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tube power amp - Boom!

Discussion in 'Amps and Cabs [BG]' started by Fealach, Dec 21, 2005.


  1. Fealach

    Fealach Guest

    Apr 23, 2003
    Gone to a better place
    So I've come into the possession of these 2 Marshall 9005 EL34 stereo (2X50 watt) poweramps. The good one I'll probably trade in for some odd hunk of old junk at a music store eventually. But the other one...

    2 sides, A and B. I have tried swapping tubes, and even having the bias set as cold as possible to see if the problem went away. It didn't.

    Problem: side B works fine, sounds great. Side A sounds great for 5 seconds after it is switched out of standby, then the power tubes outshine the sun and a thermonuclear reaction begins. Angry red, dark at first but gets bright quickly, from the center of the 2 power tubes on side A. I know, take it to a tech. But techs want money, they weren't happy the last time I tried to pay with a handful of mud and some twigs. I'm thinking it's fixable, but expensive. The previous owner had a preamp blow up - smoke and everything - and hasn't used it since, I got it free as junk.

    What I am wondering - is it safe to use just side B? Pull the tubes and fuse from A, have no speaker plugged in, leave it on standby forever?

    Any thoughts?

    Thanks!
     
  2. tadawson

    tadawson

    Aug 24, 2005
    Lewisville, TX
    I would guess that it would be pretty cheap to fix - it sounds like nothing more than a leaky cap, probably on one of the sreens, causing the bias to go nuts. The trick is knowing how to find and replace it without killing yourself, and I can't tell you if you are capable . . . . but those caps are typically $2 or so . . .
    Bottom line is, that no matter what the "boutique" shops may claim, there is really nothing that expensive in a tube amp, with the possible exception of the transformers . . . . . everything else is pretty much off the shelf, generic parts.

    - Tim
     
  3. Fealach

    Fealach Guest

    Apr 23, 2003
    Gone to a better place
    I was worried it might be a transformer, my uneducated sense being that would make it an expensive repair. I'll probably hang onto it until I can get it looked at, I know a guy does great work for cheap but an expensive part is an expensive repair.

    Kinda hoping I can pull the tubes and fuse for that side and use it as a mono 50, that would be more useful to me than a stereo rig anyhow.
     
  4. Groundloop

    Groundloop

    Jun 21, 2005
    Toronto
    I'd take a different tack here. Why not keep and use the good one, and keep the light show amp for spare parts?
     
  5. Fealach

    Fealach Guest

    Apr 23, 2003
    Gone to a better place
    $.

    I need.

    I figure I can get some $, or at least some credit which I will eventually use if only on strings and such, for the good one.
     
  6. Groundloop

    Groundloop

    Jun 21, 2005
    Toronto
    Point taken. I know where you're coming from.

    Let me preface this by saying that I am by no means a tube amp expert.

    But, after looking at the manual it seems that Marshall built the amp as a true dual mono design, right down to output and impedence switching for each side, so I'm pretty sure that just using one side as you've described would be safe.
     
  7. It is not a transformer problem, I agree with tadawson that this should be an easy and relatively cheap fix. For now, you could pull the tubes and fuses from the bad side and run mono on the other side.

    But a competent tech should fix this easily. Especially when you have a working channel as a reference. (Pull the tubes from both sides, measure voltages at all pins, see if something's way out, that by itself may narrow the problem down.) Caps are cheap. With a basic knowledge of electronics you could maybe even fix it yourself...but be forewarned, there's potentially lethal voltage involved.
     
  8. Fealach

    Fealach Guest

    Apr 23, 2003
    Gone to a better place
    Sounds good. I'll hang onto it, and may use it as a mono 50 amp as described, with my guitar preamp until I can get a better practice guitar rig going. I'll add it to the list of sick tube heads I got cheap that need attention by the tech, but it'll be at the end of the list. Gotta get my Sound City fixed first, and he's still restoring my 60's tube console stereo.

    Thanks for the help everybody!