Ok Replacing a feild coil speeker say 3.2 ohms with a high end pm speeker 8 ohms, i am not concerned with the feild coil,only the match of the out put transformer. in order to keep the sound clean i need to be at 3.2 ohms output. can i place a 8 ohm resister across the speeker leads, that will drop the resistance to 4ohms, and if so will the wattage of the resister have to be the rated wattage of the anp and or the speeker.? Has any one else tryed this and how did it sound.Terry

: Ok Replacing a feild coil speeker say 3.2 ohms with a high end pm speeker 8 ohms, i am not concerned with the feild coil,only the match of the out put transformer. in order to keep the sound clean i need to be at 3.2 ohms output. can i place a 8 ohm resister across the speeker leads, that will drop the resistance to 4ohms, and if so will the wattage of the resister have to be the rated wattage of the anp and or the speeker.? Has any one else tryed this and how did it sound.Terry
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:: Ok Replacing a feild coil speeker say 3.2 ohms with a high end pm speeker 8 ohms, i am not concerned with the feild coil,only the match of the out put transformer. in order to keep the sound clean i need to be at 3.2 ohms output. can i place a 8 ohm resister across the speeker leads, that will drop the resistance to 4ohms, and if so will the wattage of the resister have to be the rated wattage of the anp and or the speeker.? Has any one else tryed this and how did it sound.Terry

Terry: Just use the 8 ohm speaker without any matching R. The R will just convert the audio power into heat energy and will not affect the quality of the sound whatsoever. Speaker Z varies with frequency and that 3.2 ohms Z was only at a frequency of 400 hz anyway.
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If you haven't purchased the speaker yet, 4 and 5 ohm speakers are pretty easy to come by. The SABA "greencones" of the 1950s were some of the highest-regarded speakers ever put in tabletop radios (yes, that's somewhat akin to advertising "the finest wine that you can buy in a box"!) and they were 5 ohm units. 4 ohm speakers are still sold new for auto radio applications.

The idea of matching the voice coil impedance comes from something called the Maximum Power Transfer Theorem, which (executive summary) basically says that the most power is transferred to a load (speaker) from a driving source (output transformer) when the impedances are matched. It's good design practice to match the speaker impedance with its driver.

Having said that...

A 2:1 mismatch will result in an imperceptible loss of sound volume and will not introduce distortion- assuming that (because you are posting here) your project is a 3-to-10 watt tube radio and not an 800 watt earthquake machine.

:If you haven't purchased the speaker yet, 4 and 5 ohm speakers are pretty easy to come by. The SABA "greencones" of the 1950s were some of the highest-regarded speakers ever put in tabletop radios (yes, that's somewhat akin to advertising "the finest wine that you can buy in a box"!) and they were 5 ohm units. 4 ohm speakers are still sold new for auto radio applications.
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:The idea of matching the voice coil impedance comes from something called the Maximum Power Transfer Theorem, which (executive summary) basically says that the most power is transferred to a load (speaker) from a driving source (output transformer) when the impedances are matched. It's good design practice to match the speaker impedance with its driver.
:
:Having said that...
:
:A 2:1 mismatch will result in an imperceptible loss of sound volume and will not introduce distortion- assuming that (because you are posting here) your project is a 3-to-10 watt tube radio and not an 800 watt earthquake machine.
:

::If you haven't purchased the speaker yet, 4 and 5 ohm speakers are pretty easy to come by. The SABA "greencones" of the 1950s were some of the highest-regarded speakers ever put in tabletop radios (yes, that's somewhat akin to advertising "the finest wine that you can buy in a box"!) and they were 5 ohm units. 4 ohm speakers are still sold new for auto radio applications.
::
::The idea of matching the voice coil impedance comes from something called the Maximum Power Transfer Theorem, which (executive summary) basically says that the most power is transferred to a load (speaker) from a driving source (output transformer) when the impedances are matched. It's good design practice to match the speaker impedance with its driver.
::
::Having said that...
::
::A 2:1 mismatch will result in an imperceptible loss of sound volume and will not introduce distortion- assuming that (because you are posting here) your project is a 3-to-10 watt tube radio and not an 800 watt earthquake machine.
::
:No its not a earthquake machine, There are a lot of speakes at thrift stors that were vary high end in there day that may get that old consol rocken with a cd pluged into it, thanks for all the info, Terry
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I will just add this little tidbit. A vacuum tube radio or amplifier must have a load at all times or the output stage(s) could be damaged.
This is the opposite of a transistor amplifier, which can be left on all day with no speaker, no load across the output terminals.

So as the others have said, you won't damage anything with an 8 ohm speaker, but make sure that speaker has some resistance, is not shorted, or it can damage the radio. You don't want to cook the output transformer.

You might find a low watt speaker will sound better than a high watt unit. A low watt radio might not have the power to get the most from a high watt speaker. But again, no harm in trying it. The higher watt speaker is able to dissipate heat better than the cheap little speaker.

John, I agree with the "tube output stage damage guideline" but I don't think that it's much of a concern for most consumer tube radios, which were manufactured robustly enough, and with low enough output power, to tolerate almost any kind of dead short/open speaker abuse heaped on them by clueless consumers. For rock concert high-wattage amp tube equipment, it's definitely a concern.

One way to QUICKLY burn out an early solid-state amp was to put a dead short across its speaker terminals and then crank the volume control to max. The two technologies have different sensitivities to abuse, but the tube radios were far more abuse-tolerant than their semiconductor counterparts.

Of course, the higher-end modern solid-state amp makers have acknowledged that smart people (the customers who bought their products) do stupid things, and have incorporated active and passive protective devices into the output stages to save the amp in case the speaker leads get shorted.