? about ohms

Dave Zirbel · Post by **Dave Zirbel** » 5 Feb 2001 10:56 am

I have a Fender amp that has an 8 ohm speaker. Would powering a 4 ohm speaker hurt the amp?

Dave Z

Glenn Austin · Post by **Glenn Austin** » 5 Feb 2001 3:31 pm

Yes. You can definitely burn out the output transformer by doing this.you need 2-4 ohm speakers in series to make 8 ohms.

Dave Zirbel · Post by **Dave Zirbel** » 5 Feb 2001 3:42 pm

Thanks Glenn!

Dave

Donny Hinson · Post by **Donny Hinson** » 5 Feb 2001 5:10 pm

If the amp does not have an extension speaker jack, it probably means that the manufacturer doesn't recommend running the amp with a lower output impedance load on it. In this case (as Glenn said) get an 8 ohm speaker.

But, if the amp does have an extension speaker jack, the manufacturer assumes you're going to use another 8 ohm speaker (two 8-ohm speakers in parallel = 4 ohms). If this is the case, then you can probably safely use one 4 ohm speaker. If you do, just remember not to use that "extension speaker" jack!

Also, make sure the 4 ohm speaker will handle at least as much power as the 8 ohm speaker...preferably more. In other words, don't replace a "75-watt 8 ohm speaker" with a "60-watt 4-ohm speaker".

{This next paragraph was edited...see my next post (below) for an explanation...}

One last thing. Remember that two "50-watt speakers" hooked together (in series, or in parallel) may not handle 100 watts. [This message was edited by Donny Hinson on 06 February 2001 at 02:16 PM.]

Hamilton Barnard · Post by **Hamilton Barnard** » 5 Feb 2001 6:51 pm

...but they will still only handle 50 watts...not 100 watts! Speakers need to be hooked in parallel to increase power handling capacity.

Donnie, watts is watts. You're gonna have to explain that one.

To make my point, Marshall cabinets are wired parallel/series, yet the cabinets are rated for the combined total wattage of the speakers, i.e.- four 25 watt Celestians is a 100 watt cabinet.

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Ole Dantoft · Post by **Ole Dantoft** » 6 Feb 2001 12:18 am

Donny, Hamilton
2 identical 50-Watt speakers combined gives a total power-handling capacity of 100 Watts, in series OR in parallel, doesn't matter.

The power distributes itself across the two loads evenly, but of course only when the speakers are exactly the same impedance.

Mr. Ohm proved all of this long time ago, in his Law.

Ole[This message was edited by Ole Dantoft on 06 February 2001 at 12:30 AM.]

Vernon Hester · Post by **Vernon Hester** » 6 Feb 2001 2:24 am

Power = E2/R, If you measured 10 volts AC across the speaker,squared=100 with 8 ohm speaker, load would be 12.5 watts. 4 ohm load with same voltage 25 watts. But you have to have good test equipment and monitor points to keep from blowing up the amp. Be careful.
Vern

patrick donovan · Post by **patrick donovan** » 6 Feb 2001 5:44 am

If the speakers are in series it is a voltage divider network, assuming equal speaker impedance and Vern's 10 volts (RMS should be used, Root Mean Square, with AC) you would have a 5 volt drop across both speakers, so use 5 volts in the power equation. In parallel the drop would be 10 volts on each speaker.

Stick with the manufacter's specifications/recommendations to avoid problems.

Regards, Patrick

Donny Hinson · Post by **Donny Hinson** » 6 Feb 2001 2:00 pm

I spoke to an "expert" today, and he told me that what Ole and and Hamilton said is essentially true. <BLOCKQUOTE>quote:<HR>If identical speakers are hooked together (in series, or in parallel) they will handle the combined wattage (power rating) of the two speakers.

