Output impedance in Fender Twin with Low Power Option

[IceTre]

Greetings,

I have a 1990 Fender Twin (not the Twin Reverb, but the "Red Knob Twin" or "Evil Twin"). It is a 100W amp, but there is a HI/LO Output switch that allows you to reduce the maximum output to 25W. You can also remove two of the 4 output tubes to reduce the power further.

Here's what the manual says:

"Low Power Option: The Fender TWIN amplifier can be run with only two output tubes instead of four. This is done by removing the two inner 6L6-GC tubes... and setting the IMPEDANCE SELECTOR switch to one-half of the total speaker load impedance... (Remember-- half the tubes, half the impedance.) This will produce 60 Watts R.M.S. in the HI power setting and 15 Watts R.M.S. in the LO power setting.

Also from the manual:

"The Fender Twin is supplied with two 8 ohm speakers connected in series therefore the total load impedance is 16 ohms..."

The problem is, if I remove the two tubes, and change the impedance selector from 16 to 8 ohms per the instructions, now I have an impedance mismatch, because the speaker impedance hasn't changed; it's still 16 ohms. I've always heard that the amp and speaker impedances should match.

I think the answer is this: yes, if the speaker and amp output impedances match, you get the maximum efficiency; the maximum transfer of power to the speakers. However, if the speaker impedance is more than the amp impedance, there will be a larger voltage drop across the speakers and less in the amp. Less efficient, less power transferred, but not harmful. However, if the amp impedance is higher than the speakers, that's bad, because more voltage will drop across the amp's output stage, and will cause it to overheat.

Am I correct? Or not?

[Hwoltage]

I would assume the labeling of the impedance selector pertains to the stock setup of four tubes.

Your amp is running two sets of tubes. Within each set you have two tubes running in parallel. These tubes are in part biased by the impedance load presented by the output transformer.

The wire of both coils have a resistance. When a voltage is fed through the resistance of the primary wire (via the output tubes), a measurable current exists. This current produces an Electromotive Force (EMF) that is radiated from the coil and this EMF induces a voltage in all adjoining "secondary" coils.
This voltage flows through the resistance of the secondary coil so there is a measurable current. A guitar speaker of a selected ohm value is connected to the secondary and limits the current. The current in the secondary is also "reflected" back to the primary, appearing in the primary as an impedance load.

As an example...

The output tubes amplify the AC guitar signal from the preamp. this signal is assumed to be 120 volts
This voltage exists in the primary winding of the output transformer.
A smaller voltage of 4.8 volts is induced in the secondary.
120v / 4.8v = a 25:1 ratio between the two windings.
A 4 ohm speaker is connected to the secondary.
4.8v / 4ohms = 1.2 amps flowing in the secondary.
This current is divided again by the ratio of the windings.
1.2amps / 25 = .048 amps in the primary.
The voltage in the primary is divided by the current to determine the impedance load placed on the output tubes.
120v / .048 = 2500ohms.

If you double the speaker load to 16ohms, you only get .3 amps in the secondary and .012 amps in the primary for 10,000 ohms.

When tubes are run in parallel, and biased by a single impedance you need half the load you would normally need if you were biasing only one of those tubes with the same single source. When you pull two tubes and run only one tube per side, you need twice the impedance load.

The stock setup is a 16 ohm load on the 16ohm tap which sets the correct impedance for two sets of parallel tubes.
If you connected an 8 ohms speaker with the selector on 16 the tubes would see half the load.
If you were to select the 8 ohm tap with a 16 ohm load while running all four tubes, these tubes would then see twice the load than is needed because the 8 ohm tap is designed to provide four tubes with adequate impedance when only half the load is reflected through the transformer due to the 8 ohm speaker.

So you see that with two single tubes needing twice the load over four tubes running in parallel (running 16 ohms into the 16 ohm tap), the 8 ohm tap puts them right where they need to be because it increases the impedance through the transformer.

[IceTre]

Thanks for explaining. I don't fully understand, but understand better than I did!

[Guitardave]

Your combo speakers are still wired for a 16 ohm load. But if you pull the two inner tubes it "changes" the setting your amp needs to match things. It cuts it in half - so the 16 ohm setting that you normally used with all 4 power tubes would be a mis-match, you have to change it to 8 ohms with the 2 power tubes.

Think of what happens when you plug in an extension cab to your twin. With the 4 power tubes installed if you plug an additional 16 ohm speaker cabinet into the parallel extension speaker jack then you have two 16ohm loads running in parallel and the amp needs to be set at 8 ohms to match. With 2 power tubes installed and 2 16ohm speaker cabs in parallel you would cut that in 1/2 again and use the 4 ohm setting.

Read up on parallel and series wiring and it'll start to make more sense! It took me awhile to get a handle on it but once you do it becomes simple.

Hope that helps out!

[Ronsonic]

The problem is, if I remove the two tubes, and change the impedance selector from 16 to 8 ohms per the instructions, now I have an impedance mismatch, because the speaker impedance hasn't changed; it's still 16 ohms. I've always heard that the amp and speaker impedances should match.

Fender knew what they were talking about.

Transformers don't have an impedance so much as they match the impedances on either side. When you remove the tubes you change the impedance on the primary side of the transformer, this changes the impedance on the secondary side. By "mis-setting" the impedance to half the usual setting it matches the fact that pulling tubes has produced twice the usual impedance on the other side. (if you think about that for a moment it sounds really wrong, but if you think about it longer it makes sense)