Need advice changing the L-Pad in an attenuator...

Discussion in 'Amp Central Station' started by sirdavy, Oct 1, 2020.

  1. sirdavy

    sirdavy TDPRI Member

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    I bought a 8ohm/15w Tube Juice Attenuator (which I think have been discontinued) and I've been using it with my 4w Vox AC4TV...and it's been OK-ish, not great sound when you dial it up. However, I suddenly realised that the Vox is 16ohm I should be really using a higher rated attenuator. (btw I understand that the AC4TV can be used with a 8ohm load, so this may all be for nothing)

    I've bought a 16ohm/100w L-Pad - is it just a matter of swapping the L-Pads?

    I can see two 'things' - this should give you an idea of my electronics know-how - stuck to the enclosure with the numbers '4' handwritten on them, which I think are big resistors. Do I need to upgrade those too?

    Also, I'm going to add a brightness switch switching between a:
    6.8uf 50V Alcap Bipolar Axial Electrolytic Capacitor
    ...and a...
    2.2uf 100V Alcap Bipolar Axial Electrolytic Capacitor
    ...but I'm a bit concerned about the 50v/100v disparity - is it ok to use them?
     

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  2. radiocaster

    radiocaster Poster Extraordinaire

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    If you just wanted a different impedance going into the attenuator, you didn't need to change the L-pad at all, just the resistors. You are going to have to change at least one of the resistors.

    If you wanted to use a speaker of a different impedance, you didn't need to change anything at all, but it's too late now.

    You probably won't be able to fit 100W resistors in there, you might need to consider a new case unless you want to keep it at 15W. You might be able to keep one of the resistors, but you'll have to rewire it in the position of the other, and it would remain a 15W attenuator.
     
    Last edited: Oct 1, 2020
  3. sirdavy

    sirdavy TDPRI Member

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    I don't really understand what you are saying, in every discussion about L-pad based attenuators the value of the L-pad is important, the one I've bought is 16ohm and rated up to 100w, which is obviously much far higher than I require. I've patched in the attenuator in my 4watt AC4TV combi between the amp and the 16 ohm speaker - I don't have a choice of speaker.

    I'm guessing the existing L-Pad (see the photos in the original post) is rated 8ohm/15w. If those two resistors are 4ohms then, again guessing here, maybe I should replace them with 8ohms?
     
  4. radiocaster

    radiocaster Poster Extraordinaire

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    What I am saying is that the classic layout is one resistor in parallel with the L-pad, and one in series. So with the 8 ohm L-pad, it would be an 8 ohm resistor to make it 4 ohms, then a 4 ohm resistor in series so the amp sees 8 ohms. With a 16 ohm attenuator, you'd double those values.

    That is if you want to copy the circuit of your attenuator in 16 ohms. And I am just assuming about the circuit, you'd probably need to draw a schematic, and I don't know what those little resistors are for.

    However, since you just want to use your AC4, those big resistors are not needed with a 100W L-pad, just wire the L-pad without them as per the instructions that came with the L-pad.
     
  5. sirdavy

    sirdavy TDPRI Member

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    Ah, I think I'm getting closer to understanding, thank you.

    I've just taken a closer look at the existing dial-thing inside the attenuator and what I've been calling an L-Pad is actually a...

    "TRU-OHM POTENTIOMETER 350 OHM, 25W RHEOSTAT" (Part no. RP101FD351KK)

    Am I right in understand that this component combined with the two resistors does the same job as my new L-Pad (see photo). (There were no instructions that came with the new L-Pad btw) IMG_4466 copy.jpg IMG_4465 copy.jpg
     
  6. radiocaster

    radiocaster Poster Extraordinaire

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    Not sure if that's what you mean, but the big resistors provide extra protection so you won't fry the L-pad. If you were to put 15W through a 15W L-pad, you'd fry it quick despite the nominal value.

    By the way, I am quite familiar with L-pads as I've built one attenuator and also have parts for others. The one pictured looks like a 50W model, not a 100W model.
    [​IMG]
    https://www.parts-express.com/speaker-l-pad-attenuator-50w-mono-3-8-shaft-16-ohm--260-254


    100W looks like this, may have long or short shaft.
    https://www.parts-express.com/parts...ttenuator-100w-mono-3-8-shaft-16-ohm--260-261
     
    Last edited: Oct 1, 2020
  7. sirdavy

    sirdavy TDPRI Member

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    My mistake, you are absolutely correct, it's a 50w.

    I'm still a bit confused. What in the attenuator is a rheostat/pot attached to the big (wire wound?) resistors glued the enclosure; the rheostat/pot doesn't have any resistant load of it' own, does it? That's why it's connected the load of the resistors, isn't it?

    But that new L-Pad does have a load of its own inside, no? Aren't there two wire wound resistors inside linked up in order to produce 0 to 16ohms? I'm not doubting you, I'm just confused...

    I'm looking at this DIY attenuator video

    and they are using the following L-Pad schematic - no extra resistors.
    Screenshot 2020-10-01 at 17.06.37.png
     
  8. radiocaster

    radiocaster Poster Extraordinaire

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    Those are L-pads, not sure there is much of a difference between the rheostats except maybe in construction, never seen a rheostat inside. It works the same as a potentiometer.

    I didn't see any wire wound resistors in your attenuator, they look like ceramic power resistors, but it's kind of hard to see.
     
  9. sirdavy

    sirdavy TDPRI Member

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    According the the video, the L-Pad - very similar to my new one and from the same supplier - has coils inside which provide the load.

    This is what's inside the attenuator currently...(along with the ceramic power resistors)
    Screenshot 2020-10-01 at 17.21.11.png
     
  10. sirdavy

    sirdavy TDPRI Member

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    I went ahead and swapped the pot, which was attached to two (ceramic?) 4ohm resistors for an L-Pad along with a bright switch (2.2uf & 6.8uf).

    I've only tested it so far with a 6.5"/16ohm speaker (which I made using a speaker from a vox ac4tv mini) and it sounds GREAT, really good and the difference between both settings on the bright switch is clearly audible to me. This is the bedroom volume tube amp I've been trying to achieve.
     
  11. avspecialist

    avspecialist Tele-Meister

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    tubegeek likes this.
  12. tubegeek

    tubegeek Friend of Leo's

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    The issue with those is, it is basically unloading the secondary of your output transformer. I would think you'd run a similar risk of damage (less but still a risk) as working into an open circuit - 100K impedance is a load but not much of one.

    Inside, those units are usually a stepped switch connected to a bunch of taps on an autotransformer winding. So they act as a variable step-down ratio which is how they adjust volume. They present to the input, a high impedance load so as not to degrade the system signal.

    They can serve as a really pretty decent "passive preamp" for peanuts, talking audio listening system here. Not sure they'd make a reasonable amp attenuator unfortunately.
     
  13. avspecialist

    avspecialist Tele-Meister

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    Hi Tubegeek, i was just wondering if they would work. I was thinking it could be an inexpensive way to cut the volume down f a speaker. I have been using these style controls for over 40 years in large house systems, some of them with McIntosh tube amplifiers with no issues.
     
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