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Thread: Peavey Classic 400 schematic ?

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    Peavey Classic 400 schematic ?

    Anyone have Peavey Classic 400 schematic I could peruse? the one that does 400W with 8 6550s

    Yes, I did ask peavey.

    It's 5:08 and I missed the boat. Got the answering machine

    Please post if you have it or if not I'll just contact them tomorrow

    WOuld like to try to work on this tonight
    Last edited by nsubulysses; 11-02-2017 at 12:49 AM.

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    Supporting Member The Dude's Avatar
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    The Classic 400 all tube bass head

    Schematics towards bottom of page.

    I hope you're feeling strong today.
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    I just took a nap so this dead lift will wake me up.

    Maybe I am not good enough at using a computer but when I clicked those schematics I got a blank page

    Edit: AH, duh. it is a pdf so I must open it in photoshop or some other program that can do PDF.

    thanks for making me give it another try

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    Supporting Member The Dude's Avatar
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    Hmmmmm,...They load fine on my computer. Maybe they're large enough that it takes more time. Try waiting it out.

    Or maybe try this. Here are the direct links.


    http://www.hartsafire.com/classic400/Peavey_Classic_400_service-1.pdf


    http://www.hartsafire.com/classic400..._service-2.pdf
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    Out of curiosity, can you open it by clikcing the link and viewing in your web browser? I get blank page that way and also blank page when I view it with Preview even though it is supposed to support PDF.

    If I open in photoshop there is no issue other than annoying toggling between page #s rather than scrolling up and down like I would in the other scenarios. Maybe there is a way to scroll pages in photoshop as well, hmmm

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    Supporting Member The Dude's Avatar
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    Most people have Adobe Acrobat Reader installed on their PC. PDF files will then automatically open if configured normally. Do you have that installed or another .pdf reader?

    FWIW: I use Foxit Reader instead. It does everything Adobe does and is also free. The advantage it has over Adobe is that it also allows you to do some editing of .pdf files. You can fill out forms, add signatures, etc., etc.
    Last edited by The Dude; 11-02-2017 at 12:59 AM.
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    I can use Preview to read many books and whatnot I have in PDF. Not sure why it won't open some schematics. Maybe I gotta check out Foxit. Thanks Dude

    I have the same problem with these 5150 schematics. 5150 combo can be opened in seemingly anything, but I must open Peavey 5150 II schematic in photoshop or I get blank pages. I must be missing something. they are both PDF. I'm not a good tech geek in the computer realm
    Attached Files Attached Files

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    Does this amp really do 400W? I would believe it does, and it even lists itself as doing 56.5V into 8 ohms for 400W, but I only get this one to do 280-290W at clipping. Tubes are strong and good, even replacing 6 of the 8 power tubes with new ones it still does same power output.

    B+ is 670V and sags to about 590V at clipping. No screen grid resistors are open. The amp had a shorted 12AT7 phase inverter but I found no shorted/open resistors in the gain stage. Also I guess it doesn't really matter because when you plug into the power amp in the amp does the exact same, 34V into 4 ohms. DDT button has been pushed in and out so I'm not overlooking that. Anything come to mind that might be worth looking into? I have never gotten one of these for repair before so hoping I might be missing something simple
    Last edited by nsubulysses; 11-17-2017 at 07:45 PM.

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    Did you go through the bias procedure?

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    Is it anything more than setting the bias to -53 to -53.5V?

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    Old Timer J M Fahey's Avatar
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    I never expect tube amps strictly meet rated power if **clean** , zero clipping.
    real "Hi Fi" almost 300W RMS looks acceptable to me.

    Just for peace of mind, do it the other way: rise output until you have rated 56.5V "RMS" out (actually whatever you have, but read on rthe "V RMS scale), then look at the screen, if possible take a pic and upload it hgere.

    I bet we´ll have the classic power tube output waveform, sort of skewed similar-to-crossover type mixed with top clipping (but not a full squarewave either) and to boot: horizontal parallel line tops which actually show ripple .

