1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  

B-15 rebias

Discussion in 'Amps and Cabs [BG]' started by joplin, Sep 19, 2008.


  1. joplin

    joplin

    Aug 31, 2008
    NYC
    Who is recommended for doing repair work on B-15 heads? The tubes in my B-15 have a slight red hue on the plates which makes me think they are drawing too much current.

    I would adjust the bias current myself but I do not know how this is done on a B-15 (mine's 1972 era). I'm assuming this involves changing a resistor.

    The main reason I ask is i'm considering replacing the no-name chinese tubes in my amp with some NOS Sylvania 6L6GC or 7581A/STR387 and believe I will need to rebias the tubes. I emailed Jess Oliver but have not heard a reply back.

    I could take the amp to a local guitar store (the techs there apparently are reputable) but I really don't want to spend hundreds of dollars to change a resistor.
     
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Any amp tech should be able to do it for fairly cheap. It does involve adding a resistor because it's a fixed bias amp, but it's not a $200 job. Just call your local tech and tell him you need to rebias new power tubes in a fixed bias B-15N and he should be able to give you a price over the phone.
     
  3. joplin

    joplin

    Aug 31, 2008
    NYC
    it's humiliating to pay someone to do that - i've been working on my tube-based ham radio gear for years... oh well, live and learn. thanks for the info.
     
  4. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Well if you know how to bias tubes, what's the problem? Is it a different procedure for measuring fixed bias amps?
     
  5. joplin

    joplin

    Aug 31, 2008
    NYC
    I'm skittish about doing that kind of stuff on a B-15. I've got dozens of ham radios and lots of people to call on for help. Doing a B-15 cold without anyone to call for help makes me nervous :) Plus, I think there's a lot to be said for the experience someone would have who has done B-15s and knows that while the math says one thing the reality for a good tone is something else.

    Do you know how to contact Jess by phone? I may just send it up his way if he has time to do it.
     
  6. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    Can't remember his number offhand, but that's way overkill to send it to him. It's going to cost you $40 each way. You could probably get it biased locally for that much and it wouldn't be at risk in UPS trucks.
     
  7. jimbilly

    jimbilly

    Apr 19, 2006
    My local tech made my '70s B15 head 'adjustable fixed bias', the adjuster is on the back of the chassis and locks after adjusting, so I don't have to take anything apart to adjust bias. -I can't remeber if he did that to my '66 B15 too, -lol. I've got an Allesandro home bias meter, -one of those and a tube tester will get you a long way. My tech tells me that it's not too tough to make your own bias meter, and what you're looking for is getting idle currents fairly matched, you've just got to figure out which pin to use your multi meter on. Here's a link to the Allesandro meter, it's pretty cool, it's supposed to work with 7027s, but I have found that it doesn't (most Ampegs don't really need 7027s anyway), and it works great with other common octal power tubes. http://www.alessandro-products.com/accessories.html
    IMO, a tool like that (and there are other good brands) will pay for themselves in no time. You just plug the bias-meter sockets into the tube sockets, and the tubes into the meter sockets, let it standby for about 2 minutes, then flip it off-standby, the meters tell you right away what the idle current is, and how close the tubes are matched. It would tell you right away if your tubes are running too hot.
     
  8. joplin

    joplin

    Aug 31, 2008
    NYC
    I couldn't bear the shame :)

    I spent some quality reading the schematic that is glued to the chassis tray.

    Long story made short this amp (1972) is fixed bias - the 1K 10W WW resistor is fine.

    I noticed that the 100K 2W resistor that ties to the 10D6 diode and pin 4 of the 5AR4 has changed dramatically. It has become very hot in the past and the value is around 40K. The resistor body has discolored and has a few bubbles.

    Tomorrow i'll have to see what the grid voltage is - i'll bet it's nowhere near -50V (point "K" on the electronic version I have). Got to check the 80uF 100V cap as well.
     
  9. joplin

    joplin

    Aug 31, 2008
    NYC
    cool - will have to check that out.

    Have you ever tried putting 7581A in a B-15? I don't think i'm going to pay the extra expense but was curious what the sound difference was, if any.
     
  10. Main problem with swapping out tubes is the possibly different heater currents. That can cause the power transformer to fail. If you are a Ham then you should have no problem in checking the tubes are getting the voltages they need. ANY burned or off value components should be replaced. A pair of good 6L6GCs will bring the amp to life. Check the 6SL7s if you have a tube tester or know someone who does.

    Paul
     
  11. joplin

    joplin

    Aug 31, 2008
    NYC
    hi - yep, went through the whole thing today. spent the last 6 hours checking it out and fixing things. the biggest bonus was replacing the resistor for grid bias. it was way way off. -31V to the grids before, -49 now and a tremendous reduction in heat from the 6L6 tubes. The plates no longer have a dull glow in the corners. I took a bunch of photos, etc. will send a message here later. the wife wants to go have dinner now.
     
  12. fireincairo

    fireincairo

    Jul 8, 2008
    I'm curious, is anyone making any new production 7581A's?
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.