maintenance costs

dbw

Senior member
Messages
4,531
How much should I expect to pay to have my amp biased?  How about re-tubed (not including the tubes)?  It's a 100W Carvin head, 4xEL34.
 
R

RLW

Guest
That is in the top ten of skills every guitarist should learn.
 

dbw

Senior member
Messages
4,531
'kay, thanks for nothing guys.... :p  I know I could bias it myself but there's something wrong with the bias... one of my tubes is redplating... I want a pro to look at it.

Edit: What are the other 9 skills, Rich?  Seems like tuning should be number one :)
 

jimh

Senior member
Messages
1,344
Sounds more like a bad tube to me.  They should be biased at least in pairs (well, at least my Marshall clone is) if not all four.  That is to say, there is a bias splitter network (usually a set of resistors, all in parallel leading to each tube) so that you only adjust one bias pot and that sets up the bias for all the tubes.  So in theory each tube gets the same "amount of bias". 

If only one of your tubes is beginning to fail, then I'd say it's that tube beginning to fail, unless the bias network is breaking down or failing.  I might be wrong, or have an incorrect notion in my head (which is likely), as I haven't done any biasing for a while. CB will be better able to explain this, or at least corect me if I'm wrong.


dbw said:
Edit: What are the other 9 skills, Rich?  Seems like tuning should be number one :)

Surely you need to be able to actually string your guitar before you can tune it.  So there's your number 1 right there ;-)
 

dbw

Senior member
Messages
4,531
But tuning is a more valuable skill than stringing.  You can always have someone string it up for you if you're retarded.  :p

Back on topic, I'm just gonna ignore you guys and take it in.  If he tries to charge me something unreasonable I'll have a friend look instead.  :)
 

Patrick from Davis

Senior member
Messages
2,197
Rebiasing the amp is not difficult, and Carvin makes it rather easy.  What you will pay for is the time spent to do it.  If the guy charges 75 bucks an hour and it takes 90 minutes for him to tear it open, bias it, and put it back together, then you are looking at over 100 bucks.  The process is not hard or expensive, but the time someone spends on it is generally expensive.
Patrick

 

-CB-

Senior member
Messages
5,427
Generally......

On a push pull Class A/B guitar amp, which you have, they use a common bias voltage, isolated from each half of the output with some decent size resistors - in the 220k-470k range.  There's very little grid current, so the bias voltage can go thru the resistors with almost no I/R drop, but its enough to isolate the AC signals from each half.

Most larger amps have a bias control of some sort.  Most do not require getting into the chassis to adjust.  Some older Fenders in the late 60's up till early 80's had a bias "balance" whereby the overall level was set, but could be varied to "match" the two tubes.  Most techs dont like that approach, prefer to use matched tubes, and convert the balance circuit back to overall level by swapping a few wires and adding a small resistor.

If you have one tube going red and the other not, its likely a bad tube or grossly mismatched tube, as both tubes receive the same bias voltage.  If you blow a screen grid resistor, the tube will usually self destruct and you might have done that... reverse the two tubes, and see if the problem follows the tube.

Get or make a bias probe (PM me if you want one, I have a few left over).  Use it with a cheap volt meter and set the level to about 35milliamps for a good ballpark middle of the road.  Done.  No need to go inside.  No need to deal with danger.  Takes like 10minutes.  I'd bias amps FREE just to look over the rest of the amp for paying work... scratchy controls, bad jacks etc. 

 
G

guitlouie

Guest
Speaking of scratchy controls.....no wait, don't want to hijack.  But seriously, I've tried contact cleaner to no avail.  I'll go start another thread, though. like I said, don't want to step on toes here.
 

jackthehack

Senior member
Messages
5,630
+1 on CB's post, most likely it's a tube gone bad.  You can follow his advice to check, but probably simpler just to get a matched duet to replace what's there

Guitlouie - If you soak down a pot with contact cleaner and it's still scratchy, it's toast, just go ahead and replace it
 

dbw

Senior member
Messages
4,531
Okay, thanks guys.  To complicate things a little, this is a 100W head with 4 EL-34s.  I can hit the 50W switch and then only the middle 2 tubes are on.  Do I need a matched quad, or two matched pairs?
 

dbw

Senior member
Messages
4,531
OK that's what I thought.  I will contemplate the amp some more tonight, updates to come...
 

-CB-

Senior member
Messages
5,427
jackthehack said:
Guitlouie - If you soak down a pot with contact cleaner and it's still scratchy, it's toast, just go ahead and replace it

There is another cause for scratchy pots....  that is, leaking (electrically) capacitors in the coupling of the stages.  Normally, the DC of the plate is blocked by a capacitor, that also serves to limit the low end response.  If the pot does not clean up with spray, check before you replace it.  There normally is no DC on a volume or tone pot.  Presence pots are the exception.  Turn the pot fully one way, test both end connections.  Make sure there's no DC.  Then turn it the other way fully and see if there's DC.  If there is, the new pot will only last as a scratch fix for a short time.  I used to replace caps that were leaking more than about .1 or .2 volts.  Much above .5 volts is gonna be a candidate for scratchies.

Or it could just be the pot, esp on a newer amp.
 
Top