Can I use an 8 Ohm speaker with 16 Ohm head?

[Jack]

My little AC4TVH only supports a 16 Ohm load. However, I have several speakers I'd like to try just to see how they sound with this head, but they're all 8 Ohm. If I keep it at low bedroom levels and not go past say halfway on the volume, do you think I could check them out safely?

[neocaster]

Never underload your amp. You'll burn it up.

[Big Mike Simpson]

You could use 2 8ohm speakers in series to give you a 16 ohm load.

http://www.bcae1.com/spkrmlti.htm

[Wally]

I am a believer in matching impedance loads. However, I have to disagree with NEocaster's warning as applied in a general sense. There is nothing to support this thought that underloading any and all tube amp/s will put the amp/OT in jeopardy, and I don't know where this philosophy comes from. IF proof to support his claim can be provided, it will a refutation of all that I have read on the subject and of what I have seen in my experience.
IN fact, certain amps are designed to accept this mismatch.....almost every FEnder amp from '61 through '82 for example. They are provided with the ext spkr jacks that will create a load at 50% of the 'ideal'. Fender would never have put their amps at risk. I have known Fenders to operate safely at 50% of the OT's spec'd impedance, and I have known of FEnders that have operated safely for years at 200% of the specification. That is a 100% mismatch in either direction...safely and with acceptable sonics.
There are certain amps that don't deal with mismatches well at all. I was taught to never mismatch old Marshalls. Others say that they have run Marshalls mismatched with no problems. IF an amp has a switch for matching impedance as do old Marshalls, I figure that the manufacturer is suggesting matching impedances. If the manufacturer provides the ability to mismatch, I figure they have designed the amp to tolerate these mismatches.
IF a manufacturer doesn't provide switching for impedances or the obvious ability to mismatch safely, then I don't mismatch. So, to the OP, if that amp doesn't provide these options and if the manual doesn't specify that the amp can be safely mismatched, I would suggest not mismatching. Tube amps function best sonically with a match of the load to the OT's design specifications.

[T Prior]

I would agree with Wally, It is a somewhat common understanding that going LOWER with tube amp rated impedance is generally safe, going higher is not as it may cause current flashback which may , or may possibly take out the output transformer, but that's all on paper.

I have used countless speakers with my previous Fender amps from way back then, we never even considered load impedance, if there was a speaker, we plugged it in, if there were two, we plugged them both in. There are also countless players on this forum who have run mismatched loads on their Fender Tube amps and had no issues for years.

I can speak to Fender amps from the era and not to other brands, I would also agree that it is best to match impedance. With Fender amps, we have lots of history, lots to go on, many of those amps are still being used everyday on the bandstands, lots of data. I don't think I ever actually knew anyone that blew an OT, but we sure blew a bunch of speakers !


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[Wayne Alexander]

In general, Fender amps can tolerate a 100% mismatch (but not more), but Vox and Marshall amps cannot tolerate any mismatch. It's VERY expensive to blow output tubes, output transformers and related components. Do not do this with a Vox, period. Match the load to the OT.

[fendertastic]

I read once where leo designed all his amp to work with an impedance mismatch. I have a weber 15' 4ohm speaker in my SF vibrosonic (which began life as a super six reverb.) the AMp is supposed to 8 ohm, but the original super six reverb (six 10 inchers) was 5.2ohm or something. I haven't had a problem in 2 years, (thats when I matched the chassis up to the speaker)

[Jack]

Thanks for the responses. I'll stick with 16 Ohm speakers. Now I'll need to trade off some of my 8 OHm speakers.

[eugenedunn]

As Big Mike said, you need to take 2 of your 8-ohm speakers and wire them in series, in a single cabinet. That ought to resolve your problem and give you a nice full sound.

[gpower]

I'd go with the dual speaker option as well. What speakers do you have?

[stantheman]

I had a 16 Ohm EVM-15L in my '64 Pro for years and it sounded real nice.
Never incurred any problems.
Going "in the other direction" I must confess complete ignorance.

