A quick tube question

Discussion in 'Glowing Bottle Tube Amp Forum' started by spellcaster, Jun 13, 2021.

  1. spellcaster

    spellcaster Tele-Holic

    Posts:
    732
    Joined:
    Sep 8, 2009
    Location:
    Duncan, B.C.
    I have a 1970 Twin Reverb that developed an issue quite a few years ago. A tech diagnosed it as a failing power tube, so I removed the inner pair of 6L6's and have been using as a two power tube amp since then. I've decided to restore it to a four power tube amp, and plan to order a replacement for the bad tube. However, I neglected to mark which is the bad tube. Can I simply replace one of the good pair that are in the amp with the tubes I took out one at a time to find out which one is defective? I've heard that bad power tubes can take out the transformer, and don't want to put the amp at risk. I don't have access to a tube tester, so if there's a danger in trial-testing to find the bad tube, I'll order a matched pair.
     
  2. Torren61

    Torren61 Poster Extraordinaire Gold Supporter

    Posts:
    7,666
    Joined:
    Mar 12, 2013
    Location:
    Humboldt County, CA
    I wouldn’t run that amp that way. If you had one tube fail, I’d buy a new quad.
     
    Twelvefrets and radiocaster like this.
  3. spellcaster

    spellcaster Tele-Holic

    Posts:
    732
    Joined:
    Sep 8, 2009
    Location:
    Duncan, B.C.
    Since the amp is my living room practice amp, finding ways to tame it has been important. I've been using two power tubes instead of four, plus a 12AY7 in V2. The amp's only run at very low volumes, and I have a Weber Z-Matcher showing the amp an 8 ohm load when it's only running with two power tubes.

    I'm a low income pensioner, so spending a couple of hundred bucks for a new matched quad of power tubes isn't in the cards.

    Can anybody answer my original question?
     
  4. Mike_LA

    Mike_LA Tele-Afflicted

    Posts:
    1,571
    Joined:
    Apr 27, 2010
    Location:
    Just past 11 on the Dial
    How about that tech you used, sure he would test a tube or 2 for you, no?
    Sounds like a good errand for a pensioner with more time than cash . . . lol
     
  5. ScottJPatrick

    ScottJPatrick Friend of Leo's

    Posts:
    2,524
    Joined:
    May 12, 2011
    Location:
    Stirling, Scotland.
    Why bother if it's just for living room use?
     
    SnidelyWhiplash and Wally like this.
  6. spellcaster

    spellcaster Tele-Holic

    Posts:
    732
    Joined:
    Sep 8, 2009
    Location:
    Duncan, B.C.
    The tech I was using at the time has since retired and has sold all his equipment. I could probably get a technician in Nanaimo or Victoria to test the tubes for me, but it's a 40 mile drive north or south.

    Incidentally, not a lot of spare time. I'm working as a volunteer at the job I retired from five years ago to replace an employee hurt in a bad car accident. I can't take a salary because it would negatively impact my pension, so money's still an issue.
     
  7. spellcaster

    spellcaster Tele-Holic

    Posts:
    732
    Joined:
    Sep 8, 2009
    Location:
    Duncan, B.C.
    If I was living in the same circumstances I had over the last 17 years, I'd just live with it the way is is, but I'm now in an over-55 apartment building and can only practice with headphones. The Twin's not of much use to me here, so I've thought about selling it and buying something more practical. I just don't want to have to try selling the amp in less than optimal condition, so it makes sense to me to buy a power tube and bring it back to it's four power tube configuration.

    I'm using T.A.D. 6L6WGC-STR short bottles and they came with a sticker on each tube base that has the tube specifications. I presume that would give me the means to acquire a single tube that would be a reasonably good match for what's in the amp. I just need to know if it's safe to test the two tubes that are out of the amp one at a time to know which one's the throw-away and which one's the keeper.

    [​IMG]
     
  8. VintageSG

    VintageSG Friend of Leo's Ad Free Member

    Posts:
    4,634
    Joined:
    Mar 31, 2016
    Location:
    Yorkshire
    What was the original fault?.
    You can replace one of the pair at a time to see/hear which is the bad one. Do it in a low light room, or rather, turn the main room light off before powering the amp up so you can see easily if the one you replaced is lighting up the heaters, arcing internally or redplating. You want the heaters to glow. Arcs and redplates are not good.

    The original valve may well be fine. What diagnostic work was done to the circuit to ensure the valve hadn't exhibited failure signs because of a dud component or dry joint. How long ago did it have a cap service?.
     
  9. spellcaster

    spellcaster Tele-Holic

    Posts:
    732
    Joined:
    Sep 8, 2009
    Location:
    Duncan, B.C.
    The amp was recapped when I acquired it in 2006. When the tech called to let me know it appeared to be a bad tube, I went to the service shop......I first pulled the inside pair of tubes for a brief time to verify that the problem disappeared once that pair of tubes was pulled out. Then I briefly installed a pair of new Groove Tubes 6L6 Whites to verify that the amp worked well with them. Because the chassis's in a head cabinet, full-size tubes weren't an option long-term because the heat of tall tubes would have melted the reverb tank bag, so I pulled the tall tubes back out and have used the amp as a two power tube amplifier for years.

