[Collins] [collins_radios] Re: help needed

[Carl]

----- Original Message ----- 
From: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
To: <collins_radios at yahoogroups.com>; <geraldj at weather.net>; "Carl" 
<km1h at jeremy.mv.com>
Cc: <collins at mailman.qth.net>; <collins_radios at yahoogroups.com>
Sent: Tuesday, January 31, 2012 1:14 PM
Subject: Re: [collins_radios] Re: [Collins] help needed


>
> ----- Original Message ----- 
> From: "David Knepper" <collinsradio at comcast.net>
> To: <geraldj at weather.net>; "Carl" <km1h at jeremy.mv.com>
> Cc: <collins at mailman.qth.net>;
> <collins_radios at yahoogroups.com>
> Sent: Tuesday, January 31, 2012 3:50 AM
> Subject: [collins_radios] Re: [Collins] help needed
>
>
>> Gentlemen, what wonderful treatise that you presented.  I
>> may use this
>> material in the next Collins Journal if you permit.
>>
>> I would only add that when modifying the conventional
>> power supply from tube
>> rectification to solid-state rectification defeats the low
>> noise floor of
>> the Collins S-Line receiver and also makes these receiver
>> susceptible to
>> spurious oscillations.  Unless, of course, one takes
>> appropriate measure as
>> follows:  I would only add that the line voltage be no
>> more than 115 VAC and
>> if one is inclined to go to solid-state rectification in
>> the power supply,
>> that he reduce the output voltage using a dropping
>> resistor.  It's
>> phenonomel what a difference there is when a 75S-3 or
>> KWM-2, etc. is
>> operating on 110 VAC - for example -  using tube
>> rectification.
>>
>> Using a dropping resistor in the center tap of the
>> secondary or even in the
>> B+ line may seem good "engineering practice" but
>> completely ignores the rise
>> in filament voltage when the transformer "sees" 120+ volts
>> on the primary
>> winding.
>>
>> As everyone knows by now, there has been this rush to go
>> out and buy these
>> solid-state kits from vendors which I will not name.
>> However, when I am
>> asked whether to replace the tube rectifiers in the
>> Collins 516F-2 power
>> supply, I am rather emphatic - NO.  My response is to
>> recommend reducing the
>> line voltage using a thermistor, variac or bucking
>> transformer
>> configuration.
>>
>> IMHO
>>
>> David Knepper, W3CRA/W3ST
>>
>     In the mid-1940's RCA published a graph showing
> filament life vs voltage. The chart was meant for
> transmitting tubes with tungsten or thoriated tungsten
> filaments or for lamps. I don't know how valid it is for
> indirectly heated tubes like most receiving tubes. With this
> in mind the chart shows that a 5% reduction in voltage about
> doubles the life of the filament. The chart goes only to a
> 5% increase but shows a significant decrease in life
> although not as extreme as the increase from reducing
> voltage.
>     Currently (no pun intended) mains power in the USA
> seems to be higher than the design value for most older
> equipment. A few decades ago AC line voltage in most places
> was standardized at about 115 or 117 V, it is now more
> nearly 120V and in some places up to perhaps 125V. The
> reason is probably that line losses are lower as voltage is
> increased so the increase probably puts off having to rewire
> the grid for higher capacity. Of course distribution
> transformers could be used to keep the voltage to individual
> homes constant but there are millions of them and they would
> have to be replaced.
>     Reducing voltage to equipment meant for a lower voltage
> is not as simple as it might seem. Variable transformers
> like the Variac are expensive. Installing a bucking
> transformer requires either modifiying the equipment or
> constructing an external housing. Using a line dropping
> resistor is wasteful of power, creates heat, and the drop is
> dependant on the amount of current drawn. This last is
> important for many pieces of equipment which have variable
> power demand. Among these are transmitters and tranceivers,
> I think a simple dropping resistor is not acceptable for
> either.
>     In the same way a dropping resistor in the B+ supply
> can cause problems with regulation. As pointed out by David,
> a dropping resistor in the B+ does nothing for the filament
> voltage but such resistors are often applied where a tube
> rectifier is replaced with a solid state one. In this case
> the loss of regulation may not be so significant simply
> because vacuum tube rectifiers have an internal resistance
> so that the voltage drop is depenant on the current draw.
> The drop will depend on the particular kind of rectifier and
> on the type of filter. However it can be on the order of 50V
> for HV rectifiers. In comparison a silicon diode has about
> 0.7V drop which is fairly constant with current (but can increase to 1.5V 
> for some diodes under load). In most cases a
> simple dropping resistor will do to maintain the output at
> the right value. since the vacuum tube diode also looks like
> a resistor the regulation will be no worse. Where mercury
> vapor rectifiers are used, as was very common for high
> current supplies in transmitters and similar equipment a
> simple resistor is not suitable because mercury vapor diodes
> also have a constant drop, about 15 volts. Probably the best
> way of compensating is to stack silicon diodes so that the
> drop of each adds to the right value. Since one is usually
> dealing with a high voltage supply this may be a way of
> solving two problems at the same time.
>     One can also use diodes to drop the voltage when
> replacing vacuum tubes with diodes by using a regulator
> diode with suitable voltage drop and current capacity. This
> will also improve the regulation of the supply.
>     An expensive but satisfactory solution to the high line
> voltage problem is the use of voltage regulating
> transformer, like a Sola constant voltage transformer. These
> work quite well but can be mechanically noisy and should be
> operated at near full capacity for reasonable efficiency.
> Where line voltage varies a lot the Sola transformer is
> quite valuable.
>
>
> --
> Richard Knoppow
> Los Angeles
> WB6KBL
> dickburk at ix.netcom.com
>

Eimac has similar filament life specs in Care and Feeding and includes 
external anode tubes a s well as those indirectly heated.

When my AC was approaching 125V and bulbs were short lived I called the 
local utility.
Their response was to change to a different tap on the pole pig about 200' 
away which resulted in a nice 118.5VAC

While hams are thought of as communicators I think many have forgotten how 
to use the telephone!

Carl
KM1H