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Peavey Century Problem

Discussion in 'Amps and Cabs [BG]' started by groovaholic, Aug 15, 2012.

  1. groovaholic

    groovaholic The louder the better.

    Sep 19, 2004
    Grand Rapids, MI
    I have an OLD Peavey Century (the Bass 120 Series) that is a ridiculously great sounding amp.

    Not great for the price, just great.

    The problem is that there is something going on in the power section that makes an odd buzzy noise on low notes; it almost sounds like a loose dust cap on a speaker.

    I suspected the amp's distortion circuit, but using the Booster Out indicated that the issue is coming farther downstream: with the Booster Out into another amp, then into the same speaker, the sound is pristine.

    Are there any common components that could make this sound when they start to fail?

    Other than this buzzy tone, the amp plays LOUDLY and doesn't cut out, blow fuses, or shut itself down...although I do get the occasional hiss/pop like a worn preamp tube.

    I can't afford to sink a ton of money into repairing an amp that I bought for $80, but I am good with fixing stuff, if I can streamline the troubleshooting a bit...

  2. craig.p


    Sep 28, 2008
    New Hampshire
    First thing to determine is whether it's vibration-related. Put it on a folded blanket or a pillow, not on top of the cab, and try 'er again. If the problem goes away, there's probably something loose/corroded in the power amp section.
  3. Has this amp been recapped? One symptom of dying power supply caps is occassional weird sounding notes, an A for example. Why? The 120 Hz ripple on the amp's power supply rails is no longer being attenuated, and this can induce funny resonances on notes that are harmonics of that 120 Hz (or close to it).

    Of course, there can be other causes, but a 40 year old amp that hasn't been recapped is long overdue.
  4. If it has a bias adjustment on the power amp after that many years the bias must have drifted. The "buzzy sound" on low notes is a symptom of improper bias, the hiss/pop is not.
  5. groovaholic

    groovaholic The louder the better.

    Sep 19, 2004
    Grand Rapids, MI
    I will look into it some more tonight.

    Physical vibration was PART of the problem; there is a cable running from the preamp board to the power amp board and it's connections were a little dodgy. I clipped off the multi-pin connector and soldered the connections, and that improved things.

    As far as caps and bias go...the caps are definitely original (and HUGE) and I'll have to get a schematic to see what components set the bias in the power amp.

    I didn't realize that solid-state amps were that susceptible to bias drifting...

  6. groovaholic

    groovaholic The louder the better.

    Sep 19, 2004
    Grand Rapids, MI
    Further examination revealed that one of the RCA 62792 output transistors had been replaced with an MJ2955..which appears to be a suitable sub, but AFAIK -- these need to be replaced in sets.

    I noticed because it was missing a screw, when I put a screw in there & tightened it down, I got a loud hum.

    I pulled the "odd man out" transistor altogether and the amp sounds the same as it ever did -- makes me think it wasn't working when it was in there.

    I've ordered new transistors and caps from Mouser -- hopefully those changes will clean up the output stage.

    Seriously, the preamp out-signal from this amp sounds fantastic: it can go from sounding clean and clear to sounding like an all-tube head on the verge of breakup. If I get the power section working properly, this will be a GREAT little amp

    Date codes on the ORIGINAL transistors indicate that this amp was made in 1975...

    Seems like bass amps have only recently gotten back to having distortion circuits; Peavey was FUNKY back in the day!

  7. When you reinstall the transistors, make SURE you have installed the mica insulators behind the transistor body. Clean the old heat sink grease off and apply new heat sink grease--sparingly, you only need a tiny bit, more is not desirable!

    After reinstalling the transistors, I use an ohmeter to double check and make sure the transistor bodies are not grounding out to the chassis.

    Have you cleaned the pots with Deoxit? Could be the source of the pops and ticks. Also, jacks may not make good contact internally (some of them are internaly closed until a plug is inserted).

    And don't forget to recap those old electrolytic caps...

    p.s. I've got the Century's cousin, called "the Bass" (Series 400 version B) with distortion AND fuzz! Two separate effects.
  8. bumperbass

    bumperbass Supporting Member

    Jun 19, 2012
    What transistors did you order? The Peavey number is 70484140. I hope you ordered Motorola transistors. MJ15052 would be a good substitute. I'd have called Peavey and ordered them. I restored a Centurion and a Mark IV (400 BH module). Nothing but a Motorola would do the trick. Be VERY WARY of any Mexican transistors.
    My $ .02

    The transistor cross-reference issued from Peavey says that the 62792 is actually a 2N3055.
  9. groovaholic

    groovaholic The louder the better.

    Sep 19, 2004
    Grand Rapids, MI
    I ordered MJ15003's on the recommendation of Enzo from the Peavey forum.

    He seems exceptionally well-versed on any Peavey amp I've ever heard of.
  10. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    Man, my Century Bass 200 is one of my all time favorite bass amps. It's a monster for 100 watts and that distortion circuit has got to be made of awesome. The clean/dirty blend on that amp is just about perfect to get it to react like a tube amp overdriving. Best $60 investment I've made in my playing yet.
  11. groovaholic

    groovaholic The louder the better.

    Sep 19, 2004
    Grand Rapids, MI
    Exactly! Everyone who has heard it has been amazed that there aren't tubes in it.

    Seems like a really simple, straightforward circuit--I'm tempted to try to build it into a 1U rack enclosure and make it my go-to bass preamp..
  12. bumperbass

    bumperbass Supporting Member

    Jun 19, 2012
    Good for you. Didn't want you to have to repair it twice.
  13. groovaholic

    groovaholic The louder the better.

    Sep 19, 2004
    Grand Rapids, MI
    Thank you! I also do not want to fix it twice. Or, do even MORE damage then have to replace the transformer...

    As is, I'm spending over 50% of what I originally paid to get new parts into it.

    The main (3700 uf, 80 vDC) filter cap and the 4 transistors were the bulk of that.

    Still, I figure if I end up spending a total of $130 for the amp + parts, that's still a VERY cheap path to an amp that sounds the way I like and should be good for another 37 years.

    I'll be almost 80 by then and probably not want to play so loudly all the time...
  14. Enzo won't steer you wrong. :)
  15. PawleeP


    Oct 8, 2012
    are there any usa made ones being made anywhere? not sure which country is making the good transistors and how do I know which ones from the places bill had recommended to me are ok... I do wanna steer clear of el cheapo transistors
    Last edited: Jun 28, 2015
  16. PawleeP


    Oct 8, 2012
    thanx everyone for the much needed help on transistor issue..

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