New tubes rebiased sounds like crap. Please help

Discussion in 'The Workbench' started by 12AX7, Feb 4, 2020.

  1. 12AX7

    12AX7 Member

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    I know this isn't a Marshall but was hoping y'all could help me with it the Carvin forum seems to be down. I Just re tubed and biased a Carvin Legacy 100 Steve Via head for a friend. I was told the amp had issues before hand with the volume coming and going and then it just quit altogether. The tubes did show signs of past red plating on 2 of the EL34 power tubes. I went through the test procedures to gather the info needed to bias the tubes and set the bias to 180mA across the standby switch. The amp now works and shows no signs of red plating but sounds like crap! On the clean channel you can hear a very noticeable distortion not a good distortion but a nasty distortion like a blown speaker sound. I did try another speaker cabinet and the other output speaker plug and no change. The same nasty sound can be heard in the background of the distortion channel too and it sounds like its farting out also. I'm thinking there has to be a bad component in the circuit somewhere but I didn't see anything obviously burnt up or worked on before. I just downloaded the schematic but I need some advice on where to look for the problem (somewhere in the pre amp stage maybe). Would it be a resistor or capacitor issue? Here are my test results I got when biasing. The plate voltage was a little high .pdf file said it should have been around 450 and the dropping resistor voltage was a little high .pdf file said it should be around 10vdc. Plate voltage was 483vdc, Dropping resistor voltage was 12.1vdc, current across Standby switch was 160mA, current across the dropping resistor was 34.57mA.
     
  2. Matt_Krush

    Matt_Krush Well-Known Member

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    I have never heard of setting bias by measuring current across the standby switch.
    So...you may want to research other methods of setting bias and get either a Bias King probe set, or learn how to do it with measurements at the tube sockets.

    If you run another device into the effects return or power-amp in (if equipped) does it sound ok?
     
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  3. Gunner64

    Gunner64 Well-Known Member Premium Member

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    The factory service manual that I have seen For the only Carvin I have any experience with, the x100, uses the standby switch as bias test points as well if I remember correctly. Perhaps the legacy is the same.
     
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  4. Trapland

    Trapland Well-Known Member

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    It sounds like it’s running really cold. Can’t speak to how you bias that particular amp, but generally ugly distortion when playing clean is a sign of cold bias (overbiased or too much negative voltage). Plate voltages will also run higher with a cold bias.
     
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  5. Esa Martikainen

    Esa Martikainen New Member

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    If there are four valves then each use about 19,5W quiescent power when those measures are used to calculate.

    You measure voltage across resistor(s) so you need to know its resistance. If resistor is one ohm the millivolts come milliamps direct! But not all amps have one ohm resistor(s) and there can be one, two or four of them. Some don’t have and then a socket which comes between valve and base socket, which has a resistor and voltage measuring wires, comes handy and safer as well to use.

    Esa
     
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  6. South Park

    South Park Well-Known Member

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    You bias the amp at the power tubes at the socket so you can see what that tube is doing . So what is the bias set at
     
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  7. Gunner64

    Gunner64 Well-Known Member Premium Member

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    I'm telling you, The Carvin service manual has instructions, and Like I mentioned, that x100b service manual did reference the standby switch as a test point..that was a factory service manual.
     
    Last edited: Feb 4, 2020
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  8. Gunner64

    Gunner64 Well-Known Member Premium Member

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    What about the preamp tubes? I'd swap them one by one with a known good one and see if maybe one is toasted.
     
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  9. 12AX7

    12AX7 Member

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    Here's the link to the manual I used to bias the amp. It is for a Legacy 100 and there is 3 methods for setting the bias in it and I used the standby switch method the first one in the manual. i haven't tried to run anything through the effects loop. What would you suggest trying to plug into it?
     
  10. 12AX7

    12AX7 Member

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    I set the bias to 180mA across the standby switch after taking all the readings and doing the math that this manual said to do. http://www.carvinmuseum.com/pdf/Ka-Boom_v1a.pdf
     
  11. 12AX7

    12AX7 Member

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    All the tubes are new I just installed all new pre amp and power tubes.
     
  12. Gunner64

    Gunner64 Well-Known Member Premium Member

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    I didn't see where you mentioned the preamp tubes, so I just assumed retubed and biased meant just the power tubes were replaced.

    It's not out of the realm of possibilities that a new tube could be bad too. Or an issue with a dirty socket.
     
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  13. Esa Martikainen

    Esa Martikainen New Member

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    First method does not seem very good if it cuts the Cathode current and tube can cool at least some. And there is danger when Anode circuit is cut and brobe can get plate voltage. Also those calculations do not consider Screen current as tube Power. It is not much but it can be up to 5mA.

    Esa
     
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  14. 12AX7

    12AX7 Member

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    I cleaned all the sockets with contact cleaner. I will try some different tubes. Also here are my notes I took when I had it apart tacking measurements and figuring out the bias setting. Maybe I did something wrong but I did exactly what the manual that I posted the link to said to do.

    How do you add a pic here I tried creating a link to the pic from my dropbox account?
     
  15. 12AX7

    12AX7 Member

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  16. South Park

    South Park Well-Known Member

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    The only current you want is the grid current. Grid current controls plate current. Should be around 34 mA . Try to clean tube sockets you can try to roll some preamp tubes to see what happens
     
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  17. Esa Martikainen

    Esa Martikainen New Member

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    Cathode current is best quieszent messure to calculate power because it contains Plate AND Screen currents.

    Esa
     
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  18. 12AX7

    12AX7 Member

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    You are talking about the control grid pin 5 right? Not the Screen Grid or the Suppressor Grid correct. If you will give me the list of test you need I will get you the results. I have the pin out for the tube right here in the manual just need to know which pin to test and what I'm testing for vdc/mA ect.. I have already cleaned the sockets with contact cleaner before installing all new tubes.
     
  19. South Park

    South Park Well-Known Member

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    Yes it is pin 5 you need the tube socket .
     
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  20. 12AX7

    12AX7 Member

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    Ill take the head back apart tomorrow evening and get you the current reading for pin 5 in mA. I'm sure y'all need more info than just the current of pin 5 so list the other test and i'll post all the results tomorrow. While I'm in there I will try cranking up the bias some and see if it helps any. I had calculated an adjustable range of 179.49mA-200.20mA 70%-80% and I went conservative and set it at 180mA 70%. Also if someone could give me step by step instructions on a better way than I was doing to calculate the bias I'd appreciate it. I dont have bias probes all I have to test with is a fluke 87III thanks.
     

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