Re: Battery bias directly to grid

in article 1129510760.869425.23050@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, Andre Jute
at fiultra@xxxxxxxxx wrote on 10/16/05 8:59 PM:

***** I'm responding to a couple of points, not to be argumentative, but
because I really don't understand.

> 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!

***** Granted, a power supply is a signal path, but intuitiveIy it seems
that a component directly in line with a signal would have more effect on
the sound than something "on the side."

***** I could accept on faith that a battery might have less noise than a
resistor, but how is it less variable than a cap, especially with respect to
time? Are you comparing it to a film or an electrolytic cap? An
unregenerated NiCad has a degradation cycle with a much longer duty cycle
than an (electrolytic) capacitor's discharge cycle, no?
>
>> 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.

***** OK. That changes the entire perspective. But how exactly does a
battery get charged in a cathode circuit? The charging EMF across the
battery's terminals must exceed the voltage generated by the chemical
reaction, for charging to occur.
>
>> 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.

***** Easy to test if you've already prototyped the circuit.
>
>> 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.

***** I didn't know there were four grids in that tube. Now it makes
sense.
>
> 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.

***** I've recently been cautioned about the importance of layout and
topology in sonic "signature." Stuff from a fellow who has spent his life
building and testing both tube and SS amps. He has hundreds of thousands of
dollars in sophisticated test gear. After he makes his measurements he
listens. Some of his observations run against the grain of "accepted"
wisdom. Like a star ground is not optimum. And never ground the signal to
the chassis.
>
> Thanks for polite input.

***** Well, being an ass wasn't getting me anywhere . . . .

;-)
>
> 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
>>>
>

.

Relevant Pages

  • 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)
  • Re: Battery bias directly to grid
    ... Using the battery *instead of* the grid leak gives you a 1ohm shunt ... in the absence of a grid leak, and the circuit would be just as ... > third possible position, in the cathode circuit, simply because I ...
    (rec.audio.tubes)
  • Re: Battery bias directly to grid
    ... The entire circuit is ... >>> attenuator, tube, battery and a couple of pieces of wire and maybe a ... >> the tube? ... > Battery bias circuits act just like fixed bias circuits. ...
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  • Battery bias directly to grid
    ... What are the implications of this circuit? ... What, theoretically, makes it better than the standard battery bias ... application in the cathode circuit? ...
    (rec.audio.tubes)