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Using attenuator to play 50w amp through 20w speaker?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Aug 9, 2018.

  1. itsGiusto

    itsGiusto TDPRI Member

    Age:
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    Dec 31, 2017
    02141
    I'm about to build a 50 watt plexi amp head. I already have a combo amp that has a 1x12 16 ohm celestion greenback that sounds great, and it's plenty loud. I'd rather just reuse that, instead of having to build a new cab, and get new speakers, etc. I have no need for an expensive new cab, and certainly no need for a ridiculously loud 2x12 cab to get the wattage rating correct. That speaker is only rated at 20w, however.

    I wanted to get an attenuator for my 50w amp anyway. Could I just use an attenuator like the Weber Mass 100 to plug the 50w head into the 20 watt speaker? If so, how do I know I have it set at a safe value that will not exceed the speaker's rated power capacity?
     

  2. W.L.Weller

    W.L.Weller Tele-Meister

    264
    May 20, 2014
    Queens
    You'd prefer to get a $260 attenuator (and increase the number of output tube replacements you'll need during the amp's lifetime) over buying a $100 speaker?

    How loud do you plan on playing the 50w amp you are building? What combo amp is currently housing the 16 ohm Greenback, and how loud do you turn that combo up?
     
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  3. itsGiusto

    itsGiusto TDPRI Member

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    It's not just a speaker I'd have to buy, I'd have to buy a whole new cabinet for it, and find a place to keep it.
    And the attenuator was something I needed anyway. It's very useful to keep the amp tone and gain the same but be able to turn the volume down. The attenuator I have for my Marshall 1974x has been completely invaluable for gigs.

    Why would I have to increase the number of output tubes I have to buy? As far as I'm aware, attenuators don't cause output tube failure, they just allow people to play at higher-gain settings, which in turn causes tubes to go faster. But the whole point of my building a plexi is to get the cranked power-tube-distorted tone, so it'd be pointless to have the amp if I could not play it with the gain up.

    The combo that has the speaker is a Marshall 1974x. I'm able to blast the 1974x through the speaker fully without an attenuator when I want, though I always use my attenuator heavily at home, and pretty much always use it lightly for gigs, too, unless the gig is outdoors.
    I plan on playing the 50w amp at bedroom levels most of the time, with the occasional gig. Hopefully one day when I have a home-studio I'll be able to blast it outright. But that'll be a while from now.
     
    Last edited: Aug 9, 2018

  4. W.L.Weller

    W.L.Weller Tele-Meister

    264
    May 20, 2014
    Queens
    So your 18w Marshall combo is already too loud for your use cases, and your plan is to build a 50w amp. Seems like a really expensive space heater to me.

    My replacement speaker suggestion was for your combo, you could sell the Greenback and use the new one for both the combo and the head. But I suppose you could also sell the old attenuator, and use the new one for both the combo and the head.
     
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  5. Phrygian77

    Phrygian77 Tele-Afflicted

    Apr 30, 2016
    Crawfordville, FL
    Skip the attenuator and get an Eminence Maverick. I can crank the Trainwreck Express channel of my blackface Bassman build without waking the dead. I wouldn't quite call it bedroom level, but it seriously tames a 50 watt amp.

    Why not build your Marshall with EL84s or 6V6s?
     
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  6. jimgchord

    jimgchord Tele-Meister

    Age:
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    Apr 3, 2018
    victor,ny
    Anytime you run the amp hard you are shortening the life of your tubes. The reason you attenuate is to run the amp hard, therefore you get shorter tube life. You are going to need quite a bit of attenuation to run that mismatched speaker so you will likely experience some loss in tone.
     

  7. itsGiusto

    itsGiusto TDPRI Member

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    I want to keep my 1974x stock, because I like the sound of it. Having two attenuators might be useful to me, though if the new one works as well for the low-wattage amp as the old one, I'll consider selling the old one.
    A plexi sounds pretty different than an 18 watt. While I like the 1974x, I've found it to be too nasally, as is the case with EL84 amps, so I figured I'd like to build a real plexi, an EL34 one. The fact that I don't have a plexi has caused issues for me when I have done some recording in the past, because I'm unable to get the sound I want.
    Besides, 50w vs 18w is not actually a tripling in volume, just tripling in power. It's only somewhat more loud.

    Thanks, but I'm not really not interested in getting another speaker. As I said above, I'd have to get a cab too. And besides, I'm kind of into vintage, low-wattage speakers.
    Looking up the Eminence Maverick, I'm not sure why it'd be better than an attenuator. An attenuator seems way more flexible to me, in that I can use it on any amp, and play into any speaker, and potentially run it all the way to a dummy load if I want.

    I've had pretty good luck with my weber attenuator on my Marshall 18 watt, using the +3db treble switch - it sounds really good, even though I'm usually running it in my apartment on the lowest 1/8th of the attenuation dial. I'm hoping I'll have similar luck with this one as well.
    Sure, it's not the same sound as you'd get running it outright, but it's pretty good, and most of the tone-loss I'd simply attribute to the fletcher-munson curves meaning that bass is the first thing to go. It's not a problem with the attenuator per se, just a problem with the fact that human hearing sucks at picking up quiet bass frequencies.
     