However, the key word here is identical. Though speakers of different types and different manufacturers are the same "rated" impedance, they may not be the same electrically. The reason is that they are a "reactive" load, and their actual impedance varies over frequency. In other words, a speaker "rated" at 8 ohms actually has an impedance of somewhere between about 2 and 30 ohms, depending upon the frequency of the signal applied to it. Speakers usually are rated at a frequency response of something like 40 Hertz to about 16,000 Hertz, and the true impedance will vary many times across this bandwidth. For this reason, manufacturers usually recommend (and use) identical speakers. Mixing speakers with different impedance curves, and then running them near (or at) their maximum power ratings might cause a problem. <HR></BLOCKQUOTE> The only exception to this rule is when audio engineers decide to "smooth out" frequency response anomalies by using different sized speakers. This is usually done only in Hi-Fi gear, and the speakers are specially chosen or designed to work together. Most always, crossovers are employed to assure that each speaker is being operated in its intended frequency range.

So, if you use identical speakers, you should have no problems.

[This message was edited by Donny Hinson on 06 February 2001 at 02:22 PM.]

Matt Farrow · Post by **Matt Farrow** » 7 Feb 2001 9:44 am

I think we are getting away from the actual question:

<BLOCKQUOTE>quote:<HR>
I have a Fender amp that has an 8 ohm speaker. Would powering a 4 ohm speaker hurt the amp?
<HR></BLOCKQUOTE>

We need more information to correctly answer your question. Please look at the back of your amp, at the speaker jack and tell us what it says. Usually, it will indicate the total acceptable load impedance, and the output wattage. If it's a Fender tube amp, then if you tell us the model name, and preferably the year or schematic code (ie AB763, AA768, etc.) then we can tell you what the manufacturer thinks is the right answer to your question.

Tube amps are generally more forgiving of an impedance mismatch. I know a player who used a blackface Bassman with two 4-ohm cabs (2-ohm total load) at full volume for years with no damage. And I have seen solid-state amps that were run at too low an impedance for only a few hours with burned up output transistors and blown power transformers, so go figure.

What kind of amp is it, what sort of speaker is in there now, and what kind of speaker do you want to use?

Thanks
Matt Farrow
Pharaoh Amplifiers.

Dave Zirbel · Post by **Dave Zirbel** » 7 Feb 2001 11:30 am

Matt, it is a '73-'74 Fender Deluxe Reverb, 22 watts, one 12 inch 8 ohm speaker. The back has an extension speaker jack. I tried running a 15' 4 0hm speaker to get better bass response and possibly more headroom. I wasn't really thinking of using it for steel(maybe for real quiet gigs), but primarily for guitar, although the steel sounded pretty good and loud with the 15" speaker. Lead solos cut through and sound good but it needs more body when playing chords. The Twin is too loud and heavy. I'd like to make the Deluxe work. I'm tossing the idea around of turning the tremelo channel into a midrange control.

Thanks, Dave

Matt Farrow · Post by **Matt Farrow** » 7 Feb 2001 12:29 pm

Dave, are you using the stock 6V6s in your amp? One thing you may want to think about is switching to 6L6s. This will give you a little bit better clean headroom. Also, the tubes will last about 4 times longer. Keep in mind that there are lots of speakers out there with better bass response than the stock Fender speaker. A JBL, EV, or Weber speaker will provide you with a richer, deeper, warmer sound without having to lug around a bigger box. Also, you could have a new cabinet made for your Deluxe that would accommodate a 15" speaker. I think that you are safe with a 4ohm speaker with this amp, but if you play it at high volumes, your power tubes will die a lot sooner. Are you using a 5U4 (tube rectifier?) If you use 6V6s, then you should use a tube rectifier. If you switch to 6L6s then you can put in a solid-state rectifier, which will give you a faster attack, and will help eliminate "sag" in the power section of your amp.

As far as midrange, do you want less mids or more mids?

