Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Nope...
If B+, screen and filament voltages are applied, and the cathode is
grounded, the finals will draw however much current the power supply can
put out until either the fuse blows, the cathode resistor(s) open up,
the plate choke opens up, or the tubes melt far enough down to
disconnect the plate cap!! :-) Absence of drive has absolutely no
effect on this scenario.
Probably the weakest link is the 15 ohm cathode resistors. They are 1W,
which would reach 2W dissipation at about 360 mA current through each
tube, which is about 230W in each tube. Unfortunately, low value carbon
comp resistors often go DOWN in value when overheated, so the
dissipation could actually go down as they heated, enabling them to last
longer!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Chuck Grandgent wrote:
"Chuck Grandgent" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Alright, here's a thought.
If I have the gain / drive level control all the way down (counter
clockwise), and the bias were to fail, would my finals be safe ?
Chuck, K1OM
On Dec 16, 2007 11:17 AM, Garey Barrell <[EMAIL PROTECTED]> wrote:
Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Geoff -
No, the problem in this case is purely the result of the finals
"saturating" (to use a SS term) when there is no grid bias applied.
They do NOT have an I-DSS like a FET either!
Put another way, a SS final is normally "off" with no bias, and a tube
final is normally "on".
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Geoffrey S. Mendelson wrote:
"Geoffrey S. Mendelson" <[EMAIL PROTECTED]> made an utterance to the drakelist
gang
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On Sun, Dec 16, 2007 at 10:24:48AM -0500, Mike Bryce wrote:
Do you want to put the protection inside the power supply?
That brings up a question in my mind. Is the actual cause of damage
to the finals the failure of the bias circuit, or is it due to the
finals being overdriven? In that case wouldn't a device in the power
supply which limits the power supplied to the rig to a safe level
be a better idea, or an "overdrive" cut out?
For example, in a transitor rig there is often some sort of
protective circuit to prevent the finals from being destroyed
if they draw too much current. Many Japanese rigs use limiters
o "fold back" circuits.
Ten-Tec does not use one, instead they includea saftey cut out in their
power supplies.
Could a similar circuit be devised and used in this case?
Geoff.
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