[Boatanchors] Line Voltage issues

[D C _Mac_ Macdonald]

I "suspect" that there is a world of difference between receiving tubes and high power transmitting tubes regarding any tolerances in filament voltages. 
 
Receiving tubes are installed in a large variety of devices with varying quality.  Consequently, those tubes are DESIGNED to operate safely over a fairly wide range of applied voltages. 
 
On the other hand, high power transmitting tubes are generally used in devices with a higher dependence upon being operated within a quite small range of filament voltage.  I think I have read that a 10% overvoltage may cut the life of a filament by 50%!  My Dentron DTR-2000L's 3CX1500A7 went bad in a terribly short period of operation with line voltage at around 126-127.  Investigation after the failure showed the applied filament voltage of around 5.15-5.20 or so.  Addition of a homemade wire resistance with three twisted strands of stainless steel wire brought the filament voltage down to 5.0 and the filament of the replacement tube is still good. 
 
It definitely pays to operate any transmitting tube at or a bit below the manufacturer's rating. 
 
* * * * * * * * * * * 
* 73 - Mac, K2GKK/5 * 
* (Since 30 Nov 53) * 
* k2gkk hotmail com * 
* Oklahoma City, OK * 
* USAF & FAA (Ret.) * 
* * * * * * * * * * * 
 
 
 
> Date: Tue, 6 Jan 2015 23:57:12 +0000
> From: wa1rkt at comcast.net
> To: boatanchors at mailman.qth.net
> Subject: Re: [Boatanchors] Line Voltage issues
> 
> 
> From: "Robert Spooner" <rls19 at psu.edu>
> Sent: Tuesday, January 6, 2015 5:40:37 PM
> >>
> >> The problem with autotransformers such as the Variac
> >> is that they are high enough impedance that you may
> >> see inordinate voltage fluctuation with changes in
> >> load unless the current rating is high relative to
> >> the current draw.
> 
> I have a 15-amp variac that I use on the bench for testing and bringing up boat anchors, and I have found that to be the case.  When I set the variac for 110 volts and transmit into a dummy load with the KWM-2, I see a definite dimming of the dial lights, considerably more so than when just plugged into the wall.  I haven't actually measured the voltage drop but it is significant.
> 
> >> My favorite solution is to use a filament transformer
> >> to buck the higher voltage down to the rated voltage
> >> for the equipment.
> 
> Have you found that the voltage drop through a bucking transformer, especially near its current-carrying limit, is significantly less than a similarly-sized variac?
> 
> Yesterday I bought one of those Rat Shack DMMs that someone here suggested, that will save the maximum and minimum readings, and between last night about this time and now, I set it to store the max and the min and plugged it into the wall.  Over 24 hours' time I saw a stored maximum of 122.9 and minimum of 114.3.  Another DMM was connected at the same time to provide real-time readings, and every time I looked the reading was between 117 and 119, meaning that the 114.3 and the 122.9 were probably momentary excursions.
> 
> In receive mode on the KWM-2 powered by a 516F-2 power supply in stock configuration, at 118 V in I measured 6.17 volts at the filaments of several of the tubes.  LV at the power supply measured +320VDC (haven't yet found a good place in the transceiver to measure it) and HV at the top of the RF choke in the HV cage measured +900 VDC (on receive, with no load on the HV).  So, while LV and HV are a little high (they should be nominally +275 and +800), looks like filaments are right where they need to be.
> 
> Not real sure I need to do much, here.
> 
> Rick WA1RKT