Fixed resistance pad for tube amp, can it be done

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by samdjr74, Jan 12, 2006.

  1. samdjr74

    samdjr74 Supporting Member

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    Hi All,

    I'm trying to make a 5W tube amp even quiter. I spoke with my amp tech yesterday nad he said that installing a resistor across the legs of the speaker would accomplish this. After doing some research it sounds like I'm trying to make a fixed resistance pad, ok no problem but the data I keep finding is for a microphone and not a speaker, any ideas on how I'd go about doing this?

    Thanks,
    Sam
     
  2. John Phillips

    John Phillips Member

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    What he said will work up to a point, but can only give up to a 3dB reduction before you start to get out of the safe mismatch range for the amp, and will change the tone (making it duller) both because of the mismatch and because a speaker is not a uniform resistance over the audio range.

    But it may not be too bad - given the cost of a 5W resistor it's worth trying to see. You need a higher value than the speaker's impedance because the average impedance of the speaker over the audio range is higher than the nominal value - typically about twice - so for an 8-ohm speaker you should probably use a 15-ohm resistor.

    If you want more precise impedance matching or a different volume level, you're getting into designing an attenuator, and you need to be able to work out combined impedances and power distribution - you need resistances both in parallel and in series with the speaker.
     
  3. samdjr74

    samdjr74 Supporting Member

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    Hi John,

    Thanks for the info. Here's what I got, an 8 ohm Jenson from 1960, original to the amp, and I'm guessing the amp puts out 5 watts or so with the tube combo it has. So if I go to my local electronics shop all I need to get is a 5 watt 16 ohm resistor, I'm guees the ceramic coated type for heat disoppation and solder that between my two speaker terminals at the speaker itself?

    Thanks,
    Sam
     
  4. John Phillips

    John Phillips Member

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    Sorry, I should have asked what kind of amp. It may not be a good idea - a lot of old amps like this have very undersized transformers, and the winding insulation may be weakened with age too. It isn't really a good idea to mismatch, even downwards which is normally safe for a tube amp. Even a slightly increased current draw may be a risk.

    I think you need to go further toward making a simple attenuator. Don't worry, it isn't complicated. If you only want one level of volume reduction, you only need two resistors, one in parallel with the speaker and one in series. A good place to start would be the same 15-ohm resistor in parallel, plus one in series; the value is a bit more tricky to judge since you don't want the total to be too high at any frequency - I'd probably use 4.7 ohms (giving a total resistance of about 8.5 ohms to 16 ohms over the whole audio range); this will give a better match and slightly more volume reduction, around 5-6dB. I would use a 10W for the series, to give a bit of safety margin, and definitely use ceramic-cased wirewounds as they are the most sturdy and reliable.


    If you want to calculate other values, the speaker will run from about 5 ohms at DC (zero frequency) to maybe around 50 ohms at 10KHz. So in this case the range of the speaker+resistor parallel pair is 3.75-11.5 ohms (1/Rparallel = 1/R1+1/R2), then add the series resistance (Rseries = R1+R2) which gives 8.45-16.2 ohms, which is definitely safe for an amp designed for an 8-ohm load.

    Hope this hasn't complicated things too much!

    I would also build it on a little bit of tag strip and secure it with one of the speaker mounting bolts or somewhere, rather than solder the resistors to the speaker tags - otherwise the series resistor will be unsupported at one end. You really don't want any of this to break, as then the amp will suddenly be running into no load, and that really will endanger the OT.
     
  5. samdjr74

    samdjr74 Supporting Member

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    Hi John,

    Thanks again for the advice but I already purchased the resistor, 5W 18 Ohm 5%, two to a package at $1.00 so it's not a total loss.

    The amp itself is a 1960 or so Lectralab R200B with a built in isolation transformer. The cabinet is garbage but everything I've rea on this so far say's it's actually a decent amp. Plus I just retubed it so now the thing is exetremely loud over the original tubes, and I do mean original!!!

    As far as you explanation on the attenuator, it's not complicated but I just want to make sure I have it right;

    The 5W 18 Ohm resistor will go in parallel to the speaker, meaning it would be just as I desribed before, bridging the two terminals, just not directly to the terminal, that I understand but the second resistor 4.7 ohms (I'm guessing 5 watts also?) Would go in series attached to one leg of the terminal, again not directly to the terminal; but remotely mounted in the cabinet, is this correct? Also if this is correct which terminal or it doesn't matter?

    Sorry for all the questions

    Thanks,
    Sam
     
  6. samdjr74

    samdjr74 Supporting Member

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    I just drew a really basic schematic, is this what you mean?



    [​IMG][/IMG]
     
  7. John Phillips

    John Phillips Member

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    Yes, exactly. It's fine to use the 18-ohm resistor, it won't put the calculation out by enough to matter.

    It's also fine to mount the 18-ohm resistor directly in parallel across the speaker terminals, but the series resistor is best fitted elsewhere, since if you just attach it to one terminal and the wire to the other end, it will vibrate about and may snap its lead wire. The best thing to do is to get a small piece of terminal strip, and cut a three-tag length with the middle one being a ground tag, and connect the resistor across the other two. Then you just need to mount it somewhere convenient and run a wire to the speaker - it doesn't matter whether it's in the ground or hot connection.

    No worries for all the questions :).
     
  8. samdjr74

    samdjr74 Supporting Member

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    Hi John,

    Thanks again, I'm heading out to Radio Shack tonight to see if they have the other resistor, if not the shop by my office should tomorrow morning. You've been a huge help, thanks again!!!

    Sam
     
  9. ZiggY!!

    ZiggY!! Member

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    I've used an L-Pad to reduce output on more than one occasion... works pretty good...
     
  10. samdjr74

    samdjr74 Supporting Member

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    Well that was an easy quick way of getting less volume from my little amp, it worked out perfectly and the amp still sounds great. Thanks for the help and advice John
     
  11. John Phillips

    John Phillips Member

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    Great! :)

    The interesting thing is that although in theory a purely resistive attenuator "shouldn't" sound good, because the speaker is a varying impedance not a constant resistance, in practice they do. The Airbrake is purely resistive and is well known as one of the most transparent and natural-sounding commercial attenuators, and the Marshall Powerbrake (which mimics the impedance curve of a real speaker quite well) one of the least good.

    I'm now using a very simple one I built a long time ago when I didn't know enough to even realise that the impedance "should" duplicate that of a speaker, and then forgot all about for about ten years... I found it again when I moved house last year, and it sounded so good that I actually sold my 'proper' attenuator :eek:.
     
  12. samdjr74

    samdjr74 Supporting Member

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    Cool, now the only thing that remains for my little amp is a proper cabinet for it, hopefully that will be taken care of next week :)
     

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