Dismiss Notice

Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

Pretty glowing blue tubes

Discussion in 'Amps and Cabs [BG]' started by oo0o00o0oo, Nov 14, 2000.


  1. oo0o00o0oo

    oo0o00o0oo

    Apr 30, 2000
    Chicago
    I got a really old (1950's) tube amp the other day. It sounds ok, exept it needs a major bias job. One of three 6L6's glows with a blue sheen. The other 2 6L6's and 3 12AX7's cherry plate when turned on. I really would prefer biasing them myself, can anyone tell me how?
     
  2. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    If the 12AX7's plates are glowing red, you have something more than a bias adjustment wrong. I also can't figure what type of amp would have 5 6L6 tubes. I wouldn't use it until someone looks at it. When tube plates are glowing, the power transformer is being stressed to the max. Psyco could probably steer you in the right direction.

    Pkr2
     
  3. oo0o00o0oo

    oo0o00o0oo

    Apr 30, 2000
    Chicago
    Oh sorry, I should have been more specific. There are 3 6L6's total. Any advice PBG?
     
  4. B3Nut

    B3Nut

    Oct 28, 2000
    Central Michigan
    You better look again....6L6's (and all other power output tubes) work in pairs. A low power amp may have a single 6L6 or 6V6, but after that you'll have them in multiples of 2. If you have 3 6L6's, the fourth is missing. More than likely you're mistaking the rectifier tube for a 6L6. Have a careful look in there again and tell us what the tube lineup is, and if there's a tube chart see if what's on the chart and what's actually there match.

    First off, what is the make and model of your amplifier?

    If your 12AX7's are going cherry red (never seen a preamp tube do this) then take the amplifier to a qualified repair tech. Tube amplifiers are definitely not a project for newbies, indeed many repair techs don't know what they're doing with them either.

    The 6L6's going cherry red in the plates is usually a sure sign that the tube needs to be replaced. Beam-power tubes go into thermal runaway when the grid becomes deformed and starts to move in the direction of the screen grid, which is positively charged. This causes the control grid to acquire more of a positive charge, causing the tube to conduct more current than it can safely handle. The result is that the plate begins to glow red. However, do not rule out the bias as an improperly-biased amplifier can send the power tubes into runaway as well.

    As for biasing, leave that to a technician - and don't operate the amplifier until it has been repaired, or you'll do some serious damage to the power transformer and associated components, causing a major pain in the wallet.

    HTH

    TP
     
  5. b30307

    b30307

    Nov 18, 2000
    ....and lastly, even low powered older amps can produce big voltage . Do not "poke around " in any amp
    unless you're sure of what you're doing . You can be KILLED !
     
  6. B3Nut

    B3Nut

    Oct 28, 2000
    Central Michigan
    Also, don't assume it's safe if the amp is unplugged. Tube amps operate at high internal DC voltages, up to 600 volts or so in the Ampeg SVT and 350-450 in most mid to high power tube amps, 250-300 in lower-powered units. The filter capacitors in the power supply retain this charge even when the power is off unless the power supply has bleeder resistors across the filter caps. A jolt from a charged filter cap *will* light ya up.

    Don't poke around in a tube amp unless you are familiar with proper technique for working on them.

    And when measuring voltages with a meter, clip a ground lead to the chassis and connect it to the meter's negative terminal, plug the test-probe into the meter's positive terminal, and take measurements with one hand in your pocket. Don't hold the chassis with your free hand - if you slip with your probe hand and touch a B+ rail, the current will go up one arm, through your heart, and down the other arm. You won't like the usual outcome of this. Keep your free hand in your pocket when measuring voltage.

    TP
     
  7. I think its safe to "pour the current " by playing the amp for a couple of minutes and then just pull the plug while playing, this will drain the caps and transformers and you will be fine. Am I wrong?
     
  8. pkr2

    pkr2

    Apr 28, 2000
    coastal N.C.
    Most techs wouldn't consider it safe untill the filter caps are bled off through a resistor. A power supply cap can develop a memory voltage even though its been bled off and left unplugged. The memory voltage is high voltage but low current. The biggest danger with getting shocked by it is in jerking your hand into danger from pure reaction. I sliced my hand badly when I jerked my arm back and broke a tube off once.

    My advice to any one is simply stay out of it unless you have enough knowledge and experience to stay safe.

