[Amps] Ameritron (et. al.) filament voltages

[Joe Subich, W4TV]

> I put the question to the list in hopes that someone could 
> explain why commercial amps always seem to be designed for 
> higher filament voltages. 

I don't know that you can say that commercial amps are 
always designed for higher filament voltages.  They are 
designed to handle a certain minimum line voltage but 
many areas regularly exceed that ass utilities push the 
higher limit of the tolerance band to minimize current 
and losses in the system. 

An amplifier might easily be designed for 220 V (common 
residential voltage 20-30 years ago) with the filament 
transformer/winding for a 3-500Z was designed for 5.1V 
(just a bit extra to account for losses in a filament 
choke).  Now put that in service today in an area where 
the line voltage is running 244-245V and that same tube 
will see 5.6V.  

The problem with amateur amplifiers, unlike broadcast 
transmitters they simply don't have multi-tapped primaries 
on their transformers.  In the RCA BTA-1R2 for example, 
all of the transformers were tapped for 208 and 240 
with additional +/- 11V taps.  The transmitter could 
be matched to the line voltage within 5V (+/- 2%) from 
197V to 251V.  In addition, I believe the modulator and 
final amplifier supplies included a filament rheostat 
to further adjust filament voltage at the tubes (4 x 
4-400C). 

Transformer taps cost money ... the added cost isn't all 
that significant for a high price broadcast transmitter 
but every amateur manufacturer in the world is looking 
for ways to reduce their parts cost by $50 - $100 (and 
the final list price by $250-$500).  Separate taps for 
105, 110, 115, 120 Volts on dual primary transformers 
for both high voltage and filament/control (plus maybe 
a screen transformer) is a lot of extra cost that most 
hams simply will not accept. 

If the manufacturer built the power supply for the 
highest possible line voltage, filament voltages that 
are 15% below spec will cause as many problems as the 
excessive voltage.  The manufacturer knows the average 
ham won't put 500 hours a year on a set of tubes and 
will never consider filament voltage when they have to 
replace tubes every four or five years.  After all, 
they can always blame "turn on shock" to blame for the 
premature failure. 

73, 

   ... Joe, W4TV 
  
tubes just as easily by  



> -----Original Message-----
> From: amps-bounces at contesting.com 
> [mailto:amps-bounces at contesting.com] On Behalf Of Rob Stampfli
> Sent: Thursday, January 28, 2010 11:25 PM
> To: Alek Petkovic
> Cc: amps at contesting.com
> Subject: Re: [Amps] Ameritron (et. al.) filament voltages
> 
> 
> Hi Alek,
> Thanks for the response.  Personally, I agree with you that 
> it is better to keep the filament voltages in check for a 
> number of reasons.  However, here's a pointer to a post (from 
> this list) that takes the opposite
> perspective:
> 
> http://lists.contesting.com/archives//html/Amps/2007-07/msg00367.html
> 
> Tom is Tom Rauch, who had a hand in the design the AL-80B, 
> and who certainly knows more about building amps than I ever 
> will.  Note he's not disputing the Eimac claim, only 
> counter-claiming that the hazards of ICAS operation will 
> typically kill the tube before an over hot filament will.  He 
> may well be right, but I still can't see any advantage to 
> running excessive voltages on the filament.  That's why I cut 
> mine back.
> 
> I put the question to the list in hopes that someone could 
> explain why commercial amps always seem to be designed for 
> higher filament voltages. The only advantage I can see is 
> that it gives the amp more headroom in the event of low line 
> voltage or soft line voltage that falls excessively under 
> load.  Does anyone know of any additional advantages?
> 
> Inquiring minds...
> Rob / KD8WK
> 
> On Fri, Jan 29, 2010 at 06:05:43AM +0800, Alek Petkovic wrote:
> > G'day Rob
> >
> > It is not any "camp" that makes the claim of shortened 
> emissive life 
> > due
> > to high filament voltage. It is Eimac, in its "Care And 
> Feeding" book. 
> > For every 3% increased in filament voltage, the life is 
> shortened by 50%. 
> > In the case of Ameritron's 10%, one would expect around 12% 
> of normal 
> > tube life. (Didn't someone recommend Ameritron here a 
> couple of days ago 
> > on a different thread? Ha ha)
> >
> > Yaesu have the same problem with the 2100 series amps. Even when the
> > mains input voltage is reduced to the specified 230 Volts, 
> the filament 
> > voltage still needs to come down.
> >
> > Like you, I have made the correction with every amp that 
> has ever had
> > the lid removed in this house.
> >
> > 73, Alek
> > VK6APK
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