FPGA VAX update, now DIY TTL computers
ajp166 at bellatlantic.net
Wed Nov 9 09:46:52 CST 2005
>Subject: Re: FPGA VAX update, now DIY TTL computers
> From: Paul Koning <pkoning at equallogic.com>
> Date: Wed, 09 Nov 2005 09:58:19 -0500
> To: cctalk at classiccmp.org
>>>>>> "Chuck" == Chuck Guzis <cclist at sydex.com> writes:
> Chuck> ... the other was a low-voltage tube
> Chuck> used in automobile receivers that were specified for 12.6
> Chuck> volts on the plate (e.g. 12AE7 dual triode). These would be
> Chuck> coupled with a solid-state driver and power amplifier for a
> Chuck> auto radio with no vibrator supply.
> Chuck> ... the second would seem to substantially reduce
> Chuck> the power requirements. To anyone's knowledge were either of
> Chuck> these two components ever used in digital applications?
>I doubt it for the 12 volt tube case. For car radios they make sense,
>for the reasons you gave. (I built a radio for our camper using
>those, as a boy scout project. Tubes for the RF/IF stages, a very
>early (1971 or so) op amp as AF preamp, and totem pole transistor AF
This is somewhat off topic. To directly answer it however...
The additional components of the power supply (vibrator or transistor driven)
are overhead and high drain even for auto use as efficientcy is at best 80%.
12V plate voltage car radio tubes were a considerable savings in drain by
deleting the HV supply. The heater power required for the 12V vs the more
usual high voltage cousins is the same or only slightly higher, some cases
it was lower depends on the specific tube. the real difference was
generating the HV had a standing 10W load that the tubes maybe used
less than 70% of.
>But I don't think the electrical parameters were all that good, given
>the unusually low anode voltage. And I don't see any reason for the
>power consumption to be less. Lower voltage, sure, but the current
>may go up in proportion. And the filaments were still the usual,
>which accounts for a fair chunk of the power.
Correct. The transconductance is lower and the lower max plate current
means lower power and overload thresholds. For computers that also
translates to slower switching speeds. For AM car radio use it's
acceptable but for higher performing radios it really is problematic.
The serious limitation for tubes has always been the heat and power
from the filament rather than the heat and power used for functional
circuits. Computers suffered using them as heat removal and protecting
other components from that heat were the issues. Never minding even with
135A 6.3V filiments used in the subminis say two hundred of them is a
mere 27.A @ 6.3V or 162W of power and no small issue with bussing around
that kind of power. when you consider they may run at 100V on the plate
and maybe 2ma (.2W) and that same 200 (assuming all conducting the same)
is only 40W power dissapated. It's the heaters that are at issue.
Transistor systems removed the heater power and lowered the per device
dissapated power by a typical factor of 5 so 200 transistors were running
in the 8-10W range. The latter is important as now heat is less an issue
and power distribution simplified. With heat being less an issue more
compact and modular systems are easier to design and assemble.
After transistors the second evolution in computers was packaging.
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