[drdos]

Originally Posted by Wayne Alexander

In general, Fender amps can tolerate a 100% mismatch (but not more), but Vox and Marshall amps cannot tolerate any mismatch. It's VERY expensive to blow output tubes, output transformers and related components. Do not do this with a Vox, period. Match the load to the OT.

Agree on this as well as on Orange amps.. Seems the Brit amps are a little temperamental..

[Jack]

This is for my Vox AC4 V112TV cab. I don't have and don't plan to buy a 2X12 so wiring two 8 ohm speakers isn't an option. I just wanted to see if I could safely try some of the 8 Ohm speakers I have in my little Vox half stack. I have other amps, but I was just looking to improve this rig.

Thanks.

[fezz parka]

Originally Posted by neocaster

Never underload your amp. You'll burn it up.

Yep.

Ok, so let's discuss the problems associated with mismatches. When you use a load that is lower than the intended load, the output has to drive the load (speaker) with more current because it is a lower impedance than is expected. Two inherent problems associated with transformers are flux leakage and regulation. Flux leakage is also referred to as leakage inductance. It is related to the current in the secondary, and these problems increase as the current increases. As the current draw in the secondary increases, the primary has a more difficult time transferring the signal to the secondary, so the secondary signal to the load gets squashed, or 'soft-clipped'. This soft clipping is called regulation. While regulation is desireable in a power supply, it is undesireable in a transformer. In other words, in a power supply, if the input voltage or the output load current changes, we don't want the output voltage to change. In a transformer, we want the output voltage to follow the input voltage and not regulate at all. When you put a heavier load on the output than was intended, it will pull the output voltage down, hence regulation. The leakage inductance problem arises because the current from the heavier load causing the regulation to occur reduces the efficiency of the transformer by not allowing the output to follow the input. Transformer designers simulate or view this problem as having extra inductance in series with the primary. The extension of this idea then, is that with the heavier load, you could affect the efficiency of the transformer, alter the frequency response (due to the extra leakage inductance in series with the primary), and cause other distortions to occur. OK, on to mismatching the other way. A speaker is a current operated device in that it responds to the current through it to generate a magnetic field that works against the magnetic field of the speaker magnet to make the cone move in and out. Thinking in very short amounts of time, when the output charges up the voice coil with current, then the signal goes away or gets reduced, the cone system moves the voice coil back to its home or resting position. As it is moving back, it generates a voltage that is fed back up the line into the transformer and appears in the output circuit of the amp. This generated voltage is often referred to as flyback voltage, because we are charging up an inductor, then when we disconnect or stop charging the inductor, the magnetic field in the inductor collapses and induces this big voltage into itself. This big voltage then 'flies back' to the source of the charging current. There is a mathematical formula to determine how big the voltage is and it is related to the inductance of the voice coil, the amount of time it was fed current, and how much current it was charged with. The bottom line is that the voltage fed back to the output circuit is oftentimes much higher than the voltage that was used to drive or charge up the voice coil initially. This voltage gets transformed up by the turns ratio of the output transformer, and in many cases can be over 1,000 volts. What happens then is that arcing can occur between the pins on the output tube socket. Once this has occured, a carbon path forms on the tube socket between the pins. The carbon path allows a steady current to flow between the pins and eventually burns up the socket due to the heat that is generated. For example, it wouldn't be too uncommon to see a transformer turns ratio of 30:1. If we had a voltage fed back from the voice coil that was around 50 volts, 30 times 50 would be a 1,500 volt spike at the plate of the output tube. This is why you often see designers connect diodes in a string between the output tube plates and ground. They are trying to suppress these spikes and dissipate the energy in the diodes rather than allowing an arc to occur at the tube socket. So, when you use a higher impedance load on a lower impedance tap, the turns ratio is higher and resulting fed-back (flyback) voltage gets multiplied up higher than what it would have been with the correct impedance load.