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Fender Twin Reverb extension speaker question

TheGrooveking

Member
Messages
2,209
A friend of mine has a 1978 Fender Twin Reverb and wants to connect a closed back 212 cab with two Celestion Vintage 30's to the amp, to use along with the 2 internal speakers. The question is in regards to impedance, the amp says 4 ohm total, but the two internal speakers are 8 ohms each and wired in parallel for 4 ohms, so therefore how can an extension speaker be connected without causing problems?

The extension cab is wired at 4 ohms also, which can be changed over to 16 ohms by changing the wiring to series, and I can do this for the two internal speakers, but then the amp would always have to run with the extension cab, otherwise it would be running on a 16 ohm load.

What do you guys think or have done in similar situations?
 

cochese

Member
Messages
2,170
My suggestion would be to try it with the extension cab first to make sure you like it. You could just solder two jacks to the internal speakers and plug them both in the amp when using just the amps speakers. When you want to use the extension cab you would need a parallel adapter for the two internals. 2 jacks to 1 and then plug both sets of speakers in. The jacks on the back of most Fender amps are in parallel. Just make sure the adapter you use is capable of a speaker load.
 

TheGrooveking

Member
Messages
2,209
Thanks for the info, I do have a few of the Reverend made Y Speaker cables, but what about the load issue, two 4 ohm loads are going to result in a 2 om load, and I don't think the Twin Reverb will do that, for very long.

TheGrooveking
 

cochese

Member
Messages
2,170
No. You need to unsolder the two internal speakers from each other and give them both their own jack. When you plug these two 8 ohm soeakers into the two jacks on the back of the twin it will be 4 ohms. When you need to hook up the extension cab you need to hook the 2 internal speaker jacks to a Y adapter and then plug that into one of the jacks on the back of the amp and the external cab to the other.
 

TheGrooveking

Member
Messages
2,209
I understand the connection of the speakers and using both speaker outputs for running both internal speakers when only using the internal speakers and then using a "Y" cord for when using an extension cabinet also. But if he does that, that will result in a lower than 4 ohm load being placed on the amp, can the Twin Reverb run a 2 ohm load?

TheGrooveking
 

cochese

Member
Messages
2,170
I see what you mean. What he would have to do is wire the Twins speakers to their own seperate jacks and plug them both into the back of the amp when only using the amps internal speakers. This would be 4 ohms. To use the extension cab he would need to wire an adapter for the Twins two speakers that would put them in series which would be 16 ohms. Then when he plugs in the extension cab set for 16 ohms it would bring the load down to 8 ohms.

He could also check with a tech to find out if the Twin is capable of running at 2 ohms. In that case he would just use the parallel adapter on the Twins speakers.
 

John Phillips

Member
Messages
13,040
Be careful what you assume!

If this is a '78 (135W/Ultra-Linear) Twin, the speaker jacks are wired in series. The Ext. jack has a switch in it which changes the amp to 8 ohms. Yes really... that's why it says 4 ohms below each speaker jack, not 'Total Load 4 ohms' below both of them, like the older models do.

:)

Which makes things very simple, because you can just plug your 4-ohm cab into the Ext. jack and everything will be fine.


(Actually you could even on an older Twin, because like all BF/SF Fenders it was intended to handle a downward mismatch of up to half the matching impedance - which is usually safe with any tube amp, in fact - and is quite capable of taking a 2-ohm load.)

Do not wire the cab in series for 16 ohms, because then it will be combined in series with the 4-ohm combo speakers giving 20 ohms, which is not safe from an 8-ohm amp.
 

jmjohnson40

Member
Messages
21
Geniuses,

I am wondering the same thing, except I have 90s "The Twin." Red knobs. Speakers are stock wired. Looks to be in series (the speakers are connected to each other once, other wires go to chassis)

Can I run an 8 ohm (7.5, actually) ext cab with the main speakers? Note: "The Twin" has a selector switch for 4, 8, 16 ohms as well as two ext. jacks, one says series, the other says parallel. What do I choose?

To complicate things further, my ext cab is a series/parallel 6x10.

I'm terrible at this kind of thing, any advice would be GREATLY appreciated.
 




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