Bias range resistor question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by samwheat, Feb 19, 2006.


  1. samwheat

    samwheat Member

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    I have my Plexi clone from a Laney Pro Tube 50 almost complete

    With the bias maxed out I'm getting around -11 volts

    The Laney transformer has a separate winding for the bias and the bias range resistor is 22K according to the Laney schematic

    I used a 220K from the Hoffman amps schematic which is typical Marshall

    If I change it to what the Laney schematic says, will this cure the issue?
     
  2. Dave C

    Dave C Gold Supporting Member

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    Can't say it will cure the issue for sure but a max of -11 won't work . I'd be looking to see -35 to -45 or a little wider for el34s and a range of -10v more for 6l6.
    Dave C
     
  3. Gordon

    Gordon Silver Supporting Member

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    Since your power tx has a separate bias winding you will want to use the 22k. The 220k is for old marshall where they didn't use a separate lower voltage winding and just took it off the tx high voltage. Be sure to check when done and you should be ok. Sometimes stuff needs a little tweak when done but you should be in the range you need.
     
  4. samwheat

    samwheat Member

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    I have 15K and 33.2K's in my supplies

    Whch one should I use?

    BTW, I have 1 ohm cathode resistors and am running RFT EL34's
     
  5. John Phillips

    John Phillips Member

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    Try one of them and see.

    The bias voltage you are looking for should be about 8-9% of whatever the plate voltage is, for EL34s, or -30 to -45V which should pretty much cover any situation.
     
  6. samwheat

    samwheat Member

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    John,

    The 33k did the trick

    I have a 1 ohm cathode resistor on each tube

    What do I set my multimeter on to measure current thru them?

    BTW, it sounds really good and I have the rich mod MV ..... I can turn it up alot before becoming deaf

    Both jacks on channel 1 work however channel 2 doesn't
     
  7. Gordon

    Gordon Silver Supporting Member

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    You must set your meter to read millivolts. How ever many millivolts you read is how many miiliamps of current are flowing, assuming that your resistors are exactly 1 ohm. That's the drawback to this method of measuring current.
     
  8. samwheat

    samwheat Member

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    John,

    I tried jumpering the channels ..... still no sound off the bass channel

    Tried a different V1 .... still the same

    BTW, I have a sylvania in V1, rft in V2 and tungsram PI

    When I plug into the hi of the bass channel and plug in the jumper to the lo, I get a squelching sound
     
  9. Gordon

    Gordon Silver Supporting Member

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    I'm not John, but here's what I would do. Check for a voltage drop across your ch2 v1 plate resistor. If none exists then that half of the dual triode is not working. Since you've already tried substitution the next step is to eliminate bad connections at the tube socket. Remember. you must also have a good filament connection, if no filament current is passing then the tube will not function. If a voltage drop exists. Then you must check every component in the signal path (i.e. coupling cap, vol pot, blend rx) up to v2. BTW you never did say if you had a hiss in ch2 when vol turned up. This would be helpful to know since it tells you where to start looking.

    Good Luck,

    Gordon
     
  10. Gordon

    Gordon Silver Supporting Member

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    CAUTION!!! The things I just said in that post involve working on a llive amp with lethal voltages inside. It is extremely dangerous if you don't know what your doing and make a mistake and aciidentally touch something you shouldn't touch. Having said that, the easy way to do the component check that I suggested is to just touch the probe on you voltmeter to the place you want to check and listen for a popping noise in the speaker. To do this turn up the Ch2 vol. and start at the grid of v2. You should hear a pop here since your ch1 is working. Next go back to the ch2 vol pot. Is there a pop? If so, then you mix rx is Ok if no then it's the mix rx that's bad. Use this method to find the bad component. When you hear no pop you know that's the bad one. This kind of a methodical diagnostic procedure can show you the problem in just a matter of a few seconds.

    Good Luck,

    Gordon
     
  11. samwheat

    samwheat Member

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    Gordon,

    The amp has a 180K/3w bleeder resistor from B+ (preamp) to ground

    I always have them in my build and let the amp drain b4 working on it

    Thanx for the warning tho
     
  12. Gordon

    Gordon Silver Supporting Member

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    The bleeder Rx is great for making sure you don't get shocked when you're soldering or working on the amp when it is unplugged. However the stuff I was talking about you must have the amp powered up and connected to a speaker so the bleeder won't save you since the amp is actually running. If you want me to walk you though it over the phone send me an email and i'll give you my number.

    Gordon
     
  13. samwheat

    samwheat Member

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    Thanks for the offer Gordon,

    I believe my problem is that the bass channel jacks are not wired correctly. I will change and report back.

    On biasing .... I have mini clip leads for my multimeter .... all I do is clip them where needed when the amp is off and drained and then turn the amp on .... take reading, turn amp off, let it drain then move them if I have to ..... This takes longer than normal ....... I refuse using the standard probes when taking measurements.

    If i have to adjust the bias .... i do it when the amp is drained .... i have a 20 turn 3/4w bias pot
     
  14. samwheat

    samwheat Member

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    the bass channel jacks weren't wired correctly

    its not that loud compared to what it was in the laney form

    it may need bias adjustment

    does the ppimv reduce the volume when at 100% open?
     

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