Re: Battery bias directly to grid


Sander deWaal wrote:
> "Andre Jute" <fiultra@xxxxxxxxx> said:
>
> >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.
>
>
> My instincts tell me to keep the battery or rechargeable battery in
> the cathode circuit.
> A battery in the grid circuit via a potmeter would mean DC on the pot,
> which introduces noise.
>
> Elsewhere you mention the use of a stepped DACT attenuator. Good.
> That ekiminates pot noise.
> But from the way you describe it, it seems that the wiper of the
> attenuator is connected via two AA cells to the grid.
> This means you can't adjust -Vg, you just vary the resistance to
> ground. -Vg is determined by the 2 cells, which is 3 volts.

There are basically three places you can inject battery bias into a
stage. One is into the cathode circuit, where the battery takes the
place of the resistor and the bypas cap; this is the standard way I
have always used, elegant and goodsounding. The second place is in the
input circuit directly between the wiper of the pot at the input and
the grid, as already described. The third is in the place of the grid
leak resistor. The third one is the only place where small multiturn
pot is really conveniently introduced to adjust the negative bias
voltage from a battery. My idea of simplicity has always been simply to
use the battery without adjustment. The voltage is around 2.4V for two
AAA batteries, not the nominal 3V you mention. That is ideal for
running my favourite 417A hot and hard (it doesn't come alive at
textbook dissipation but it is a sturdy little tube that one British
custom amp maker operated at 200V for amps he sold without a problem)
with the full 2V available from CD players.

>
> A battery in the grid circuit means excellent frequency response in
> the very low frequency region, but is that a wise thing in an amp
> where I assume there is no global feedback?

I'm talking about battery bias for an input tube. I take care of the
frequency choking in the driver tube's cathode arrangements.

> This means the only limiting factor is the output transformer, which
> now has to deal with very low frequencies, and that will pose some
> problems by itself.

Not to mention, if this were true, what it would do to my modestly
sized horns!

> As a rule, I let the first driver stage of an amp cutoff the lowest
> frequencies, by deliberately choosing a small cathode cap.
> Bear in mind that I always build PP amps though, the sonical effect
> may well be very significant in a SE amplifier.

A good trick with simple SE amps is to decide your cutoff and tune all
stages, including the power, to the same time constant. It is worth a
little extra clarity in the sound.

> >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?
>
>
> The Ri would increase, to the point where the grid may float a bit
> free.
> Reverse grid current would come into play, which usually isn't a good
> thing.
>
> Of course, this is all armchair engineering on my part :-)

Well, no, I don't see that as a negative. I'd rather not build a
circuit that will give trouble of any kind. I'm not an experimenter who
never switches his soldering iron off. I like knowing how my amp will
respond before I build it. Of course, we are at the margins of audible
improvement here.

> I've never tried batteries in either the cathode or grid circuits, but
> it seems to be a simple and elegant solution.

Simple and elegant is always good for me. That is how I first started
on battery bias, looking for parts I could elimate. That gaincard guy
is a piker without the courage of his convictions.

> --
>
> "Audio as a serious hobby is going down the tubes."
> - Howard Ferstler, 25/4/2005

Poor Howard might be into silicon, and quite unaware of coining an
awful, awful pun.

Andre Jute

.

Relevant Pages

  • Re: Battery bias directly to grid
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  • Re: Battery bias directly to grid
    ... the grid leak resistor, between grid and ground. ... I republished the T39 KISS Ultrafi circuit with the batteries in the ... hear no difference with the battery in the grid leak position, ... >>> low impedance voltage source, no matter what the setting of the DACT, it ...
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  • Re: Battery bias directly to grid
    ... >I constantly use battery bias in the cathode circuit. ... A battery in the grid circuit via a potmeter would mean DC on the pot, ...
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  • Re: Battery bias directly to grid
    ... >>> I constantly use battery bias in the cathode circuit. ... No part of a tube stage is without ...
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