- Amps [BG]
|BBKINGBASS ||08-21-2013 09:36 PM |
Ampeg V4 loss of bass output
Recently I just fixed up my early 70s V4 head. I modded the c3, c6, and c7 to make it match a V4B. Everything has been sounding great until I had a prolonged practice 3 hours last Friday.
During that practice its seems like I lost the lower fundamentals of my bass tone. I also noticed a super high freq. hum as well. Where I used to have a nice pillow like low end with my 810, now it seems to be severely reduced. Cranking the bass doesn't really make a difference. My low mids are still there but my "shake the walls" low end is gone.
I noticed that (looking from the back) my right two tubes are glowing with more blue than they did before. I had the amp serviced about 3 months ago and it all checked out well. When I bought the amp it still has JJ 7027A installed but since I go it used I have no idea the age of them. The tech said at the time they looked fine.
My overall volume didn't really decrease but my low end has really disappeared. Could my output tubes be starting to go?
|JimmyM ||08-21-2013 09:41 PM |
Could be. Could also need new caps, but bad tubes would likely be a suspect as well, and not necessarily just the power tubes.
|BBKINGBASS ||08-21-2013 09:44 PM |
Thanks jimmy, caps were replaced within the past 6 months on it. I did tube roll the preamp tubes, 12ax7 and 12au7 in the preamp. I didn't touch the 6k11.
|JimmyM ||08-21-2013 09:53 PM |
Could be one of the others, yes. Tried gently tapping them with a chopstick or butt end of a wooden spoon while it's on? Be very careful not to touch other stuff, and don't use anything but a wooden stick of some sort to do it, no metal or plastic. Usually bad tubes will exhibit bad behavior when you do this.
|solodb ||08-21-2013 09:54 PM |
Try moving the hum balance abit one way or another to see if affects the hum
|B-string ||08-21-2013 10:36 PM |
Are the four output tubes glowing the same amount of "blue" haze? If not try changing the 12AU7 Driver/PI first.
|hibachiduck ||08-21-2013 10:50 PM |
My guitarist one day said "My amp isn't sounding right" followed by "One of my power tubes is glowing blue!" followed by "damn, I blew my power tube." I now fear blue tubes after seeing the charred remains of the tube he pulled out of his nice Marshall amp.
|B-string ||08-21-2013 10:56 PM |
Don't fear blue, that is NORMAL. Fear the outside metal part of the tube glowing any color especially red.
|WretchedExcess ||08-21-2013 11:38 PM |
yes, blue = no worries.
super high pitched frequency hum? how high?
a parasitic oscillation will thin out your power like that, but they're usually ultrasonic.
you need to rule this out since you went mucking around with caps. the quickest way to figure out if your amp is oscillating would be to look at it on a scope.
|SactoBass ||08-21-2013 11:59 PM |
V-4B owner here.
I would take the power tubes out and have your tech test them. If one or more of those tubes are bad, bingo, problem solved. If all of those tubes test good, then it's time to have your tech re-evaluate your amp and fix it.
Good luck to you.
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