How to tell if your amp is Class A???

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by willhutch, Jul 16, 2006.


  1. willhutch

    willhutch Member

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    Here's an opportunity from the esteemed amp gurus who hang out here to enlighten some of us normal guys.

    How can you determine if your amp is class A? Using a multimeter, is is possible to determine an amp's class of operation? How do I do it?

    In order to head off certain responses, let me say I'm not caught up in the class A hype. My amp maker's marketing says my amp is class A (Savage Rohr 15 - dual EL84, cathode biased, push-pull). Whether it is true or not, I do not really care. However, it would be fun/interesting/informative if I could identify operating class myself.

    In asking this question, I'm trying to learn about amplifiers. This is not about exploding marketing hype or about being sure I buy a real class A amp!
     
  2. John Phillips

    John Phillips Member

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    If it's cathode-biased you can tell very easily. Meter the voltage across the cathode resistor (it should be around 10-12V or so in an EL84 amp). Measure it with the amp at idle, then keep watching the meter while you increase the volume up to the loudest clean sound the amp will make. If the voltage rises, even a little bit, the amp is not Class A.
     
  3. danieldroukas

    danieldroukas Member

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    If the manufacturer says it's class A then it's class A. Never question the manufacturer, ever.
     
  4. Riscchip

    Riscchip Member

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    I'm curious...if the amp maker says "(Savage Rohr 15 - dual EL84, cathode biased, push-pull)" doesn't the "push pull" part immediately rule out class A?
     
  5. danieldroukas

    danieldroukas Member

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    nope. class A can be SE or Push-pull... doesn't matter as long as the tenets of class A are still met.

    follow aiken's tech info for more: http://www.aikenamps.com/ClassA.htm
     
  6. VacuumVoodoo

    VacuumVoodoo Member

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    Shouldn't we be able to tell the difference between class A and AB by ear too :confused::D
     
  7. Wakarusa

    Wakarusa Member

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    If you keep tryin' to pick fights we're gonna send you to your room without supper.
     
  8. Swarty

    Swarty Member

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    Why would someone ever buy an amp that wasn't class A?
     
  9. Chrome Dinette

    Chrome Dinette Silver Supporting Member

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    Some amps have no class.

    [​IMG]
     
  10. clothwiring

    clothwiring Supporting Member

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    I feel that Class A amps fatigue the ear easier. I've never had a problem with any AB amp, but with my Matchless...it's hard on the ears.
     
  11. donnyjaguar

    donnyjaguar Member

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    I always consider true class A amplifriers to be single-ended. When it comes to musical instruments amplification the even-order distortion products of a single-ended amplifier are really what one should be striving for. If its push-pull of any description these even-order products are going to be less. So in short there is no substitute for single-ended.

    Triode amplifiers sound more clinical to my ears and I think this is really what you're gonna hear on a heavily biased push pull tetrode stage.

    An interesting use, well to me at least, of true class A single ended are the AM/MW car radios of yesteryear. These used a single output transistor (in TO-3 case) and I think the rationale here was to take the limited frequency response of AM broadcast (typ 5kHz) and create some even overtones that made it sound more "musical".

    DJ
     
  12. willhutch

    willhutch Member

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    Here are the results of my experiment:

    1) I clipped the probes of my DMM to either side of the cathode resistor. and turned the power and standby on. The meter stopped at 11.96 volts. The preamp and master volumes were all down to about 9:00 o'clock.

    2) I turned up the master to noon and saw a rise in voltage across the cathode resistor to 12.00.

    3) I turned the master all the way up and the voltmeter climbed to 12.02.

    4) diming the preamp volumes knobs (this is a 4-hole, 2 channel amp with channels jumpered) increased voltage to 12.05.

    The above procedure was done with no guitar signal applied. Playing thru the amp, the voltages jumped around. With everything dimed, I could get 17 volts across the cathode resistor.

    Conclusion: This amp is not running in class A.

    Hey John Phillips or Old Tele Man. Do you agree? Did I do this test right? Do you have any observations or insights to share?

    I'll give Old Man Tele's test a try and post results later.

    Thanks!!
     
  13. Swarty

    Swarty Member

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    The Matchless amps I've been in have not been class A, including Spitfire, Lightening, Jorgenson, Skyliner SC/HC/DC 30 and Chieftain. These amps simply have too high of plate voltage. So far, the only push/pull amp I've come across that is legit class A is an Allesandro English Coonhound... The term for the most part is just a marketting buzzword.
     
  14. clothwiring

    clothwiring Supporting Member

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    I've read Sampson saying "mostly Class A" design. After that...who knows. Not I...I'm not an amp wizard by any streatch of the imagination.
     
  15. donnyjaguar

    donnyjaguar Member

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    Like in a single-ended amplifier? :)

    Like a push-pull amplifier? :)

    Like no guitar amplifier I've seen, and let's face it irrelevant to the topic. :)

    That just may be, but the topology is more relevant than pedantry of electronics theory. :D

    Its like vehicles. If you had a 350BHP turbo-charged, rear engined, aluminum bodied, road machine with 3" exhaust. It could be an expensive European sportscar or it could be a city bus.
     
  16. Wakarusa

    Wakarusa Member

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    MusicMan RD-80 with a grounded grid power amp design straight from the world of Class B transmitters. :)
     
  17. Swarty

    Swarty Member

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    I guess "mostly class A" means because there are more preamp tubes in the amp that are running class A than there are power tubes running claass AB that it is "mostly Class A design".:rotflmao
     
  18. willhutch

    willhutch Member

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    I played into the amp, with the following results. With master volume at 9:00, the voltmeter reading would jump from idle value of 11.96 to 12.05 or so when playing. Up at gigging volume, with MV at 12:00, the reading would jump up to around 13.25 when I hit big chords. This is probably the "loudest clean" I could get. Diming all volume controls, 17+ volts were showing across the resistor.

    Does this still suggest to you this is not actually a Class A amp?
     
  19. donnyjaguar

    donnyjaguar Member

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    Hey Wakarusa, I'd like to see that schematic. I'm not doubting you, just curious is all. Most grounded-grid power amplifiers are class C though. If its indeed class B I will consider myself summarily bitch-slapped. :)

    DJ
     
  20. VacuumVoodoo

    VacuumVoodoo Member

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