Hello Pierre,
If I remember right, the big iron potted case contains a device is called a
"saturable reactor" (or something like that) and is used to regulate the AC
voltage applied to the vertical amplifier tubes. I think this was an
attempt to minimize the effects of cathode interface which looks like a
spike on the leading edge of a fast-rise square wave. Actually, it is a
DECREASE in low frequency gain of the vertical amplifiers.
In a scope without regulated filament voltage, cathode interface is always
worse at low line voltages which means lower filament voltage and therefore,
lower filament temperature. If you observe a fast-rise square wave while
you lower the line voltage on a scope with bad cathode interface, the
leading edge spike seems to grow over about one minute of time. If you look
very carefully, it is actually the trailing edge, or flat portion of the
square wave, that is DECREASING in amplitude over time as the filaments cool
down. You can verify this by simply plugging in the scope calibrator and
watching the gain of the vertical change as the calibrator signal appears to
decrease in amplitude. No spike seems to grow on the leading edge because
there are no high frequency components in the scope calibrator signal.
There is actually a filament voltage adjustment in the 555 power supply and
the list of equipment needed to calibrate a 555 includes an iron vane AC
voltmeter to accurately set the AC filament voltage. I think the 517 is the
only other scope Tek made that has adjustable filament voltage . . . but I
could be wrong about this since it has been years since I looked at my
517's.
Stan
_____
From: TekScopes@... [mailto:TekScopes@...] On Behalf
Of coresta
Sent: Saturday, December 30, 2006 11:53 PM
To: tekscopes@...
Subject: RE: [TekScopes] Re: Bringing up a 555
Hi Stan,
As i see this new thread, i always asked myself about the use of Kalotron in
the PSU of the 555 ? What's the big iron potted case too ?
I broke a pair of those PSUs many years ago and those are somewhere in the
basement . I never saw the schematic .
I have a 551 working that doesnt contain these parts .
Ah, happy new year to all of you !
And long life to our old old tubed scopes ;-)
Pierre
