Re: Battery bias directly to grid

In article <4353F476.257BE164@xxxxxxxxxxxxxxxxxx>,
Patrick Turner <info@xxxxxxxxxxxxxxxxxx> wrote:

> bruce seifried wrote:
>
> > In article <4353775E.5B755A29@xxxxxxxxxxxxxxxxxx>,
> > Patrick Turner <info@xxxxxxxxxxxxxxxxxx> wrote:
> >
> > > But the transients you speak of don't occur any more than with any other
> > > biasing method.
> > >
> > > Patrick Turner.
> > >
> >
> > With the Dact 20K input attenuator, and a +2.4 volt dc source connected
> > to its wiper, you will see something on the order of a few tens of
> > millivolts being generated when changing the gain setting at the low end
> > of the attenuator. Feeding this to the input of a dc-coupled power amp
> > will give you low level, audible transients. Not a deal breaker, but not
> > good design practice.
> >
> > Switching the attenuator from its lowest gain setting to off, or vice
> > versa, will give you a 2.4 volt step signal into your input. I would
> > call this a rather large transient, and not especially good for the
> > health of any speaker attached.
> >
> > -bruce seifried
>
> I don't understand how switching gain settings changes the tube bias;
> there is always -2.4V at the grid regardless of the gain setting.

Well, bias really isn't the issue here. Because the battery is a very
low impedance voltage source, no matter what the setting of the DACT, it
will still be pretty close to +2.4 volts... except when the attenuator
is set to 'off', and then it will rudely drop to zero volts. The issue
is the (small) step-change in the dc level, which manifests itself as an
ac signal at the input grid.

> The only chance of transients being generated by gain level change is if the
> input grid draws dc current and a small dc voltage exists across the resistances
> of the DACT, so that when switching, you hear the switch steps.

Are we talking about the same circuit?

The battery is constantly supplying current to the attenuator, in fact,
way more than any normal or abnormal grid current would be. I don't have
a 20K DACT series step attenuator here to measure, but roughly speaking,
the resistance to ground at the -60 dB setting (just above off) will be
on the order of 100 ohms. With a pair of 1.2 volt NiCad batteries in
series, you'll see a current of 24 ma flow through the attenuator.
Assuming that the collective source impedance of the batteries is around
1 ohm, you will see the voltage drop to +2.376 volts. Change the setting
a step higher, and the reistance is roughly around 300 ohms, for a
current flow of 8 ma. The voltage will change to +2.392 volts. You will
see a step-change of roughly 16 millivolts at the input grid. Yes, it is
small, and yes you will hear this.

> I have heard this in amps with a DACT. But its a tiny effect compared to
> music transients.

Please, wire this up for yourself and see what happens. I did, last
night, just to reaffirm what I already knew. Turn knob, see step-change
in voltge on scope, hear click in headphones.

This would all be just a moot conversation if the original circuit were
designed better: place a resistor in series with the batteries (say
100k-500k) so that that input impedance is somewhat normal ( and not 1
ohm!), and then place a good quality cap after the input pot. This would
work quite well.

cheers,

-bruce seifried

-bruce seifried
.

Relevant Pages

  • Re: Battery bias directly to grid
    ... > With the Dact 20K input attenuator, and a +2.4 volt dc source connected ... > millivolts being generated when changing the gain setting at the low end ... > will give you low level, audible transients. ... I don't understand how switching gain settings changes the tube bias; ...
    (rec.audio.tubes)
  • Re: Battery bias directly to grid
    ... With the Dact 20K input attenuator, and a +2.4 volt dc source connected ... will give you low level, audible transients. ... Switching the attenuator from its lowest gain setting to off, ...
    (rec.audio.tubes)