Tube bias?

[Brendo]

Ok, so I've been fooling around with my guitar amps and swapping tubes in and out, etc.

The amps are a Marshall DSL401 and a Crate VC30.

I've improved the dirt sound in the DSL by swapping in a pair of Sovtek's from the Crate, and putting the chinese junk in the Crate. But I'd like to clean up the clean channel a bit and lower the overall gain of the entire amp just a touch, so I was thinking a 12AT7 or something in the first position might work.

I called a guitar store and asked the guy on the phone if a lower gain tube would help, and he said that whenever I'm changing tubes I should get the amp rebiased. Even for preamp tubes.

Last time I had the Crate in somewhere, we were swapping tubes in and out like crazy, trying to find the combination where it sounded the best.

Is rebiasing preamp tubes a necessity, do they even give you the option? Am I going to damage my amps if I keep this up?

Also, which of these tubes is which?
http://www.piller.at/music/dsl201/chassis.jpg
I'm assuming the one under the cover is the first tube, and the 12ax7 near the power tubes is the phase splitter?

[Spock]

You can always try it and see if you like it.

But as far as bias, my thoughts are if it is not the output tubes you are changing, or the driver right before them, your bias will not need to be touched.

That driver tube is given a bunch of different names, but in most cases it doesn't add that much gain to the system, it makes sure you have an in phase and out of phase signal to drive the output tubes.

So try the early stage and see if you like what you hear, much of it will be very dependant on the way the circuit is designed. But I would touch the bias one bit.

[Baddo]

...I called a guitar store and asked the guy on the phone if a lower gain tube would help, and he said that whenever I'm changing tubes I should get the amp rebiased. Even for preamp tubes.

I have never known an amp where you needed to adjust bias when changing the preamp tubes. My guess is the guitar shop guys just wanted to charge you for maintenance.

Power tubes are different animals and bias should be checked/adjusted even when changing brands. (At least I do)

[Brendo]

Well, from a discussion with Spock, I worked out it's probably a good idea to rebias if you change the driver, as well, seeing as the driver and the poweramp are so closely linked...

[Baddo]

Well, from a discussion with Spock, I worked out it's probably a good idea to rebias if you change the driver, as well, seeing as the driver and the poweramp are so closely linked...

This is interesting. Would you please elaborate?

As I understand, preamp tubes are self-biasing. So I guess what you're saying is that by changing the driver you would be affecting the idle current of the power tube stage? I always pictured preamp and power amp as separate sections.:Confused: :Confused:

[Brendo]

As I understand it... the driver tube splits the signal into the regular and polarity inverted versions to feed the push/pull output stage. The tube, then, needs to be spitting out equal signal each way, right? But it probably never does... so bias does affect this too.

[ggunn]

Ok, so I've been fooling around with my guitar amps and swapping tubes in and out, etc.

The amps are a Marshall DSL401 and a Crate VC30.

I've improved the dirt sound in the DSL by swapping in a pair of Sovtek's from the Crate, and putting the chinese junk in the Crate. But I'd like to clean up the clean channel a bit and lower the overall gain of the entire amp just a touch, so I was thinking a 12AT7 or something in the first position might work.

I called a guitar store and asked the guy on the phone if a lower gain tube would help, and he said that whenever I'm changing tubes I should get the amp rebiased. Even for preamp tubes.

Last time I had the Crate in somewhere, we were swapping tubes in and out like crazy, trying to find the combination where it sounded the best.

Is rebiasing preamp tubes a necessity, do they even give you the option? Am I going to damage my amps if I keep this up?

Also, which of these tubes is which?

I'm assuming the one under the cover is the first tube, and the 12ax7 near the power tubes is the phase splitter?

Biasing an amp doesn't do anything to the preamp tubes, and it doesn't do anything to the primary high voltage supply that feeds both the preamp tubes and power tubes. The biasing circuit is a separate supply that only affects the power tubes; it is a negative voltage applied to the grid pin.

Here's some stuff:
http://www.hans-egebo.dk/Tutorial/biasing.htm

[Spock]

In talking with Brendo I said, that the pre-amp tubes , or a chnage in them will not require you to check or adjust the output tube bias.

Now depending on the exact circuit layout the driver tube MAY make a bit of difference, so it wouldn't hurt to give things a check if you chnage out that tube.

[Brendo]

My next question is, in a cathode bias amp, is it important to have matched tubes? Can you have two matched pairs (which are different, like say a matched pair of sovtek, a matched pair of EH)?