Will i be ok to use this speaker cab with this amp?

RazzerRazzer Posts: 22Member
I have a speaker cab made  from a line6 spider 2 amp combo which i think was 120 watts ,   I think both speakers are 50watt -  8 ohms, the two speakers are wired in parallel  , well my question is with a Blackstar ID60 TVP-H head which is 60watts  will i be ok to use this speaker cab with this amp? Or should i just use one speaker from the cab two run the amp?

Comments

  • JockoJocko Posts: 7,009Member, Moderator

    The Blackstar ID60 TVP-H needs speakers with minimum impedance of 4 ohms. Two 50 watt, 8 ohm speakers, in PARALLEL, give 4 ohms, 100 watts. Present speakers should work fine.

  • RazzerRazzer Posts: 22Member
    Originally Posted by Jocko:

    The Blackstar ID60 TVP-H needs speakers with minimum impedance of 4 ohms. Two 50 watt, 8 ohm speakers, in PARALLEL, give 4 ohms, 100 watts. Present speakers should work fine.

    Hi . I don't know that much when it comes to matching amps with speakers so am i right in saying  a 60 watt amp head with push two 50 watt speakers   ?

  • JockoJocko Posts: 7,009Member, Moderator

    You add the powers when you connect speakers in parallel.  Two 50's can handle 100 watts.  Two 30's, 60 watts, etc.

  • RazzerRazzer Posts: 22Member
    Originally Posted by Jocko:

    You add the powers when you connect speakers in parallel.  Two 50's can handle 100 watts.  Two 30's, 60 watts, etc.

    Thank you for explaining that  , So my worry is if i am using a 60 watt amp to push two 50s with is equal  to  100watts will this not blow the amp  ?

  • JockoJocko Posts: 7,009Member, Moderator

    The power rating of the speakers has no influence on the amp.  What does affect the amplifier is the impedance of the speakers, measured in ohms. There is a law in physics called Ohms law which states that the current flowing in a circuit is equal to the Voltage applied divided by the impedance or resistance of the circuit (in a simple dc circuit).  If, as a made up example, an amplifier has an output of 40 volts, driving into a 8 ohm load (speaker combination) then it will draw 5 amp.  However if you use a 4 ohm load it will try to draw 10 amps, and if the amp is not designed for that current it will be damaged.  Using a 16 ohm load will only draw 2.5 amps. so too high an impedance will not damage the amp.  It will just not give its best.  This is a very simplified example.

    As an aside, Power=Volts x Amps, so this example amp would be rated at 200 watts into an 8 ohm load.

  • Reg SoxReg Sox Posts: 3,121Member

    Jocko,  I found your reminders of schoolboy physics lessons very useful.

     

    Ever thought about doing a series of short posts on electronics for guitarists?  Just basic stuff like the above.  I know you've done some pieces on guitar wiring in the past but it might be nice to pull it together into on blog series?

     

    Another area I might suggest, because I can't remember enough theory about, is changing the resistance of pots by adding resistors across the terminals - this would be useful for the amateur tinkerer in fine tuning.  As a real life example, on my next build I'll be using P90s that sit between ordinary Fender type Single Coils and Humbuckers.  So using the conventional wisdom of 250k for singles, and 500k pots for Humbuckers I'm caught between two stools.  What value resistor do I use to change a 500k pot to a 375k pot, and which terminals do I put it across?  Do I have to do the same for tone and volume or just one of them?  It would all be useful stuff.  Not suggesting you cover any mains voltage level stuff, just signal path.  Obviously if you feel this request would be a big imposition fell free to tell me "off" preceded by your own choice of word - no offence whatsoever would be taken!

     

    Razzer - sorry for the thread hijack.

     

    Cheers, Reg.

  • JockoJocko Posts: 7,009Member, Moderator

    If there was enough interested it might be an idea otherwise it is easier just answering individual questions.  With regard to your query, I don't think you'd notice much different what pot, of the two, you used.  I wouldn't bother with resistor in parallel.  Formula is 1/Rtotal=1/R1+1/R2

    or in this case 1/375000-1/500000= 1/Radded therefore

    Radded =1/(1/375000-1/500000) or 1500K.  So you could add an 1800K (preferred value) resistor across the two outside terminals.  Or you could just source a 390K pot.

    You would only want to change the volume control.

  • JockoJocko Posts: 7,009Member, Moderator

    Can only find 300K and 470K pots between 250K and 500K.

  • Reg SoxReg Sox Posts: 3,121Member
    Originally Posted by Jocko:

    Can only find 300K and 470K pots between 250K and 500K.

    Yep!  Apparently Fender do do a 375k pot as a spare part (it's factory fitted in some of their guitars with humbuckers and/or "hot" single coils), but it seems that the general consensus in other forums is that it is of poor quality having been a custom build for them rather than something from CTS or whoever.  And they are like hens teeth to find in this country (I just prefer to give my business to British first, European countries second).  Hence why I was looking at the pot/resistor combo.  Plus it means I can fine tune it to what works best. I was thinking of experimenting using a temporary set-up with crocodile clips when I get round to building the guitar to see what works best.

     

    Thanks for your post.  But my rationale for suggesting a blog is while you've answered my question here, the likelyhood of me finding your post again in three months time is going to be pretty remote

     

    Again, apologies to Razzer for the hijack.....

     

    Cheers, Reg.

  • RazzerRazzer Posts: 22Member

    I got my amp head to day and hooked it up can't get any sound from the speaker cab . On the Amp there are two inputs for the speaker cable to go one says 1 x 4 ohms and the other 2 x 8 ohms , I tried both  i have two speaker that are 8 ohms but in parallel  they make 4 ohms . not sure what to do?

     

  • Reg SoxReg Sox Posts: 3,121Member

    Start the process of elimination.

     

    First up check the cabling.  99 times out of a 100 it'll be the cable.  Is it a fixed cable at one end in the cab?  If not just try a different cable.  If it's a fixed cable out of the cab is it easy to take the back off the cab and check for continuity of wiring all the way between the jack socket and the speakers (you can also check they are wired in parallel rather than series as well). Obviously an easy check is to just unscrew the barrel of the jack plug and check both wires are securely soldered to the terminals.  If you have a multimeter, and know how to use it, it'll make life easier.  If it is wired in parallel at least one speaker should work as it's unlikely both speakers will have developed a fault at the same time (unless they've been seriously abused) so you should be able to discount the speakers at this point.  However, getting another sound source could help.

     

    Can you use the cab with any other amp  - do you still have the original line 6?

     

    Is the head brand new?  Is it a known working unit?  If new can you take it back to the shop (if not bought online) and ask them to try it with an appropriate speaker?  Does the output stage on the Blackstar have any external fuses (i.e. mounted on the back of the amp and obviously designed to be user servicable) that you can check?  Does the amp actually turn on?  If you aren't 100% sure what you are doing whatever you do don't take the cover off the amp - there are life threatening voltages inside amps even when they have been turned off and disconnected from the mains for some time.  Do you or a mate have another suitable speaker your could try the amp with?

    Hope that helps as a first stage.  I'm sure other will chip in with suggestions as well.

     

    Cheers, Reg.

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