Guitar amplifier speaker impedence question.

[starfieldwild]

I just bought a used fender hot rod deluxe 8 ohm 40W tube amp... and took it back to the shop i got it from because the fender eminence speaker was blown, they dropped in a Celestion 100W speaker (G-12T 100) but its only rated at 4 ohms.

Does anyone know if this is a good or bad thing, i suspect that its not good as the amplifier is rated for an 8 ohm speaker (but maybe that only applies for solid state amps), i'm concerned about risking damage to the speaker or to some components(s) internally within the amp, or perhaps less power that the amp will drive or possible distortion at higher volume level???

Unfortunately on my fixed budget this is the only amp i currently own and need it to do some recording with my band.

[Scodiddly]

It's not ideal, but it isn't a big enough difference to cause any damage.

[soundguy]

presumably bad bad bad.

you want to have a speaker that is equal to greater than the impedance that the output transformer wants to see, but never less. If the output transformer only has an 8 ohm tap you can damage the output transformer which is probably the most expensive single part on your amp. you want an 8 ohm speaker, or a 16 ohm speaker or if you keep the 4 ohm, you need to drive it off a 4 ohm tap from the output transformer. Ive never melted an output transformer, but I think it boils down to a heat disappation issue which can melt the transformer...

dave

[Scodiddly]

Hmmm... my understanding of tube amps is that they're actually happier with too low of an impedance than too high, and also that a 2:1 mismatch is within general tolerances. Certainly I've seen some Fender designs that mismatched impedances slightly, using the same exact chassis (transformer #s the same) with different cabinets.

[starfieldwild]

I found this statement on Fender Amps from Mr. Gearhead.com:
#2 applies to my case, but what is considered too large of an impedence mismatch is my next question i guess...

[i]Q. "How does speaker impedance relate to the amplifier output impedance?"

A. This answer will differ depending on whether you have a solid state amplifier or tube amplifier. The term "speaker impedance", refers to the load which either a speaker, (or the combined load of multiple speakers in a cabinet), presents to an amplifier. The load may vary depending on whether the speakers are wired in series, parallel or series/parallel.

1. In the case of solid state amps, there will normally be a "Max Load" (also called "Minimum Impedance") rating for the amp. The load plugged into the amp should never go below the minimum impedance or serious damage may occur to the amplifier. Maximum power will be obtained when the speaker load matches the minimum impedance. It is perfectly safe to use a higher impedance speaker load, but there will be a corresponding drop in output power (i.e. an 8 ohm speaker load will result in approximately half the output power of a 4 ohm speaker load).

2. In the case of tube amps, it is best to match the speaker load as closely as possible to the amplifier output impedance. In theory, there is some small amount of mismatch (either larger or smaller) that will not hurt the amp. It is, however, always recommended that you match the speakers as closely as possible to the rating of the amp to avoid potential problems. [/i]

Thanks for the responses thus far, it seems i'll be giving this shop another call tomorrow...

EDIT: I'm reading from the Fender Discussion Page (FDP) from misc. impedence issue threads that several people mention that they think or heard that 100% mismatch is ok w/ Fender Amps (tube).

Also on the Hot Rod Deluxe (HRD) there is a jack to plug into an external cabinet so that both speakers run in parallel at 4 ohms. I don't think i'll be able to do that since i already have one speaker at 4 ohms (i didn't plan on running an external cab anyways) well the belief is when using an external cabto drive 2 speakers at 4 ohm the output is louder but not as tight which is what i think i'll be getting now, not big deal for me at the moment as long as i don't destroy anything until i get a matching speaker to be on the safe side. but i first want to try plugging the internal speaker to the external jack to see if the sound changes at all (which would diffentiate the 8 to 4 ohm).

[daede]

As I understand it, you can put a 16 ohm speaker to an amplifier that wants 8 ohms with no issue other than a loss of volume due to speaker inefficiency. I wouldn't recommend hooking a 4 ohm speaker to an amp that wants an 8 ohm load.

The shop that did that work should either take it back and not charge you, or replace it with a speaker with the correct rating for your amp. In theory they could put a 16 ohm speaker in there and you'd be better off, but I find you get the best tone with a speaker properly rated for your amp.

[RayCathode]

Also on the Hot Rod Deluxe (HRD) there is a jack to plug into an external cabinet so that both speakers run in parallel at 4 ohms. I don't think i'll be able to do that since i already have one speaker at 4 ohms...

Yes and maybe NO. The old tweed amps external cab output was simply parallel to the internal speaker so adding one more 8ohm speaker would reduce the total load to 4ohms. Tolex models often used a seperate winding within the output transformer that often got roached by people running only on that. But still, those amps run 4ohm loads fine.

I dunno your amp exactly but 4ohm load isn't likely to hurt anything. Tubes may wear a little faster but the OT should be fine unless it's a POS and probably gonna die sooner or later anyway.

