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Elementary resistance question

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Although the Soldano Astroverb 16 guitar amplifier was made for 16 Ohm, the manual says it's possible to connect it to 8 Ohm, but will that reduce the headroom and the dynamics? If so, would it be possible and advisable to add an 8 Ohm resistance?
I may have to hook the AV up with two 16 Ohm speakers, resulting in an 8 Ohm parallel wiring.


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If you put a series resistor to bring the *nominal* impedance back up to 16 ohms you also drop the output by 6dB. Otoh, when you run an 8 ohm impedance speaker out of the 16 ohm tap, there are a few effects. First the load line for the output tubes is shifted, so the output power is reduced. How much will vary a bit depending on the actual plate impedance of the tubes and the actual primary impedance of the transformer as that impedance is reflected back based on the actual impedance sitting on the end of the secondary (output).

If you are using the *same* speakers: going from one speaker to TWO speakers in *parallel* gets you 3dB more SPL. The change in impedance will not get you the extra 3dB that you would with a solid state amp whose output Z is very low, and so produces extra current (thus power) when presented with a lower Z load. So you gain 3dB vs. the lower output power of the amp running into the "wrong load".

You can figure this out graphically, and mathematically on paper, but it is far easier to actually test the amp on the bench and see what it is doing.

As long as the tubes are not being made to produce too much current (bad) nothing negative will happen.

IF ur playing the amp full out, and ur playing a lot, maybe ur a semi-pro player who makes money doing this, then buy an 8 ohm transformer from the manufacturer, or have that work done by a shop that is able to source original factory parts, not just general purpose replacement parts. General purpose replacement output transformers, or even purpose made replacement OT will work fine, but may or may not effect the tonal quality of the amp - sometimes better, sometimes worse.

I'd advise against the resistor solution, since it eats power. Unless of course, you want to play at a slightly reduced volume OR want more "crunch" from the output stage by playing the amp up in volume more than you usually might.

You want to use a wattage resistor in *series* with the speaker that is *equal* or greater in power rating than the speaker or the amplifier's maximum *power output rating* (not the rating of the AC mains).

Wire wound ceramic resistors are easier to find in the relatively high wattage size you want. But fact is they can be any resistor or combination of smaller power resistors that do the job.

You can get those resistors that come inside an aluminum case, easy to mount, but they DO NOT meet their power specs unless strapped to a heat sink of some sort, and there is air flow. Of course you can always get one way over rated too... but there is thermal buildup in them, and I have seen them explode out of either side of the aluminum case due to that. I stay away from them in most cases. If ur playing a gig for 3-4 hours they will get hot.

Then you want to MOUNT that resistor securely so it has both air flow around ALL of it, and it doesn't FLOP around and break.

Yes you can use a switch. BUT the switch needs to be RATED for a LOT of current, and preferably should have more than one pole, AND NEVER switch it when sound is being produced.

Also, get on a users forum for guitar amps or those amps and see what others experiences have been, ur not the only one to want to do this...

".........The Astroverb 16 is designed for a 16-ohm speaker. You may also operate this amplifier in conjunction with additional speaker cabinets. An 8-ohm or a 16-ohm speaker cabinet will work fine with the Astroverb 16, but we do not recommend using a 4-ohm cabinet.........."

Almost all Fender tube amps from the early 1960s forward were built with an optional "ext" output in parallel. fyi

Thanks so much for all the great replies, that's quality information. Special thanks to _bear_. The primary reason for my question was to know if the resistor would be safe. I have a Carvin V3M amp that can switch between 4/8/16 Ohm, with an 8 Ohm cabinet, so without realizing it I already knew what it would do soundwise. Basically you're changing the dynamic signature to something somewhat unnatural.
For Guitarists Soldano is like the Ferrari among amplifiers, as in quality that comes at a price. So they developed a no-frills 20W amp for recording and for the infamous prosumer who plays at home. They left out all the non-essentials, it has no stand-by switch, no effects loop, just one channel and two outputs for 16 Ohm only. The only "luxury" is a very good spring reverb.
No-frills as it is, the avarage speaker doesn't do justice to this amp, in my opinion, so I'm eyeing certain alnico speakers of which I may be able to get a demo pair at a reduced rate, but the're 16 Ohm. Four of them would fill a 4x12 cabinet @ 16 Ohm, but would cost almost as much as the amp.

Dilemmas galore.

I did post to several guitar forums, among which Soldano, but the're like: "Buy the Soldano cabinet". But I think I have a pretty picture of the situation, thanks to you.

Keep up the good work.

I think that the issue of speakers is complicated.

Alnico itself doesn't determine very much of the subjective sound compared to the dustcap, the cone itself, the surround, the spider, etc...

So, don't get sold on some mumbo jumbo.

Fwiw, Eminence is currently making virtually*every* combination of cone and magnet possible. Really. Figuring out which one does what is a bit tricky.

I just bought one for a single 12" cabinet and picked it based on the sound clips online. It is a bit different than the online clips, but wasn't terribly expensive and if anything it's a bit more laid back and smoother than I expected but within range... then I happened into a music store while out and about and they had a box with 4 different eminence speakers... one of which was actually more like I would have picked. But they have a number of similar lines, and likely horizontally across the lines there are speakers that are likely the same or almost the same (same cones, magnets, etc...). Which makes it a bit more confusing.

