Plate voltage? Low plate voltage?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by RUSHFANnLV, Aug 27, 2005.

  1. RUSHFANnLV

    RUSHFANnLV Member

    Messages:
    623
    Joined:
    Dec 5, 2003
    Location:
    Las Vegas
    What are the symptoms of low plate voltage? I was told my Marshall 4140 has "really low plate voltage" by the guy who worked on it for me.

    All I can tell you is that my Peavey classic 30 with a 2x12 Yamaha cab (with eminence speakers) blows my Marshall 100w running through a RedBear 4x12 cab away in terms of volume.

    Any help would be appreciated.
     
  2. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Ask him what he means? greater the 20 percent?

    What is spec for the amp?
    What he actually measured?
    What about the stages?
    Screen, Driver, Pre-amp voltages...

    ...are they also low?

    Generally, highter output tube plate voltages yield
    greater output watts.

    There are a bunch of other things in the equation too, one
    example is speaker efficiency--Given a 100 watt amp where
    you install a 3db more efficient speaker, it is like doubling the
    the power of the amp to 200 watts.
     
  3. John Phillips

    John Phillips Member

    Messages:
    13,080
    Joined:
    Mar 17, 2002
    Location:
    Scotland
    Could this be associated with some of that switching on the back panel?

    One way to lower the output power is to lower the plate voltage. In an amp with 'stacked' filter caps - which that one is - a way to do this is to switch the plate voltage supply to the mid-point of the stack, not the top. This gives half the plate voltage (which would definitely qualify as 'really low', it would measure about 240V on that amp), and one quarter of the output power, since that's dependent on the square of the voltage. So you now have a 25W amp.

    This is exactly the way it's done on the modern Fender Twins, and if I remember correctly I did to my 4140 as well. Whether that's what's been done to yours, I don't know... it's very difficult to help further without at least seeing good pics of the inside.

    But yes, something is wrong if it's quieter than a Classic 30, with just about any possible arrangement of speakers. Those amps are a full 100W, the same as any other Marshall of the same period... ie LOUD.
     
  4. TheAmpNerd

    TheAmpNerd Member

    Messages:
    1,062
    Joined:
    Sep 23, 2004
    Location:
    Tejas
    Hey John,

    I always enjoy reading your posts because I always learn
    something. I hadn't thought of that, but have measured
    it plenty of times going through amps.

    Talk about a simple implementation of "power scaling".

    Perhaps..."Power Scaling for Dummies" in a little yellow book.

    Ciao,

    TAN
     
  5. RUSHFANnLV

    RUSHFANnLV Member

    Messages:
    623
    Joined:
    Dec 5, 2003
    Location:
    Las Vegas
    You guys freakin rock. I 'm printing this thread off and taking it to my tech to ask him a few questions about it. I'm also coing to take some close ups while we have the amp opened up.

    Thanks again guys.:)
     

Share This Page