Re: Battery bias directly to grid


Jon Yaeger wrote:
> in article 1129488747.358162.79260@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, Andre Jute
> at fiultra@xxxxxxxxx wrote on 10/16/05 2:52 PM:
>
> >
> > I constantly use battery bias in the cathode circuit. It gives you
> > better bass, as all kinds of fixed bias do. But John Byrns, the last
> > time we discussed the sound shaping of capacitors, said that the
> > battery is itself a soundshaping element and should be removed from the
> > cathode into the grid circuit.
>
> *** I'm not an engineer, so I don't have the benefit of backing up my
> opinions with good quantitative data.
>
> Nonetheless, if simplicity is your goal I can't imagine how you could
> benefit from putting something as complex and as variable as a NiCad in the
> signal path (could it be considered an "active" passive device?)

A nicad is less variable than a cap and less noisy than a resistor. The
question we're answering is, in which part of the signal circuit should
it be put? No part of a tube stage (or amp for that matter) is without
the signal path!

> NiCads have a non-linear discharge curve and internal impedance increases as
> charge decreases. This changes the setpoint of your input level control and
> linearity overall. NiCads also have the potential to actually reverse
> polarity if they are allowed to discharge fully. You can imagine what a
> positive grid bias would do to the current drawn by the tube . . . .

That's merely an argument for putting the battery in the cathode
circuit where it will be charged.

> As pointed out by others, there is no mechanism for charging the cell.

Not in the direct signal path between pot wiper and grid. But in the
cathode circuit the battery is constantly charged.

> NiCads are supposed to be charged with DC (pulsating DC is fine) but I
> believe A/C is inadvisable.

There is no AC on two out of the three possibilities, and the AC signal
on the third possible placement of the battery, between pot wiper and
grid, has a very low current.
>
> What kind of artifacts, distortions, or noise are introduced by a cell in
> line with a signal? This ought to be observable with a scope and a signal
> source. I would certainly know what the waveforms look like before I did
> anything else (although I personally would not employ one in this manner).

The battery (in the cathode anyway) looks better on the scope than the
resistor and cap. I have no idea how it scopes in the other two
positions.

> I wish someone would kindly explain the alleged benefit of using four grid
> stopper resistors where one would do. On the face of things, it would seem
> to add complexity and noise and little else.

You'd be surprised how many tubes are radiophonic, because, surprise,
surprise, they were designed to be radio frequency tubes. The tube I
use so often, the 417A, is an RF tube. It has four grids and RF has be
stopped right up close to the pin, so it is convenient to use four
resistors and to use the unconnected centre pin of the tube for the
other ends of these resistors and the input. The 417A is also tricky in
layout, because if the paths are fractions of an inch too long the tube
picks up rubbish from the air. Furthermore, you'd better not put hang
more than 10K on its grid. It is one of those tubes you just have to
know about before you start implementing it, or your life could be very
frustrating for a very long time as you debug the circuit and layout.
But the sonic qualities make the troubles worth the result.

To summarize, battery bias directly in the cathode circuit is proven
and preferred to autobias by many audiophiles, and the other two
possibilities for placing battery bias have serious downsides, as
listed in posts by others and summarized by me in a recent post.

Thanks for polite input.

Andre Jute

> Jon
>
>
>
>
>
>
>
> >
> > I used direct grid battery bias briefly about fifteen years ago but it
> > involved me in screaming match with the Magnequest Scum. That was
> > before I learned that whatever I did they turned into a screaming
> > match, and that they and their hangers-would lie about electronics to
> > keep the screaming match going. I can't even remember what it sounded
> > like.
> >
> > But questions remain to be answered before I entrust valuable tubes to
> > the topology. It is much more useful to work them out in RAT than to
> > put my mind in gear and hug my findings to me, wouldn't you say? (Or
> > has everyone now given up on the ARRL dream of sharing information,
> > which motivated the founders and early members of RAT?)
> >
> > Battery bias directly in the grid circuit between pot wiper and the
> > tube grid uses the downwards leg of a pot as the grid leak. What would
> > happen in case of a loose connection? In fact, what would happen if
> > your suggestion of a battery running out of juice came true?
> >
> > Are there any other questions to be answered with this circuit?
> >
> > Andre Jute
> >

.

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