However, I would not be happy with a store sticking whatever they had laying around, unless you gave them permission to get it working for the least amount of money? The best amp/guitar will sound like shit with a bad speaker! That's fine if that's what you want but if you want to find a good sounding guitar speaker for your style check this site - www.webervst.com

[puls]

you could maybe go to harmony central (www.harmonycentral.com) & check out what the folks there have to say. Lots of guitar players there....

jwp

[SKEETER]

Most newer amps use a LM3886 op amp that can handle 4 ohms. 4 Ohms will cause the amp to run a little warmer, but it may not be enough to cause probs. If it is another chip find the rating for the chip. A lot of amps out there will run at 4 ohms, some Crates do 2 ohms. If the chip is designed for 4 ohms, the worst case scenerio is that 4 instead of 8 could cause premature falure of the chip, because of it running warmer. It could also have no effect, depending on the chips rating.

[starfieldwild]

I'm fairly new on tube amps, so i don't quite understand what LM3886 op is or the chip that will tell me the rating. i printed the schematics from Mr Gearhead for the Hot Rod Deluxe and i don't see anything, its hard to read too.

I've gone on Harmony Central and the Fender Forum and pretty much the consensus is that i should get an 8 ohm speaker... and its somewhat foggy as to the effect of using the 4 ohm speaker, it could cause problems and it may not, the amp does run on 4 ohms in parallel when running w/ a cab which tells me that it may not be too detrimental (at least short term) to have this 4 ohm speaker. and if it has a multi tap transformer which i'm not if it does or not then 4 ohms should be ok.

[SKEETER]

My fault, I assumed because it was a Fender that it was solid state. If it is a tube amp, getting info out of Fender will not be easy, find out who in your town repairs tube amps, almost every town has a guy or two that works on them, some of them have figured out the parts in Fenders (from what I hear they are pretty proprietary and parts in them are unmarked, not sure how true it is, older Fenders I have had had unmarked parts). Perhaps one of the local techs can tell you if the output transformer will handle 4 ohms ok. If it does, that is a good thing most likely because it will make your amp a little more efficient, and you may get some more beef out of it.

[starfieldwild]

Skeeter, thanks for the input which is something i'll look into...

However at the moment I have found a temporary solution to run the amp safely with the 4 ohm speaker at the suggestion from someone at the Fender Forum Discussion Page:

The Hot Rod Deluxe has both an 8 ohm and a 4 ohm tap on the output transformer, as it runs with an internal 8 ohm speaker and is able to run with an additional extension speaker cab at 4 ohms parallel . To use with a single 4 ohm load (as presently) a "blank" 1/4" plug must be inserted in the Main Speaker output (the one furthest from the tubes) and the plug for the speaker goes into the External Speaker Jack (closest to the tubes). This is necessary because of the way Fender wired up the jacks.

I've done this and the amp seems to work just fine i don't hear much compensation volume wise and i haven't used it enough to determine how the tone has been affected but it gets me thru the immediate need for band practice this weekend without risking damage to it.

[SKEETER]

I am glad you got it resolved. I do think it is pretty bogus that if the amp is under warrenty it was not repaired with exact spec replacement.

[starfieldwild]

well i bought this amp used which pretty much makes any warrenty thru Fender null and void as i understand it... perhaps it is pretty bogus but i guess the worst case scenario was if the shop just said oh well you want a replacement speaker we'll order one at your cost, because basically i am slightly at fault here. i did try the amp out in the store and bought it as is which ultimately is their policy. i did learn from this discouraging experience.
yet they should know better about using the proper impedence speaker in amplifiers, and they could've at least told me about it, but something tells me that they just didn't even think much about the impedence issue.

[Scodiddly]

My fault, I assumed because it was a Fender that it was solid state. If it is a tube amp, getting info out of Fender will not be easy, find out who in your town repairs tube amps, almost every town has a guy or two that works on them, some of them have figured out the parts in Fenders (from what I hear they are pretty proprietary and parts in them are unmarked, not sure how true it is, older Fenders I have had had unmarked parts). Perhaps one of the local techs can tell you if the output transformer will handle 4 ohms ok. If it does, that is a good thing most likely because it will make your amp a little more efficient, and you may get some more beef out of it.

Ummm....

1. Most people think "tube" when they hear the name Fender. Fender does make solid-state amps, but their many classics, including ones that are still in production, are completely tube-based.

2. Getting Fender info is ludicrously easy, especially for tube amps. Grab a copy of the "Groove Tubes Tube Amp Book" for a ton of schematics, or just go over to the Fender Field Guide. Parts are indeed marked, although by today's standards some of the capacitor markings might seem a bit cryptic. At worst just compare to the schematic - compared to other companies, Fender was extremely good about having accurate schematics.