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Any reason not to get one of these for adjusting Bias?

Discussion in 'Amp Input - Normal or Bright' started by BlacknBlue, Nov 15, 2019.

  1. bob_66

    bob_66 Strat-Talker

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    Bill i read the instruction that were with the probe and i thought it was a mistake at instruction 9-10 where it says set meter to mv and gives reading as ma . SO does it work like you suggest with a resister somewhere in the probe?
    I just tried with the probe switched to plate current and the meter set to mv .I get a reading of 27mv . Is that what your suggesting is 27 ma? As in the direct reading in the instructions attached at no10.
    I only got 7.88 ma when probe set at plate current and meter set at ma.
     

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    Last edited: Dec 5, 2019
  2. Bill Moore

    Bill Moore Strat-Talk Member

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  3. Triple Jim

    Triple Jim Senior Stratmaster

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    I assume you'd have to do that to get to the 6V6 plate's pin, but maybe you could pull the tube out of its socket just enough to get the meter probe on it. But if in any doubt, don't mess with it! I have no idea what experience you have working around high voltage.

    Edit: I see you've correctly measured plate current now. The plate voltage you posted is below spec though, of course.
     
  4. bob_66

    bob_66 Strat-Talker

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    I`am getting confused everytime a read something about these amps. Some mention class A single ended as fixed bias and other mention cathode biased in the same discussion . As far as i know my amp is Class A single ended and has cathode biasing with a resistor and capacitor between pin 8 on the 6v6 and ground via a bias pot on the rear of the chassis. Can someone explain what the bias pot on the rear exactly does as im confused as to whether i have to change the resistor or turn the pot or BOTH.

    I have a schematic if that helps ,which is accurate i believe apart from i have an effects loop in it as well. I have red arrowed what i think is the cathode bias resistor?

    Jim i assume you mean putting the red probe on the pin 3 and the com lead fixed to the chassis/ground via black lead with a crocodile clip on the end?
     

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  5. Triple Jim

    Triple Jim Senior Stratmaster

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    Your amp has an adjustment pot in series with the main cathode resistor. It lets you make adjustments to the plate current.

    That's correct as far as measuring plate voltage. I'd think your probe could do it to, but since it's so far off it makes sense to measure it right at pin 3 to double check. Make sure your meter is rated for that much DC, keep one hand in your pocket, etc., or have someone who has done this before help you. Again, forgive me but I don't know how much practice you've had in this area.
     
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  6. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    Ohm's Law, E=I/R... Voltage = Current divided by Resistance
    If R=1, then E=I... Voltage = Current.
    So yes, reading voltage across a 1 Ohm resistor gives a direct reading of current... that is why a 1 Ohm resistor is often used in the original amp circuit to facilitate taking current readings.

    Do not attempt to read plate current directly from Pin-3. To read current, you have to measure in series. Connecting the meter to Pin-3 and ground, with the meter set to ma (or any current range) will result in a direct short to ground and smoke your meter.
    The bias adapter tools intercept Pin-3 between the OT and tube pin and allow a proper series reading to be taken.
     
    Last edited: Dec 5, 2019
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  7. bob_66

    bob_66 Strat-Talker

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    Hi Jim, my meter reads to 1000 vdc, i dont have much experience with poking around in amps , but im not stupid and realise the dangers. Messed with alot of machinery 3 phase vfds and motors etc... but not valves etc...
    If the reading is still 305Vdc for the plate voltage what do i do to rectify it?
    The manual i have says it should be in the 350-480v range.
     
  8. bob_66

    bob_66 Strat-Talker

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    Rich, i think i got it now very simple explanation. I dont expect to measure plate current directly from pin 3 ,but thanks for the warning anyway. To think i used to be chemist but forgot most of it,but this is all physics to me:)
     
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  9. Triple Jim

    Triple Jim Senior Stratmaster

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    That's the best pun I've heard all day!

    If the power supply isn't getting the correct voltage out, I guess it's time for general troubleshooting.
     
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  10. rockon1

    rockon1 Strat-Talker

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    Wow this is a confusing read. Your probe states to test the bias in mv. That means it has a 1 ohm resistor built into it. Test the current draw in mv as you did. 27 mv = 27 ma draw. Running at 305PV that means your bias is set at about 70% which is low for a cathode biased amp. 12 watt/ 305PV = .039 for 100% .039 x .7 = .027 where your at.
    As noted your PV is below spec according to your probe reading.
     
