Re: Battery bias directly to grid



bruce seifried wrote:

> 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?

There is no chance of transient problems with the schematic Andre sent me,
it has a 20k DACT attenuator, then the wiper goes to a battery, then
to the grid.
So at all levels of gain the bias remains and there are no transients.


>
>
> 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.

Huh?
I don't follow you at all.

You must be talking about some other circuit.

Patrick Turner.

>
> 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

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