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Two Letters about … Well, Twins!

Jeff,

I took out one of the 12" JBL speakers in my 1970 Fender Twin Reverb and put it in an old vintage Band-Master cabinet to spread out my sound when we play outside. I then plugged the cabinet into the external speaker output in the back of the Twin. Is playing with only one 8 ohm 12" speaker hurting the amp? Personally, I didn’t think so since they’re supposed to be in parallel. But is playing with both speakers that way affecting anything? Thanks.

Mark Roberts
Santa Barbara, CA


Hi Mark,

Thanks for reading the column. From everyone here at Premier Guitar, we appreciate it. Let me just start by saying that I like the idea of removing one speaker from your Twin and placing it in an external cabinet for occasional use. Removing one speaker, as you mentioned, reduces the load on the output stage to only 8 ohms, which reduces the available clean output power in realistic terms by 30 to 40 percent. For playing smaller indoor gigs, a 60–70-watt Twin should be just fine—I know I’d prefer it. It allows the amp to soften up slightly by compressing a bit more due to the reduced output power. Granted, you won’t move as much air as you would with two speakers but that could be a good thing too, especially for people in the first row! Plugging the additional speaker into the extension speaker jack, since they’re in parallel in this amp (not so with the 135-watt version), returns the load to 4 ohms and the power back to its original 100 watts, so you’ll have a bit more clean headroom for larger outdoor gigs. Also, the additional cabinet can help make the sound more spatial. As far as playing the amp with only an 8-ohm load, I personally don’t feel that it causes any harm, nor can I say that over the years I’ve seen any damage that I could directly attribute to this. Over the years, players have mismatched the heck out of those amps and that output transformer (p/n 022889) and the amps still keep kickin’, so I don’t think I’d be concerned.

Hey Amp Man,

Can I have two Celestion Vintage 30 16-ohm speakers in my 135-watt Silverface Twin Reverb? The amp has two speaker inputs and they each have “4 ohms” labels. Years ago I had the amp in the shop and I asked them to put the Vintage 30s in there. Well, I pulled the amp out recently and noticed there were two 16-ohm speakers in there (makes 8 ohms) so I pulled them out. But I really liked the sound better. I’ve read lots of confusing stuff about this 135-watt version of the Twin, specifically about how it can handle the different ohms. But I don’t want to mess it up and I hope you can set me straight. Thanks.

Mark


Hi Mark,

The short answer to your question is yes, but there may actually be two different configurations possible here, one being more optimal than the other. I’m assuming your statement, “But I really liked the sound better,” means that you preferred the sound of the Vintage 30s in your Twin over some other speaker options. Since you stated that you had the Celestions installed “years ago,” I have to assume that you’ve been playing the amp for years with no issues (otherwise you’d be writing me about some catastrophic meltdown of your amp!). Since it’s been fine up until now, I wouldn’t expect, nor would I normally expect, that you’ll be having any problems due to this setup. That said, let’s explore another possibility.

In looking at the schematic for the 135-watt Twin (which, by the way, was the same chassis used for the Showman Reverb, Quad Reverb and Super Six Reverb of that era), one of the most notable differences is that the two speaker output jacks are not in parallel, and that there is an additional output tap on the output transformer. Very interesting! In following the output wiring I see that when a plug is inserted into the main speaker jack a connection is made to the 4-ohm transformer tap through the switching contacts of the extension speaker jack. This would be perfect for the stock 4-ohm load of the standard speakers. When a plug is then inserted into the extension speaker jack the extension speaker is connected in series with the main speaker load and they are connected to the 8-ohm tap on the transformer. Perfect scenario, and optimal if you’re using a 4-ohm extension cabinet. But here’s where it gets interesting: if no plug is inserted in the main speaker jack and only the extension speaker jack is used, a connection is made only to the 8-ohm tap of the output transformer with the ground connection being made thru switching contacts of the main speaker output jack: optimal if someone had a pair of 16-ohm Vintage 30s in their amp. Using this jack, you would have full power transfer to the speakers and a proper impedance match, but if you prefer the sound of the amp with the speakers connected to the main speaker output— possibly due to the 30- to 40-percent reduction of output power—I’d say continue to run the amp that way and enjoy.

I hope that helps the twin Marks with their twin questions about the Twin Reverb.

‘Til next time…


Jeff Bober
Jeff Bober, Godfather of the low wattage amp revolution, co-founded and was the principal designer for Budda Amplification. He can be reached at pgampman@gmail.com.

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