The Deluxe Reverbs have the standard Fender tone stack, but instead of a midrange pot, they have a fixed resistor. There's a 6k8 resistor soldered from one lug of your Bass pot to ground. If you want less midrange, lower this value, and if you want more, raise it. The best thing to do might be to put a pot in there temporarily, dial it in and then take the temporary pot out and measure it. You can probably find a resistor close the the measured value. De-solder the 6k8 resistor from the back of your Bass pot. Take a 10K pot, and using a pair of clip leads (these are wires with "alligator clips" on each end) clip the middle lug of your 10K pot to the lug on the Bass pot where the 6k8 resistor was connected. Clip one of the other terminals of the pot to the chassis ground. (There are lots of little bare ground wires, just find one that goes to the brass plate that the control pots are on.) Once you find the "sweet spot," take out the pot and measure it. Put in a resistor of the same value across the Bass pot where the 6k8 resistor was. Don't take the 6k8 resistor all the way out, instead leave it hooked up on one end and put some heat-shrink tubing over the free end so it doesn't short out. This way you can return the amp to stock if you decide to sell it later. If you have any questions, email me.

Matt Farrow
Pharaoh Amplifiers

Jerry Gleason · Post by **Jerry Gleason** » 7 Feb 2001 5:24 pm

Matt's excellent post above assumes a certain level of technical knowledge. It 's worth mentioning here that there are potentially lethal voltages inside an amp chassis like this, even after it has been off for a long time. It you don't know what you're doing, it's probably best to take it to someone who does.

Matt Farrow · Post by **Matt Farrow** » 7 Feb 2001 5:36 pm

Yes, of course! I apologize for not mentioning that, that was inconsiderate of me. At any rate, the filter caps in your amps may be discharged by unplugging the amp while it is still on, with the standby switch still on. Amps with tube rectifiers have resistance inherent in the powe rsupply and will discharge themselves if allowed to. I usually discharge amps with solid-state rectifiers by touching a pointed probe from pin 1 of any preamp tube socket with the other end of the probe lead earthed to the chassis for a minute or two. Most Fenders have "bleeder" resistors that will automatically discharge the power supply if the standby switch is left on and the power is turned off.

Moral of story: If you've never been inside your tube amp, get some help from someone who has. Tube amps contain potentially LETHAL voltages and currents even when switched off and unplugged, so be careful.

If you need help, email me.

Matt Farrow
Pharaoh Amplifiers

Vernon Hester · Post by **Vernon Hester** » 8 Feb 2001 12:45 am

Back to your original question,a Fender deluxe will drive 2 to 24 ohm load,but the best sound for the Steel is a JBL-D130,I used one from 1951 till 1977 if need more level for a Gig put a good mike in front of it.you can see my old one in the Sho-Bud Gallery We are playing in an 13000 seat room that day in 1969.Leave the 6V6 tubes in the amp.E-mail me and we will get technical.
Vern

Jack Stoner · Post by **Jack Stoner** » 8 Feb 2001 3:58 am

Not everyone has the luxury of being able to mike their amp if they need more volume level. Also the volume level has increased over the years in clubs and what was "more than sufficient" is underpowered today. Back in 61 I played through a stock Fender Bassman amp, but that amp wouldn't cut it in many venues today.

Dave Zirbel · Post by **Dave Zirbel** » 9 Feb 2001 7:23 am

Matt, I tried 6L6's in the Deluxe. Sounded pretty cool but I didn't get more headroom, maybe because the tubes were old and I didn't adjust the bias. It sounded really fat and warm at a lower volume, good if you're playing Rolling Stones. Doing that made me wonder what would happen if you swapped the 6L6's in a Twin Reverb with 6V6's.

What do you think? Dave Z

Hamilton Barnard · Post by **Hamilton Barnard** » 9 Feb 2001 1:27 pm

6V6s are only rated for 400 vdc on the plates.

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Donny Hinson · Post by **Donny Hinson** » 9 Feb 2001 4:10 pm

Using 6V6's in a Twin Reverb is a bad idea. The plate voltages here are WAY over what a 6V6 was designed for. Also, using 6L6's where a 6V6 is called for is probably a bad idea. They draw about twice the filament current of the 6V6's. If used for a long time, they could ruin the power transformer, which is expensive...and maybe hard to get.