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Old July 17th, 2008, 02:24 PM   #1 (permalink)
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Do preamp tubes need to have a correct bias?

This is probably an easy question to answer. With the DRRI are the power tubes the only ones that you need to bias or do you have to bias the preamp ones too? I have a bad hiss coming from my amp and i think it is one of the preamp tubes. can i just swap those out?

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Old July 17th, 2008, 02:39 PM   #2 (permalink)
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Every tube needs to have the correct bias to work best, but preamp tubes are nearly always cathode-biased, which means they take care of themselves for the most part.

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Old July 17th, 2008, 02:40 PM   #3 (permalink)
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Yes, you can swap out preamp tubes, and you don't have to bias them.
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Old July 17th, 2008, 07:30 PM   #4 (permalink)
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Which leads to an observation/question.....
This bias situation for preamp tubes has a lot to do with the difference in the way different tubes act/sound in a circuit, right? Different tubes of the same type...say two 12AX7's....can have different operational/performance specs and therefore the bias point will be shifted and the sonics change?????
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Old July 17th, 2008, 07:45 PM   #5 (permalink)
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As Scott S says, tubes need to bias or idle at the ideal place for the tone you're looking for, and for their spec range.

That said, some folks like hot or cold biasing, others like the opposite.

Preamp tubes are cathode biased (often called "self bias"), which means a fixed resistor sets it's bias. They're "plug and play" (no rebiasing required). Messing with the bias resistor does yield different results... as does changing the cathode bypass capacitor (which determines the frequency range passed by the preamp tube).

Typical Fenders use spec resistors, per tube design specs. Usually 1500 ohms. Often Fender went with the widest frequency response possible, so they went with larger bypass caps (25uF, 50uF, or even larger). Marshall used an unusual approach for "Bright" channels by using 2700 ohm resistors and small caps (0.68uF), and this still carries over when they went high-gain (to keep farty or boomy bass at bay).

In any case, preamp tubes in a DRRI are biased within spec, and with great tone.
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Old July 17th, 2008, 10:00 PM   #6 (permalink)
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Quote:
Originally Posted by Wally View Post
Which leads to an observation/question.....
This bias situation for preamp tubes has a lot to do with the difference in the way different tubes act/sound in a circuit, right? Different tubes of the same type...say two 12AX7's....can have different operational/performance specs and therefore the bias point will be shifted and the sonics change?????
yes! I have a fishing tackle box full of 12AX7-types: ATAX7, 12AT7, 5751,
7025, 12AU7, ECC83, etc - these are NOS, used old tubes, new production,
y'all get the idea.. anyway: I've noticed that there's a pretty huge difference
not only from brand to brand but even tube to tube.

preamp tubes tend to be strong or weak, noisy or quiet, microphonic or
not, and/or they can color the sound of the signal and/or boost or cut
whole chunks of the frequencies as well as affecting the gain as well.

I don't think any of this (on the preamp end of things) has an effect or
is affected by the bias for the power tubes.

I could be completely wrong, I'm no poindexter about this stuff.
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Old July 18th, 2008, 01:01 PM   #7 (permalink)
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Johnny Crsash wrote: "Typical Fenders use spec resistors, per tube design specs. Usually 1500 ohms. Often Fender went with the widest frequency response possible, so they went with larger bypass caps (25uF, 50uF, or even larger). Marshall used an unusual approach for "Bright" channels by using 2700 ohm resistors and small caps (0.68uF), and this still carries over when they went high-gain (to keep farty or boomy bass at bay)."

It is true that when Marshall separated the cathodes...1970....that they went with a .68 cap with a 2700 ohm resistor on the cathode of the Bright Ch. Prior to that, the set-up was exactly that of the 5F6A BAssman....the cathodes of the preamp tube shared a 820 ohm resistor with a 250mfd cap.
The tone caps for a 5F6A are .022 for the bass and mid and Marshall followed this topography. These tone caps have a great deal to do with the sound of all of these amps....decreasing the bass compared to the .1 bass and .047 caps in 'typical'...read post tweed...Fenders. Changing these tone caps in a BF Fender relieves much of the low end boominess/fartiness and yields more 'cut' for some players.
FWIW, on the Marshallls that have the separated cathodes on that first preamp tube, bright guitars, such as Jimi Hendrix' Strat, benefit from daisy chaining the channels together to yield bigger, meatier sonics. Hendrix did this on his rigs. A Les Paul through the Bright Ch might not need the extra lowend that is gained when daisy chaining.
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