Comparing tubes - some common mistakes

Discussion in 'Amps and Cabs' started by myles111, Sep 25, 2008.

  1. myles111

    myles111 Member

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    This is a question I received over at Guitar Player and I thought it might be of interest to some here ....

    Hi Myles!

    Newbie alert! I have recently purchased a Rivera K-Tre 55-watt combo purely on the basis of the spectacular clean channel. When I got the amp home, I discovered it actually had a dirty channel too -- one for which I hold no particular love. I've tried a number of things with greater or lesser success:



    - I returned the combo to a near-original tube setup (12AX7WBs across the board), and the clean was great, but the dirt was staccato as opposed to smooth

    -I replaced the tubes with 12AX7EHs across the board, and the whole thing went dark

    -I put back the 12AX7WBs and added a 12AX7EH in position 1. It all got better, but I still find it a bit staccato-sounding

    For what it's worth, my other rig is driven by a JMP-1 in which the dirty preamp is run through a 12AX7LPS. It is by far (to my ears) the smoothest gain/overdrive I've heard so far, and is more or less the quality I'm going for in the drive side of the Rivera. Alas, the LPS does not do particularly well in combos.

    I'm about to order another raft of tubes (probably just in pairs for position 1 and 2) and I'm looking for advice. I'm considering Tung-Sols, JJs, and in an interesting twist, 5751s.

    Anything else I might like to try?

    Joe


    Joe,

    The biggest problem in swapping preamp tubes is one does not know what an off the shelf preamp tube is doing. Trying to compare a 12AX7EH to a 12AX7WB will yield untrue results if the WB is close to design spec and the EH is not close. You need to compare tubes that have their specs close .... transconductance, true gain, plate resistance and current output.

    You said that the EHs made the amp dark and this tells me you had some weak ones as the EH is brighter and more articulate than a WB.

    A recent test of a lot of EH's showed this:

    1.0 1160 0.014 82.9
    1.1 1430 0.016 89.4
    1.1 1510 0.017 88.8
    1.3 1610 0.018 89.4
    0.8 1260 0.014 90.0
    0.8 1120 0.013 86.2
    1.3 1580 0.0165 95.8
    1.3 1560 0.0165 94.5
    1.1 1540 0.017 90.6
    1.1 1540 0.017 90.6
    0.8 1310 0.015 87.3
    1.2 1570 0.0165 95.2
    1.3 1540 0.017 90.6
    0.9 1370 0.015 91.3
    1.1 1470 0.017 86.5
    1.2 1590 0.017 93.5
    0.6 1160 0.014 82.9
    0.8 1280 0.014 91.4
    1.0 1430 0.0155 92.3
    1.4 1670 0.018 92.8
    0.8 1200 0.014 85.7
    0.9 1290 0.014 92.1
    1.1 1480 0.0165 89.7
    1.2 1490 0.016 93.1
    0.8 1340 0.015 89.3
    1.0 1540 0.0165 93.3
    1.0 1370 0.015 91.3
    1.3 1620 0.018 90.0
    0.8 1300 0.0145 89.7
    0.6 1170 0.013 90.0
    1.0 1500 0.017 88.2
    0.9 1370 0.0155 88.4

    Output TC Gain
    1.0 1418 90.09
    84.9% 88.6% 90.1%


    1610 100.6%
    1120 70.0%
    30.6%

    1.4 116.7%
    0.6 50.0%
    66.7%

    95.8 95.8%
    82.9 82.9%
    12.9%

    This might not make a lot of sense but the important factor here is that these vary 66.7% in regard to spec current and the average tube is down 15% from spec and that is like turning a 50 watt amp into a 30 watter depending on position in the circuit.

    Transconductance has a 30% spread and this is not good either.

    You need to be able to compare apples to apples as today's production preamp tubes are a crap shoot and the odds are NOT in your favor.
     
  2. FrankieSixxxgun

    FrankieSixxxgun Member

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    EH tubes are typically very gainy and mechanical sounding, I can see why you didn't like them. If the stock tubes were Sovtek's you'll be disappointed as they sound kinda flat and lifeless. JJ's are a good choice although a little dark, and a lot of people like the Tung Sol RI's for classic sounding tubes. FWIW nothing on the market currently will make up for a high quality NOS or at least used tube. For the price you can get JAN Philips 12AX7s I would strongly recommend you try those. They're very full, albeit a little boomy when cranked, and deliver a fine tone. For me, nothing can replace the B-code Mullards I have loaded into my preamp, but you'll end up paying for those.
     
  3. myles111

    myles111 Member

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    At the moment, the EH tubes are only a hair more gainy than average production offerings. They are still lower than spec in true gain.

    The post I inserted was my reply to another person's point so I was trying to help him with some data. I have no like or dislike for any tube ... they are all tools ... and using the right tool for the job in a given amp for a given requirement is the key.
     
  4. doublee

    doublee Member

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    Great information, thank you.
     
  5. myles111

    myles111 Member

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    You are quite welcome.
     
  6. nomadh

    nomadh Member

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    Good info. Thanks. If you had 1 ax7 to throw into an amp in an emergency what would be the best generic bet? I'm thinking maybe the tung RI. I used to think the JJ but I think it isn't the most likely replacement after reading up and my own personal experience. Maybe for toughness and general plug in and go possibly the std sovtek. Just as it seems many OEMs have decided.
     
  7. myles111

    myles111 Member

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    Can't really offer help here as production tubes vary +/- 50% or more off the shelf and that range falls below spec not around spec. Anybody I work with has tubes where we know what they are doing so it is not a crapshoot.
     
  8. Scott Auld

    Scott Auld Staff Member

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  9. go7

    go7 Member

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    IMHO Rivera`s are very tube sensitive. My Quiana Studio loves Tesla Rosnov E83CC pre amps and a pair of GE 6l6`s. Different amp. Good luck!
     
  10. myles111

    myles111 Member

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    The 6L6GE is a great tube in the amp but one time I tossed a set of KT66s in a Quiana at Paul's shop and he said it made the amp sound even more vintage and loved it.

    Bias at 32mA if you toss in KT66s .... they will require a rebias but to me it is worth the effort.
     

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