Replacing power tubes - rebias?

[Roybot]

What tools do I need to rebias an amp? If it's just a multimeter then I want to learn how to do my own amps. The Marshall that started this whole thing was rebiased at the last tube change and one of them shorted out after about 30-40 hours of playing time. Putting it in the shop is a 5 hour ordeal (2 round trips to electronic repair shop next to Chuck Levin's) plus downtime. Theres gotta be a smart, safe, and sensible way to do this on my own...how do I learn?

 

[Thatupstateguy]

Here's some help on that front....


http://www.diyguitaramp.com/bias.html


and there are other sources as well.....



like... http://www.hans-egebo.dk/Tutorial/biasing.htm


and..... http://www.diyguitarist.com/GuitarAmps/PT-Biasing.htm


hope you find what you need at one of these.....

Steve

 

[Lt. Bob]

in answer to the original post yes you can use the same rating and expect it to work reasonably well... that being said IDEALLY ya want to rebias each time you retube an output... since someone already used an auto analogy let's stay with that... bias is like setting the idle...if it's too cold the amp doesnt respond quickly... if it's too hot they tend to burn up quickly... what you are doing is introducing a negative voltage at the tubes input to litterally shut it down... with out it the amp thermals... incedently dont think that you can just test one tube and get away with it ya really gotta do them both...
if you've been lucky so far good for you...

Well yes .... anything can happen. For that matter you might get killed in a car wreck on the way to the tech.
And I do agree that the ideal is to do it although I don't bother to do so myself especially since almost all my amps have fixed bias. Old Ampegs are usually fixed and so are Mesas and I don't hear of them blowing up because of not being able to set the bias. I'm a tech myself although not really practicing because I mainly went the piano tuning route.
But the fact remains that in the context of his original question he can indeed drop a replacement tube in that amp and the chances of it causing a serious problem are practically non-existent.

 

[dementedchord]

$ .02

fixed bias should be done as well it's just harder to do as it involves changing resistors instead of adjusting a pot... and not to be confused with selfbiasing amps... and yeah i've done plenty of ampegs including svt's ... a simple mod can make it easy to check... turn off and let the filters discharge... if you check your tubes you'll find one pin is at grnd thats the current source for the tube and all the current flows through it... if you take a precision 1 ohm resistor (i use 2wt for overkill) and wire it between the pin and grnd then you can read the bias very accurately across the resistor...1mv=1ma...