    Funny thing is, such waveforms do not "sound distorted" to the average Rock player, but "just a little grungy" or "with character" or some other colourful description.
    Juan Manuel Fahey

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    Senior Member Enzo's Avatar
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    In my view, the power rating of a guitar amp is about how much power the amp can put into a speaker. Distortion? So be it. My 100 watt speakers will not turn away 150 watts of signal just because it gets distorted after 50 watts.
    Education is what you're left with after you have forgotten what you have learned.

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    Not to disagree with anyone, but I seem to recall generally being able to get that kind of power from SVT's, with 2 less power tubes.
    You said the screen resistors were all good, but how about the screen supply voltage?
    Otherwise, maybe that amount of sag in the B+ is more than the SVT's sag. Or maybe it's running out of drive to the outputs? (12AX7 driver vs. 12BH7 in the SVT)
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    G1 is correct. Or as far as I know.

    power supply voltages are the same as SVT plus two power tubes so yes, i think this amp should do more power than an SVT

    I'll be at the shop again tmrw and can give it another look

    Thanks for the insights so far

    Tomorrow I will post the scope image. 34V at clipping into 4 ohm load with a nasty crossover look. Plugging into input jack on front panel or plugging into power amp in, the output looks the same. It has to be some sort of power supply problem.

    If the amp is hitting clipping too early isn't that basically controlled by how high the B+ and screen voltages are? It seems the only other thing that could drag this down are weak power tubes
    Last edited by nsubulysses; 11-19-2017 at 05:15 AM.

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    I wonder if the Peavey techs could provide what the signal levels should be at various points in the signal path. It would be nice if that was in the service manual.

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    Supporting Member Jazz P Bass's Avatar
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    According to the owners manual, the amp ts rated at 400 watts, 4 or 8 ohm.
    420 watts at clipping.

    The overall power consumption is 1000 watts from the mains.
    That is where I would look next.

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    Old Timer J M Fahey's Avatar
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    In a very simplified way, "power tubes are inherently current limited" .
    It´s very hard to pull many electrons from metal surface and to make them travel in VACUUM!!!!
    Which is good as an excellent built in short protection and helps having a "Rock" guitar sound and clipping but is a PITA when you try to pull more power than usual from them.
    And current capability only goes *down* every hour that tube is ON, even faster if abused.

    I trust Peavey, and they have shown to be conservative in general, so I bet that amp can put out 400W RMS "barely clipping" or not much less (say 350W+?) when leaving Factory, which means fresh tubes and settings.

    After a few years?
    Who knows, but power out spec can only go down, never up.
    Juan Manuel Fahey

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    Senior Member Enzo's Avatar
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    I wonder if the Peavey techs could provide what the signal levels should be at various points in the signal path. It would be nice if that was in the service manual.
    It is just another tube amp, the voltages within will all be similar to any other amp.
    Education is what you're left with after you have forgotten what you have learned.

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    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by nsubulysses View Post
    Does this amp really do 400W? ... ...
    In the beginning to understand.
    Each factory stands behind the power specified for its amplifier, provided that all tubes are 100% new, from the same manufacturer and that the bias is set to the specified value for the types of power tubes that are installed.

    When replacing power tubes, the change complete set (in this case 8 x 6550) because the amplifier does not know the "strong and good" power tubes, the amplifier knows only new tubes.
    What we call "strong and good" power tubes does not have to mean that all 8 x 6550 are matched as an octet and that for bias (for example -56.5V) they have approximately the same current (+/- 5%) because the power decreases with the square current.

    In other words if output tubes instead of 100% have a 90% emission, amplifier gives 80% power output. Power tubes generally change when their emission drops to 70% because the amplifier drops power to 50%.
    The bias is properly adjusted to the quiescent current (n x 25-35 mA) because it is high probability that all power tubes at high power will have approximately current.
    It is desirable not to mix power tubes from different manufacturers, because during exploitation emission do not drop at the same time

    Useful link: http://music-electronics-forum.com/t8291/
    Last edited by vintagekiki; 11-20-2017 at 11:51 PM.

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    Sorry no scope output pics yet

    These two 100 ohm resistors to ground in filament supply are open. They look normal but if you see the underside it's burnt.

    Would this make filament supply low and tubes will have weak emission ?

    img_2053.jpg

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    Supporting Member Jazz P Bass's Avatar
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    The 100 ohm resistors are like a 'hum balance'.

    They will tend to fail if a tube fails B+ to heater.

    If they are open, then the heater supply should go up.

  23. #23
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    I'm a bit surprised your B+ is a tad low, rather than higher which is what we usually see with modern line voltage.
    Have you looked at AC ripple on the main caps at idle and at full tilt?
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    I forgot to mention I measured power consumption as JPB suggested and it was 240W at idle, 760W when amp is driven to clipping.

    This was before I found open 100R resistors.

    I am checking this thing out now so I'll check for ripple when I put it back together too. However, if there was excessive ripple how would this make the power output less than expected? just asking because I wouldn't think to look for that when an amp doesn't put out full power. thanks

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    Excess ripple will basically reduce the B+ available to the tubes.
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    Here's what the amp does at clipping. About 34V into 4 ohms. it says it in RMS in bottom left side
    img_2054.jpg

    And the 400W output as Enzo requested. So yeah it can put 40V into 4 ohms for 400W but hmmm
    img_2055.jpg

    B+ and screen supplies have about half a volt AC at idle and about 2VAC at clipping.

    B+ and screen supplies drop about 100V each from idle to clipping, 680V B+ drops to 598V and 392V screen supply goes to 298V

    Power tubes are doing about 100-110mA at clipping. Seems like amps usually do 120-150mA at clipping typically. maybe this is less so since there are 8 power tubes and it's only doing 290W at clipping.
    Last edited by nsubulysses; 11-21-2017 at 04:28 AM.

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    Old Timer J M Fahey's Avatar
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    Ok, what you have is *reality* .
    300W RMS as is, period; *might* do (or approach) 400 RMS with all fresh power tubes, if good family.
    It´s hard for tubes to put out clean high power, easy task for SS.
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    img_2056.jpg

    Maybe I am at war with reality

    I have an SVT-II here so I might as well give it a look for comparison

    here's how it does 34V into 4 ohms

    B+ is 655V and drops to 565V at clipping. Screen supply drops less compared to Peavey Classic 400, 385V at idle and 327V.

    SVT power consumption is 220W at idle and 650W doing 34V into 4 ohms.

    SVT power tubes (I only measured one) are doing 132mA when amp is clipping

    Maybe this Peavey classic 400 is different because it has a saggier screen supply and 2 additional power tubes so it does similar power to SVT but is supposed to be easier on the tubes.

    Seems disapppointing but possibly real??
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    Supporting Member The Dude's Avatar
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    Did you ever check for ripple as g1 suggested?
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    yes in post 26.

    I checked with DMM on AC on power supply nodes

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    Supporting Member The Dude's Avatar
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    Gotcha! I missed that part (obviously).
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    img_1695.jpg

    Good news an SVT can do 400W also. They just don't call it that so you don't get confused


    Is this all because of the 400R dropping resistor in series with the screen supply in PV classic 400??

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by nsubulysses View Post
    Click image for larger version. 

Name:	IMG_2056.JPG 
Views:	24 
Size:	2.19 MB 
ID:	45784

    Maybe I am at war with reality

    I have an SVT-II here so I might as well give it a look for comparison

    here's how it does 34V into 4 ohms

    B+ is 655V and drops to 565V at clipping. Screen supply drops less compared to Peavey Classic 400, 385V at idle and 327V.

    SVT power consumption is 220W at idle and 650W doing 34V into 4 ohms.

