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NTD - EL84's & 12AX7's

Discussion in 'Amp Input - Normal or Bright' started by gjohnson441496, Aug 9, 2018.

  1. gjohnson441496

    gjohnson441496 Strat-Talker

    Age:
    61
    285
    Jun 6, 2017
    TN
    I wasn't happy with my Strat sound through my Vox NT15c. It was a bit too bright/chimey and changing strings didn't seem to help. Being new to guitar when I bought the amp, I didn't know that Vox was known for their chime. I found the amp used for $200 out the door, and fell head over heels in love with it after hearing the overdrive/gain channel. My Gretsch sounds great through it as is so I didn't want to sell the amp or get a new one. I considered modding the Strat with different pickups but I've done mods on my bass guitars which were hit or miss (mostly miss), so I really wanted to avoid modding the strat.

    It just so happened that about a month or so ago I had to have a preamp tube/valve replaced and I noticed how much of a difference it made and, the difference it made depending on where the tube sat in the lineup. After much research I decided to buy all new tubes. I bought (2) Tung-Sol EL84 New Production Power Vacuum Tubes and (3) Electro-Harmonix 12AX7 Preamp Vacuum tubes, and the difference is like night and day. I can't put the Strat down now. I only stopped playing to take a break and decided to post these findings.

    If you're not getting the sound you're searching for, installing new tubes may be the answer. The original 2 EL84's just said China on the tube and the 3 12AX7's were mixed (1 China, 1 Ruby, and a JJ). TubeDepot had a sale on tubes and delivered the new ones in 3 days.
     
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  2. Strat Jacket

    Strat Jacket Senior Stratmaster

    May 11, 2018
    Illinois
    I've heard the same from others. After replacing, did you have your bias checked?
     
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  3. The-Kid

    The-Kid Most Honored Senior Member Strat-Talk Supporter

    Age:
    28
    Feb 15, 2014
    SO CAL USA
    El84 amps dont require bias if at all.
     
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  4. gjohnson441496

    gjohnson441496 Strat-Talker

    Age:
    61
    285
    Jun 6, 2017
    TN
    No. I just replaced the old ones with the new. I've heard of having them biasd but I'm not familiar with the process. I also read about having them "burned in".
     

  5. Triple Jim

    Triple Jim Senior Stratmaster

    Feb 27, 2018
    North Central NC
    What? They're just like any power pentode... if they're running cathode bias, they're pretty forgiving, but if they're running fixed bias, they certainly do need to have the bias set correctly. I don't know abut this amp in particular, but the Laney Cub 12R and my Monoprice 15w amp both use fixed bias, and have a pot on the board to adjust it. I've changed EL84s in mine and definitely had to readjust the bias to get the plate current reasonable.
     

  6. gjohnson441496

    gjohnson441496 Strat-Talker

    Age:
    61
    285
    Jun 6, 2017
    TN
    I didn't see any pots on my NT15c during install. Can you explain power pentode, cathode bias and why these items should be readjusted if necessary?
     
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  7. RJ Squirrel

    RJ Squirrel Strat-Talker

    Age:
    64
    427
    Jan 28, 2018
    Frostbite Falls
    Even cathode biased amps may need the cathode resistor swapped out for a different value of resistance to properly bias the amp. It all depends upon the current draw of the tubes.
     

  8. heltershelton

    heltershelton ASKED TO LEAVE THE STAGE Strat-Talk Supporter

    Jun 5, 2013
    Not Florida
    all i know about that stuff is my ac30 and ac4 are self biasing and my marshall is not.
     
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  9. The-Kid

    The-Kid Most Honored Senior Member Strat-Talk Supporter

    Age:
    28
    Feb 15, 2014
    SO CAL USA
    Pretty much this is if its a Peavey Classic 30 or ACVox.....dont worry about it.

    Evrything else.....worry about.
     
