Re: T68bis "Minus Zero" potato amp circuit and layout now online


Chris Hornbeck wrote:

Andre Jute wrote:
2. The circuit:
http://members.lycos.co.uk/fiultra/t68mzwe417acircuit.jpg

WRT the circuit: looks pretty bullet-proof. Naturally,
quibbles follow. Arf!

Bow-wow.

A large-value resistor from the wiper/tap of the
input attenuator to ground will minimize noise as the
control/switch contact ages. About a half a meg
should do it.

AKA grid leak. Perhaps necessary with a cheap attenuator but this is a
DACT, a Swiss medical switch unit with SMT resistors on little gold
boards of the utmost precision and quality.

Anyway, it should be kept in mind that in my hands this is not a
permanent amp. It is merely the driver stage of another amp, being used
to prove a few points about the driver and its circuit. The final amp
won't be build on this chassis either.

The bit that really bugs me, because I know just
enough to be dangerous, is about the NiCad charging
and discharging.

Tell us what you know that's dangerous. Then suggest alternatives and
evaluate them. We went through this on the Joenet and decided that
nicads were the least bad choice. Persuade me different.

What regulates this? (obviously
rhetorical, but raised as an issue).

Negative feedback, actually. <G> The two batteries produce 2.4V and
that, in conjunction with the 175V available at the plate, fixes the
current across the battery at 20mA. If the batteries with age produce a
lower voltage the quiescent current will rise but those are sturdy
tubes and 20mA is a long way below maximum draw.

In practice nothing has ever gone wrong with a nicad battery bias
installation in my hands or with anyone I know of.

Again, tell us what you fear. I'd be delighted to hear I am living
*dangerously*.

And, as a related quibble, can an unbypassed battery
of any type be a low enough impedance at high frequency
for good stability with a high transconductance
device with long wire leads?

The leads that matter are very short indeed. The input transformer,
intended to provide another stage of isolation, was not used at any
stage (you can see the holes for mounting the ISTs on one of the
photographs.

Vacuum valves are inherently high bandwidth devices
with moderately inductive connections to the outside
world; God and the Devil dwell in the details.

On the 417A you can either take care of the details or you can be
monstrously lucky. Only the clowns on RAT think I'm lucky; Pasternack
is in his ninth year of hanging around like a bad smell wishing for my
luck to run out.

All good fortune,

Wu to you too.

Chris Hornbeck

Andre Jute
Visit Jute on Amps at http://members.lycos.co.uk/fiultra/
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