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My Voltages

Discussion in 'Shock Brother's DIY Amps' started by jh45gun, Apr 28, 2009.

  1. jh45gun

    jh45gun Banned

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    TP 1 381 V
    TP2 352 V
    TP3
    TP 3&4 ??? Them resistors are not on my turrent board

    TP 5 304 V

    TP6 20 V Is this where the bias is checked if so it is then -20 with the leads reversed

    TP7 236 V
    TP8 56.5 V
    TP9 186 v
    TP 10 272 V
    TP11 .52 V

    TP12 6.4 mv

    TP 13 through 17 my digital meter bounced around so much I could not get a reading as it was not stable OK now I know it was supposed to be AC

    So will these help any?
     
    Last edited: Apr 28, 2009
  2. jh45gun

    jh45gun Banned

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    OK since this was a head build with the board over the tube sockets I am going take the nuts off the board to loosen it off of the stand offs so I can access the tube pins. I need a refresher course on Octal tube sockets which pin is 1 where the Key is and which pins are the Cathode and the Plate. Once I find that out I will take readings and post. Cathode to ground then using the meter with Positve to ground and Negative to cathode will give me the Bias readings right?
     
  3. FiddlinJim

    FiddlinJim Tele-Holic

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    The voltages on TP13, 14, etc. are measured in AC, so what they're doing is measuring the signal as it gets amplified along the way. The schematic says that they're measured with a 100 mVAC, 1Khz sine wave. So, to measure those, you need a way to inject that signal into input Jack 3 and need to set your multimeter to AC.

    Looking at the DC voltages you listed... I really hope you have TP 9 and TP 10 mixed up. TP 9 is should be a plate voltage of around 165 volts and TP 10 is a cathode voltage of about 1.25 volts.

    All of the other voltages look high, but not ridiculously so. An exception is TP 11, which at 4.6 volts is way higher than expected 1.33 volts. That can really change how that first tube responds. Can you make sure that resistors R7, R8, and R9 are the correct values? These will set the operating conditions of the tube and affect the cathode voltage TP 11.

    Also, TP 12 is again an AC voltage, dependent on having a 100mVAC signal in the Mic 1 jack.

    I'm an amateur, so take my comments with a little salt (lime and tequila).
     
  4. jh45gun

    jh45gun Banned

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    Sounds like you know more than I do as far as the resistors go I bought this board preloaded from Watts so I hope they are right.

    Ok 186 for TP 9

    tp11 .52 with the meter set at yt 20 vdc setting
     
  5. FiddlinJim

    FiddlinJim Tele-Holic

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    186 is better for TP9. The cathode voltage of 0.52 is strange. It really should be pretty close to the 1.33 volts as listed on the schematic.

    Overall, the voltages you read indicate that either your power transformer is pumping a little more juice in to the rectifier, and/or the rectifier is supplying a little more voltage than the Fender schematic's test voltages, but well within reason as far as I can tell.

    The voltages (with the possible exception of that V1 cathode weirdness) won't explain your problem that the amp stays clean. I assume you've got the 1 Meg volume pots all wired up right, don't you?

    Now I'll let the smarter guys chime in with some more comments...
     
  6. jh45gun

    jh45gun Banned

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    Cathode Voltages are 19 VDC if I am supposed to reverse the leads on the meter then it is -19 VDC Plate Voltages are 370 VDC one one and 399 VDC on the other
     
  7. jh45gun

    jh45gun Banned

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    OK so there are my Measurements guys any ideas????
     
  8. pchilson

    pchilson Friend of Leo's

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    What are you measuring?
     
  9. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Hey jh, I assume this is your modified 5E3, and you're using the test points that Fender uses in their reissue schematic?

    If your power transformer has a center tap for the filament winding, then you won't need the resistors that feature TP 3&4.

    It's possible that your V1 tube simply isn't drawing as much current as it ought to, which would explain the low cathode voltage -- what tube are you using there?

    The voltages are a little high overall, but probably not hurting anything.

    - Scott
     
  10. jh45gun

    jh45gun Banned

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    Yes this is my 5e3 clone I made out of a Zenith Hi Fi Chassis. Scott VI is a Baldwin Marked 12AX7. According to my tech they were not quite as hot more like a 12AY7 or to be more specific kinda inbetween the two.By Modified do you mean that I changed the chassis stripped to a 5e3 with a preloaded board or that I modded it as I did not my question was before how do you mod it to use 6l6s but I never did as I do not know what they do this is amp using the stock tube set up for a 5e3
     
  11. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Ah, I see. Some modern 12AX7's can be a little weak on the current draw, too. Not necessarily a problem in the V1 slot, since the guitar signal is too weak to push it into clipping. What are your voltages on pins 1 and 6 of V1?

    - Scott
     
  12. jh45gun

    jh45gun Banned

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    So any ideas ? I am a bit confused on the bias V is that what I checked on Pin 8 for the Cathode Voltage and if so is that within range???
     
  13. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Yeah, 20V is good, give or take a volt or two. Means your power tubes are drawing about 80mA of current.

    - Scott
     
  14. jh45gun

    jh45gun Banned

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    OK Scott so if every thing is reasonably in line though some voltages are a bit high why does my amp have clean headroom compared to the normal 5e3?
     
  15. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Hard to say! It's possible that the stars have aligned, the tubes are biased perfectly, and the output transformer has the right primary impedance to give you a lot of voltage swing before clipping. The phase inverter looks fine, but it just seems like the preamp isn't producing enough gain. Have you tried other 12AX7's in the first slot? Perhaps the Baldwin is mislabelled!

    My 5E7 would barely break up when I loaded the preamp slots with 12AU7's and other low-gain tubes.

    - Scott
     
  16. jh45gun

    jh45gun Banned

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    Yea I have tried several 12ax7s in that slot in fact I took the amp to my tech friend and we tried several to see what tube sounded the best and non of them really broke up like the clips I have heard on the net. This amp seems clean right up to being dimed. Go figure????
     
  17. jh45gun

    jh45gun Banned

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    So now my question is what do the guys do that hot rod these using 6l6s ? I suspect a heavier voltage capable OT what else? Not saying I will do it but I am curious
     
  18. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    Part of it is deciding the right primary impedance, to get the most power at the voltages you intend to use. People tend to associate higher voltages with more headroom and power, but you can ruin that effort with the wrong primary impedance on the OT!

    - Scott
     
  19. jh45gun

    jh45gun Banned

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    I will probably just leave it for what it is since I have my 5f4 build though that too is a bit different as it has a massive OT from a Peavey Amp that used 4 6l6s though I am only running two. The bias voltage had to be rigged too since my PT did not have a negative bias tap. So my tech rigged a new bias circuit for it plus we put a pot in for the bias. Works and sounds good now and I decided to run the amp using a 4 ohm load even though the OT is set up for 8 ohms as the guys here and the Peavey Customer service told me to use the 4 ohm tap as it would read more like 8 ohms to the amp. However since the OT is so large and heavy duty the tech and others told me I could run a mismatch of sorts and the OT would not even blink at that. I also have a Bogen MX60 PA head (7581 output tubes though this one is running 6l6s)


    I am slowly giving a cap job to when I can afford the caps I got a couple changed out already. At 60 watts this will be a pretty ballsy amp and sound good with my Digitech pedal providing reverb and effects.
     
  20. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I'm slightly confused.

    Is this same amp a 5E3? The fixed bias "borrowed" voltage (a common technique), is this for a "5E3" as well? The four 6L6 OT, is this also for the 5E3?

    A fixed bias 6V6 amp with a heavy duty OT built for four 6L6's will definitely be part of this headroom issue. If you were referring to a 5F4, then that makes sense. I'm just a bit confused.

    Otherwise, the voltages all seem to fit the 5E3 setup - otherwise the tubes, input circuit, or OT (or impedance issue) has to be the culprit. 5E3's get dirty quick - almost filthy dirty quick too.
     
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