Rob's 1/4 power mod - in a box?

Discussion in 'Shock Brother's DIY Amps' started by jsnwhite619, Jan 14, 2020.

  1. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I was looking at this, and I don't want to go through the trouble of drilling and modding my chassis, and then it's only going to work for one amp.

    Could you do it with an external box where the 8R resistor is in series between two 1/4" jacks (in & out of the box), then the 16R resistor connected as a dummy load to the ext. jack? Then run two speaker cables out of both speaker jacks, and plug the speaker into the 8R out from the box?

    So,

    8R MAIN OUT --> 8R(50WATT) --> 8R SPEAKER
    EXT. OUT --> 16R(50WATT DUMMY LOAD)
     
  2. BigDaddy23

    BigDaddy23 Tele-Meister

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    Why not make the simple attenuator from Weber’s book? That fits in a large stomp box enclosure and gives you multiple options in terms of different levels of attenuation. I built one of these and a more complex reactive load attenuator and they are both great.
     
  3. Prophetsnake

    Prophetsnake Tele-Meister Silver Supporter

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    I did my version of Rob's device in a box. Works well, bu tit doesn't cut the volume down as much as I had hoped.
    I simply wrapped a box out of a scrap sheet of aluminium. That 'mini chassis' also works well as a heat sink.
    I left a pair of flanges on the edge so I could screw it into the cab.
     
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  4. jhundt

    jhundt Doctor of Teleocity

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    My experience was similar. I built the circuit - copied from a Fender Eric Clapton Tremolux - into a box. It works, but the difference between 1/4 and 1/1 power is not so much as I had expected. I never use the device.
     
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  5. BigDaddy23

    BigDaddy23 Tele-Meister

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    I had a chance to pull out Weber's book 'All about vacuum tube guitar amplifiers' today. Page 107 has the schematic for a simple 2 switch attenuator that gives a choice of bypass, 6dB or 12dB attenuation. Options for various output impedances are provided plus the theory behind them. It works exceptionally well with my amps and is small enough to leave in the cab permanently if you want or move it between amps as required.

    I'll take some pics of mine for you tomorrow if you like. Circuit etc.
     
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  6. chas.wahl

    chas.wahl TDPRI Member

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    Last edited: Jan 15, 2020
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  7. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yeah, I've seen the posts about it not being enough reduction, but I think it would have to help some. I'm not looking for "quiet" playing as much as just knocking a few dB's off of it. My office/music room is ridiculously loud - hickory floor, 9' ceiling, and bare walls. Everything is too loud in here.

    I guess it essentially gives your amp the same output as a 4x10 with 3 speakers unplugged? And without the catastrophic failure... :D
     
  8. King Fan

    King Fan Friend of Leo's

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    Didn't someone scale up (?down?) Rob's resistors to get 1/10th power? Do I recall they said it gave a more noticeable volume cut?

    I'd like to see a discussion about the pros and cons of different attenuator designs. I've seen arguments and opinions, but not too much evidence or even theory (much less demos) of actual differences.
     
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  9. jsnwhite619

    jsnwhite619 Friend of Leo's

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    The guy on the Marshall forum - he's posted it here as well - used inductors I think that supposedly help save the tone through the real low powered options. All the frequency response charts look exactly the same throughout.
     
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  10. DanceCanse

    DanceCanse TDPRI Member

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    I built one I really like with a switch to bypass it as well as a switch for 1/4 power or 1/10 power. This thread is where I got the design from, credit to @robrob @FenderLover and @mcentee2 :https://www.tdpri.com/threads/5e3-1-4-power-switch.839543/page-3

    I copied the design exactly from that thread. I use it on my 5E3 and the 1/10 setting gets me close to bedroom volume. Here's what mine looks like: IMG_1268.jpg
    IMG_1766.jpg
    I liked it so much I built a second one for my Weber 6A14. That amp is still pretty loud even on the 1/10 setting though. So I'm probably going to put Rob's pre PI master volume on it. For my second one I moved the speaker jack over to make the wiring neater. The layout for it looks like this:
    Dual attenuator w bypass.png
     
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  11. Andy B

    Andy B Friend of Leo's Silver Supporter

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    I've recently built some of these simple attenuators. Mine are -3db and -6db. I built 25 watt one to use with a Deluxe Reverb and 2 Blues Jr's. I also built a 100 watt -6db one that I'm using with my SF Pro Reverb.
    Ted Weber's site has a calculator that lets you figure out exactly what value resistors to use for the amount of attenuation you desire. Mouser was my source for the resistors. All the attenuators were housed in reject pedal enclosures with extra holes drilled for ventilation.
     
