How important is re-biasing?

[TelePaul]

Ordering new tubes tomorrow and I'm wondering how essential re-biasing actually is, given that I don't know how to go about it.

[famous beagle]

It depends on the amp. What amp is it?

Just about all preamp tubes (the ones nearest the input) don't need biasing.

Some amps have output tubes that are self-biasing as well (which is also called "cathode-biased"). But if your amp has "fixed bias" output tubes, they need to be biased.

Proper biasing is very important and will totally change the sound of your amp.


Also, you may not necessarily need all new tubes. Some tubes, like those used for tremolo (mistakenly labeled as "vibrato" in old Fender amps), only need to be functional, period, as they don't pass any audio signal.

In fact, you usually don't need to replace the preamp tubes with every tube change, because they don't wear out nearly as quickly as output tubes, except for the phase inverter tube, which is the one next to the output tubes. That one wears out almost as quickly as the output tubes and should probably be changed along with them.

[tom18222]

essentially what ive heard is if it doesnt have enough voltage, the amp will sound week. if it has too much, the amp will sound incredible but it will burn out the tubes faster.

[dementedchord]

ok so if you've seen my comments in other threads ya know ithink it needs to be done... and fixed bias doesn't mean non-adjustable... ya just have to swap out some resistors till ya get it right...

IMO... the only time you dont have to rebias is a cathode biased amp... or if ya are using the same brand and rated tube each time you retube...

[Henry Mars]

Originally Posted by famous beagle

It depends on the amp. What amp is it?
"Also, you may not necessarily need all new tubes. Some tubes, like those used for tremolo (mistakenly labeled as "vibrato" in old Fender amps)"
Just about all preamp tubes (the ones nearest the input) don't need biasing.

Some amps have output tubes that are self-biasing as well (which is also called "cathode-biased"). But if your amp has "fixed bias" output tubes, they need to be biased.

Proper biasing is very important and will totally change the sound of your amp.


Also, you may not necessarily need all new tubes. Some tubes, like those used for tremolo (mistakenly labeled as "vibrato" in old Fender amps), only need to be functional, period, as they don't pass any audio signal.

In fact, you usually don't need to replace the preamp tubes with every tube change, because they don't wear out nearly as quickly as output tubes, except for the phase inverter tube, which is the one next to the output tubes. That one wears out almost as quickly as the output tubes and should probably be changed along with them.

Except that it is normally part of a phase shift oscillator. If the gain of this tube is to low it won't oscillate. Fact is that this oscillator normally produces a signal in the low audio frequency range. This signal is used to vary the bias on one of the gain sections of the amp (this may vary from one amp design to another). The amplitude of this signal determines intensity and frequency determines the "speed" of the tremlo. So if your tremlo seems a little weak this tube may be the problem. Usually preamp tubes last a lot longer than power tubes. On the other hand it is a mistake to think that they never need to be replaced. Tubes that drive reverb units should be replace more often in my opinion.

[dementedchord]

Originally Posted by tom18222

essentially what ive heard is if it doesnt have enough voltage, the amp will sound week. if it has too much, the amp will sound incredible but it will burn out the tubes faster.

Wrong !!!

bias is a Negative volyage applied to the input of the tube to turn it off...

so if it has too much bias that's when it sounds weak... cause the amp is at best slower to turn on...

and when it's too low then it tends to burn up... as it's not turning all the way off...

think of it as setting the idle on your car...

[TelePaul]

Hmmmmm. It's a TSL 601. Might leave it to someone who knows.

[dementedchord]

seems to me someone posted a link to biasing info for that amp not too long ago... look at the back and/or the top of the metal chasis... is there 2-3 little L shaped connectors sticking out??? those are the test points...