Basic speaker load question

[mort]

Deluxe Reverb Reissue says 8 ohm total load for the speaker outputs and comes with an 8 ohm speaker. The manual states that higher or lower than 8 ohm total load and the amp will not put out maximum power before distortion occurs, and it doesn't say anything else on the subject.

Am I to take that as it's safe to the amp to add an 8 ohm cabinet to the ext jack, making the total load 4 ohms? That it might behave a little differently but that's it?

I've always gone by the old rule of "over but not under" the rating is safe for the amp.

[g1]

That is the standard way Fender has done it on all the old vintage amps.
So the re-issue is faithful to the original in this respect. All the old Fenders got mismatched to a lower impedance load when the "ext.spkr" jack got used.

[mort]

So then what I've read about low load being bad for the OT is not as critical as I had assumed? And perhaps that only applies to an extremely low load, such as 2 ohms or less?

[g1]

I think it will run the tubes a bit harder, but I don't see an issue with the OT. I guess if you were really pushing it maybe?

[Leo_Gnardo]

Deluxe Reverb Reissue says 8 ohm total load for the speaker outputs and comes with an 8 ohm speaker. The manual states that higher or lower than 8 ohm total load and the amp will not put out maximum power before distortion occurs, and it doesn't say anything else on the subject.

Wish I could remember where I saw this maybe 2 years back, a claim that Deluxe Reverb would be happier with a 16 ohm speaker rather than 8. So . . . the next couple of Deluxes that passed my repair bench, I tried with 16 ohm as well as 8 and 4 ohm load resistors, and in fact the power was up a bit at 16. The definition of a matched load is that which enables the maximum power transfer. Even with this, nobody seems to be in a rush to swap in 16 ohm speakers for their Deluxes.

In another case, just last night I was working on a Traynor YCV40 2x10 combo. With a pair of 6L6's it was turning out a meager 22 watts at clip into a pair of Celestion G10E-30 8 ohm speakers, factory wired for 4 ohms. Fresh output tubes raised this figure 30% into the high 20's. I ran the amp into 4, 8 & 16 ohm resistors, found it maxes 35 watts into 8 ohms. The owner had me install a pair of early 90's 16 ohm Celestion G10S-50 I was lucky to have in the store room. Not only do we have a power jump over 50%, the tone of this amp is out-of-this-world on the overdrive red channel, very throaty like a good opera singer. I expect the owner will be astonished when he gigs with it tonight.

So why not test your Deluxe with load R's, see where your power max really is, and don't be afraid to try a 16 ohm speaker. Granted, it isn't the ohms that makes the tone, OTOH there's something to be said for the maximization of good tone when the impedance between amp & speaker is properly matched.

[mort]

In this case I'll never see the amp. An online acquaintance was inquiring about having me build him an amp. He loves the DRRI but said it was unpleasant when it breaks up and also wanted t get a little more clean volume out of it. Turns out it still has the stock speaker so I suggested getting a louder and better speaker. He's gonna try a Swamp Thang in it (I had used one in a DRRI and it did exactly what he's looking for), and he wants me to build him an extension cab, so I just wanted to remove any doubt on what speaker to order for it.

[Leo_Gnardo]

In this case I'll never see the amp. An online acquaintance was inquiring about having me build him an amp. He loves the DRRI but said it was unpleasant when it breaks up and also wanted t get a little more clean volume out of it. Turns out it still has the stock speaker so I suggested getting a louder and better speaker. He's gonna try a Swamp Thang in it (I had used one in a DRRI and it did exactly what he's looking for), and he wants me to build him an extension cab, so I just wanted to remove any doubt on what speaker to order for it.

If you're going to build him an amp, you can put in a multi tap OT then he can have a proper impedance match with or without the ext cab. Similarly you could put a multi tap OT in his existing DRRI. AND there's nothing like experience with a speaker to get a recommendation so Swamp Thang it is. Slosh away! Good all around.

[Richard]

Does this mean that I should try my amp into different speaker loads and whichever yields the highest output is closest to ideal and I should use that? I just want to confirm that this testing is safe. Also would testing with a smaller signal (and therefore output) give similar results?

[Leo_Gnardo]

Does this mean that I should try my amp into different speaker loads and whichever yields the highest output is closest to ideal and I should use that?

That would apply if you have committed to a particular output transformer that has only one output. You send in a sine wave, look at the output on a scope while driving load R's typically 4, 8, & 16 ohms, figure out which gives you the max power to the resistors at the clipping point, then you know what the ideal load is.

I just want to confirm that this testing is safe. Also would testing with a smaller signal (and therefore output) give similar results?

As long as your connections are secure (no dodgy wires wrapped around wires OK?) and you don't run your amp for extended periods into mismatched loads, all is good. You should be able to figure it out in 5 or 10 minutes of testing, including calculations. Simple stuff, P = V squared R, good ol' power law.

I don't see what's the use of testing at lower signal levels, you're trying to find the best impedance match & that's done at amp's clipping point.

Again, you can select an OT with 4, 8 & 16 ohm output taps so your friend can get the best out of his amp. Even with that, it still pays to check once your amp is built.