Re: Battery bias directly to grid



Wessel Dirksen wrote:

> Andre Jute schreef:
>
> > Seems to me, my dear fellow, that your letter illustrates either that a
> > Marxist can make a meal out of very little or that you too can be an
> > "ultrafi fetishist". Those self-charging advantages of the battery in
> > the cathode circuit you are so keen to dismiss are huge benefits for no
> > cost. Your better ultrafidelista is not an audiophool. He is imbued
> > with the conviction that simplicity means better sound. The battery in
> > the cathode is the simplest safe implementation, requiring no other
> > components.
> >
> > Once more, the three circuits considered in these discussions:
> >
> > 1. Battery in cathode circuit. Grid leak is taken care of by leg of
> > attenuator to ground. This is the common version. The entire circuit is
> > attenuator, tube, battery and a couple of pieces of wire and maybe a
> > grid stopper resistor.
>
> A quick newbie question about this circuit #1 if I may:
> This looks interesting and I would like to try it but I don't know how
> for sure how to calculate/implement this. Is the DC bias voltage of the
> batteries all there is to it in determining the idle current through
> the tube?

Yes.

Battery bias circuits act just like fixed bias circuits.

> Or does the value of Ra (in the absence of setting a specific
> value for Rk) need to be calculated differently than a normal bypassed
> grounded cathode circuit?

The value of RL ( and not Ra, which is plate resistance )
is chosen so RL = approx 3 x Ra at least, then if you wish you can trim the
B+
supply to suit the best point for working with that RL and that bias current
due to that grid bias.
Most ppl choose the B+, choose the RL, then adjust the applied grid bias for
the wanted dc anode voltage
and best working point, or they bias g1 from 0V, and adjust Rk to get the
wanted
Ia, hence Ek.
Once cathode bias is set up set, this is much more stable than fixed bias
because the cathode bias voltage isn't fixed,
and there is a state of negative current feedback which tries to keep the
tube
current the same.
Should more Ia flow, there is more Ek, to the tube tends to
be cut off because its as if the grid is being driven -ve with a rise in Ia.

However, fixed bias for input tubes usually is a long lasting fix
until the tube wears out or the battery goes flat if there is battery bias.


Patrick Turner.

>
>
> Thanks, Wessel
>
> > 2. Battery shunted in grid leak position between grid and ground,
> > parallel to leg of attenuator to ground.
> >
> > 3. Battery seriesed in line between wiper of attenuator and grid.
> >
> > Your idea of a battery in series below the attenuator would be a fourth
> > version and has the disavantage that it would vary the bias with volume
> > setting. How the battery could be charged by the signal after the amp
> > is switched off, as you claim, beats me. Maybe you know some magic that
> > I don't, which doesn't quite seem likely.
> >
> > My source in all these experiments was a Quad 67 CD player, which has a
> > capacitively coupled output. I don't have any sources without a cap on
> > the output. Very few audiophiles can buy such a thing, and no competent
> > DIYer would build such a thing.
> >
> > It is true that I already know what the ultrafidelista think. Most of
> > them think what I think and change their when I change mine; it's like
> > a ballet. But I was wondering if there is something I overlooked about
> > battery bias that might be worth some margin of extra silence. Seems
> > not.
> >
> > On the subject of noise, you're on a hiding to nothing. A battery is a
> > more silent component than anything else including a straight piece of
> > wire. What I was reaching for was a more, even a marginally more,
> > silent implementation of a battery. The truth is that I couldn't hear
> > that the battery in the grid circuit was superior to the battery in the
> > cathode circuit, so therefore the more convenient and conservatively
> > safer cathode circuit it is.
> >
> > Of course, by the nature of tubes, where there is very little that is
> > not already known, most experiments do not lead to improvements. On the
> > other hand, it became to clear during the course of this discussion
> > that the vast majority of RATs don't know about battery bias and that
> > only a few have ever tried it. It was definitely a worthwhile
> > discussion for me, and I hope it has given those who have not tried
> > battery bias (in the cathode, gentlemen, in the cathode) something to
> > do this winter.
> >
> > HTH.
> >
> > Andre Jute
> > Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
> > "an unbelievably comprehensive web site" -- Hi-Fi News & Record Review
> >
> >
> >
> > Ian Iveson wrote:
> > > I can't speak for other heads, but the inside of mine finds itself
> > > no further forward. It would help if you were to say what you are
> > > using as a source in your listening test.
> > >
> > > Using the battery *instead of* the grid leak gives you a 1ohm shunt
> > > across the lower leg of the attenuator, which is not a good test of
> > > the principle. Perhaps you mean *in series with* the grid leak?
> > >
> > > All methods of grid bias in the absence of DC blocking will result
> > > in biasing the source, obviously. Here again it is not clear whether
> > > that in itself may have degraded the sound, so again the test is
> > > inconclusive.
> > >
> > > A battery at the foot of the attenuator would not discharge itself
> > > in the absence of a grid leak, and the circuit would be just as
> > > simple as the one you have chosen. The battery could also be charged
> > > through the signal input with the amp off, so recharging could be a
> > > switched function of a dedicated preamp. I dare say there would be
> > > objections to this option, and I wonder what they would be.
> > >
> > > What I would really like to know is would it be better, leaving
> > > aside the issue of DC blocking and recharging, to bias via the grid
> > > or the cathode? And would a series or a shunt connection to the grid
> > > be more appropriate?
> > >
> > > Do batteries have the same impedance in both directions? Are they
> > > more noisy when discharging, or charging, or in equilibrium? What
> > > are the mechanisms of distortion and will it be less in the
> > > high-impedance low current input or the low impedance high current
> > > cathode circuit. Will it be less by a factor greater than the
> > > relative gain?
> > >
> > > I would be surprised if these issues had not already been resolved,
> > > considering you say that the cathode connection is so common. It
> > > seems unlikely that the obvious alternatives were not explored, or
> > > that the merely practical charging and DC blocking issues won the
> > > day with the "ultra fi" fetishists.
> > >
> > > And don't forget all of your famous guru friends, who I am sure
> > > would be happy to advise you. I would be interested to hear their
> > > views.
> > >
> > > cheers, Ian
> > >
> > >
> > >
> > > "Andre Jute" <fiultra@xxxxxxxxx> wrote in message
> > > news:1129658943.962078.103860@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > > > 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

.

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