The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Dynaco VTA tube amp kits, all products and all products

    ST70 rolloff from 300hz



    Posts : 13
    Join date : 2013-03-05

    ST70 rolloff from 300hz

    Post by JackF999 on Wed Mar 06, 2013 4:54 am

    Okay guys....

    I have a pristine ST70 Dynakit ( yes with selenium diode I know
    I have to replace do it ,,, do it ) but it starts rolling off
    the bass at about 300hz or so.
    How do I know... I have a Umik-1 and use REW with a pair of
    Spica TC50 and REW says a big NO to anything below approx. 300hz.

    With another amp bass shows up fine.

    What is the cause of this ?
    I would like to step through the solution instead of just
    upgrading to newer parts, just because I'm curious.
    I have a suspicion it would be some magnetic flux capacitation..

    Maybe the tubes are old ???
    Like I said this thing has not one replacement part since it
    was assembled....

    What should I do...
    New cap can first from tubes4hifi ??

    Speak to me...

    hahaha thanks for reading


    Posts : 650
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST70 rolloff from 300hz

    Post by peterh on Wed Mar 06, 2013 5:01 am

    How do you know that you have a rolloff at 300hz ?
    Any measurements ? And what is the load and inputs ?


    Posts : 13
    Join date : 2013-03-05

    Speaker measurements from Umik-1 and REW

    Post by JackF999 on Wed Mar 06, 2013 5:05 am

    Measurements are from my Umik-1 ( calibrated microphone from MiniDSP )
    feeding into REW ( Room EQ Wizard, a sound capture program )
    that captures sweeps from my Spica TC50 speakers.
    The other amp does not display any such anomaly of frequency.

    Thanks for responding.......


    Posts : 650
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: ST70 rolloff from 300hz

    Post by peterh on Wed Mar 06, 2013 5:29 am

    Maybe your speakers are ill-suited, i have no knowledge of Spica TC50.
    You could try 4-ohm output if speakers are very resonant.

    To be more systematical load the st-70 with
    resistors and feed it with sweepgenerator. Then you would
    also have an opportunity to see power limits.

    What is needed here is a simplification of your object. You have a bass rolloff,
    how come ? Well that's why you should simplify in order to measure each
    component at a time.
    Measuring the st-70 to figure out if it fullfills the performance stated
    is one step. If it does not - well time to fix it. But if it
    fullfills all it's spec then it's a waste of money and time to start
    replacing components.

    Maybe your bass-rolloff comes from a interaction of amp and speakers.
    Maybe the other amp has a huge bass-increase ( it could have loudness control
    active). There is lots of reasons for the perceived rolloff.

    Start by ruling out the components that does not causes bass rolloff.



    Posts : 722
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: ST70 rolloff from 300hz

    Post by GP49 on Wed Mar 06, 2013 1:38 pm

    To accurately determine what the amplifier is really doing, you need to measure the output with an AC voltmeter or on a scope while it is driving a noninductive dummy load matching the output tap to which it is connected. Otherwise your microphone is measuring not only the amplifier but the loudspeaker and the room acoustics.


    Posts : 13
    Join date : 2013-03-05

    Why I ask questions

    Post by JackF999 on Wed Mar 06, 2013 2:05 pm

    I guess some would question why I would ask questions
    without much previous thought or action about my amp.
    Well, firstly I have no experience with really troubleshooting
    ( as in signal generators, scopes, DMMs )

    I was asked to work on a Cadillac CTS v6. This car had a
    p0305 code ( misfiring on cylinder 5 ). Conventional wisdom
    points to inspecting and/or replacing spark plug/ignition coil.
    Well one guy out of thousands on a YouTube video just happened
    to mention one thing that fixed this car. ( its a big job
    to replace spark plugs, the intake top plenum has to come off )
    He said that oil pressure regulates the tension on the timing

    Low oil pressure causes the timing chain to jump around to produce
    2% - 10% variance and up pops a code.

    Soooo, I learned to ask, because instead of taking hours to fix
    a problem, there may be just that one expert that has just that
    one bit of advice that can make life soooooo much easier.



    Posts : 45
    Join date : 2013-03-15
    Age : 48
    Location : Birmingham Alabama

    Re: ST70 rolloff from 300hz

    Post by Bigron865 on Tue Mar 19, 2013 9:40 pm

    I'd also like to know how different parts affect soundstage, dynamics, freq response etc.. Like a choke? Does replacing that affect the sound? Once all pets are new, is it just a matter of different tube combos for noticeable changes. Thanks for your patience.

    Sponsored content

    Re: ST70 rolloff from 300hz

    Post by Sponsored content Today at 3:02 am

      Current date/time is Sat Oct 22, 2016 3:02 am