[Amps] 3-500Z tube problems

[Jim Thomson]

Date: Mon, 13 Jun 2016 14:44:48 -0700
From: Bill Turner <dezrat at outlook.com>
To: Amps group <amps at contesting.com>
Subject: Re: [Amps] 3-500Z tube problems

------------ ORIGINAL MESSAGE ------------(may be snipped)

On Sat, 11 Jun 2016 16:55:55 +0000 (UTC), Jack wrote:

>Also wondering about design additions that you may have included in amps to protect the circuitry from catastrophic failures.? 

REPLY:

Proper meter protection is easy if one understands the basics. Too
often someone just puts a diode or pair of diodes across the meter and
thinks it is protected. Not so.

Typical silicon diodes do not conduct until the voltage across them
reaches about .7 VDC. By the time the voltage across the meter
movement reaches that level, the meter will be toast. 

The answer is to put a small value resistor in series with the meter
and then put the diode(s) across the combination of meter plus
resistor. Simple Ohm's law will tell you how much resistance is needed
to reach .7 VDC at the meter's full scale current. Add about 10-20% to
keep it linear at full scale and you're done. Don't forget to include
the meter's resistance in the calculation. 

This is normally only needed for current meters. Voltage meters rarely
need this kind of protection since voltages do not spike like currents
do.  

73, Bill W6WRT

##  Interesting.  IF  you had a  B+  to chassis arc, the safety diodes  would conduct asap, and assuming one had
safety diodes located between B-  and chassis, the event would over in a split second,  with all the energy   dumped, directly 
between the B+ and B-....... via the safety diode. 

##  The problem is, if you slowly  buried the grid / and /or grid meter to the peg...and also had safety diodes across each meter,
you are correct, the meter would be pegged long before the diodes  across the grid + plate meter conducted.  Typ grid + plate
meter shunts  are a fraction of an ohm, like .09 ohm  etc.    A  1 ohm resistor in series with  a 0-1 Amp  plate
meter  would do the trick.... with the safety diodes across the entire mess.    Problem is... the safety diodes conduct before the
meter has reached full scale, diverting dc plate current through the safety diodes instead of the meter itself,  resulting in inaccurate
plate current meter readings. 

##  If a 1/2 ohm resistor used, the meter would be pegged, b4 the diodes conducted.   if a .7 ohm resistor used, the diodes
would conduct just as the meter is maxed out..and would work.   So the concept works, but the exact value used would have to be
carefully thought out. 

##  I have never used the small resistor value in series with the meter concept.... just safety  diodes across grid + plate meter...
and more safety diodes between B-  and chassis.    For added safety,  on GG amps, I also installed a fast 3AGC fuse, wired between
chassis and positive terminal of grid meter, with a fast grid fuse.   Draw too much dc grid current, 3AGC fuse opens off,  no more dc
grid current.   PO drops to zero.  Amp cant be driven.  Input swr, from xcvr to  amp..rises to infinity,  xcvr goes into shut down mode. 

Jim   VE7RF