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I think I made a rookie bias mistake?

Discussion in 'Amps and Cabs [BG]' started by JohnnyChatter, Oct 10, 2018.


  1. So my Marshall blew a fuse the other day. So I took it home and of course I had a power amp tube blown. No big deal. I replaced the tubes and noticed my meter was on MA and not MV. So I switched it over and went on with my business. Marshall amp sounds great. Got to thinking and walked in and checked the voltage on the last amp I put tubes in and it’s off the charts. Switch it back to MA and reads where it should. So I switch my meter to MV and I can’t even get the tubes to come down to the 80MV it suggests. When I bias it to 80 in MA and balance it at 0 it works just fine? Did I screw those tubes up some how. They have maybe an hour on them tops? It’s a fender bassman 300 pro. Amp seems fine. Maybe a little hot but nothing mind blowing for a tube amp. Thanks.
     
  2. So I just put the old tubes I had in the amp and it’s the same situation. Amp runs fine. Sounds good. I can set it to 80 when I’m in MA but can’t even go under 120 on mv.
     
  3. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Are you measuring voltage or current?
     
    nbsipics likes this.
  4. Wasnex

    Wasnex

    Dec 25, 2011

    When your measuring mV at the bias TP in the Bassman Pro 300, you are measuring across a 1 ohm bias sense resistor connected between the cathodes of three tubes and ground. The voltage dropped by the resistor pushes the cathodes more positive in relation to the control grids, which should reduce tube current a bit. The meter connected as a voltmeter is high impedance, so it shouldn't load down the voltage.

    When you connect the meter as an ammeter, it essentially presents a low impedance across the bias sense resistor. This low impedance will load down the voltage, so effective bias is reduced a bit. As a result, the tubes will conduct slightly harder, although probably not much, since the resistor is already really close to 0 ohms.

    Here's your bottom line:
    When you set the meter to mA and take the reading, it is connected in parallel with a 1 ohm resistor. This means the resistor will carry some of the current, so the mA reading is probably not very accurate.
     
    john m likes this.
  5. I’m supposed to be measuring MV.... in short I know something is wrong. I set it to MA by mistake I think originally. When I set my voltage meter to MV both sets of tubes I have won’t go down to 80... or even less than 120 mv... I have played an hour tops with the tubes set up the way they were. I don’t know how I could have messed up the tubes but maybe? The amp isn’t any hotter than normal and isn’t blowing fuses or making strange noises. My meter works fine. I have another 300 pro at my practice spot. I’m gonna pull the back off tomorrow and see what that one reads. I have been running that one 8 hours a week for 9 months now so I know that one isn’t having any issues. I just feel like I would be blowing fuses left and right if i has my bias set super high?
     
  6. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    With a 1 ohm bias sense resistor, you really need to follow the instructions and measure the voltage across it. The error will be quite large otherwise, especially on the low mA scale.

    Are you measuring on a Fender Bassman 300 or a Marshall, I can't follow your OP.
     
  7. Fender bassman 300. My Marshall is running great. Where I’m confused is why is my bias would be running so hot. When I have my probes in and my bias know turned all the way down it reads 130 on one set of tubes and 150 on the other. I’m just hoping I’m missing something. One set of tubes is brand new. The other set I just pulled out because I didn’t know how old they were when I bought the amp used.
     
  8. I would be taking it to a tech to look it over but I don’t trust anyone currently in Denver...
     
  9. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    That's a pretty standard monitoring circuit, have you been adjusting the bias pot and not the balance pot???

    Since the bias is injected via V4, and the new power tubes might very well need a higher negative bias voltage applied, it V4 is weak there might not be enough range in the pot to account for this.

    I would REALLY recommend a qualified tech to take a look at this. Give Fender's consumer service guys a call and ask them what they would recommend, especially who they would suggest looking at it. Is it still under warranty?
     
    JohnnyChatter likes this.
  10. Wasnex

    Wasnex

    Dec 25, 2011
    Per the Bassman Pro 300 schematic, B+ is 742V. If the meter is accurate, the 120mV reading is approaching 80-85% dissipation. Good tubes should operate biased this hot, but they will probably age pretty fast. Factory setup of 80mV comes out at <55% dissipation.

    IMHO, try another meter if you can. If the meter is confirmed good, the amp needs to be repaired or the output tubes are either trashed or not graded well for the amp's operating characteristics.
     
    JohnnyChatter likes this.
  11. Wasnex

    Wasnex

    Dec 25, 2011
    Perhaps I misunderstand the bias circuit. Wouldn't a weak driver tube allow the bias to increase (more negative)? If the driver stops conducting, the bias should go to it's maximum negative value and cut off the output section. So, with a weak driver you would not be able to set the current high enough.
     
  12. Thanks guys. I’ll try another meter and check my other 300 pro. If my meter is acurate I have been running the tubes at like 350 my.... I know a lot about amps but just enough to be dangerous. If anyone has any good tech suggestions in Denver let me know. I’m pretty jaded by shotty techs around here. I’ve had dudes close shop with my amp and others take 8 months and others yank Brand new NOS preamp tubes from my head and act dumb about it.
     
  13. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    The driver is a cathode follower, so there's no voltage gain (otherwise if it were a common cathode VA without a lot of feedback, the bias would not be very stable). If the cathode follower was weak, it's likely that the bias voltage would sag (more positive or less negative). This is just an educated guess, I still would recommend having a QUALIFIED tech take a look at it.

    I would not run this amp without the V4 tube installed, I would expect that it might saturate the output stage and cause damage. [edit: I see that there is back-up bias in the event of a V4 failure that would or should protect against catastrophic failure. There are several mechanisms happening simultaneously, but the impedance of the V4 driver tube playe a big part in how the circuit works.)
     
    Last edited: Oct 10, 2018
  14. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Are you sure yo have a 12AT7 in the V4 position and not a 12AU7? The plate impedance difference will also scale the bias.
     
    Wasnex likes this.
  15. I’ll check when I get home. I never change my v4 tubes. Just v1 and v2 so I’m assuming it’s what is original to the amp.
     
  16. Wasnex

    Wasnex

    Dec 25, 2011
    If this is the amp you bought in Aug this year, I wouldn't assume it has the original tubes.
     
    agedhorse likes this.
  17. Ummm. I think it is the one I bought in August. I have had a few of them. It’s a 12at7 in v4 and appears to be original. Should I change that out before taking it to a tech? I never change that tube out of any of my amps.
     
  18. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Does the measured voltage change with rotation of the bias control?

    Does the balance change between tube sides as the balance control rotates?

    I suspect that more is wrong with the amp than just the bias, that's just the symptom.
     
  19. Wasnex

    Wasnex

    Dec 25, 2011
    IMHO confirm the multimeter is good first. If you get the same measurement with a different meter, then swap out the 12AT7. Before you do, set the Bias and Balance controls for minimum tube current. I assume you have already done this.

    I am not sure why Fender used the term Bias and Balance control. Each control actually sets the bias for three tubes.

    The left test point is ground. The center test point measures across the bias sense resistor for three tubes. The right test point measures across the bias sense resistor for the other three tubes. Keep this in mind when you are adjusting the Bias and Balance controls for minimum tube current.

    If adjusting the controls does not change the voltage at the test points, then you probably have bigger problems.
     
    Last edited: Oct 11, 2018 at 1:14 AM
    JohnnyChatter likes this.

  20. Yes and yes. It acts just fine. I have biased not this amp but the same model amp many of times. It just won’t go less than 130 MV. I can make it go wayyyy higher but not lower. The balance goes to zero like it should. The amp sounds like it should. Both set of tubes I have do the same things. I’m almost tempted to pull the tubes from my super bassman and see what they do but I’m afraid I would mess something else up.
     
    Wasnex likes this.

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