I'm looking of some advice here, as I am new to SFDR's. I recently purchased a 1973 Deluxe Reverb that is already running the 6L6 mod. The amp was sold at a reasonable sale price since the OT had been replaced with a modern Fender replacement. The other work that has been does is the tubes changed to JJ Tesla throughout, rebiased, re-capped, diagnosed, cleaned, lubricated (dials aren't even scratchy) and then the replaced output transformer.

Anyways, I'm looking to replace the original oxford speaker with a Weber 12F150B, but I'm confused on now what the ohm of the amp is and what ohm to order a new speaker at. The original owner told me, "from what I have read, the impedance with the 6L6 mod is effectively 4 ohms - but the original speaker is 8 ohms." (??).

Any advice and input is very much appreciated. Thank you!
Google the codes on the OT and see what it actually is?
Gilchrist custom
Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band
would it be possible to measure this at the speaker to see what the amp is doing?

couldn't a multimeter at the speaker tell you what the amp is doing putting out at the speaker.

it may become obvious that i am not an electrical engineer. so take that all with a grain of salt and do what Cath said first.

good luck.
I wondered why the frisbee was getting bigger, then it hit me.
What Cathbard said is probably the best bet, find out what the transformer does. It should have a number stamped or inked in it, the first 3 digits are usually the manufacturer code.

The original amp was 8 ohm, with 6V6 tubes. A 6L6 swap shouldn't change that, the tubes don't set the output resistance, the transformer does. Some 6L6 amps are 8 ohm, to handle one speaker, usually standard for Fenders, others are 4 ohm, when they used two wired parallel.

This is the problem with modified amps, when you don't have good documentation on what was done. My Super Reverb has been modified a little, but I won't be selling it in my lifetime...Champ too. I'm using a 6L6 power tube instead of the original 6V6. Minor mods to do that, mostly reset the bias voltage.. But if anyone else ever gets these amps, they won't know just what has been done. But the impedance hasn't been changed on ether one.

If the modern Fender replacement was used for the transformer, it's probably still 8 ohm, the newer 65 reissue is also an 8 ohm amp so it's probably the same transformer. The Fender Amp Field Guide lists the original Deluxe Reverb as using the AB 868 circuit, nothing listed for the reissue, but the rectifier tube is different so it may have a different transformer. The output transformer is the one that makes a difference, it's the one that sets the impedance. Many also have multiple wires, which is how they build some amps with selectors for 4, 8 or 16 ohms.

If I were to gueshttp://www.thevintagesound.com/ffg/s, I'd say it's probably still 8 ohm, I don't know of a way to check it with a meter. You can check the speaker itself, but that only tells you the impedance of speaker, not what the amp is delivering. Just pull one wire to isolate the speaker and check impedance, on the 20 or 200 ohm scale. (whichever is your lowest range)

Fender Amp Field Guide - http://www.thevintagesound.com/ffg/

Reading the codes - http://www.triodeel.com/eiacode.htm
Hmmm...I wonder what this button does...