Is dsl20hr a “class A” design?

Discussion in 'The Workbench' started by tomykoz, Feb 22, 2020.

  1. tomykoz

    tomykoz Active Member

    Joined:
    Mar 13, 2010
    Messages:
    157
    Likes Received:
    58
    I know that a EL 34 is capable of 25 watts...So I was wondering since the DSL20HR Has two of these tubes in it that possibly maybe it was a Class A design?
     
  2. FracStrat

    FracStrat Well-Known Member

    Joined:
    Jan 22, 2020
    Messages:
    257
    Likes Received:
    648
    Location:
    Water Side of WA State
    Nope ……It’s PP
    Low plate voltages, maybe larger screen grid resistors, cathode bias.
    Probably Lower Voltage OT
    I think @RickyLee has looked at the layout as well
     
    Micky likes this.
  3. ampmadscientist

    ampmadscientist Well-Known Member

    Joined:
    Jun 26, 2014
    Messages:
    16,538
    Likes Received:
    10,452
    Location:
    Bio-Isolation Lock Down
    The amp appears to be Class A, switchable from Pentode 20 watt to triode 10 watt.
    Cathode biased "is" class A.
    In Class A the output tube runs 100% all the time -unlike larger powered amps which run class AB 70% at idle.
    Because of the increased heat class A usually has lower plate voltage (that's why the wattage is lower - lower voltage because it's continuous 100%).

    The EL34 may be capable of Class AB 25 watts dissipation, but not when in class A operation.
    In class A, the wattage is de-rated/reduced because of the increased heat.
    I think in class A, the max rating is about 11 watts per tube.
    Also when the tube is switched into triode operation, the power is again de-rated.

    The voltage of the power supply limits the output wattage to 20 watts.
    In triode the tube becomes less efficient, that's how it reaches 10 watts.
     
    Last edited: Feb 22, 2020
    tce63 and marshalltsl like this.
  4. tomykoz

    tomykoz Active Member

    Joined:
    Mar 13, 2010
    Messages:
    157
    Likes Received:
    58
    Thanks!...that explains why I like the tone...i think class a sounds better to my ears than an push pull.....I think Marshall artist heads were class a too and I liked tbose
     
    tce63 likes this.
  5. RickyLee

    RickyLee Well-Known Member VIP Member

    Joined:
    May 22, 2010
    Messages:
    9,817
    Likes Received:
    3,257
    Location:
    Riverside CA U.S.A.
    I do not have a DSL 20HR schematic. But Ampmad is the man and knows this stuff, and explained it well for us.

    The Artist series amps (if you are referring to the 800 3203/4203) are not Class A. The use a long tail paired phase inverter and two EL34's push pull in fixed bias. But like Ampmad explained, they are also limited by their power supply. If my memory is there, they run around 430V on the EL34 plates. Very small output transformer as well.
     
    tce63 and marshalltsl like this.
  6. tomykoz

    tomykoz Active Member

    Joined:
    Mar 13, 2010
    Messages:
    157
    Likes Received:
    58
    Wow...all those years i used the 3203 head i thought it was class A ......not being a tech i was under the impression that any amp i saw that had less than 50 watts with 2-EL34's was class A......guess im still learnig......

    Heres another thing i was told somewhere in my life.....''class A amps are louder than push pull amps of the same rating''....any truth to that?
     
    tce63 likes this.
  7. Georgiatec

    Georgiatec Well-Known Member VIP Member

    Joined:
    Apr 23, 2010
    Messages:
    5,287
    Likes Received:
    4,243
    Location:
    Looking for something I don't remember what.
    You don't have to use EL34's either. Because of the reduced voltage you can use any octal based output valves, even 6V6's (just use a matched pair).
     
  8. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    2,351
    Likes Received:
    570
    Location:
    flo rida
    class a it's a great marketing gimmick
     
  9. RickyLee

    RickyLee Well-Known Member VIP Member

    Joined:
    May 22, 2010
    Messages:
    9,817
    Likes Received:
    3,257
    Location:
    Riverside CA U.S.A.
    I have no experience with a true Class A amp. Never really read up on them much either. The closest thing I have would be my Vox AC 30. But then is that even true Class A? I think it might be at idle, but then under heavy load/demand it might actually be Class A/B?? Anyone on that one??
     
  10. tomykoz

    tomykoz Active Member

    Joined:
    Mar 13, 2010
    Messages:
    157
    Likes Received:
    58
    I finally called Marshall amps they told me the origins and the DSL20’s are A/B design.... thank you everybody for your input ..

    I was struggling which $500 amp head I wanted to get to just play locally if I ever had the opportunity again and in my bedroom .... I was torn between the origin 20 origin 50 and DSL 20 HR or a used DSL 50 head .... I finally decided on the origin 50 due to its brand new ,light weight and it seems to have that plexi flavor of my 1974 super lead head that I used to own ( I wish I had never sold it)
     
  11. tomykoz

    tomykoz Active Member

    Joined:
    Mar 13, 2010
    Messages:
    157
    Likes Received:
    58
    U were right it’s push pull
     
  12. printer

    printer Member

    Joined:
    Sep 30, 2017
    Messages:
    91
    Likes Received:
    93

    Might be a good thing to check the datasheet first rather than guessing.


    http://www.r-type.org/exhib/aaa0014.htm


    "Output pentode rated for 25W anode dissipation", right up at the top of the page. Not in Class AB operation, not in Class A, not in Triode, but anode dissipation. Let's look at what it says is a good place to operate the tube in Single Ended operation, which just happens to be Class A operation also.

    250V on the anode, -13.5 grid voltage, 120 ohm cathode resistor, 100 mA bias current. And it lists power output of 11W.

    Now let's do some math, cathode to anode voltage, 250 V - 13.5 V = 236.5 V. Now the power at idle, 236.5 x 0.100 A = 23.7 W. We will not bother with the screen power, subtract 10% if you are so inclined.

    SOB, close to the 25W rating of the tube. And in Class A push-pull, you can design one side separately and just double impedance as the plate to plate impedance. Does not change. In triode SE they say 375V - 26V = 349V cathode to plate, 70 mA, so 349V x 0.070 A = 24.43W.

    So no derating, the 25W is the same no matter how you use it.



     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice