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Post subject: Output impedance and Lo Power mode on Fender Twin
Posted: Sun Jan 08, 2012 8:00 pm
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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.

Could someone more knowledgeable tell me if that is correct?

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Post subject: Re: Output impedance and Lo Power mode on Fender Twin
Posted: Sun Jan 08, 2012 9:11 pm
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The transformer matches the output impedance of the tubes to the input impedance of the speakers. Two 6L6's have a different output impedance than four 6L6's, so by setting the transformer switch to a lower impedance when running half the tubes you're actually restoring the match, not creating a mismatch. When you pull half the tubes, you'd have a mismatch if you didn't change the transformer setting.

The rest of what you said is correct. With a mismatch there's less power output & more distortion. High vs low mismatch creates different stresses, but as a practical matter it doesn't really make any difference -- twice the impedance, half the impedance, neither one is likely to hurt your amp. On some amps, mismatches also make the tone a bit darker/muddier.

Generally the goal of pulling two tubes is lower volume and more distortion, so a mismatch might be better than a correct match. (Again, in the case of a Red Knob/Evil Twin running on two 6L6's, leaving the switch in the normal 16 ohm position is a mismatch that will give less power & more distortion.)


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Post subject: Re: Output impedance and Lo Power mode on Fender Twin
Posted: Sun Jan 08, 2012 9:29 pm
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strayedstrater, thanks for responding. Let me restate my question differently. The speaker impedance doesn't change, it's 16 ohms, regardless of how many poweramp tubes there are-- 2 or 4. It's two 8-ohm speakers in series-- 16 ohms. That is fixed.

So if the output impedance of the amp with 4 tubes is 16 ohms, it matches the speaker impedance, and all is well.

But if you pull two tubes, and change the output impedance switch to 8 ohms to match the new output impedance of the amp, now you have a mismatch with the speaker impedance. I don't notice a problem with the sound, so it's a probably a theoretical question; I should probably be practicing guitar rather than typing on my computer. :) Still, I'd like to understand why Fender says it's ok to pull 2 tubes and set the output impedance to 8 ohms when the speaker impedance is 16 ohms.

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Website: http://www.robroper.com
Fender Guitars: Am Std Tele, EC Strat, AVRI '62 Jazzmaster, Lee Ranaldo Jazzmaster, Thurston Moore Jazzmaster.
Fender Amps: '61 Brownface Deluxe, '65 Champ, '59 Bassman Ltd, Tweed Champ clone, Tweed Deluxe clone.


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Post subject: Re: Output impedance and Lo Power mode on Fender Twin
Posted: Sun Jan 08, 2012 10:47 pm
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The transformer's 16 ohm tap multiplies the speaker load by 500, so a 16 ohm speaker load presents an 8000 ohm load to the tubes. If you hook an 8 ohm speaker to the 16 ohm tap, it'll present a 4000 ohm load to the tubes. If you hook up a 4 ohm speaker to the 16 ohm tap it'll present 2000 ohms to the tubes.

The 8 ohm tap multiplies the load by 1000, so an 8 ohm speaker load presents an 8000 ohm load to the tubes. A 16 ohm speaker hooked to the 8 ohm tap presents 16000 ohms to the tubes, a 4 ohm speaker hooked to the 8 ohm tap presents 4000 ohms to the tubes.

The 4 ohm tap multiplies the speaker load by 2000, so a 4 ohm speaker load presents an 8000 ohm load to the tubes. A 16 ohm speaker hooked to the 4 ohm tap presents a 32000 ohm load to the tubes, an 8 ohm speaker hooked to the 4 ohm tap presents 16000 ohms.

Four 6L6's want to see 8000 ohms, but two 6L6's want to see 16000 ohms (similar to four speakers in series/parallel vs two speakers in series). Connecting a 16 ohm speaker load to the 16 ohm tap presents an 8000 ohm load, but connecting a 16 ohm load to the 8 ohm tap presents a 16000 ohm load to the tubes. So when you pull two tubes, you use a transformer tap with a different multiplication factor to turn the 16 ohm speaker load into the correct impedance for two tubes.

(You can view it the other way -- the transformer divides an 8000 ohm tube load by 500, 1000, or 2000 to present 16, 8, or 4 ohms at the output. If you change the tube load to 16000 ohms, then the transformer's output taps are 32 ohms, 16 ohms, and 8 ohms, so you'd use the middle tap to match a 16 ohm speaker load.)

All those numbers are just for example. I forget the precise impedance 6L6's want to see -- it might be 4500, it might be 9000, so the multiplication factors of the taps may be a bit different than 500x, 1000x, 2000x. But the basic principle carries over -- a transformer is about ratios. The "16 ohm" tap is only 16 ohms if the tubes' impedance is "A" -- if the tubes' impedance is "B" then the tap labeled "16 ohms" is no longer 16 ohms.


