Re: Battery bias directly to grid

Uh-uh, Bruce. What you're talking about is the battery in the place of
the grid leak resistor, between grid and ground. That's a low impedance
path to ground. I've tried that one with a DACT, a high precision Swiss
medical switch with SMD resistors and ultrashort paths, with the tube
in position to amplify the clicks if any, and the amp working into
ultra-sensitive horns, and I heard no clicks.

I republished the T39 KISS Ultrafi circuit with the batteries in the
third possible position, in the cathode circuit, simply because I could
hear no difference with the battery in the grid leak position, and the
cathode circuit position has the very great advantage of being more
failsafe than either of the other two positions, by virtue of the
trickle charge the tubes can enjoy there.

One final note on the *discussion* circuit I published (battery between
wiper of attenuator and grid of signal tubes) : A "professional"
implementation would use at least a resistor and a bypass cap in
addition to the battery, as has been pointed out by you and several
others. But I'm not interested in defending a circuit before
professional production engineers. I'm interested in simplifying the
circuit to what sounds best, regardless of expense or "professional
practice", and in this case, because I am publishing the circuit for
amateurs, in making it reasonably longlasting (there is a companion
circuit, called the T44 "Populaire", with autobias, recommended for the
newest aspirants, precisely because it is a largely self-adjusting
circuit). Thus the return to the battery in the cathode...

Thanks to all who contributed valuable insights (even if we were all
talking about different circuits!). I'm removing the discussion circuit
because it really doesn't add much to our wisdom; it may be a viable
alternative (in that it works) but it is a decidedly less desirable one
than the battery in the cathode circuit. That's the point of discussion
with one's peers, to choose the best course of action from among
possibilities.

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
"an unbelievably comprehensive web site" -- Hi-Fi News & Record Review

bruce seifried wrote:
> In article <43551ED2.2319BC54@xxxxxxxxxxxxxxxxxx>,
> Patrick Turner <info@xxxxxxxxxxxxxxxxxx> wrote:
>
> > 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.
> >
>
>
> Aha... apparently we *are* talking about different cicuits.
>
> Until it was changed several days ago, I had been referencing the T39
> schematic on this page:
>
> http://members.lycos.co.uk/fiultra/T39-KISS-300B-Ultrafi-crct.jpg
>
> I was never able to view the other schematic due to unknown browser
> demons, but its description sounded like that of the T39 input biasing
> arrangement, which I must say was bizarre, but easily fixable (...and I
> happen to believe that a good quality capacitor makes a better capacitor
> than a battery).
>
> Carry on...
>
> -bruce seifried
>
>
> > > 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

.

Relevant Pages

  • Re: Battery bias directly to grid
    ... Battery in cathode circuit. ... >> grid stopper resistor. ... Battery bias circuits act just like fixed bias circuits. ...
    (rec.audio.tubes)
  • Re: Battery bias directly to grid
    ... >>I constantly use battery bias in the cathode circuit. ... >>cathode into the grid circuit. ...
    (rec.audio.tubes)
  • Re: Battery bias directly to grid
    ... >>> I constantly use battery bias in the cathode circuit. ... No part of a tube stage is without ...
    (rec.audio.tubes)
  • Re: Battery bias directly to grid
    ... Those self-charging advantages of the battery in ... Battery in cathode circuit. ... grid stopper resistor. ...
    (rec.audio.tubes)
  • Re: Battery bias directly to grid
    ... >> I constantly use battery bias in the cathode circuit. ... No part of a tube stage is without ... Not in the direct signal path between pot wiper and grid. ...
    (rec.audio.tubes)