But the speaker in my view is extremely critical in terms of the ultimate sound.
Hi bear,

Yes, I agree the speaker is critical. I don't know what you use them for, in a guitar rig you have the chain: guitar - pickups - amp - speakers, each of which can make or break. Actually, the last step should be cabinet, but to be honest I'm not clear how the construction of the cab affects the sound.
You clearly know more of the separate components than I do, but all in all for guitarists, I think that alnicos are good speakers and I'll explain why I feel that way.
When I tested the Soldano they first gave me a Bogner cabinet, which didn't work at all. I think it had some kind of resonating tone wood. Then they gave me a cab they had made themselves, but with the speakers I intend to buy and it sounded great.
I knew nothing about speakers at that point, but when I did some research it started to make sense.
The thing with my amp, is that it has a signature sound in the lows and mid-lows and regular speakers in a regular cab don't capture that, for instance the Celestion G12H, which I have in my 4x12. Soldano's own cabinets do capture it, with avarage quality speakers, but then 90% of the price goes into the empty cabinet. They give you Russian birch.
If I buy the alnicos I pay 60% of the Soldano cab and it works in any cab. The thing with alnico is that it's smooth. I don't know if you can say that it gives more detail, but if you have a distorted guitar sound, it does give more usefull detail in the low. The Soldano works very well with a clean, almost undistorted guitar sound in combination with the Celestions, but the distorted highs worked much better with the alnicos that I tried. Which supports my suspicion that the smoothness of alnico works well with the distorted guitar. in other brands I can discern better lows only (while listening online), but with Eminence I find certain alnicos altogether better. But then, in expensive products, manufacturers are more likely to throw in other higher quality components too.
There are a few Eminence alnicos I think are very good, but they are voiced "British", which is the most popular sound for guitar. In Celestions that sounds natural, but the American sound has a certain sweetness (compare Fender) that makes the highs of American modeling of "British" somewhat kitsch, actually. The classical Soldano and the British voicing are opposites anyway.
There a guy named Eric Johnson comes in, who has his Eminence signature speakers, and has had them voiced in a largely flat, transparant way. In the beforementioned chain every element puts in a color of its own (not counting any effects devices), so what I don't want is the last step adding yet another style, so the EJs are the best for me. I also think that with unbiased lows you can drive them harder, important to the lows-oriented Soldano sound.
So there you go, my Rube Goldberg way of explaining a simple thing :)

What do you use your speakers for and do you have a background in electronics?

I hear what you are saying, and I'd say that given two *identical* speakers, except for one having an alnico motor and the other ferrite, you might prefer the alnico. But that is for otherwise *identical* speakers. If you are not comparing identical speakers than imo the magnet has perhaps the least effect on the sound.

Try to find out what cones (dustcap, surround, etc), T/S parameters and frequency response the speakers you seem to like have first. Then see if there are other less expensive drivers with virtually the same specs. Chances are they will sound essentially the same.

I use speakers for hi-end hifi and for guitar/bass. Different speakers, different applications.

You can look at my sig file and click on my now antique website and your other question is pretty much answered in some detail.

You can doubtless find a less expensive version of the Eric Johnson speakers in the Eminence line, if you look carefully. I don't know for sure, but I'd expect so. Do they show the freq response of the EJ drivers, or is that not shown?

The cabinet will effect the sound. Sometimes rather significantly. Open, closed, stuffed, unstuffed, etc... different. The highs too can be effected by reflections and standing waves...

I'd not trust the clips on the Eminence site. They're a bit misleading. They show relative differences between their speakers, but in actually hearing them, I found the clips not representative due probably to the recording technique, mics etc...


PS. there is zero guarantee that name brand mfrs will use more expensive or objectively "better" parts or speakers than you buy aftermarket...
Hi bear,

Your website is very interesting and what a background you have. That you were a Chic roadie is cool. Those guys were my heros back in the day.
You're right, a manufacturer's sound clips are always flattering and manipulated. What doesn't help either is someone playing the same lick over and over on different speakers. What does help is guys noodling spontaneously. To my utter amazement, even on your mobile phone's speaker, youtube videos can give you an accurate impression of a product. Over the past couple of years I've bought a guitar, amps, pickups and some effects and they were exactly what I thought, compared to the youtube videos.
I have to work that way, because shops in my country hardly sell the somewhat more arcane brands. The EJs aren't carried by any shops in Europe.
But the quest for tone is a fascination journey.
As for alnico, I use Dimarzio pickups, which give a detailed, rather harsh sound. Especially on the 5th and 6th strings I'm having trouble with overtones on the crunch channel, that is, the gain amplifies overtones that sound interesting on the clean channel, but appear to be unstable when amplified. My Carvin guitar's stock pickups would filter that out. I find ferrite speakers also rather harsh, while the alnicos do their own filtering, apparently. Maybe it's best for the filtering (of frequencies, I don't know if I'm formulating this correctly) to take place at the end stage (the speakers), than right on the guitar, so the amp can do its thing on a full spectrum.
You're right in that speaker quality is in the overall craftsmanship, alnico v. ferrite is a matter taste.

- Guitarski

- Guitarski
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