Tubes out of bias?

Discussion in 'Amps and Cabs [BG]' started by damien7902, Jul 16, 2007.

  1. damien7902


    May 21, 2007
    does anyone know what this means? I was talking to a friend about my svt4pro and how it doesnt have the power it should right now and he mentioned that my tubes could be out of bias. I understood what he meant but i wanted to get more opinions on what my problem could be.

    Here is what happened. I was jamming with my back up bass at practice. after a couple of songs i switched to my regular bass and i noticed it didnt have the power or volume that it normaly should. i cranked the main volume andthe gain to full and no significant change. Before if i turned everything up i wouldnt be able to carry on a conversation in the room but this time the amp had no power. I thought it could have been a dead battery in the bass so i changed that and nothing changed. TO compensate for the problem I had to boost the output gain on my compression unit. ( i dont use the 4pro compression)

    So this is my problem, could someone help?
  2. jimbilly


    Apr 19, 2006
    I believe the bias just means the tube 'output' as balanced with the output transformer. That could be it, but it could be a lot of other things as well. You'll have to take it into 'the shop' to find out for sure.
  3. anderbass


    Dec 20, 2005
    Phoenix. Az.
    Setting the bias is something that's usually only done to amps that have a tube power amp section.
    I believe your SVT-4 is a tube preamp with a solid state power amp.

    For a test, I'd temporarily skip using your compressor (and anything else) and try both your basses connected directly to your amp to confirm what exactly is malfunctioning.
    Also try substituting known-good cords, speaker-cables and speaker-cab to positively locate the problem component.
    Make sure your amps FX blend/level knob is set to full dry/off for these tests. (if equipped)

    Try this and let us know your findings...:)
  4. This is from , you really need a scope to set the bias properly

    Setting the bias current in the output tubes.

    If you have been around tube amps for a while, I'm sure you have heard of the term, "bias". But what exactly is bias? Let's take a look at this term.

    First of all, there are basically two kinds of (class AB) push-pull output stage designs. The first is called "fixed bias" and the other is called either "self-bias" or "cathode-bias".

    Fixed-bias is the term used when an amp has a separate negative voltage supply (often referred to as the C- supply). This separate power supply has its own half-wave rectifier (usually a single diode) and a smoothing section made up of 1 or 2 filter caps. The bias supply may or may not have a "bias pot" to adjust the negative voltage of the grids of the output tubes.

    It's the (negative) voltage on the grid of the tube (with respect to the cathode) that causes (due to the transconductance of the tube) plate current to flow through the tube. The higher the negative voltage on the grid, the lower the plate current. Thus if you increased the negative voltage on the grids by going from -35 to -40, then you have effectively reduced the plate current (made the output tubes run cooler.) In this design, the cathodes are usually tied to ground.

    In a self-bias or cathode bias design, there is no separate bias supply. In this design, the grids are at 0 volts (DC) and a cathode resistor is used between the cathodes and ground. Now, the plate current flowing through the tube goes out the cathodes (along with the screen grid current). This current flows through the cathode resistor to produce a positive voltage on the cathode. Since the grids are at 0 volts, they are now effectively "less negative" than the cathodes producing an idle current because it's the effective grid-to-cathode voltage than determines the plate (or bias) current.

    The sound, the harmonics and the compression effects are quite different with fixed-bias and cathode bias circuits. Fixed-bias designs will give a bit more power and be a little "tighter" and more efficient. Cathode bias designs have a bit "sweeter" sound due to the natural compression of the output stage. As you drive the output stage harder, more plate current starts to flow. This then increases the voltage on the cathode (the increased plate current flowing through the cathode resistor). This then increases the grid-to-cathode voltage which begins to the turn the output tubes off. Natural compression!

    There are several ways to set the bias current in a fixed-bias amp. Note, you do not have to set the bias in a self-biased amp, the amp designer has already done that for you with the selection of the cathode resistor! One method is called the shunt method and another is the scope method. Both are equally valid and can produce good results. Only an experienced technician should perform these settings.

