Simple power tube shoot out

Discussion in 'Amp Central Station' started by alexwilds, Aug 24, 2019.

  1. alexwilds

    alexwilds Tele-Meister

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    I recently acquired a matched pair of 'holy grail' old RCA blackplate 6L6GCs. With breathless anticipation, I swapped out the 7581-A tubes, adjusted the bias, and cranked her up.

    An hour later the 7581-A's went back in. Clearer, quieter, louder, tighter, stronger bass, way more headroom, way more chime, way prettier all the way around. I have tried a lot of tubes. For my tastes and my money, I will be going with 7581-A's from now on.
     
  2. aerhed

    aerhed Friend of Leo's

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    I've listened to them and liked them. I don't think I'm liking old RCA 6L6 %50 of the time.
     
  3. Wally

    Wally Telefied Ad Free Member

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    Are you using new production Tung Sol 7581A’s? I just need to make sure what tube you are talking about.
     
  4. 11 Gauge

    11 Gauge Doctor of Teleocity

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    I had a pair of them (Philips) in my old Super Reverb for a few years, and I loved them, until one of them redplated and absolutely failed.

    I had intended to replace them with other 7581-As, until I saw that the cost of them has crept up a bit (over $100 a tube, and IDK how easy it is to get matched pairs now).
     
  5. alexwilds

    alexwilds Tele-Meister

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    Yes, the new Russian Tung Sol 7581A's. I would like to try NOS Phillips, but I am way too cheap!
     
  6. Obsessed

    Obsessed Telefied Ad Free Member

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    Once I slipped in an old RCA 6L6GC, I was done. This is all about personal taste for the most part though.
     
  7. deus56

    deus56 TDPRI Member

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    Never tried the Phillips, went with GE.
    They are very nice.
    [​IMG]
     
    Jerry J and tubedood like this.
  8. Tonetele

    Tonetele Poster Extraordinaire

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    I had an old Fender Twin with 6L6s and found it too clean. Had the set replaced and it sounded worse.
    Anyway, it's your amp, your choice as to what you prefer.
     
  9. muchxs

    muchxs Doctor of Teleocity

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    Don't make me come down here!


    Simple "tube shootouts" are like comparing apples to Elvis memorabilia.


    While some tubes were created nearly equal, real 1950s Tung-Sol 5881s for example... modern tubes are wildly inconsistent. Also, while true NOS tubes will last nearly forever in their boxes, "NOS" is the most mis- used term in the vintage electronics industry. Your chances of getting genuine NOS or even "closely matched tubes" are approaching zero. How tubes perform relates to how hard they've been used in their previous lives. While the goal was to make all tubes of the same type equal they were only nearly equal when they were new or true NOS in the original boxes. The enormous variable is how worn they are compared to true NOS.

    If you're incapable of testing tubes on a calibrated tube tester you shouldn't be doing "tube shootouts". What's the point of more subjective results rather than objective observations?

    Further, your calibrated tube tester should be something like a TV-7 or a 539C. Then you can compare your transconductance readings to other user's results.

    To be objective we need to compare our tubes in similar amps if not the same amp. A '60s Super Reverb would do. We need to measure plate voltage and set bias correctly and uniformly, every time.

    Simply swapping tubes without re- biasing introduces variables.


    Thus, if we test our tubes on a calibrated tube tester every time, set bias every time and objectively compare results with basic instrumentation we might generate some useful information.

    Don't assume the amp's supply voltage stays consistent from tube set to tube set. It may not. Worn tubes may drag the supply voltage down without parity in performance.


    For a valid comparison without extensive instrumentation, the lowly 5F1 Champ or a reasonable facsimile is our friend. The 5F1's cathode bias allows tube swapping without re- biasing. Mind you, what we're hearing may be related to bias. Still, we can roll tubes with relatively impunity. The Champ's simple circuit tends to bring out tonal differences in tubes. The downside? Your subjective observations only relate to how individual tubes perform in that particular amp.
     
    aerhed and Wally like this.
  10. Jwalker99

    Jwalker99 TDPRI Member

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    Besides tubes the biggest effect on my vintage amps has been thr Amp RX Brown Box



    vintage amps were not designed to run at current wall voltages (120v-125v). The BrownBox lets you easily step down the voltage to levels that you"re old Fender and Marshall"s are supposed to see.
    The feel of the amp is where you really see the biggest difference but I also think it does positively affect tone as well.



    Sent from my iPhone using Tapatalk
     
  11. Wally

    Wally Telefied Ad Free Member

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    One thing to take into consideration is that when that voltage is changed, the entire amp has been rebiased. It is very interesting to maintain the plate dissipation numbers at each voltage so that one comes to understand what the difference in voltage really is....imho. Why? That is because one could simply revise at one certain voltage and change what the amp does in another way.
    The demo here is of interest, I agree; but ime it is an incomplete view of what needs to go on.
     
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