Speaker/amp impedance mismatching to reduce volume?

[Cliff Schecht]

I've been running my Express at 8 Ohms through a 16 Ohm cabinet at band practice for a good while now to cut the volume down a bit. It doesn't really kill the headroom with EL34's and the distortion sound business as usual, just a bit quieter. I don't find it has a terrible impact on tone either, you lose a bit of the balls that the Express has but it'll still rip and cut through the mix if everyone else is staying at a reasonable volume. 4 Ohms into 16 Ohms does sound like crap, very weak and thin sounding.

So what's the down and dirty on doing this? My intuition tells me that as long as a lower impedance is feeding a higher impedance than I am always understressing the output. So I'm not so worried about blowing the speaker or even the OT, but how do the power tubes react to this change? Doubling the impedance does shift the load line down a bit which, in the hifi world, means you have to drive the grid harder to get the same power output. But in guitar amp world, we're already clipping the output so instead it gets even more saturated. You increase plate resistance and decrease the available output power. Seems alright to me.. Is there a reason why more people don't use this method to cut down on volume?

[rooster]

OK, take a look at the Fender 'Tone Master' amp schematic. Here they elected to list 'Low - Med - High' instead of what must be '16 - 8 - 4' and they do so without much concern to the amp or tubes in their 100 watt amp. Kinda interesting. They also go on in the literature to say the wattage becomes '30 - 60 - 100'. ??

Now it may turn out that those impedances are not what I think they are, but then again, they could be. And actually, having heard the amp recently at all settings, it sounds pretty good in all of them - and the reduction was pretty obvious if not actually '30 - 60 - 100'.

OK, and then there is the story from Glen where he reported that Ken told him to try all the different settings, and use whatever one worked for whatever he might be doing at the time. And Glen reports that he does use the impedance switch on occassion to mismatch things to get the volume down.

For myself, I tend to get too anal about it, going for the correct matchup. But then I realize that the EL34 typically sees 3200 ohms primary from Marshall, and not the 6600 ohms it sees in th Express. Eh, which is to say a 16 ohm cab should see the 8 ohm tap if Marshall had their way... ??

[Alexo]

Ehhh I would be hesitant to do it.

But it all depends on the individual amp and the way it's played. As long as you aren't saturating the output stage, you will just lose a couple db and there's no harm done.

But if you're running into a giant load (like 13K+!!) and your driving signal is hot enough to push the plates down to such a low voltage that the screens start acting like the plates and pulling what should be a hefty dose of plate current, they ain't gonna last too long. Those 1K screen R's do give you some protection, (as long as they don't pop) but I'd advise you to keep an eye on your screens under heavy signal and scale it back if they start glowing.

The stock 'wreck is already pushing it with a larger-than-typical load (which helps it sing, imo, lotsa screen current into 1K screen R's = nice compression).

The caveat is that if your pi is already maxing out before it can push the output stage into heavy clipping, you're probably safe. Kinda depends on the bias voltage, the pi tail resistor, etc..

Basically if you go into a lower load, you're stressing the plates more, and if you go into a higher load, you're stressing the screens. The plates are typically much more durable than the screens, IME.

...so that's the reason more people don't do this, I think... that and it sorta non-linearizes your output (not neccesarilly a bad thing). Sometimes an impedance mismatch can do wonders for your tone (and sometimes not) but I personally treasure my precious old stock glass and mind the load carefully so that they can live long, healthy little lives.

....long story short, hook yer amp into a signal generator and measure your screen dissipation under these conditions. Or just use cheap/readily available glass. ...my $.02

[selloutrr]

for what it's worth. If you are experimenting with Mismatching the Impedance/ohm load. DO NOT RUN THE AMP FULL POWER or through any sort of power attentuation. The exception is if the Attentuation has a selectable Ohm matching built in. i.e go into the Attentuator at 8ohms come out selectable at 8 or 16.

honestly most larger transformers can handle the mismatch. It was common to over build the iron. But the transforner is still creating current that is not being distributed to the properly. Thus it's turned into excess heat. This can cause serious damage to the OT, tubes, and other components if it decides it's had enough abuse and fails.

Can't say it'll happen can't say that is wont.

I personally run the proper ohm load for the design of the amplifier, I double check to make sure the amp has a load before I plug it into the wall and again before I turn it on. I'm pretty OCD, if an amp has set unpowered for over 6 months I reform the caps before turning it on, or put it in series with a lightbulb. 15-20 years gets new filter caps, and if a tube tests below 80% on the tester it's replaced.

[lumox0013]

I have been running mine that way as well,from my limtd understanding this would double the load that the tubes see- ex. from 6.6k to 13.2k yes or no?

[rooster]

lumox - Technically, to reduce the primary (6.6K) to 3.3K, you would cut all the speaker cab loads in half, so 4-8-16 becomes 2-4-8. So, yeah you have this correct, putting a 16 ohm load on the 8 ohm tap would be making the primary 13.2K.

I guess if you're going to do this kind of thing, the question becomes what direction should you go, up or down? Eh, me, I could see putting a 4 ohm cab of the 8 ohm tap because this would make the OT 3.3K. As to then running the 4 ohm cab on the 16 ohm tap, well this would be like 1.65K. Hm not so sure the EL34 tubes would like that for too long.

So.... To be in the middle of all this primary adjustment, given the urge, it looks like an 8 ohm cab is maybe the best cab to use for the best overall variety. The 4 ohm tap would be 13.2K, the 8 ohm tap would be 6.6K, and the 16 ohm tap would be 3.3K.