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How to set the BIAS on the old SVT's.

Discussion in 'Amps and Cabs [BG]' started by sedan_dad, Apr 1, 2009.


  1. sedan_dad

    sedan_dad Supporting Member

    Feb 5, 2006
    Columbus,Ohio
    My tech told me how to do this over the phone and I wanted to share.
    With the cabinet hooked up,the amp on and warmed up,stand-by off ready to go,volume cranked up ,
    NO cord plugged into the input............,
    Measure mV from K1 to ground. The schematic states .072 volts,
    so use the bias pot (VR1) to set it to .072 volts.
    Then measure from K2 to ground and adjust VR2 until you see .072 volts.
    Now go back and see if K1 has drifted. If so ,readjust it back to .072.
    Go back and forth checking and rechecking.
    You won't believe the difference in clearity if it was off much.
     
  2. Chinguschild

    Chinguschild

    Apr 17, 2006
    Chico, Cali
    You have a GREAT tech on your hands if he will tell you how to do that than rather take your money and do it instead. Good advice to it sounds like.
     
  3. sedan_dad

    sedan_dad Supporting Member

    Feb 5, 2006
    Columbus,Ohio
    he's the best!
     
  4. Chinguschild

    Chinguschild

    Apr 17, 2006
    Chico, Cali
    And I just noticed your sig. What Dano bass are you playing if you dont mind me asking. I got a Longhorn I play from time to time.
     
  5. Rickenbackerman

    Rickenbackerman

    Apr 17, 2001
    Laurel MD
    Good advice except for having the "volume cranked up". It doesn't need to be.

    Also, setting the bias is only half of the equation - setting the balance is the other half which requires a signal generator and dummy load.
     
  6. johnk_10

    johnk_10 vintage bass nut Supporting Member Commercial User

    Feb 16, 2008
    Thousand Oaks, CA
    John K Custom Basses
    that's nice, but i'm not so sure that your tech is actually "giving" you anything as all of my vintage SVT's have that printed right on their front panel.
     
  7. sedan_dad

    sedan_dad Supporting Member

    Feb 5, 2006
    Columbus,Ohio
    I use a Longhorn and a U-1. Both are short scale.
     
  8. sedan_dad

    sedan_dad Supporting Member

    Feb 5, 2006
    Columbus,Ohio
    i'm not sure whats printed on mine as there is a varnish like coating obscuring alot of the text. It's a chrome chassis so it's hard to read anyways.
     
  9. Groover

    Groover

    Jun 28, 2005
    Ohio, USA
    Are the steps after setting the correct voltage required when biasing? (the part where they mention 40hz).

    Also, is rebiasing needed when replacing only the phase inverter tube, or driver or preamp tubes?
     
  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The 40Hz is for setting the phase inverter balance.

    You can set the hum balance any time.

    Biasing is needed when the 12BH7 or 6550 are changed.

    The 40Hz check is potentially helpful after replacing the phase inverter.

    For any preamp tube no attention whatever is needed other than putting it back together.
     
  11. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    you just use a regular voltmeter for this? where are k1 and k2? can any idiot do this? can you do it on fenders as well? and b-15's?
     
  12. Kindness

    Kindness

    Oct 1, 2003
    Chicago
    Yes.

    On the front panel of the power amp, behind the "grill."

    If you can follow the instructions written on the amp/schematic/in this thread. :smug:

    They don't have the same test points and you wouldn't be setting them to the same readings.
     
  13. Groover

    Groover

    Jun 28, 2005
    Ohio, USA
    I plan on buying for a digital voltmeter tomorrow if I can find one. I saw some at lowes today, but they wouldn't go down to the 0.0x numbers (hundreths?).

    I think setting up the K1 and K2 is idiot level from
    what I can tell, which is why I feel I'm totally qualified for :p

    The other steps I think require some tone generator or some other skill set, which is making me think I should take it in.

    I'm such a tinkerer, and i'll be experimenting with a bunch of tubes, so I really need to get this down instead of going to the tech every day...

    I finally put toghether my SVT-HD today, sounds awesome, dead quiet (except for fan), but Im sure it needs some bias work as I changed all the tubes from the way I got it.

    It does have a nice subtle fuzz, which I like, but from I read it needs biased? Too cold running?
     
  14. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    right, i get that ;) i just find the test points and the proper readings for them and i'm set, right? if so, this is fantastic news! i always thought it had to be done on a scope, and the closest tech is an hour away. terrific!

    btw, i'm assuming this is for all svt's, or if it has 6146's would that make a difference?
     
  15. Kindness

    Kindness

    Oct 1, 2003
    Chicago
    I know plenty of pros that have never once biased an amp with a scope. Doesn't mean it's the best way, but it is a standard way. I have a scope here that I use on my SVT, but I have done it using a meter and the instructions on the chassis and got good results.

    I don't know if they had test points on the chassis, do you have any pics with the front grill off?
     
  16. The spec was .72 volts for both JimmyM. Its a safe ballpark setting. Not always the "best" for any set of tubes but close. Those circuits had a 1ohm resistor between the cathode and ground to provide the test point and would also burnout if the tube shorts. The resistor blowing saves the output tranny and lets you keep playing through the night till you can get it serviced.
     
  17. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    very cool...so i've been looking at meters and most of them seem to only go like either 1 decimal place or none at all. or am i just imagining things? anyone know of a couple not too expensive ones i can check out?
     
  18. Kindness

    Kindness

    Oct 1, 2003
    Chicago
    Here's an example of a a bias calculator. You measure the plate voltage, select the tube type and it outputs a chart with bias currents. If you have an adjustable bias control, you just dial in your target dissipation. http://www.ax84.com/contrib/biascalc.php
     
  19. Kindness

    Kindness

    Oct 1, 2003
    Chicago
    There are some suggestions in here: http://acapella.harmony-central.com/showthread.php?t=2232266
     
  20. Surly

    Surly

    Feb 2, 2007
    South Florida
    Did mine with a dummy 4 ohm load and a scope, following the instructions on the chassis. I also recall using a frequency generator to drive the amp (maybe this was the last step?). Also calculated power output in the end. I got 290 watts RMS.
     

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