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Fender amp techs...2 specific questions...

Discussion in 'Amp Central Station' started by LesTele, Nov 10, 2020.

  1. JamesAM

    JamesAM Tele-Meister

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    Your 57 deluxe is self biasing in that the current and voltage draw of the tubes work with the cathode (positive side) of the power supply to “self-regulate” the power they receive. The only way to adjust this is to swap out the cathode resistor of the power supply for one of a different value and re-measure the voltages. It’s kind of like spinning plates- the plate voltage (ha), power supply voltage, negative current draw of the tubes, and resistance of the output transformer all act together to influence the bias current the tubes receive.

    If you are not comfortable working around the hundreds of volts of dc current that can remain in your power supply’s filter capacitors, even after turning the amp off, you may want to just leave it how it came from the factory and enjoy. Your tubes will be safe and your warranty will stay intact.

    adjustable-bias amps have a potentiometer in the power supply to raise or lower the current supply to the power tubes. I’ve never heard of a tech who was willing to swap out cathode resistors while you play, but I’m sure crazier things have happened!

    As for the smell, Id go with some of the others here who are much smarter than me- it’s fine until it’s not. Putting the amp in a dark room and looking for red plating is the best way ahead- if it’s not red plating, it sounds good, and the smell isn’t getting worse, you’re good.
     
    Last edited: Nov 14, 2020
    Jared Purdy likes this.
  2. Jared Purdy

    Jared Purdy Friend of Leo's

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    You're already in over my head. But thanks, I read what you posted, and I do make an effort to understand. I'm a college prof, but there's a very clear reason why I don't teach math or electronics. I'm comfortable with my limitations, and I'm glad that there's people like you and others here who are patient (mostly) enough to explain to us that don't have these skills listed on our CV's.
     
  3. JamesAM

    JamesAM Tele-Meister

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    No worries bud! You mentioned biasing it yourself and just wanted to be sure you knew what that entails with a cathode biased amp like the 57 deluxe. Step 1: change power tubes. Step 2: you’re done. If tubes redplate or voltages are weird, the only way to adjust the bias is by changing component values in the circuit.

    edited to say my explanation of bias was probably wrong anyway and it’s only a matter of time before someone smarter than me corrects me- it’s all good! Best part about music is you don’t need to be an EE to have fun playing it!
     
    Last edited: Nov 14, 2020
  4. Wally

    Wally Telefied Ad Free Member

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    There is a way to bias the amp that does not involve changing the bias resistor. This is true not just for cathode biased amps but fixed biased as well. You buy power tubes that bias either colder or hotter in the circuit than do the tubes that are in the amp that you want to change out for new ones. So, if this Deluxe is running hotter than you want, you buy colder tubes....they draw less current in the circuit than does a hotter tube. This is also how one maintains sonics from one set of tubes to another if the amp is not adjustable bias..fixed or cathode. This is one advantage of buying burned in and matched power tubes. The burn-in assures that the tubes’ operating parameters do not drift in the early use in the circuit, and during matching the tester finds out how the tubes work in a circuit...and put tubes with similar performance together in pairs, quartets, sextets...hey, the Mesa Bass 400 runs 12x6L6s.
    ‘Biasing with tubes’ is actually how Mesa advises using their tubes in their amps. They grade their tubes to their circuits. All others they sell in bulk in the wholesale market, as I understand it.
    Biasing is a very interesting part of the amp, ime. A Class AB amp, which this 5E3 and most other push/pull Cathode-biased amps are, can be biased below that 100% max plate dissipation and sound good. Pure Class A by its nature lives in that 100% max plate dissipation world. All single-ended amps are Class A. The Fender Pro Sonics I have owned operated at 100% MPD in the Class A setting. Hey....the Blues Junior is fixed biased and is known to operate at 100§ mpd...cruelty for the power tubes..when in operation they are above that. In cathode biased, idle is as hot as the tubes run.
    Plus one on sitting in the dark and observing that amp, Jared. Feel the PT. Get to know it. Hopefully nothing is wrong.
     
    Last edited: Nov 15, 2020
    JamesAM likes this.
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