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HELP! New Power tubes for an Ampeg B-15N

Discussion in 'Amps and Cabs [BG]' started by stingray56funk, Nov 29, 2005.


  1. stingray56funk

    stingray56funk Supporting Member

    Feb 16, 2005
    Atlanta
    Quick question guys! I just bought a 1968 Ampeg B-15N and I think the power tubes are bad. They are the 6L6 GC's and I want to know if I can just go and buy a pair of matched 6L6 GC's and put them in (the B-15n has a fixed bias...but has it wavered?) or do I need need need to take it to a tech and have them install the new power tubes and check the bias? It has a set of Electro Harmonix 6L6 EH power tubes...so I know that they were changed within the last decade...but I really just want to buy a new set of power amp tubes and just put them in myself without having to pay for a tech and his bench fee. Let me know what you guys think!!! I appreciate it!
     
  2. What is your amp doing that makes you think its tubes are bad? BTW, a B15N is cathode biased not fixed.
     
  3. Also BTW, I see you're from Birmingham. My B15N and Wizard Bass 500 came from Highland Music.
     
  4. jvb

    jvb

    Jul 11, 2005
    Canberra, Australia
    It's a 37 yr old amp. If it hasn't been maintained regularly it's more than likely that the power supply caps will be on their last legs, and quite possible that valves/tubes are on the way out, although these tend to get changed more often.

    I think it would be worth taking to a tech to get checked out, with a view to getting capacitors replaced, and checking the idle currents because resistors do change in value, particularly old ones. Valves may need to be replaced as well. But at the end you should have a great sounding amp which will go for another 30 yrs (but not without the odd service...).
     
  5. stingray56funk

    stingray56funk Supporting Member

    Feb 16, 2005
    Atlanta
    Thanks guys for the info! I got the idea the B-15n had a fixed bias from www.fliptops.net. So what does it mean for it to have a cathode bias? Also...I did not think about the caps going bad...oops. How much do you think it will cost for that? For Psycho Bass...I did buy mine at Highlands Music! I had seen this B-15n there for close to a year and had played it twice and I finally said...what the hell...I need it! That is my story! Any more help would be greatly appreciated!
     
  6. Tapp

    Tapp

    Aug 29, 2001
    USA, Mississippi
    Does it say B15NF on the top? Mine is a '65 and is fixed biased. A lot of people just leave off the F designation when talking about the B15N. I didn't think the later 60's B15's were anything but fixed biased and the early 60's models were cathode biased.

    Tapp
     
  7. stingray56funk

    stingray56funk Supporting Member

    Feb 16, 2005
    Atlanta
    All it says on the face plate and on the back of the amp is B-15N. So which is it...the cathode or fixed? I love the thing...but I want it to sound the best it can and last the longest it can as well. Thanks guys!
     
  8. It means the output tubes pretty much bias themselves and the amp will have significantly more spongy feel and put out less power than the same amp in fixed bias.

    Fliptops has the entire cap kits, IIRC. A good tech can do the job in an hour or two barring any unforseen problems.

    Good question. According to this schematic, identified as a 68 B15N, it's cathode biased, but this schematic shows it as fixed bias. A good tech can tell you which yours is with one look at the output tube socket wiring. FWIW, I have a late 70's B15N and it's cathode biased! :eyebrow: