External Speaker Jack on a Fender amp

[ColForbin]

Alright fellas,

I need some myths either proven or dis-proven here......here is the situation I am looking at:

It's regarding the ext. speaker jack on the silverface twin.  So that amp, has 2 x 8 ohm speakers in it, making it 4 ohms.  It says on the back of the amp, if I'm not mistaken, total 4 ohm load.  But then there is this ext. speaker jack.  What I am reading online, is fender has some sort of switch built into their amps, that if I plug something into the ext jack, the amp itself becomes 8 ohms.  Therefore, if I plug a 8 ohm cab into that jack, my impedance should match up to a total of 4 ohms.

Am I completely of the edge of the map here?  Does anybody have any expertise on this?

Thanks dudes.

Edit****

I have found out that ext speaker jack is wired in parallel with the main speaker jack.  So plugging an 8 ohm cab into a twin reverb, would yield 2.66 ohms.  Now the question remains, would this be safe?  You hear everything from, "fender amps are very forgiving with impedance mismatches" to "dude, you're gonna start a fire on stage".  Looking for your opinions.....

[cactuskeeb]

Found this, which basically says don't unplug the Fender speakers if you're using an (in this case, 8 ohm) extension cab because, if you insist on mismatching load impedance, you always want to end up going lower than what the amp is actually rated for, impedance-wise. If not, you'll torch the resistors on the power tube sockets and perhaps also kill your power tubes and output transformer. So, yes, you might actually start a fire on stage.

Q: Can I plug my 8 or 16 ohm Marshall as an extension cabinet into my Fender Twin Reverb? (Don B.)

Most Fender Twin Reverb amps, including the Blackface reissue, operate only on a 4 ohm load and have no means of selecting any other impedance. Exceptions to this are the last original silverface issues that are rated at 135 watts, and later channel-switching models. Operating both together brings the speaker load to 2.7 ohms which is not a problem at all for the Twin Reverb to handle. If, however, you use only the 8 ohm Marshall cab and unplug the Fender speakers, then the amp could be at risk if you play at loud volume.

Whenever you mismatch a tube amp with a much higher load (speaker) impedance, a higher signal voltage is produced across the primary of the output transformer. One loud popping note on the high E string and you could arc-over the output tube sockets, fry the tubes, or zap the output transformer. This is why you cannot run a tube amp without a speaker connected to it. If you must mismatch the load impedance use a lower than rated one, as you are doing now by connecting both cabinets.

Having said this, and making it very clear that I do not advise anyone to connect a higher than rated impendence to any tube amp, generally there is no problem when operating the amp at 8 ohms if your amp is well designed and in good working condition. Connecting a 16 ohm load to a 4 ohm amp is, of course, more risky.

http://www.rickonslow.com/FAQ-LoudSpeakerImpedance-8-29.html#q14

[ColForbin]

Thanks Cactus,

When I use my cab w/ my DRRI, I always plug it into the main speaker jack.  After talking with Glen @ Hard Truckers he suggested this:



https://taweber.powweb.com/store/zmatch.htm

The Weber Z-matcher, impedance matcher.  Get this and I can do a variety of things, and has two outputs.  For the above situation, I can go:  amp-->input on the zmatcher (set the amp knob to 4)-->run one output to my amp speakers and the other to my cab, and set the speaker knob to 2.6.  Easy peasy as far as I'm concerned, and most importantly, it keeps the amp happy.  If I'm running my cab with my DRRI, I can use it to run the onboard speaker and amp together, instead of having to unplug the main speaker jack.

The 100 watt version, for $110 seems like a good investment.  Glen highly recommended it, he told me that they put one in Keller Williams rig and he loves it.

[sour d]

Is the twin not loud enough with just it's own speakers? Mine is ear splitting loud.

[ColForbin]

Is the twin not loud enough with just it's own speakers? Mine is ear splitting loud.

Oh God, of course it is.  I was just curious for the most part.  Also, we play outdoor festivals in the summer and stuff like that, I thought it would be super amazing to rock the twin reverb with a Hard Truckers cab.  Very retro, very 4x12, very awesome.  I played a show with it recently and it was loud, never turned the twin above 4.