    Last edited: Aug 10, 2018 at 9:23 AM

  8. Phrygian77

    Phrygian77 Tele-Afflicted

    Apr 30, 2016
    Crawfordville, FL
    Well, I think you'd be happier with an adjustable flux density speaker than with an attenuator. Think about it this way, attenuators are going to introduce, at least to some degree, a flatter impedance curve. A speaker doesn't have a flat impedance curve. Even the Weber MASS models with the speaker motor introduce more fixed resistance as you turn them down. So, basically you get more midrange, and less bass and treble the more you attenuate. That's the opposite of the Fletcher-Munson curve that your ears and brain hear. The FDM speakers do the opposite. The impedance curve gets amplified as you turn it down. So, the bass and treble stay relatively strong.
     

  9. itsGiusto

    itsGiusto TDPRI Member

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    IDK, it could be good, but like I said, I've quite enjoyed my Weber MiniMass for many years. If I keep it such that the amp sounds at bedroom-levels, it sounds really great. If I turn it down to be super-quiet-middle-of-the-night bedroom levels, it doesn't sound great, but I don't know what does sound good at mosquito-buzz levels.
    And besides I'm just not interested in trying other newfangled speakers. In my mind, EL34 amps sound great through a greenback.
     
    Last edited: Aug 10, 2018 at 9:25 AM

  10. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    Why don't you just include a pentode-triode switch in your Plexi? The schematic exists already in the 25/50 watt Silver Jubilee Line. Lot easier, cheaper and probably better sounding than cranking the tubes to get o/t saturation then squashing it back down with an attenuator.

    Doubtful you'd blow a new Greenback which are rated 25 watts due mainly to better adhesives with it. My Mini Jubilee's single GB is hanging in just fine, cranked with gain through the front
     

  11. itsGiusto

    itsGiusto TDPRI Member

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    I considered that, but in my experience, pentode-triode switches usually change the tone a lot.
     

  12. VintageSG

    VintageSG Friend of Leo's Ad Free Member

    Mar 31, 2016
    Yorkshire
    1) Build a little box that introduces a series resistor of 16 Ohms and a parallel of 32 Ohms and use that in your speaker cable line [amp]--[box of Ohmage]--[cab]. That'll cut the power the speaker sees.
    2) Jet City Jetenuator or the Behringer/Bugera clone. These units always show a minimum 3dB reduction and are actually really rather good. I've got the Harley Benton clone (their original one ) and I couldn't get my amps to their sweet spot without it.
    3) Other attenuator and keep an ear on the volume.
    4) Can you get a VVR in there?

    Any reduction in valve life is a matter of personal acceptance. Tone is for life, valves wear out anyway. I'd rather have the sweet sounds than an extra year on the glass. I read it as you would too.
     
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  13. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    To each his own and of course it's your amp.

    OTOH, a Plexi squashed back to 20 watts......

    You know you could build a 18 tmb with EL34s and a 4kohm. OT. Probably sound better.

    Pretty much what Marshall's done with the Origin 20.
     

  14. itsGiusto

    itsGiusto TDPRI Member

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    02141
    Thanks for the advice! I'll certainly check out the attenuators you recommended to possibly save some money. But sticking with the weber for the moment, here's the schematic:

    [​IMG]

    Is that to say that the setting on the 50w resistor at which there's 17 ohms in series and 33ohms in parallel should be safe? That's only 1/3 of the way down on the knob, and I'd probably be running it like that most of the time anyway, just to get it within comfortable listening range. IMO my current Weber attenuator's tone-loss doesn't even begin to start until I'm 2/3 of the way down, and still sounds good for a bit. So that's promising! Unless I'm misinterpreting what you suggested or how the attenuator works exactly.
     
    Last edited: Aug 10, 2018 at 1:12 AM

  15. VintageSG

    VintageSG Friend of Leo's Ad Free Member

    Mar 31, 2016
    Yorkshire
    The series/parallel resistor method for reducing power is tried, tested and safe. It's also cheap as the resistors needed are only a £/$/Euro each from Ebay ( or similar )
    It cuts the power to 1/4. You lose some of the interaction you get from the voice coil and transformer, but compromises are part of the deal. The Weber shows a voice coil to try to get that 'push-back' back.
    A fair bit of the resultant tone loss using any form of attenuator is due to the way we hear anyway. Check Fletcher-Munson for further details. You can alleviate some of this with an EQ pedal, but not all. There's nothing like moving air to get the tone. For me, and I suspect you, there's nothing like moving air to get the noise police knocking on your door too.

    One of the most useful pieces of equipment I own is my attenuator.

     
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  16. itsGiusto

    itsGiusto TDPRI Member

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    Small question, though: Why is it 1/4 power, not 2/3? How did you derive that power-calculation? Isn't the point of the resistors that we're creating a voltage divider, burning off the voltage from the 16ohm resistor? So wouldn't it actually be a 32 / (16 + 32) = 2/3 power switch?
     

