Wiring a Marshall 1936 2x12 for mono

[Stevepage]

I've just got my hands on a 1936 2x12 which has had the celestions replaced with a pair of Jensen G12-70's. The cab is wired for stereo at the moment but I have a mono poweramp. Each speaker (both 8 ohms) are wired to their own jack. How do I wire the speakers to run mono? And if I wire the cab mono would that mean I would have to use the 4 ohm socket of the poweramp?

[Frank]

Depends how you wire the speakers. Two 8 ohm speakers in parallel equals 4 ohms, two 8 ohm speakers in series gives you 16 ohms.

Some calculators and wiring diagrams here

http://www.bcae1.com/spkrmlti.htm

[Stevepage]

Cheers mate. I was fairly sure I knew how to do it but I wanted to make sure, it's been a while since I've messed around with wires and speakers.

[Frank]

Careful you get the speakers in phase too!

Parallel wiring = hot from socket to both +tags of the speakers, then both -tags back to socket sleeve

Series wiring =  hot from socket to  +tag of the speaker 1, then -tag of speaker 1 to +tag of speaker2, then -tag of speaker 2 back to socket sleeve

A 9V battery across the jack will tell you if they're in phase, both cones should move in together and out together when the battery is reversed.

[Stevepage]

I''ll make sure I take a picture when I've done it just to make sure I haven't got the socket tags mixed up  :lol:

[Frank]

If you're doing parallel then it's easy, disconnect one set of connectors from the socket and connect them to the other socket exactly the same as the other speaker's wiring

EDIT: or connect the two socket hots together and the two socket grounds together, voilá!

[Stevepage]

Ok I've just finished rewiring and it's all working great. I'll be getting together with a couple of mates later at a rehearsal room so that'll give me a good chance to turn everything up. Exciting  :D

[Frank]

As an idiot check, I always put a meter across the plug to check the DC resistance before connecting it to an amp

[Stevepage]

Had a rehearsal and it sounded fantastic. Even the other guys (including a drummer) complimented me "that's one powerful sound" were their words  :D

[DoomBuggi]

What is the effect on series vs parallel? 

 Pardon me, this is an area completely foreign to me.

 I have a Marshall 412 Vintage Bottom cab.  I have flirted with the idea of replacing the V30s with the Red Coat Governors.

But alas, I would like to know about the parallel/series subject.

[Frank]

The main factor governing choice of series or parallel speaker wiring is the overall impedance (effectively AC resistance) of the combined speaker load seen by the amp's output stage. Simply put, series wiring adds all the speaker impedances together so four 8 ohm speakers would be a total load of 32 ohms. With parallel wiring the formula is more complicated

(1/R_total) = (1/R₁) + (1/R₂) + (1/R₃) + (1/R₄)

So for four 8 ohm speakers, parallel wiring them all would give you

(1/R_total) = 1/8 +  1/8 +  1/8 +  1/8

(1/R_total) = 4/8

R_total = 8/4 = 2 ohms

For the average valve amp with a choice of 4, 8 or 16 ohm outputs these are not useful choices so you would wire speakers 1 & 2 in series (8 + 8 = 16 ohm) then 3 & 4 in series (another 16 ohms) then wire the two 16 ohm loads in parallel to get 8 ohms.

Ultimately the choice depends on what speakers are fitted in your cab and what the output stage of your amp can drive.