    SVT power tubes (I only measured one) are doing 132mA when amp is clipping

    Maybe this Peavey classic 400 is different because it has a saggier screen supply and 2 additional power tubes so it does similar power to SVT but is supposed to be easier on the tubes.

    Seems disapppointing but possibly real??
    No, that´s not war but agreeing with reality.

    There´s 3 points to be considered here:

    1) it is "assumed" that "if 6 tubes give me 300W, then 8 must give me 400" ... clearly not the case.
    Tubes are *one* factor in the equation, but there´s more, with the same level of importance:

    2) as certain "Enzo" once said: "the amplifier is that thingie between the power supply and the speaker"
    IF Peavey PSU drops more than Ampeg then so be it.

    I always say that tubes are strongly current limited, so in principle Peavey "should" have 33% more peak current available.
    Fine ... if supply can supply it, if screens make tube pass it, and grids can be fully driven.
    If not ....... we only have 2 extra tubes, which look killer but maybe are not fully used.

    3) having the tubes is fine, but fully driving them is just as important.

    Waveforms show that Peavey is struggling to drive those power grids, there´s lots of poorly named "crossover distortion" which actually is grid rectification and bias shifting.

    Remember that RMS voltage is one "averaging" parameter, considering the area under the curve.

    Compare both waveforms: even IF they were to reach same peak voltage into the load, area under the Peavey waveform is peakier, narrower and smaller than area under the fatter rounder Ampeg waveform.

    Peavey area being smaller (means less RMS even if everything else is same) than Ampeg one.
    Or in this case, having 2 extra tubes sort of compensates poor waveform but provides about same RMS power, not 33% more as expected.

    Ampeg waveform can brag of effortless drive, not very common in tube power amps which require exaggerated overbiasing to compensate for that.
    Attached Thumbnails Attached Thumbnails tubecros.gif  
    Last edited by J M Fahey; 11-21-2017 at 06:08 PM.
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    Old Timer J M Fahey's Avatar
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    This is what I mean by "more area under the curve":
    I poorly drew 2 half "sinewaves", the red one being what it becomes when amplified by most tube amps, unless Class A or at least heavily biased at idle.
    Look at the red painted area.
    tubecros.gif
    The other, more perfect sinewave, is what all SS amps and a few tube ones (such as Ampeg SVT) put out.
    For the exact same width and peak current/voltage , area under curve, which is Red PLUS Green, is clearly larger.
    There are "more electrons during more time" pushing those voice coils and obviously, both Average and RMS power are higher.
    Please don't nitpick that "RMS power does not exist" and such, we are using the popular name here.

    By the same token, that´s why squarewaves provide highest power (and also burn speakers), "area under the curve for a square wave is maximum possible": "all the current possible during all the time, continuously"
    tubecros2.gif
    Squarewave area under the curve is red + green + blue: maximum possible.
    Last edited by J M Fahey; 11-21-2017 at 06:17 PM.
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    Quote Originally Posted by nsubulysses View Post
    Maybe this Peavey classic 400 is different because it has a saggier screen supply and 2 additional power tubes so it does similar power to SVT but is supposed to be easier on the tubes.
    Both have separate windings for screen supply. PV has 400ohm common screen resistor that SVT does not. And only 50uF filtering vs 100uF in the SVT. So these may account for some difference.
    JM showed how the PV waveform is peakier, leading to lower RMS readings. The only question is whether the peakier waveform is normal or not.
    I'm not opposed to these only being capable of 300W, except PV is usually pretty honest, and, as Jazz P Bass mentioned, they say 420W at clipping.
    The only other things you might want to check:
    SVT is biased around 17W per tube. Not sure about the PV, you mentioned -53V but the only thing I see listed is -50V ?
    You said the 12AT7 phase inverter had shorted. The AT7 is actually the driver, and it does need to drive 4 power tubes per triode. You checked all the associated resistors that could have been stressed by the shorted AT7? Have you tried any other AT7's?
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