Preamp Tubes? - Who, What, When, Where, Why, and How?

Discussion in 'The Workbench' started by joshuaaewallen, Jan 29, 2009.

  1. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    There is a probable cause. The 7027 tubes use 0.9 amps each at the filament (heater) where the KT88 uses 1.6 amps each. I do not have specifications on the PT but the filament windings are being taxed heavily maybe causing hum.

    There is (should be) a hum adjust control in the filament (heater) circuit. Make sure it is adjusted for least hum.
     
    Last edited: Apr 13, 2012
  2. Drake

    Drake Member

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    Thanks a lot. I'll check in to it ASAP.
    Cool thread!
     
  3. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Welcome. :thumb:
     
  4. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    Are your tubes happy to see you when you get home?

    [​IMG]

    You can thank my kids for this one.

    [​IMG]
     
  5. MartyStrat54

    MartyStrat54 Well-Known Member

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    If I had a nose like that, I'd have a lot of girlfriends.
     
  6. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    A guy gave me a box of pretty much useless military tubes and that was one of them. I got a couple of 6U5 eye tubes that are pretty cool. There's a 2050 and a 6Y6G that are semi-useful. I have no idea what to do with the rest. I sent Matt a 6080 that has to be the biggest twin triode tube I've ever seen. It's supposed to make a decent audio amplifier. I figured he could use it more than I could.
     
  7. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    Just got back from checking out a batch of tubes, two caddies worth to be exact. I found one RCA 12AX7 with the short plates and flat-sided micas. I also picked up three more 6BQ5's (they're everywhere!). I picked up a couple of 6LQ6's for my friend with the HAM radio addiction. I got a 300B just for the hell of it. It's a massive tube.

    On top of those, I also bought a Hickok 800 tester that appears to be working fine and he threw in a big box of loose tubes. There may be a few gems in it. I have to dust them off good before I'll know.
     
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  8. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    Matt seems to think this one might be worth a buck or two, so I thought I'd post it. Here it is. Now I'm going to go dig through that box of loose tubes.

    [​IMG]
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Dude if that tube is good you're looking at over 1G for sure.
    That tube tester is mighty nice too.
     
    Last edited: Apr 19, 2012
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  10. MM54

    MM54 Well-Known Member

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  11. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    Wow. This is kinda like the feeling when the slot machine starts ticking off $$$ and you wonder when it's gonna stop! :fingersx:

    Mickey, a quick question about the Hickok. On the roll chart, it shows the MU of a 12AX7 at 1250. Is that minimum good or new? I didn't get any paperwork with it. I think I've gotten everything figured out except that. I've got a 12AX7 that tested at 80% per side on the B&K 707 and it reads about 1100 on the Hickok. How do I find out what's minimum good?
     
  12. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The roll chart on a Hickok 800(A) gives the average or nominal Gm for amplifier/power tube value which is their "as new" relative to tube manuals.

    info for 800A (has nuvistor and compactron), similar to 800:
    http://bama.edebris.com/manuals/hickok/800a/
    You can always get more info on the 800 itself. You may want to find some sort of calibration information as well.
     
  13. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    It seems to be dead-on with my 12AX7's. I just tested a Tungsram that was tested at 90-92 on a friend's calibrated B&K 747. I read 1100 and 1150 on the Hickok. I mainly want to know what is nominal good. I could probably find out by checking tube listings on eBay and see what others are saying. I got the BAMA stuff as soon as I got home with it ;)

    I noticed in the 800A manual that there is a way to switch the tester to the 100% scale (green = good). I can't figure out how to do that with mine.
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The 800 models use the dot system.
    Tube 12AX7 is loaded into socket
    LEAKAGE selector set to TUBE TEST
    Selectors and bias are set according to chart
    LINE ADJUST should be set at meter LINE TEST while pressing P7
    When testing a 12AX7 for merit (bad-good) the SHUNT should be set to 58 (I believe) and press P4
    When testing a 12AX7 for Gm the shunt should be set at 73 (dot) for 3000 scale and press P4
    Same procedure for other tubes.

    Not positvely sure but I believe that on a 800 model a 12AX7 with 1250 Gm would read about 2000 on merit.
    So a 12AX7 that reads 1100 Gm should read about 1760 merit. Minimum merit will read to the left of the question mark which should be about 800 Gm for JAN minimum. (If calibrated correctly.)
     
    Last edited: Apr 20, 2012
  15. markslespaul

    markslespaul New Member

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    Telefunken tubes and placement

    I have a ? about preamp tubes.I have aquired some nice Telefunken preamp tubes.First a 12ax7 with ribbed plates and a 12ax7 with smooth plates also a 12at7.Where should I put the ribbed and smooth plate versions in a Marshall jcm800 2210 made in 1987? also would the 12at7 be good in the P.I. socket.Any help would be greatly appreciated.Also is v1 the tube for the boost channel as when I pull the v2 tube the boost channel still works and when I switch to the regular channel there is no sound till I plug in a tube.Any suggestions?:fingersx::fingersx::fingersx:
     
  16. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    Congrats on the Tele's - they are definitely one of my favorites. I think the ribbed plates are a little more aggressive and the smooth plates are a little cleaner or more hi-fi. Both sound fine in a Marshall.

    I'm not familiar with your amp. You definitely won't hurt anything by trying the 12AX7's in any of the sockets. Keep the 12AT7 for when you have a Fender amp that needs a reverb driver. A 12AT7 PI tube will make your amp sound a little more like a cranked amp at lower volume, but IMO you sacrifice a lot of tone and dynamics.
     
  17. Inkalen

    Inkalen New Member

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    Hallooooo - anybody home ????
     
  18. RiverRatt

    RiverRatt Well-Known Member VIP Member

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    Yeah, I'm still lurking around. Some of the other guys need to get on here and post again. I've tried to get things rolling time and again again and it just ain't happening. I've been thinking about building another amp or starting some other project. The forums have gotten boring.
     
  19. Inkalen

    Inkalen New Member

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    Well since I was here last time there are about 150 new pages, so it is not always this quiet!
    Unfortunately, I have to force myself to take breaks since I have a tendency to get a bit carried away at all the great offers at ebay - that site can be dangerously addictive if one is not careful
     
  20. Inkalen

    Inkalen New Member

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