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Guitarski 12th February 2013 10:41 PM

Elementary resistance question

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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DUG 13th February 2013 03:23 AM

Yes, you could add an 8R to the 8R of the two 16's in parallel.

Your sound will change and the output will not be as high.

Guitarski 13th February 2013 08:15 AM

Cool. Then I would be looking to add a switchable 8-16 Ohm resistance. Are there quality differences in that, as in low noise, or whatever?


DUG 13th February 2013 12:03 PM

use wire-wound power resistors

bear 13th February 2013 02:48 PM

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...


Tom Bavis 13th February 2013 04:05 PM

Since the manual mentions the option, I'd assume that there is an 8 Ohm tap available on the output transformer, and you would just swap two wires to change from 16 Ohm to 8, with no loss in power or efficiency.

bear 13th February 2013 04:09 PM


jjman 13th February 2013 05:14 PM

".........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

Guitarski 13th February 2013 11:52 PM


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.


bear 16th February 2013 12:03 AM

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.

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