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Post subject: Re: Output impedance and Lo Power mode on Fender Twin
Posted: Mon Jan 09, 2012 12:26 am
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To put it another way: the labels on the switch ("16 ohms", "8 ohms", "4 ohms") are only correct when the tube impedance is a certain value. If you change the tube impedance, then those labels are no longer correct. If you pull half the tubes, then the tap labeled "16 ohms" is really a 32 ohm tap, and the tap labeled "8 ohms" is really a 16 ohm tap.

Red Knobs/Evil Twins have even heftier transformers than Twin Reverbs. Red Knobs can easily handle a 1:2 or 2:1 mismatch. With two tubes pulled and a 16 ohm speaker load, the matched tap is the middle one (labeled "8 ohms"). But you can safely use the tap labeled "16 ohms" or the tap labeled "4 ohms". Those will produce less power and more distortion than the middle tap, and since less power/more distortion is the point of pulling two tubes, you might want to experiment with those switch settings. The lo-power mode with 2 tubes pulled isn't that much quieter than the hi-power mode with all four tubes, so the additional power loss due to impedance mismatches can only help (hence my earlier suggestion encouraging creating a mismatch by not changing the switch position when pulling two tubes).

If lower volume and more distortion is the goal, another thing to consider is disconnecting one of the internal speakers. A single speaker moves a bit less air than two. A single 8 ohm speaker with two tubes pulled would be correctly matched in the switch position labeled "4 ohms". Or you could use the switch position labeled "8 ohms" for a safe mismatch. One speaker, two tubes, lo-power mode, impedance mismatch -- that's as quiet as a Red Knob can get (and that's still surprisingly loud). The switch position labeled "16 ohms" would be an excessive 4:1 mismatch.

There's a slight difference in tone between a pair of speakers wired in series vs a pair wired in parallel (in series, any differences in the responses of the speakers are magnified, in parallel they're minimized). Even identical speakers have slight variations in response, although with identical speakers the difference between series and parallel will be very subtle. Even so, you could experiment with your speakers -- two 8 ohm speakers in parallel would be 4 ohms. With four tubes, obviously the 4 ohm switch setting would be the correct match, and you could use the 8 ohm setting for a safe mismatch (the 16 ohm setting would be excessive). With two tubes, there's no correct match for a 4 ohm speaker load -- you could use the 4 ohm setting for a safe mismatch, but both the 8 and 16 ohm settings wound be excessive mismatches.

With tubes, running a high or low impedance mismatch drops the power. A 1:2 or 2:1 mismatch drops the power by 30 to 50%, so you have a few more power levels available. (That's pretty much moot -- the 15 watt setting is more than half as loud as the 100 watt setting due to the nature of watts/volume, so the in-between settings are pretty subtle gradations.)

As an aside, the hi/lo power switch changes the plate voltages of the output tubes. It doesn't turn any of the tubes off or change them from pentode mode to triode mode and it doesn't have any effect on impedance. So you can run four tubes for 100 or 25 watts with the impedance labels being correct, or you can run two tubes for roughly 60 or 15 watts (the bias and plate voltage change a little when you pull tubes, so the two tubes run a bit harder) with the impedance labels being incorrect.

Red Knobs and Evil Twins (some people reserve the nickname "Evil Twin" for the second generation Twin Amp with blackface cosmetics, with the current third generation models nicknamed the "Pro Tube Twin Amp") are great amps. They have lots of "bells and whistles" -- great effects loops, external bias test/adjust, multiple power levels, flexible EQ, etc. You can have many fun years exploring all the tones available.

When I had an Evil Twin I got a matched quartet of 6L6's for it. Most of the time I ran it with just a pair of tubes and every month or so I'd swap the pairs so the quartet wore out evenly. That way when I wanted maximum clean headroom I could pop all four in and they'd all have roughly the same amount of wear on them. It ended up being way more amp than I need but it had a lot of great sounds in it.


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Post subject: Re: Output impedance and Lo Power mode on Fender Twin
Posted: Mon Jan 09, 2012 1:29 am
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To ramble some more on why the lo-power modes don't sound as quiet as their output levels suggest -- partly that's a characteristic of big transformers and stout power supplies, plus having two speakers in a good sized deep cabinet (Red Knobs, ET's, and PTTA's are all about an inch and a half deeper than Twin Reverbs). Tubes can put out much more than their rated power when they have plenty of power supply available to them and an output transformer that's too big to saturate. Tweed Fenders have a gloriously ragged sound because they have just-barely-big-enough transformers, Rivera/Zinky style Fenders (and Dumbles) have a "big iron" sound. The 15 watt setting is easily louder than a 20 watt Deluxe Reverb.

Also, the different power levels don't change the sensitivity of the amp, just the maximum possible output. If you're using the clean channel with the volume at "2", the 15 watt mode will be just as loud as the 100 watt mode. It's only when you approach the maximum output level that the hi-power modes produce more volume than the lo-power modes. On the clean channel that's somewhere around "5" on the volume (lower if you're using hot humbuckers, higher if you're using low-output singlecoils). Bass notes will get a little woolier and there'll be more compression at lower volumes in the lo-power modes, but if you're not pushing the amp hard don't expect dramatic differences in volume between 15 watts and 100 watts. The power settings are more for fine tuning between "very loud" and "way too loud".


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