Power tube question

Discussion in 'Glowing Bottle Tube Amp Forum' started by golfnut, Oct 28, 2018.

  1. golfnut

    golfnut Friend of Leo's

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    I have a question concerning 6L6 tubes. The following taken from another thread:

    If I go with a 30 watt as opposed to a 19w what difference does this make. Does it give me more headroom? Obviously I realize it doesn't increase the power of an amp.
    I have a 35 watt amp that I want to have maximum clean headroom, not early breakup. So is a 6L6GC going to provide that over a 6L6G or is there more to it.
     
  2. David Barnett

    David Barnett Doctor of Teleocity

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    Unless you've got a stash of pre-WWII tubes you're not going to encounter many 6L6 tubes out there other than the GC.
     
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  3. DCW74

    DCW74 Tele-Meister

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    Yes. You will have more headroom with higher dissipation.
     
  4. JD0x0

    JD0x0 Poster Extraordinaire

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    It means you can dissipate more heat with the higher wattage tube.



    Not necessarily. You need to factor in the tube's operating points. Just shoving in a higher dissipation tube doesn't necessarily mean more headroom. In fact, the opposite could potentially happen. If you have an amp biased up @ ~60% dissipation with a 19W tube and throw in a 30 watt 6L6GC, your total dissipation is at ~40% and the tubes could be going into cutoff territory... If you try to crank the bias up, you may wind up pulling the plate voltage down, and reducing headroom...
    No, because your amp probably doesn't run a 6L6G ( I could be wrong, but they're NOT common)

    If you see an amp advertised as '6L6' in 2018, it is 99.999999999% chance that it's running 6L6GC's and designed to run them, not any of the lower dissipation 6L6 tubes.

    If you had the plate voltage and bias to spare, you could maybe go up to a 35W 7581A tube.. but I suspect your amp's lack of headroom issue is more to blame due to the circuit. If it's only putting out 35Watts with 6L6's it's probably running cathode bias, which is significantly less efficient than fixed bias. A fixed biased amp would easily be pushing 50 watts with 6L6GC's.

    A 30-50 watt amp with the circuit designed for headroom, will be plenty to stay clean. You don't really gain much volume with increased power. For 'headroom' you want all your gain stages in the amp to be able to take a ton of signal before they start clipping. Without that, you could have all the wattage in the world, and still may have distortion. If one of the gain stages clips/distorts easily, having more output power doesn't really help compensate for that, much.



    What amp is this? You've given no info other than it's 35 watts and has 6L6's of some sort.
     
    Last edited: Oct 28, 2018
  5. golfnut

    golfnut Friend of Leo's

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    Its an Allen Accomplice.
     
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  6. JD0x0

    JD0x0 Poster Extraordinaire

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    They have that amp spec'd @ 40 watts, and they run 6L6GC's from the factory (unless you get the 6V6 version)
    If your amp does not have GC's and has one of the lower dissipation 6L6's in there, by mistake, that could negatively affect headroom, as those tubes would be massively overbiased, and would likely even be red plating.

    How are you using the volumes on that? For max headroom, you generally want to crank the master volume, and then use the preamp volume to set the levels. This will allow you to set the preamp volume a bit lower, which should help keep the preamp from distorting.

    That amp should have a decent amount of headroom considering it's based off a Fender Blackface Deluxe reverb, and it's 40 watts (so closer to a Super Reverb)
    The 'raw' control will also likely add a lot of gain and clipping, so keep that in mind as well.

    If all that checks out and the amp still seems overly dirty, I'd have the bias and power tubes checked. I know some techs like to bias up to max rated dissipation (70% class AB, fixed bias) but, the typical Blackface Fender spec is closer to 55-60% dissipation.
     
  7. golfnut

    golfnut Friend of Leo's

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    Its actually spec'd 35 watts. Hasn't got the 40 watt OT. Has the 35 watt. I imagine that 5 watts won't make any noticeable difference though. I'm not really having any issues with clean head room with 95% of the gigs I play. But I had a couple of outdoor gigs this summer where I had the Master maxed (where I always keep it anyway) and had the gain volume at 4-5. Even then had a hard time hearing it. Although that was more due to the placement of other amps, especially a steel guitar player with a twin that seemed more ideally placed for me to hear then him. With the accomplice set at those volumes clean headroom was no where to be found. Although it sounded great for some things For a clean country sound, it wasn't ideal.
    For most of my club gigs that I play, which are 1 to 3 times every weekend, I usually have the Master cranked RAW between 3 and 4 as I like the way it thickens the mids and the gain at between 1 and 2. Plenty clean there.
    I wasn't really worried about clean head room as I think the amp performs as expected. But I'm looking at picking up some new power tubes (mainly so I can pull the current ones to have a backup set)
    So since I'm picking up new ones I'm trying to understand how to buy a set that will give me maximum clean headroom.
    For the outdoor and bigger gigs I am adding a 1X12 cabinet underneath to #1 hopefully give me a little more volume and #2 help get the amp up in a better position to my ear.
     
  8. DaveKS

    DaveKS Friend of Leo's

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    Only the old real 7581a from Philips or GE might give you more headroom than the old school 6L6GC, new TAD WGC seem to have best headroom of new production tubes.
     
  9. Asmith

    Asmith Friend of Leo's

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    I think the extra cab is your best bet to get more volume, you can get near enough the same volume increase as doubling your watts. You could also try some very efficient speakers (but this could drastically change your tone.)
     
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