    I believe you've told me what I need to know anyway, and I thank you. The audible problem that originally sent me to the service shop to have it diagnosed was intermittent low output. That symptom disappeared after the inside pair of tubes was removed, so there's no doubt in my mind that one of those TAD tubes was at fault. If I can briefly install them one at a time to determine which one's the bad one, that solves my issue.
     
  10. Milspec

    Milspec Poster Extraordinaire

    Posts:
    5,900
    Joined:
    Feb 15, 2016
    Location:
    Nebraska
    When you say you had a bad tube, what was the issue? If it didn't function due to loss of gas or something, it will not glow or maybe it glows bright red...that will tell you which is bad right away. Or was it just a noisey tube in which case the old chopstick tap test will give you that answer as well.

    You already know that you have 3 good tubes so inspect them visually and see if you see the bad one. If not, pull the good outboard tubes you are running and replace them with the other pair. Then check condition of glow and/or tap test if it is a noise issue.

    I love my Twin and still run it in quad fashion, but really, I don't think you lose anything running in duo mode unless you are cranking the volume. I barely get above 2 on the volume knob...3.5 if you use the master volume.
     
    JRapp likes this.
  11. Torren61

    Torren61 Poster Extraordinaire Gold Supporter

    Posts:
    7,666
    Joined:
    Mar 12, 2013
    Location:
    Humboldt County, CA
    Ask them to buy you a quad of tubes.
     
    Rowdyman likes this.
  12. MuddyWolf

    MuddyWolf Tele-Meister

    Age:
    58
    Posts:
    488
    Joined:
    May 11, 2017
    Location:
    Paradise, Ca
    You know when you half the tubes you're supposed to double the speaker impedance. 8 ohms needs to be 16 ohms. Those amps are built for abuse but you should be aware.
     
  13. Milspec

    Milspec Poster Extraordinaire

    Posts:
    5,900
    Joined:
    Feb 15, 2016
    Location:
    Nebraska
    Disconnecting one speaker is the safest approach, but the amp should be able to handle the mismatch.
     
    Torren61 likes this.
  14. spellcaster

    spellcaster Tele-Holic

    Posts:
    732
    Joined:
    Sep 8, 2009
    Location:
    Duncan, B.C.

    The Twin is designed for a four ohm speaker load when its running four output tubes. Since I've been running it with two output tubes, it needs to see an 8 ohm load. I'm using a Weber Z-Matcher between the amplifier output and the single 15" Altec Lansing speaker, set to 8 ohms, so I'm loading the amp correctly. In addition, the amplifier has never run above a setting between 1 and 2 on the Vibrato channel volume control, so I'm being careful not to over-tax it.

    I appreciate the advice I've been given.
     
  15. Axis29

    Axis29 Poster Extraordinaire Silver Supporter

    Posts:
    6,926
    Joined:
    Jan 2, 2007
    Location:
    Beaumont, CA
    How about just buying a matched pair, instead of a quad?



    I like this idea:

    A pair of tubes can be had relatively cheaply and your old work could just call it a retirement gift? LOL
     
    Rowdyman and Torren61 like this.
  16. Ivo Conceicao

    Ivo Conceicao NEW MEMBER!

    Age:
    38
    Posts:
    1
    Joined:
    Jun 14, 2021
    Location:
    Portugal
    Hi guys!

    I have a matched pair of NOS Tubes CV25 Haltron, and I can't find any information about them. Are they also STC4211? Are they made in the USA? Are they made of graphite?
    Any clues about their value?

    IMG_20210526_102902.jpg IMG_20210526_102930 (1).jpg IMG_20210526_103227.jpg IMG_20210526_102950.jpg

    Thanks for your time!
    Kindest regards
     

    Attached Files:

  17. ArcticWhite

    ArcticWhite Tele-Afflicted

    Posts:
    1,352
    Joined:
    Apr 14, 2009
    Location:
    Portland Oregon
    clayfeat and Wally like this.
  18. spellcaster

    spellcaster Tele-Holic

    Posts:
    732
    Joined:
    Sep 8, 2009
    Location:
    Duncan, B.C.
    Still a fair amount of money in Canadian funds....99.71 plus 35.00 shipping. And, I'm hesitant to ask. I'll just end up buying a single tube myself.
     
  19. Torren61

    Torren61 Poster Extraordinaire Gold Supporter

    Posts:
    7,666
    Joined:
    Mar 12, 2013
    Location:
    Humboldt County, CA
    I have a pair of Ruby 6L6 tubes I'll send you for free. Message me your address.
     
    Blue Bill likes this.
  20. archetype

    archetype Fiend of Leo's

    Posts:
    6,234
    Joined:
    Jun 4, 2005
    Location:
    Western NY
    Welcome to TDPRI! Since you have a new question that doesn't relate to this thread, you need to start a new thread in Glowing Bottle Tube Amp Forum on this site.
     
    hepular likes this.
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.