    Pkr2

    P.S., probably just a slip of the tongue, but transformers don't hold a charge.
     
  9. B3Nut

    B3Nut

    Oct 28, 2000
    Central Michigan
    >>I think its safe to "pour the current " by playing the amp for a couple of minutes and then just pull the plug while playing, this will drain the caps and transformers and you will be fine. Am I wrong?<<

    More or less, yes. Even though you hear the sound die out, one of the reasons for this is that you've killed the supply to the tube heaters, preventing them from conducting. Tubes conduct via electron emission - a heater heats a cathode coated with thorium, an element that gives off free electrons when heated. These electrons are attracted to the positively-charged plate. If you kill the main power, the heaters go out causing a rapid drop in cathode temperature, thus fewer electrons are being emitted as the cathode cools. Current flow through the tube is thus reduced, and it is unable to drain the charge stored in the filter capacitors.

    Some designs incorporate "bleeder" resistors across the filters, but never assume an unfamiliar circuit has these in it and don't take the presence of bleeder resistors for granted in a circuit known to have them - they could fail.

    If you're going to poke around in a tube amp, you'll need to discharge the capacitors. DISCLAIMER: The author assumes no liability for injuries occurring due to attempts to follow any advice given. This advice assumes basic knowledge of electronic theory and technique. Anyone attempting to service their own equipment does so at their own risk.

    To discharge the caps, you'll need a ground-clip to clip to the chassis, a length of wire, a 1K resistor, and a test probe. Connect one leg of the resistor to the ground clip. Connect the other leg of the resistor to a length of wire. Connect the test probe to this legth of wire. Make sure all connections are insulated. Holding the probe by the insulated handle (and keeping your free hand in your pocket!!!!) touch the tip to each capacitor terminal under the chassis and hold for at least 10 seconds. Make voltage measurements with your voltmeter at each capacitor terminal before assuming everything is safe.

    Be safe, we don't want to read of any TalkBassers in the obits.

    TP
     
  10. Good stuff pkr2 & B3nut. I have a Fender 135 (mid 80's I think), and its Groove tubes 6L6s glow a real nice blue. The tubes are 5's &7's written on the tube bass, but they all appear to glow even. The transformer warms up pretty warm, but the sound of this amp is KILLER! . It sounds like a new Aguilar. There is another one in the paper, but I can't play it first. Are all these amps thaat good sounding, or do I have a special one? I don't need another amp, just obsessed. Pkr2, thanks for appreciating the "MAD AS HELL"
     
  11. B3Nut

    B3Nut

    Oct 28, 2000
    Central Michigan
    A bluish glow in a power tube is normal. As far as "Groove Tubes" go, that's more hype than fact. Bias the amplifier correctly for whatever tube you get and don't worry about "hardness" or whatever. When retubing an amp that uses 6L6's, find a good set of Sovtek 5881's or other good 6L6 variant, have the bias properly set, and you're good to go. It is claimed that if you buy a set of GT's with the same "hardness" rating, there is no need to rebias the amp. But I'd check it anyway.

    hOw tO rEpLacE tOobZ......;->

    1) Buy good tubes (Sovtek and Svetlana have good 6L6 variants), stay away from Chinese power tubes if you want to keep your amplifier intact. Definitely avoid Chinese 6550's if your amp uses that tube type. I almost burned up my Leslie with these. Svetlana 6550C's are very good.

    2) If the bias is adjustable, make sure it is correctly set. Owners of self-biased amplifiers ignore this step. As always, refer biasing to a qualifried techy-nician if you're unsure of how to bias an amplifier. :)

    3) Salt and pepper to taste. :)

    TP
     
  12. Thanks, I heard that about GT before. The amp I have has an Output Tubes Matching screw and a Hum Balance screw in the back. Probably why the tubes are even. I don't really want to retube the amp until necessary. I heard the 6L6GC's and the 6L6WXT+ are good. I think Ruby Tubes has a Tesla premium super 6L6GC too. I also have an Ampeg V4 head that 6L6GC's can replace the 7027A's directly. But I'm not ready to retube that amp yet either. I just had a full cap job done , and the tubes are OK. Te Fender sounds better, the Ampeg is more powerful, but is less punchier. It's more tubby & mushy sounding.