[AMPS] Filaments

[Rich Measures]

>> From:          Rich Measures <measures@vc.net>
>
>> >Every amplifier has pluses and minuses Herb. The reason for this is 
>> >cost.
>> >
>> I agree, Mr Rauch, cost is a factor.  However, there are exceptions.  
>> Applying more than Eimac's rated voltage to the filament or heater is 
>> definitely a minus.   {DTR-2000, TL-922, some SB-1000s, Sigma, and 
>> several models of Henry Radio amplifiers}  I do not see how this is 
>> related to cost. 
>
>That's right Rich, too much (or too little) filament voltage is 
>stupid. It cost nothing at all to get the transformer right.
>
>I just measured a new Ten Tec 811 amp, and the filament voltage is 
>nearly 7 volts with 117 volts of line voltage.

Avg. electric-mains/line potential in many parts of the country is 120v.
>
>Would I worry about it? Probably not. 
>
>The tubes will be gone from hammering the 65 watt anodes long before 
>the filaments fail. 
>
However, if the owner uses a tuning pulser, there won't be any holes in 
the 811A anodes.  Using relative emissive life = (e2/e1)^23.4,  the 
filaments would last (7v/6.2v)^23.4 = 17.1 times longer if they were 
operated at 6.2v instead of 7v.  

>> Allowing more inrush current to a 3-500Z filament than Eimac's spec. is 
>> definitely a minus.  {TL-922}.  The easy fix was to specify a 
>> current-liniting core for the filament transformer.  Cost seems to be an 
>> unlikely factor here.  
>
>You overlook the fact the filament is a series string.

Not so.  The measurement was made for the inrush current in one tube, not 
the sum of the currents in both tubes.

> The equalizing tap does NOTHING during start-up, because the tubes cold 
filament 
>resistance is much lower than the ESR of the equalizing winding.

There is no "equalizing winding" in the 922's filament transformer.
>
>What happens on start-up in the 922 is the thinnest and weakest area 
>of the weakest tube gets the most start- up inrush power. It's like 
>turning on one of the old AC-DC five tube radios. The weakest 
>filament gets all the dissipation, it gets hotter quicker, its 
>resistance increases faster than the other filaments, and it 
>dissipates even more power.
>
Not in my 922.  The voltage across each filament measures virtually the 
same.  

>Series filaments are problem enough in Christmas tree lamps, 
>they were problems in cheap-o five tube radios. They are even worse 
>in an amp with 150-200 dollar tubes.    

The 10v filament transformer CT is connected to the midpoint of the two 
series-connected 5v filaments.  Henry Radio uses the same series filament 
circuit.  IMO, there is nothing wrong with the practice, provided the 
operating voltage across the filaments is held to c. 4.8v, and the max. 
inrush current spec. is not exceeded.  .  .   Curiously, on page 71 of 
the 9/94 issue of *QST*, Mr. Rauch recognized Henry Radio as one of our 
amplifier "experts". .  

Rich---

R. L. Measures, 805-386-3734, AG6K   


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