Re: Battery bias directly to grid

In article <1129658943.962078.103860@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"Andre Jute" <fiultra@xxxxxxxxx> wrote:

> 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.

Got it. Low impedance, quite! I was looking at the T39 mk VI schematic,
since the "discussion" schematic was refusing to appear.

> 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.

Well, good on ya, mate, but I'll stand by my observations.

> 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.

I'm glad that the discussion here helped you decide to change that
schematic, since it was, shall we say, a somewhat unhealthy arrangement
of parts. A newbe building an amp based on the T39 circuit might be a
little perturbed to find that his or her batteries would be shorted to
ground whenever the attenuator was turned off. An input attenuator
having a wonky curve because it was loaded down by a 1 ohm load would be
another "surprise".

> 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.

Just for the record, again, I was making reference to the T39 mk VI.
I've made no comment on the "discussion" circuit.

A cap connected between the wiper and grid, with the batteries in series
with a 100k-500k resistor connected in the grid leak position, is what I
was proposing would work well. No charging, however. Always a compromise.

> But I'm not interested in defending a circuit before
> professional production engineers.

Which I am. Sorry, can't (and shan't) take the hat off...

> 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...

"Professional practice" sounds vaguely derogatory. You rebel, you.

> 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.

Best of luck.

-bruce seifried
eclair engineering
.

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