  12. BigDaddy23

    BigDaddy23 Tele-Meister

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    I built that one too - he was very helpful with my questions during the planning phase. It works exceptionally well and lives in a band mates' Hot Rod Deluxe where the amp's sweet spot can be used without taking heads off at 10 paces!

    The version that DanceCanse has posted is pretty much the same as the book version. I'd be surprised if you find -12dB is still too loud in that space you described. The resistors can be had from eBay as well.
     
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  13. johnDH

    johnDH Tele-Meister

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    Hi
    the Marshall forum design linked above is mine. I'm happy to discuss and help. It can be adapted to most needs and it's designed to be simple to build, while also being a proper reactive attenuator.
     
  14. dan40

    dan40 Tele-Afflicted

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    I can attest that John's design sounds great and is super easy to build if you are good with a soldering iron.
     
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  15. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Thanks for all the responses & feedback! I wasn't planning on the all out attenuator, but yall are talking me into it. Especially if the tone holds up that well.

    Sidenote, but related -- When I bought my gigging PA system from MusiciansFriend a long time ago, they included a dB meter in the whole package deal. A good one, too, like $70-$80 new these days. Never really used it on gigs, but I love it for amp stuff now. It's interesting to see how little difference there is in 5-10-15 watts in terms of actual output. I know you can read numbers on a page, but when you play and hear it and watch the numbers as you turn it up and mess around, it's crazy to see the results firsthand.

    I had a thread a couple years ago that got pretty hotly debated whether you could measure output wattage from a dB meter. I finally emailed Eminence and asked, and the tech guy there said you could probably get a very accurate measurement that way because it takes watts to make dB's. The only way to get this SPL through this speaker is with this many watts. It would be interesting to use a meter in the same way with the attenuator settings and see how they match up. Just keep flipping switches and taking readings and see how well the numbers on paper correspond with what the meter is reading.

    Here is a handy link. http://myhometheater.homestead.com/splcalculator.html

    A 20 watt amp with a 100dB speaker is 113.8dB @3ft.
    At 1/4 power (5 watts) and the same speaker, it's 107.8 dB @3ft.

    15 minutes at 100dB can cause hearing damage - at the measured 3 feet directly in front of the speaker. So, a 20 watt amp with a 100dB speaker would need a 15dB reduction just to be below damaging levels...according to CDC. Take it down to a 1/2 watt & you're there!
     
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  16. chas.wahl

    chas.wahl TDPRI Member

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    I think that I'm going to attempt the M2 design suggested by @johnDH in his MarshallForum thread (p. 27, a bit more than halfway down). He's been very helpful about adjusting the component values, and I'm going to scale it down for a ~25 W amp rather than a 50-Watter. If anyone would like to see what the component table looks like for that, I posted it on p. 35 of his thread. The M2 variation only requires one inductor (air core crossover type) rather than 2 -- the rest is resistors and switches in series-parallel stages to give reductions of -7 dB, -14 dB, -28 dB and -31.5 dB.
     
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  17. jtcnj

    jtcnj Tele-Holic

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    I'm a home player only, basement.
    Everything I have sounds thin and dull at kind of loud TV watching volume.
    So little signal barely moving the speaker cone I suppose.
    So much that when I am limited to it, I have to force myself to ignore tone completely and just focus on timing and technique.
    Surprisingly uninspiring I might add.

    I built the 1/4 power as a speaker cable, one of the thread links above.
    I have since bolted the resistors directly to the pedal enclosure, with some heat sink compound.

    I mostly use it with the 18w TMB and Eminence GB128 , sens 101.3, sometimes with the 5E3.
    I need to wire in a toggle switch to allow 4 or 8Ω on the 5F1. Even that is LOUD in the sweet spot range.

    It works pretty well at higher volumes. Simple and certainly worth the small cost of parts I had on hand and a little time.

    It is still LOUD, but not nearly as loud without it.
    I never did A/B the tone, maybe it loses some treble which is overly abundant when cranked up anyway. MV max, TMB channel Gain (vol) 2-4:00, beyond 2:00 I cant roll back guitar volume for clean-ish. Push with a Soul Food or TS or other as mostly boost or boost only. Pedals really increase volume as well.

    I am lucky my wife works some evenings so I can usually have at it.

    But, volume is also why I have Robs JCM800 Micro 1W build ready to start when I have more time.
     
    Last edited: Jan 21, 2020 at 9:09 AM
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