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  10. RJ Squirrel

    RJ Squirrel Strat-Talker

    Age:
    64
    427
    Jan 28, 2018
    Frostbite Falls
    The EL84 is an output (power) tube consisting of five electrodes (pentode), the anode, the cathode, the control grid, the screen grid, and the suppressor grid. The control grid controls the flow of current in the tube. Tube bias is the difference in voltage between the control grid and the cathode. This difference in voltage is referred to as bias. The grid must be sufficiently negative in respect to cathode to limit current flow so that the dissipation rating of the tube is not exceeded. The bias can be achieved either by applying a negative DC voltage to the control grid (fixed bias) or by placing a resistor between the cathode and ground (cathode bias). Current drawn through the cathode resistor raises the cathode voltage potential in respect to the control grid, thus making the control grid negative with respect to cathode. To a certain extent, cathode biased amps are self biasing. Tubes that draw more current will result in higher cathode voltages. Where problems arise are when amps are operated near the operating limits of the tube. In these instances, the cathode resistor may need to be changed to a higher resistance value.
     

  11. AxemanVR

    AxemanVR I appreciate, therefore I am... Strat-Talk Supporter

    Feb 8, 2014
    Minnesota USA
    `
    Simple answer:

    On amps that "need to be biased", output tubes can be set to run HOT, COLD or JUST RIGHT...

    In most cases you'll want them to run JUST RIGHT...

    `
     
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  12. Strat Jacket

    Strat Jacket Senior Stratmaster

    May 11, 2018
    Illinois

  13. Strat Jacket

    Strat Jacket Senior Stratmaster

    May 11, 2018
    Illinois

  14. Strat Jacket

    Strat Jacket Senior Stratmaster

    May 11, 2018
    Illinois

  15. gjohnson441496

    gjohnson441496 Strat-Talker

    Age:
    61
    285
    Jun 6, 2017
    TN
    You made that easy enough to understand. Those elements are inside the tube. I get it now. Thanks!
     

  16. gjohnson441496

    gjohnson441496 Strat-Talker

    Age:
    61
    285
    Jun 6, 2017
    TN
    Like with a computer, in the old days we overclocked them. If they crashed, we backed it off a bit. I'm understanding this now. Thanks.
     
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  17. gjohnson441496

    gjohnson441496 Strat-Talker

    Age:
    61
    285
    Jun 6, 2017
    TN
    I don't think I saw any bias points in the amp by the tubes but I may look inside again. Thanks for the vids.
     

  18. heltershelton

    heltershelton ASKED TO LEAVE THE STAGE Strat-Talk Supporter

    Jun 5, 2013
    Not Florida
    on that amp there isnt one.
    if you buy jj's from eurotubes, you can tell them if you want more or less headroom.
    if you want more, they will send you a cooler grade of tube, and if you want it to break up faster, they will send you a warmer grade.
     
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  19. gjohnson441496

    gjohnson441496 Strat-Talker

    Age:
    61
    285
    Jun 6, 2017
    TN
    On Tube Depot and the Tube Store websites you can select tube sets by amps which is what I selected to be on the safe side. Some sets are very expensive, ranging from $76 to $340.

    Are the more expensive tubes that much better? What do you get for more money? I know some are considered new old stock (NOS). Others have gold pins, and some may be encased in thicker glass...
     

  20. AxemanVR

    AxemanVR I appreciate, therefore I am... Strat-Talk Supporter

    Feb 8, 2014
    Minnesota USA
    '
    While I'm sure there are many who would disagree with this, in my experience NOS tubes are mainly better because the overall quality back on in the old days was more consistent, but I've found that the current production tubes are getting pretty good in that sense.

    So if you go with a reputable brand I'm sure you'll be fine.

    Sound wise I think the current tubes are excellent when you find a good one, and NOS tubes don't always sound great either.

    Anyway, I like Mesa Boogie tubes since they supposedly test them to a higher standard, and I have nothing but good things to say about them myself...



    '
     
    Last edited: Aug 9, 2018
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