Re: what the heck! DC on secondary??
- From: Patrick Turner <info@xxxxxxxxxxxxxxxxxx>
- Date: Mon, 12 Dec 2005 15:34:37 GMT
alfred wrote:
> I was just checking it out now, and the secondary has three taps:
>
> one to the speaker, centre tap to ground, the other to the cathode of
> the power tubes.
>
> the speaker tap is .7 ohms dc the NF tap is 27 ohms.
> the amp has 4.5 v on the NF tap when on.
>
> so the ground wire may be intermittant?
>
> but if the ground was not properly terminated the whole amp shouldn't
> work because I believe that the cathodes are connected through the NF
> winding to ground..
>
> strange.
>
> could it be the H.T. windings in contact? or is it probably a
> grounding/connector issue?
>
> I'm letting it warm up again as I write.
Your explanation is ambiguous and not concise enough, ie,
we don't know exactly what you mean, since we do not have a schematic to work
from.
Indeed it looks like you have a cathode feedback winding which is in series with
the speaker winding,
the NF tap is the negative feedback winding and has 27 ohms dcr.
If you have 4.5 volts dc across the 27 ohms, that's 166 mA, and there should be
only
0.1167 V across the 0.7 ohm winding, so only that much should be also across the
speaker,
which may be say 4 to 8 ohms, a lot more R than the R of the speaker winding .
But measuring low dcr of windings is tricky, some digital VM don't give accurate
readings.
Where is the schematic for your amp?
Patrick Turner.
>
>
> Patrick Turner ha scritto:
>
> > alfred wrote:
> >
> > > I have a SE tube amp that when hot makes a crackling noise on one
> > > channel and then has dc coming out of it.. about .5 volts or
> > > something.. enough to move the cone foreward a bit..
> > >
> > > what could be the cause of this?
> > >
> > > the secondary isn't connected to anything except for ground on one end.
> > >
> > > any ideas?
> >
> > Say you had a dcr of the speaker sec winding = 0.5 ohms.
> >
> > If 100mA of dc plate current was flowing due to a fault, or because the sec
> > was
> > a feedback winding, then 100mA x 0.5 ohms = 0.05V, and not a worry.
> > But you say there is 0.5Vdc, and that means maybe an amp
> > is flowing and that's horrible, how come?
> >
> > Checked all your measuring?
> >
> >
> > If there is no music, and you can't figure it out, take it to a tech.
> >
> > Patrick Turner.
.
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