  11. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    First thing I would do is pull all of the tubes and check the voltage at pin 3, then at the connection from the filter cap to the output transformer.
    If the voltage is low, then there is an issue with the rectifier or the power transformer (or some other part of the circuit is pulling the B+ down).
    If the voltage is at or above spec, then something is causing the tube to conduct and pull the voltage down. Unfortunately, that is one of the problems of measuring plate dissipation at the cathode. Yes, the cathode adjusts the bias, but leakage at the grid or screen can also tap current and cause plate dissipation to exceed the cathode current.

    Also, confirm that your line voltage is 117+. Power companies are only required to provide 104-121vac, so your line voltage could be as low as 104v and you're still "good" on line power, but that would pull your PT output down.
    If line is low, check a few more outlets around the house and see if it's only a problem with that outlet, circuit, or low at the panel. Loose neutrals are not uncommon.
     
  12. Bill Moore

    Bill Moore Strat-Talk Member

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    Lets hope his voltage is a lot more than 117+ there in England!
     
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  13. dueducs

    dueducs Senior Stratmaster

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    I like what you did with the removable (and replaceable!) power cord and ext. That seems very practical.
     
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  14. bob_66

    bob_66 Strat-Talker

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    What would be the effect of having two 1ohm resistors in series on the plate voltage taken with the probe?? The schematic shows this resistor already in the circuit and if theres one in the bias probe as well???? Or is it just a dumb question.
    A normal champ circuit doesnt have that resistor as far as i can tell.
     

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  15. bob_66

    bob_66 Strat-Talker

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    Update ive had the chassis out and for the 6v6gt i get these readings between pins and chassis ground. Pin 3 =328 vDC , Pin 8 =20.9 vDC and pin 4= 319 vDC
     
  16. Bill Moore

    Bill Moore Strat-Talk Member

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    deuducs, the chassis I bought was already cut for the IEC plug. I did have to cut an inner plate to hold the ext. plug tightly!
    bob_66 according to Rob's calculator, your bias is pretty cool.
     
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  17. bob_66

    bob_66 Strat-Talker

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    Bill so is this right,ignore the middle part as im not sure exactly how to measure the cathode resistor voltage drop or know where exactly it is in my amp.
    So how is this remedied ,just adjust the exterior mounted bias pot or what???
    Is this the cathode resistor pointed to by the yellow line.
     

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    Last edited: Dec 6, 2019
  18. Triple Jim

    Triple Jim Senior Stratmaster

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    You measure the drop of the cathode resistor by measuring from the cathode (pin 8) to ground.

    You can adjust your idle plate current (bias) with that pot you have. Whether or not you need to remedy the lowish current is up to you. If you like the sound now, then there's no need to change it. But you can experiment within the limits of the tube if you want to. Low plate current means long tube life.

    In my single ended amp, the '57 Princeton clone, I found that low-ish bias sounds better then high-ish bias, which has a harsher sound when overdriven.
     
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  19. bob_66

    bob_66 Strat-Talker

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    Thanks Jim, So to be sure ,the 20.9 VDC i read directly from pin 8 to ground is the cathode resistor voltage drop? The amp is a bit dark and slightly muddy . Ive just received a different speaker from the USA to try,dont like the ceramic c12r thats in it very much.
    The one ive just received is an original 12" 1960 Jensen P12P alnico 5 concert series with cone in very good condition.. What would i likely expect with this speaker , its rated 16watts i think .
     
  20. CB91710

    CB91710 This is a Custom Title Gold Member Strat-Talk Supporter

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    That is the total drop, including the bias pot, the 1-ohm resistor, and the 470 ohm cathode resistor upstream of the pot (which all three work together to form the cathode resistor.)

    To answer your previous question about putting two 1 ohm resistors in series, if you are only measuring the voltage across one of the two resistors, then you are still getting a correct measurement of the current in that part of the circuit. The added resistor will have a small impact on the actual bias level, since it is being added to the existing 1 ohm resistor, 100 ohm pot and 470 ohm cathode resistor, so when it is removed there will be an increase in current between 1% and 2%... pretty much insignificant given our target dissipiation.
     
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