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Discussion in 'Amps and Cabs [BG]' started by JimmyM, May 25, 2008.


  1. sloppy_phil

    sloppy_phil

    Aug 21, 2011
    Toronto, ON, Canada
    Not actually named Phil
    This was the first place I figured I could get a good opinion:

    My '67 B-18N is getting a little fussy. There's some scratchiness that's intermittent, and now a high-pitched over tone is starting to ring out after it's on for about 10mins. Regardless of there being an instrument plugged in. Trying to determine what might be the cause of that? Well, both things, really, but the shrill sound especially.

    I can see that one of the power tubes is glowing much more brightly than the other, although the bright one of the pair is glowing at the same level as the rest of the tubes. They are old Westinghouse tubes, and I can't say how long they've been in there before I got the amp (2009), but presumably a while. Many older tubes in there, including Magnavox, Sylvania, and Ampeg. I'm guessing the dull power tube is on its way out? And if so, what sort of tubes should I be looking at? JJs seem to be one of the easiest options to find, but I'm always dubious about them. There's also Tung-Sol and Sovtek options, before I start digging around in the NOS regions of the internet.

    Any suggestions or other thoughts most welcome!

    EDIT: Sorry, I just presumed this would be commonly known on the thread, but this amp has a pair of 7027A power tubes
     
  2. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Does sound like a microphonic tube. Don't know much about 7027a's except modern ones are 6l6gc's with a different pinout. Might be fine, though.
     
    beans-on-toast likes this.
  3. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The first thing to look at is the tubes.

    It’s hard to read into the glow color of the tubes. It’s possible that the power tubes are fine even though one is brighter. A tech would measure the bias oltage at the tube and the cathode current. There can be an issue with the bias voltage, it can be that the power tubes are not well matched, maybe the tube socket had gunk on the contacts that’s preventing it from working properly. The point being, there are many possibilities as to why one power tube might be running hotter than another.

    I like the reissue Tung-Sol 7027A power tubes. The amp will require having the bias checked f the power tubes are changed.

    The high pitched tone is classic for a failing tube. In a case like this, always check out the preamp tubes first.

    Here are some guidelines as to when to change tubes.

    Tubes | TalkBass.com
     
    Last edited: Feb 8, 2019
    sloppy_phil likes this.
  4. sloppy_phil

    sloppy_phil

    Aug 21, 2011
    Toronto, ON, Canada
    Not actually named Phil
    Great, thanks. Glad to hear the Tung-Sols are worth considering; they look a fair snip cheaper than any decent NOS stock. I was kind of hoping to avoid needing to get the amp re-biased, if indeed I need to swap power tubes; the thing is close enough to 100lbs, and I don't have a car... But I knew that that was unlikely to be the case.

    Maybe just bring in the head, and a 4-pin adapter, so a tech could easily run it into a cab on-hand
     
    beans-on-toast likes this.
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    One simple test you can do is swap the positions of the power tubes and see if the brighter glow moves with the tube or remains with the socket.

    Not carrying the cab into the tech is a good idea. You should note if you have an 8 or 16 ohm installed in the cab. See if there’s a label on the speaker.
     
    sloppy_phil likes this.
  6. sloppy_phil

    sloppy_phil

    Aug 21, 2011
    Toronto, ON, Canada
    Not actually named Phil
    Will do that test!

    It's an 8ohm speaker. Cleveland, I believe


    UPDATE: The lack of glow is the tube, not the socket. And after swapping their spots, the dull tube also had a purple/blue glow. Which I also believe is a sign of it failing, no?
     
    Last edited: Feb 8, 2019
    beans-on-toast likes this.
  7. sloppy_phil

    sloppy_phil

    Aug 21, 2011
    Toronto, ON, Canada
    Not actually named Phil
    Here's an idea of what I'm looking at. From R-L, it's the rectifier, power tubes, and then pre-amp.

    Thus far, after the swap, the squealy pitch hasn't returned as of yet. It's been almost 10mins since I turned the amp on

    IMG_4065.JPG
     

    Attached Files:

  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Progress!

    The purple glow is not usually a problem. See “what’s that blue glow” in the wiki tube section. This helps rule out the socket. The dimmer glow could be normal. If they both used to glow the same and now they don’t, would be a problem.