    With that said, it is very important to the overall sound of the amp to have the bias set properly! The entire tone of the amp can be manipulated (for better or for worse!) by setting the bias correctly for your particular tastes. We ask our customers what type of sound they would like to achieve and we try to translate that into the proper bias setting.
  5. Thangfish

    Thangfish ...overly qualified for janitorical deployment...

    Sep 3, 2006
    Cameron, NC USA
    You only need to bias tubes when they are replaced... or possibly if some other modifications were made to your amp, which doesn't appear to be the case.
    Ummm... actually that 4pro has a transistor power output section, right?

    anderbass beat me to it. A lot can happen when you eat supper without refreshing the page.
  6. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Fuggettaboutit...........The 4PRO does NOT HAVE a tube bias.

    All the tubes are in the preamp....... 12AX7 etc. Power section is Mosfet, and bias problems there would just make a little distortion.

    Check that the front panel settings are the same as always (check all the buttons).

    Then make sure that if you were in bridge mono that the button didn't get accidentally set back to stereo mode.

    Then get a spare 1/4" cable and plug several times into each jack on the back. Use contact cleaner if you have any.

    of course you COULD have a bad tube, or a problem in the tube power supply, etc.

    But check the obvious first.
  7. AEONmw


    Jun 24, 2007
    Chico California
    1. remove all known good components( cables, effects, and similar) until your left with the known bad component ( amplifier, and a known good cabinet). Now, at this point your setup should be the following: bass->cable->amp->cable->speaker

    2. Power off the amp.

    3. set the levels on your bass AND amplifier to settings you've used in the past that you're familiar with, and know should produce a good solid audible tone.

    4. Make sure the amp has been off for at least a good 15 minutes.

    5. play a note on your bass( maybe an open G note, such that it will sustain for a while), then immediately power on the amp....

    ...what happens ? Do you hear the note, but then it suddenly( or even not so suddenly) dies out ? This might be a tube problem.

    If no sound comes out, then it might be something in the power supply section.

    Be aware that some amps (at least the well designed amps) "sequence" the bias to the tubes, which can be a delay of several seconds from the time you power on the amp to the time you hear a sound. This is normal. Basically, sequencing means that upon power up, protection circuitry tests the bias conditions on the tubes/transistor to make sure they are correct before allowing a signal to be applied.

    If you hear the sound, and then it dies out, you might just want to replace the 12AX7's yourself. I like to keep a cheap assortment of tubes on hand just for troubleshooting like above. If replacing the cheap 12AX7 fixes it, then replace the cheap tube with a good quality 12AX7, like a grooveTube.
  8. Jazzman


    Nov 26, 2002
    Raleigh, NC
    Don't forget that you need to customize the speakon connector for bridge mode. 1+ & 2+ pins in bridge mode. That could seriously cut your output.
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Not if

    1) it WAS running OK

    2) he uses the cord that comes with the amp.


    If he has a mono bridge Speakon to Speakon cord, if plugged in with wrong end to the amp, he will get either no sound, or reduced volume, depending on how the speaker is wired.

    The Speakon connectors are great, but their real downfall is that there is no "socket" for the cord end, with matching "plug" for the cabinet.

    To avoid the problem of polarity, they SHOULD be set up like a mains extension cord, with plug on one end, "socket" on the other. That way you'd HAVE to plug them in right no matter how they are wired.
  10. jimbilly


    Apr 19, 2006
    If your SVT4pro has a solid state power section, -don't listen to your friend anymore when he gives gear advice.
  11. luknfur


    Jan 14, 2004
    Only experience I have with tubes in an amp is in a THD Univalve but to my knowledge there's no issue regarding bias in preamp tubes. I see lots of mention regarding matched pre tubes but the Uni only had input and gain pre tubes (in recall that's what THD called them) but it wasn't an issue. Maybe for some of these units with like 4 pre tubes. It's my understanding tubes wear at different rates so don't know what the matching's about but there must something to it cause you pay more for it.