• From: North Carolina
  • Registered: 2008-10-26
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Topic: Speaker question

I have a Blues jr I've modded and it put out 18 watts at 8 ohms.  I sold my 8 ohm speaker recently that I had for it and was using it as a head with an 8 ohm 4 12 cab.  Everything is too loud with this amp in my basement and I'm only wanting to use this as a practice amp.  I'm under the impression that as long as I go up in ohms it won't hurt my amp, but it will drop the output of the amp from 18 watts to about 8 or 9 watts.  Is this true?  This would work better for me because I can use the cab when I want to be loud and proud, but with 3 kids in the house I can barely turn the amp on without bothering somebody in the house.  I'm hoping that if I drop the power down I can get more use out of the amp at home.

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  • From: New Hampshire, USA
  • Registered: 2010-12-21
  • Posts: 3,650

Re: Speaker question

AD3THREE wrote:

I have a Blues jr I've modded and it put out 18 watts at 8 ohms.  I sold my 8 ohm speaker recently that I had for it and was using it as a head with an 8 ohm 4 12 cab.  Everything is too loud with this amp in my basement and I'm only wanting to use this as a practice amp.  I'm under the impression that as long as I go up in ohms it won't hurt my amp, but it will drop the output of the amp from 18 watts to about 8 or 9 watts.  Is this true?  This would work better for me because I can use the cab when I want to be loud and proud, but with 3 kids in the house I can barely turn the amp on without bothering somebody in the house.  I'm hoping that if I drop the power down I can get more use out of the amp at home.

I suggest that you keep the impedance of your speaker matched to what your amp is setup for. If it is 8 ohms match it with 8 ohm speaker. If you mis match your amp with speaker it could cause damage to your amp. I suggest you purchase an anttenuator to get your tone at low sound levels.
My two cents
Cheers Ron

"Joe B saved my soul, forever grateful Ron"
"Some people dream of worthy accomplishments while others stay awake and do them"
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3 Reply by wharris (edited by wharris 2012-04-28 10:00:34)

  • From: Ridgeland, MS
  • Registered: 2009-08-12
  • Posts: 1,248

Re: Speaker question

Get an attenuatur of put in a post phase inverter MV. Cindyron is right.

  • macg1
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  • From: St. Petersburg, FL
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Re: Speaker question

do not mismatch impedances.  even if you are off by one unit, say going from 4 to 8, 8 to 16, 16 to 8, or 8 to 4, you will put unnecessary stress in your output transformer.  If you want lower volume I would just get an attenuator, a good one like a weber MASS Lite 100, which won't break the bank.  Another option is the eminence FDM speaker:

http://www.eminence.com/2010/09/eminenc … itar-tone/

Amp: Firebird Musical Amplifiers
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  • macg1
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Re: Speaker question

btw, if you decide on an attenuator I have a Weber Mass Lite 100 that I am not using, in perfect condition and has been listed on a local craigs ad, but a fellow JB forum would get first dibs.

Amp: Firebird Musical Amplifiers
Guitars:62 LP SG , 02 FB VII, JB FB I, 76 Electra Omega, 64 Firebird V, 73 LPC, 61 Custom Tele, 59 and 60 Melody Maker
Effects: Mythos Chupacabra, Strymon Deco/Flint
  • From: North Carolina
  • Registered: 2008-10-26
  • Posts: 2,926

Re: Speaker question

I did some digging and this is what I found from the guy that sells the kits to the mods of my amp.  The biggest problem I found is it wastes power and creates heat.  I've got beefed up transformers in the amp so I should be fine.  I'm only wanting it for a little bit of use around the house and for my main use I'm going into a cabinet that is correct 8 ohms.  Won't leave it like this forever but don't have the money for any of the suggested things right now.
Roy says:
June 28, 2010 at 11:42 am
I am interested in putting in an Emminence Cannabis Rex, as I also play jazz, and my amp already has the BillM mods. Would a 16 ohm speaker work OK in a Blues Jr?
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bill says:
June 30, 2010 at 2:05 pm
A 16 ohm speaker just wastes power and creates heat in the output tubes. It’ll work OK, but it’s not optimal.

