current a short circuit is capable of flowing, in theory? Answer: A short circuit is capable of flowing infinite current, in theory. It's limited by the physical limits of the material or components the current flows through. Fuses provide an artificial limit to current flow, the fuse is supposed to open as the current rushes towards its theoretical maximum. Wire is incapable of passing infinite current, it will heat up and melt first. Resistors will burn somewhere above their wattage rating. Unprotected transistors will open their junctions.

Most modern amps have built in safegaurds to prevent bad things from happening to good people.

Vintage tube amps generally rely on their users being well enough informed to operate them with a roughly correct speaker load connected at all times.

[BigDaddyLH]

Originally Posted by Fred.T

"...According to legend and conventional wisdom, solid state (transistor) amplifiers can drive higher impedances than their rating demands. So a transistor amp with a label saying "4-ohm" will drive 4, 8, and 16 ohms with no troubles. You can go up but not down. Don't try 2 ohms.

Older legends say that tube amplifiers can drive lower impedances than their rating demands. So a tube amp with a label saying "8-ohm" may drive 4 and 8 ohms. You can go down but not up. Don't try 16 ohms. And never run a tube amplifier without speakers attached. Other legends of the tube age say that any attempt to use a tube amp with speakers of different impedance may cause damage to the amplifier.
...

Even if this is true, I worried that it has to come down to "according to legend".

[jefrs]

Instructions from Laney say that you can put an additional 16 ohm load in parallel with the internal 8 ohm speaker (i.e. now 5.3 ohm) into the 4 ohm socket, /not/ the 8 ohm one. I have had this advice for other reliable sources too i.e. that you can put a slightly higher load a valve OT on but not one lower than it is rated for (note some Fender amps are rated to take a 2ohm load on the 4ohm socket). That is because putting a slightly higher load on reduces the current, whereas a lower load will increase it and can then cause overheating. Also note the ideal load for a valve OT is one matched to about 25ohm (with a suitable transformer) because it is more efficient.

But solid state are a bit different - a solid state amp e.g. Bandit, normally running an 8ohm load, will happily take another 8ohm speaker in parallel i.e to 4ohm. In other words to take a lower load, not a higher one (note the Bandit is designed to do this, but then so is my hi-fi amp, so it is within their specification rating).

That said, guitar valve amp OT are rather conservatively rated and will generally take a lot of abuse with mis-matched loads. Neither valve not solid state will take kindly to being shorted. If you disconnect speakers from a valve amp then you have no load on the OT and that is a very bad thing too because the primary will pass more current, oops.

[muchxs]

Originally Posted by jefrs

note the ideal load for a valve OT is one matched to about 25ohm (with a suitable transformer) because it is more efficient.

The correct load is such that the reflected impedance is correct on the OT primary. Refer to widely available tube data sheets for the correct plate to plate load (OT primary impedance).

There's the Gerald Weber theory that every amp should be connected to a 16ohm load to avoid transformer losses. We might then assume that amps designed for a 2 ohm total load (tweed Bassman, Super Reverb) would sound horrible. Obviously they don't. We could then look at BF Bassman, Bandmaster, Pro Reverb, Vibrolux, Twin Reverb, etc. etc. ... amps designed for a 4 ohm load. Those can't sound good, either.

Originally Posted by jefrs

That said, guitar valve amp OT are rather conservatively rated and will generally take a lot of abuse with mis-matched loads.

For some odd reason many Marshall, Matchless and Bad Cat amps use output transformers that apply 50% of an ideal load ("by the book") to the output tubes. Those won't take a lot of abuse!

Originally Posted by jefrs

Neither valve not solid state will take kindly to being shorted.

Most tube Fenders (Tweed, BF, SF) use a shorting jack for the main speaker. Disconnect the main speaker and the jack applies a short circuit across the transformer secondary. Don't just take my word for it, crawl under the nearest vintage Fender and look.

Apparently the amp will tolerate a short circuit across the secondary better than it will tolerate an open circuit. Leo and Doc thought so.

A short circuit across the secondary is still pretty bad. It's better than an open circuit, though.

Originally Posted by jefrs

If you disconnect speakers from a valve amp then you have no load on the OT and that is a very bad thing too because the primary will pass more current, oops.

Yeah, that's a bad thing. Plates get awfully red, stuff starts to smoke...

[jefrs]

I was referring to correct reflected impedance on the primary by providing a secondary wound for 25ohm load, and loaded with a 25ohm speaker. This requires more windings on the secondary and makes for a more efficient transformer apparently. The BBC did this for transcription quality in broadcast studios. Ex-BBC amps are very good but you try finding a 25ohm speaker. Gerald Weber has a good point there, it's solidly proven science.

I can configure a speaker pair as 4ohm or 16ohm and stick them into the same amp at 4ohm or 16ohm resp., and I can easily hear the difference: the 16ohm is clear and defined, the 4ohm woolly and muffled in comparison (but can suit some applications). Greenbacks into modded Epi BC30. Marshall cabs have a similar impedance switch so you can set the correct load into the amp.

Compared to valve hi-fi, guitar amps are tractors (powerful but clunky and noisy), neither the gain nor the frequency response are very linear (wouldn't be much use for guitar if it were), speaker impedance mis-match can probably go ±50% on all of them. Marshall et al are generally striving for high power, hence the valve sacrifice - whereas the OT primary can take full mains all day. With AB2 it is actually possible get a plate dissipation to OT power ratio of 30/100 from a pair of EL34 or a quad of EL84 (which explains why "30 watt" valve amps can be very loud and can easily blow speakers rated at 30 watts). Guitar amps usually have a more conservative design and double up the valves, notable exception Vox AC30/6(TB) - 46W in standard kit and can be coaxed to double that.

Agreed a shorted secondary is bad but an open circuit secondary is very bad, hence Fender's idiot-proof output jack. At least the shorted secondary provides some load to the OT. And one reason to wire speakers pairs in parallel because if one blows then you still have some load from the other. [The opposite is true for solid state, one must not short their output but they can usually withstand open circuit].