[AMPS] filament voltage questions

[Rich Measures]

>
>Scenario:
>
>I have a Fluke 77 DVM (not RMS-reading, AFAIK).  I show 243.5 VAC on my
>dedicated 20A, 240V line in the shack, amp unplugged, no other load on this
>branch.  Run is about 45 feet of 10/3 copper Romex to the breaker panel,
>adjacent to the meter.
>
>I have 400 amp service at my home, with no other home sharing the service
>transformer.  I split the feed to two separate 200A panels, and run about
>1/2 the house off each one.  The local power substation is 1/2 mile away.
>Pretty stiff supply, doesn't budge much.
>
>After I send this message, I am going downstairs to measure the filament
>voltage on my L7 amp.  I haven't done this before; I probably will
>tack-solder leads onto the fil pins so I can meter the voltage from outside
>the amp, with the amp sitting level, and covers on.  Safety first, you know.
>
>(Why don't amplifiers have filament volt-and/or-ammeters built in, or at
>least test points for this?  Tubes ain't cheap anymore.) 
>
A great mystery.  Art Collins certainly knew the value of measuring fil. 
volts.  An amplifier without a fil. voltmeter is like a car without a 
speedometer.  Art Collins also provided a means of adjusting fil. V. 
(usually a rheostat).  ///  I saw a post where someone asked why current 
manufacturers don't provide such things.  Perhaps the answer is that they 
are not interested in providing value beyond the warranty period.  

>I intend to look at the voltage on the tube side of the choke, to account
>for any I2R loss in that component.
>
>So, there's the situation.  Here are the questions:
>
>1.  I assume the Fluke measures P-P voltage, and is this an issue?  I can't
>think of a reason why it would be, but... perhaps the 3-500Z 5-volt
>filament spec is in RMS volts?
>
yes

>2.  Suppose that the filament voltage is a bit high.  What are some good
>ways to cut it down?  I was thinking a small resistor in series with the
>primary, as the L7 has a separate fils transformer, and the primary wires
>are easy to get to.
>
...a good way.  Another is to use a rheostat in series with the fil. 
trans. primary, as suggested in "The Nearly Perfect Amplifier" (QST, Jan. 
1994).  

>3.  What if it's too low?  ;-)
>
If you have less than c.4.6v, the pep may decrease a bit, however, low 
fil. voltage will not damage the tungsten carbide emissive layer on the 
filament.  
-  later


Rich...

R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures  


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