- #31
- Publié par
lol91 le 24 Sep 2004, 17:30
J'ai trouvé ça .. assez intéressant :
MATCH YOUR OUTPUT SECTION!
Why do two Marshalls or two Fenders, of the same model, and even year, sound different?
For one thing, there was a +/- 20% or more variance in components used, but the more common reason is below.
I originally wrote this with a subject of dead spots in your sound or tone that people felt were due to dead spots in their guitar (or bass) neck.
In the last few months while blueprinting amplifiers, I have had to explain over and over about a mis-matched output section and its impact to many.
I thought I'd write a little here ....
In any class A/B amplifier, because of the NFB (negitive feedback loop ... usually labeled as the PRESENCE control), any disparity between the upper part of the sine wave (produced by half of the output tubes), and the lower part of the sine wave (produced by the other half of the output tubes), is cancelled out by the NFB circuit by design. This is the reason some notes "sing" when your amp is pushed in the output section (rather than pushing the input in a master volume amp), and other notes do not have the same magic.
The reason some of the great blues players have that tone, is that their amps are taken care of people that know how to adjust or deal with some of the issues that cause this lack of luster.
Since no tubes are even close to identical, this cancellation is always going on. The object is to limit this as much as possible.
The most common way people match an output section, is to use good quality matched tubes. The industrial spec for a match can be as high as +/- 20%. A good match by a lot of tube vendors is +/- 10%. I believe that even the untrained ear can hear the difference when a output section matched within +/- 5% is used. In the amps I set up for the folks that retain me, my spec is less than 2.5%.
The most overlooked and misunderstood part of the output section is the 12AX7/ECC83 (Marshall style) or 12AT7 (Fender style in vintage cases) Phase Inverter tube. This is the tube that drives the output tubes. A lot of folks that specialize in making amps sound great don't understand this, but fix this accidentally. They tend to use very good tubes, such as JAN spec 5751's etc., where the match is closer, and closer matched tubes in the output section. They also use tubes that sound good in the first gain stage positions, rather than the common Sovtek WA tubes which most manufacturers use (because they are sturdy, not as expensive, and ship well without developing microphonics).
When I scope an amp in the lower frequency region, the vast majority of the time, the upper and lower parts of the sinewave are not even close to equal. This is more disparent than just a slightly mismatched set of output tubes. At this point, I install a matched phase inverter / driver.
The problem with phase inverters, is finding a matched tube. You have to remember that a 12AX7 / 12AT7 etc., is NOT a single function tube as an output tube. It is TWO tubes (two triodes), sharing a single bottle.
VERY FEW TUBE COMPANIES MATCH THE A AND B SIDES OF PREAMP TUBES. They warrant the tubes to work, and warrant them not to be microphonic, but do not say they are matched. This is not any bad commentary on tube suppliers .... to do this matching is time consuming and requires specialized equipment. If you can find somebody that has a Tektronix tube curve tracer, and bring them a bunch of tubes, maybe you will be lucky and find a match. There are a few tube vendors on my feeble little website that do offer matched preamp tubes, mostly those dealing in high end audio applications.
Matched phase inverters and output tubes are one of the reasons some amps "sing" and others are pedestrian compared to their brothers and sisters.
If you seem to have a lot of dead spots, try a new phase inverter tube. This is usually the preamp tube that is the closest to your output tubes. It is a trial and error process, but you may get lucky.
