Valve Replacement & Biasing - Guitar & Bass Amplifiers
Normal Valve Wear & Tear
When valves age they tend to give a softer sound sounding like the speaker is wrapped in carpet slightly distorted and muffled. We generally say if you play every day for a few hours then 2 years is pretty good. If you are a bedroom player never thrashing it you could quite happily get 4-5 years without issue.
There are always exceptions, we have had amps that the valves last less than a year, and amps built in the 60’s still on their original set of valves.
The key way to know when it’s time is when you notice the sound changing, or performance lacking / volume issues.
Replacing the valves is straightforward, relatively… the smaller tubes often marked 12AX7 / 12AT7 or ECC81 or EEC83 can simply be swapped out without any worry or need to rebias. It’s important to notice where the missing pin is as this is the alignment to ensure the right pin is doing the right function.
The output valves often the larger ones that generally come in sets of 2,4,6 should be replaced in associated quantities as they work together to push and pull like a tug of war. In most amps they need to be biased. Amps like Mesa / Boogie however are generally fixed bias and you buy MESA tubes which are tested to comply within the manufacturer’s tolerance and as such should never need biasing. This makes it easier for a competent musician to change them without too much difficulty.
Valve amplifiers can deliver a lethal voltage when opened up. We do not recommend any work is carried out unless you are competent and have worked on electronics. Online videos and forums are great sources of information however some of the deemed boring “safety elements” may be omitted which are crucial.
Is Biasing All the Same?
The simple answer is no.
There several ways to bias valve equipment, you may have come across Cathodic “self biasing” which is a way the valve can control it’s own bias. It is often misunderstood that you don’t have to bias them, this is wrong. The valve can only self bias over a range limited by the resistor inserted. This range does sometimes need varying to accommodate different brand valves, so never assume because of an amp being cathodic bias that you don’t need to check it!
Class A amplifiers, i.e. ones with generally only one power tube (there are exceptions but commonly) need a different bias setting than if it was in a standard class A/B.
The most popular type of bias is a fixed or adjustable negative voltage which is mixed with the input signal to the grid of the power tube. voltages as high as – 60V are not uncommon to bias up something like an EL34.
The bottom line is there is no “standard” way to bias an amp if you cannot identify how it operates and would therefore recommend that you don’t get involved in this.
You can read more on the technical elements of biasing and if it’s only valve amps that need biasing. Click to read more on this article about biasing