AD3THREE's Website

  • From: New Hampshire, USA
  • Registered: 2010-12-21
  • Posts: 3,650

Re: Speaker question

AD3THREE wrote:

I did some digging and this is what I found from the guy that sells the kits to the mods of my amp.  The biggest problem I found is it wastes power and creates heat.  I've got beefed up transformers in the amp so I should be fine.  I'm only wanting it for a little bit of use around the house and for my main use I'm going into a cabinet that is correct 8 ohms.  Won't leave it like this forever but don't have the money for any of the suggested things right now.
Roy says:
June 28, 2010 at 11:42 am
I am interested in putting in an Emminence Cannabis Rex, as I also play jazz, and my amp already has the BillM mods. Would a 16 ohm speaker work OK in a Blues Jr?
Log in to Reply
bill says:
June 30, 2010 at 2:05 pm
A 16 ohm speaker just wastes power and creates heat in the output tubes. It’ll work OK, but it’s not optimal.

Good luck but I fear you will over stress your transformer and at the end of the day you would have been cheaper to attenuate. Any way I wish you luck . Cheers Ron

"Joe B saved my soul, forever grateful Ron"
"Some people dream of worthy accomplishments while others stay awake and do them"
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  • RickB
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  • From: San Diego
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Re: Speaker question

I've said this before, but from an engineering standpoint it is better to mismatch high than low. Low impedance ie. a 4 ohm load on an 8 ohm output causes high current in the output stage and can lead to catastrophic failure as mentioned by cindyron. Mismatching the other way, 16 on 8 will induce phase distortion and clipping, but not heat stress from high current. Either way it is not a good solution.
Rick

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  • From: Detroit
  • Registered: 2006-06-02
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Re: Speaker question

I've heard it's better to be even there, but it's ok if the speaker has more ohms than the amp. Although you can ruin am amp buy plugging in a speaker (or sum of speakers) that total less ohms than the amp is willing to output. hmm

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  • RickB
  • RickB
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Re: Speaker question

NPB_EST.1979 wrote:

I've heard it's better to be even there, but it's ok if the speaker has more ohms than the amp. Although you can ruin am amp buy plugging in a speaker (or sum of speakers) that total less ohms than the amp is willing to output. hmm

I will refer you all to the power formula. http://www.sengpielaudio.com/calculator-ohm.htm If you run some variable numbers through the power section, you can see the effect by halving the load resistance given the same power.

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RickB's Website

  • From: Detroit
  • Registered: 2006-06-02
  • Posts: 5,807

Re: Speaker question

hmm that site potentially has a ton of useful information, but it looks like it was created in 1995 and I cannot keep my head into it. sad

the curse of the inner (and outer) graphic designer!

- Nic from Detroit... posting on JB's Forum since 6-2-2006
Ask me about my handwound Great Lakes Guitar Pickups
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  • RickB
  • RickB
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Re: Speaker question

NPB_EST.1979 wrote:

hmm that site potentially has a ton of useful information, but it looks like it was created in 1995 and I cannot keep my head into it. sad

the curse of the inner (and outer) graphic designer!

lol Since it is all math, a pretty face doesn't change the results. The pie chart is the basics of all electronic calculations and lets the layman see the relationships without digging into the higher math behind it. The real math can get pretty hairy, needing at least a grasp of higher trigonometric functions.

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  • From: North Carolina
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Re: Speaker question

So using that chart I've got my Blues Jr clocked in at 9 watts with a 16 ohm speaker and 7.5 amps using 120 volts from the wall.  So its true I'll waste a lot of power but it will reduce the output in half.  Fantastic.  This will work until I purchase a new speaker to replace the Cannibis Rex.

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