VHT Pittbull Bias and related

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by UconnJack, Oct 13, 2005.


  1. UconnJack

    UconnJack Member

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    I posted this on the VHT Users forum, appreciate any insight here, if possible. TIA.



    Landed my first VHT yesterday, it's a 2001 Pittbull 45, 1x12 combo.

    Ran the amp for about an hour then it started cutting out. Got a little static-y and lost volume. Feels/sounds like I need to clean the tube sockets? I'm going to try than tonight. Happens intermittently, then goes away, and the volume comes back.

    I'm curious if this amp is self-biasing? If not, can anyone advise on the procedure and recommended current/voltage settings?

    Aside from all that, the amp sounds great, looking forward to getting to know it better.

    I appreciate anybody's insight and help in this issue. I'd also like to hear others experience with this amp, good or bad.

    Thanks in advance.
    UJ
     
  2. UconnJack

    UconnJack Member

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    VHT stated the following:

    Bias current is 25mA/tube - translates to -15VDC at grid with standard EL84s.

    Can anyone tell me what it means to measure the -15VDC at the Grid? I have a voltmeter and screwdriver! Just need a little direction, if this is possible!

    THanx
     
  3. doctord02

    doctord02 Member

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    Time to make friends with a tech. If you are not familiar with safety precautions in dealing with high voltage and live circuits, you would be well advised to have someone show you how to do this...

    It is as simple at putting a test probe on the correct pin on your power tube and reading the voltage; however some of those pins will have close to 400v of DC electricity on them.

    A mistake with this kind of voltage is generally fatal.

    What kind of experiance do you have in working with electronics?

    BTW, I dont believe the VHT Pittbull is self biased (cathode biased). Could be wrong.
     
  4. UconnJack

    UconnJack Member

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    Thanx for the reply. I have biased many fender type amps both using a Weber bias Rite and a volt meter. I just don't understand the terminology enough to know what "the Grid" means. I'm an Engineer, but no Technician, thanx for the heads-up on the death-voltages! VHT said the same thing.

    I can actually have a electronics tech (Power Electronics) do it at my work. I would like to be able to tell him which pins to measure across.

    VHT Confirmed that the amp is self biasing in Class A mode, but can be bias-adjusted in Class AB
     
  5. doctord02

    doctord02 Member

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    Your tech should be well aware of which pin to measure, or it's time for a new tech.

    Here is the data sheet for your power tubes:

    http://www.nj7p.org/Tube1.php?tube=6BQ5

    Pin #2 is the grid you measure your bias voltage at. Note that there is more than one grid in your typical power tube. This particular grid is known as the control grid, as it regulates tube performance.

    Again, working on a live open amp requires that you have a healthy respect (and fear) of high voltage. Measuring bias this way is nothing like using a bias-rite from outside the amp. The safest way to do this is with test probes with clips, and by connecting them while the amp is unplugged, one to pin 2, and one to your chassis ground. Then you can power it up. Be aware that the power supply capacitors can store a lethal voltage even when the amp is off and unplugged. If you dont have a perfectly clear idea of what to do, then do not do this. You also cannot allow the test clips to accidentally touch the other pins, as that would cause a short. Always keep one hand in your back pocket with an open live amp. This minimises your chances of a fatal shock, tho you can manage to if not careful.

    Be certian that your meter is set to DC volts. The amp should have a trimpot that can be adjusted using a plastic or non-conductive screwdriver to adjust the voltage range. Do Not poke a metal shaft screwdriver into a live amp.

    Again, your tech should know all this. If he's not familiar with biasing tube amps, it's time to find a new one.
     
  6. UconnJack

    UconnJack Member

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    Nice! Thanks for the input!
     
  7. Profklamen

    Profklamen Member

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    UnconnJack,

    Let me know how it goes! I'm in the same boat as you - I have a VHT 45 a voltmeter and a screwdriver - and I've biased amps using a weber bias meter before. I'm about to put new tubes in my VHT too.

    If I understand the previous posts correctly, this amp does not need to be biased when running in class A mode, right?

    No need for another pointer on the dangers of amps, I know the precautions.
     
  8. Blue Strat

    Blue Strat Member

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    Grid voltage correlates with plate/bias current, but in no way is there a direct relationship. In order for the relationship to be direct, all EL84s would have to have the same inherent idle current which is clearly NOT the case (otherwise, you wouldn't need to order matched pairs because they'd all be matched which is CLEARLY not the case).

    IOW, one pair of EL84s set at -15 V at the grids could be biased at 20mA and another pair could be biased at 40mA.

    What you need to do is measure plate or cathode current. To measure plate current you need to set your MM for current and measure between the center tap of the output transformer and the plate of each EL84.
     
  9. Bluewail

    Bluewail Silver Supporting Member

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    FYI

    I have a Pitbull 45 and I recall that when I hooked it up to a BiasRite meter and measured the plate voltage (sorry, can't recall the reading), 25ma seemed a bit too much for EL 84's - over 70% of max plate dissapation. I did some experimenting and ended up with them at 22 ma which as I recall got them below the 70% mark and did a nice job of opening the amp up and taming the ferocious low end.
     
  10. UconnJack

    UconnJack Member

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    Thanks for all the input.

    I trust what you are saying Mike (BlueStrat), not sure I'm qualified to find the center tap of my O/T and the plate pin on the tube? Why would VHT state to set the bias based on grid voltage (any conjecture appreciated?)?

    I think I'll get the Bias-Rite for the EL84's and be done with it.

    I really would like to understand this better though.

    Incidentally. Steve from VHT has been helping me troubleshoot my amp problem. As it turn's out, it's a problem with the loop I/O switch. Next step is to open the amp up and spray contact cleaner on the switch! I didn't ask him about the bias though. I love troubleshooting recently purchased equipment!


    ;)
     
  11. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    It might be cheaper to get some bias rite heads for EL84s if you're comfortable with the multimeter approach.
     
  12. Scott Peterson

    Scott Peterson Staff Member

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    +1

    That is what I did when I owned the PB45. No-brainer and MUCH safer. :D
     

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