  17. VintageSG

    VintageSG Friend of Leo's Ad Free Member

    Mar 31, 2016
    Yorkshire
    There are now four lots of 16 Ohms in total. One makes noise, the other three make heat. It's functionally no different to the way a 4x10" cab is wired, series/parallel. Each 16Ohm unit ( the 32 is two 16 combined, sort of, for the purposes of illustration ) dissipates the same amount of power, so the voice coil is seeing 1/4.

    Take the series resistor, now you have 32 Ohms. The resistor and voice coil dissipate 50% each, but your transformer melts. Not good. If we now introduce 32 Ohms in parallel, we bring the total resistance back down to a transformer pleasing 16 Ohms. Each 'module' is dissipating 50% of the power. The series 'module' is 50% voice coil/50% resistor, thus 1/4 power through the speaker.

    Note: this plays fast and loose with resistance and impedance terms, please don't shoot me.
     
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  18. telemnemonics

    telemnemonics Doctor of Teleocity

    Age:
    58
    Mar 2, 2010
    Maine
    I have a vintage non master 50w Marshall and three 18w Marshalls.

    WRT the 18w sounding nasally due to the el84 power tubes, I have 18w with el84 and with 6v6, and neither sounds nasally.
    However, I also have the 20w Greenback which I assume is the same one you have, not the 25w or 30w.
    The Celestion G12M20 is a very nasally sounding speaker!
    All mids and thick low mids! But not much top and bottom.

    I've run my 50w and 18watters through G12M20, G12H30 55hz, Gold, Blue, Red Fang, G12M65, V-30, EV SRO, and recently even added a vintage Jensen P12N.
    Some of the Celestions I have both vintage and reissue versions.

    The 20w Greenback (G12M20) is without a doubt the most nasally speaker I've ever used, and while it is certainly a cool sound, I consider it a special effects speaker, not versatile or universally useful.

    WRT building a 50w plexi, everyone should have one!
    WRT using it at bedroom levels with an attenuator, IMO any greatness of that amp will be lost to that much attenuation.
    Somebody suggested building a plexi with 6v6 power section which might sound better attenuated that much.

    WRT wanting a "Plexi sound", I gotta say, I've owned numerous plexi and early metal panel Marshalls as well as listened to a huge array of great recorded Marshall sounds attributed to Plexi Marshalls, and concluded that there is not "a plexi sound".

    Maybe you've played through numerous Plexi Marshalls and know exactly which one you want to build as well as having confirmed that the circuit is the same as a schematic you plan to build from?
    Between '65 and '68 Marshall was constantly experimenting with the designs, builders in the factory grabbed the closest parts they could reach, and then decades of techs tweaked and modded.
    IMO the "Plexi Marshall" is the most vague mythical circuit in existence!

    If I was going to build a Plexi I'd want a 67 100 or a 66 100.
    My favorite 50w distorted sound was from an early '70s Marshall.
    Every one sounded different, unlike Fender amps which are oddly consistent.
    My 50 is a '74 Bass head and has a great clean that is actually my best low volume when my wife is sleeping in the next room amp.
    Distorted not so much.

    One thing about Marshalls maybe even more than other amps is the sound is very dependent on the speaker choice.

    If you're going to spend money and labor for a second Marshall you really owe it to yourself to get more speaker variety.
    The 20w and 25w Greenbacks drastically diminish the tonal range of a vintage Marshall. But the 20 is the most limited, and as you said, nasal sounding.
    It does that one thing pretty well though.
    A Gold or red Fang does opens up the range of sounds you can dial in and still does classic Marshall when you want that, just lacks some of the GB nasal woody thing.
     
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  19. itsGiusto

    itsGiusto TDPRI Member

    Age:
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    Dec 31, 2017
    02141
    Part of this is my desire to experiment more, build something for fun, and own more cool, iconic amps. If you check out some of my other recent threads, you'll see I'm trying to build this so it covers a wide range of early-style plexi amps, ideally ranging from black flag JTM50 to the 68 model, though I think I may start out easier on myself and just build this to 68 specs, but switchable between lead and bass sounds. So regardless, I think my time, effort, and money won't be a waste here.

    My speaker-style experience has been limited, it's true. I own a few tweed-to-blackface-Jensen-style speakers in some fender amps, a blue in an AC15, and the aforementioned greenback.
    That's interesting to say that the greenback might be making the amp more nasally. I never exactly thought about that or tested it out. You might be right, though I kinda think my AC15 is a little nasally too, and it runs through a blue. And I do think that EL34s just simply sound more bassy and smooth than EL84s.
    But I'll try at some point running my 18 watt through the blue, and maybe through my Princeton Reverb's speaker (rated 75w) to see how it'd sound. Thanks!

    Interesting you mention celestion golds. I actually have no idea what distinguishes golds, and what kind of sound one can get from them. Is there an iconic "gold" sound you could point me to?
     
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  20. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    The Celestion Gold is a 50 watt alnico. It is something like a open sounding Blue with a tinge of alnico compression and not quite as efficient. Some people like it. I do. Not sure it's really Plexi flavoured.
     

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