    Chances are it’s the tubes but things should be checked out. It’s possible that the bias is fine for new tubes, but you’ll never know till it is checked.

    Tell the tech it requires an 8 ohm cab. That will be helpful. If you are going to buy tubes and have a tech bias the amp, buying them in advance could help save two trips to the tech.
     
    sloppy_phil likes this.
  9. sloppy_phil

    sloppy_phil

    Aug 21, 2011
    Toronto, ON, Canada
    Not actually named Phil
    Right on. Thanks, Beans; you've made me feel like I actually had a small idea what's going on.

    Interesting update: 40-45mins later, still no hint of that whining noise. Guess let's see what happens over time!
     
    beans-on-toast likes this.
  10. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    That is quite a difference in intensity. The tubes appear to be different makes, that could explain the difference in glow intensity. SOme tubes do not even appear to glow. This depends on the design of the tube and what sort of shielding it has.

    If you see any white powder inside the tube, that's a sign of an air leak. A bright orange glow on the plate, the metal structure running from top to bottom, is a problem. The tube can do that when it is running too hot.

    As for the whinning noise. Sometimes they do that. Removing and reinserting the tube can sometimes clean the socket contacts enough to temporarily resolve an issue.

    All amps need to a general service from time to time to deal with oxides, to lubricate the pots, and check the changing bias as the tubes age.
     
    Last edited: Feb 10, 2019
    JimmyM likes this.
  11. Rebel Rocker

    Rebel Rocker Supporting Member

    Sep 4, 2017
    Hey can I get a number.
    PF-500
    SVT-3Pro
    Heritage SVT 410HLF
    V-4B
    SVT 212AV (2)
     
  12. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    #688 Rebel Rocker
     
  13. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    Amen to that, I had to have a tech install a JBL-D-130 in my B-15N(C) a few weeks ago, and bias a NOS set of Ampeg branded Sylvania 6SL7's and a pair of Telefunken 6L6"s. Separating

    the head and cabinet were my only option. Even then, with the attached dolly, it was no picknick schlepping the amp down and back to Killer Vintage here in St. Louis.
     
    beans-on-toast likes this.
  14. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    So how did it turn out?

    These daze, my back prevents me from being a schlepper of anything. Amp doctors should make house calls like TV repairmen used to. An advantage of going to the shop is that you get to play with all the toys while you wait for your amp to be serviced.
     
    Ric Vice likes this.
  15. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    Yep, Herman Margell in Muncie, Indiana where I grew up, came out to the house and repaired Zenith, Sylvania, and RCA Color Tv's. I got to watch him DeGauss our first RCA 19" Color Set, several times. "Watch out with those magnets kids". All that work, just so we could watch "Bonanza" on Sunday evenings.

    Until that new show "Star Trek" came on to the Saturday night lineup.
     
  16. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Sullivan aired Sunday night at 8PM, Bonanza followed at 9PM.

    It was 55 years ago this past Sunday that the Beatles first appeared on Sullivan. I remember it like it was yesterday.



    The Bonanza episode that aired that night was called The Cheating Game. Angry with Adam Cartwright for constantly criticizing her decisions and refusing to take the next step in their romance, Laura Dayton becomes easy prey for a handsome grifter.

    Somehow, I don’t remember that one.
     
    Bass 45, ahc, PillO and 1 other person like this.
  17. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Everyone forgets the Pernell Roberts years...dang Little Joe :D
     
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  18. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    He never fit in with the other boys. None of them really wanted to date women.
     
    Last edited: Feb 11, 2019 at 3:54 PM
    Ric Vice and JimmyM like this.
  19. Ric Vice

    Ric Vice Supporting Member

    Jul 2, 2005
    Olivette, Missouri
    But both Paw Cartwright and Little Joe
    knew how to cook up a “Bonanza” of sizzling Steaks on the “Ponderosa”. Truly, I can’t remember any of the stories on Bonanza. The Beatles appearances on Ed Sullivan I remember vividly along with The Young Rascals, The Doors, Vanilla Fudge, Credence Clearwater Revival, The Dave Clark Five, and the Turtles. Bonanza was just something I watched wating for the Sullivan show.Sometimes an Ampeg would appear on Dick Cavett.
     
    ahc and beans-on-toast like this.
  20. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    ahc likes this.

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