[AMPS] Misc. Oscillations

[Joe Subich, W8IK]

> From: Mike Sawyer

> 5% over voltage on a tube drastically reduces its life. That's 
> not Measure's word, look at any RCA tube manual written from 
> about 1938 on. 

I'm not arguing that improper filament voltage doesn't shorten 
tube life or reduce RF output.  Simply, given the design of most 
commercially available amateur amplifiers it would not be safe 
to include a filament rheostat. 

> High voltage doesn't have to be enabled to adjust filament voltage. The 
> voltage could be taken from small probe pick-up on the side or rear. 
> The user could utilize his own DMM. No extra meter would be necessary. 

The vast majority of amateur amplifiers today use a single transformer 
for all voltages (Alpha/Power, Commander, QRO, Ameriton, etc.).  If 
power is applied for the filament, HV is applied!  

Unless the probe pickups were carefully decoupled with chokes and 
capacitors that did not introduce any stray reactance into the 
amplifier input they would be useless and their presence would 
cause significant input SWR and/or the reading at the "test points" 
would be suspect. 

> If I had an amp that used an 8877, I would squeeze every last watt out 
> that I could before having to replace it. Most people woud too, and 
> given the oportunity, would adjust the filament voltage if it were 
> made available

I would also.  However, I could certainly install a filament rheostat 
if my amplifier had a separate filament transformer ... but that 
assumes I had a homebrew amplifier to start with (and, ergo, I wasn't 
an appliance operator). 

> It sure would have been nice to have one my RCA amp since I beat my head
> against the wall and found LOW filament voltage the cause of reduction in
> power by half, (2 X 3-400Z's with filaments at 3.48VAC), and that was by
> accident.

I did not know that RCA built AMATEUR amplifiers!

> Sorry Joe, but you are using a sieve to carry water.

Sorry Mikey ... you're barking up the wrong tree. 

73,
   ... Joe Subich, W8IK
       <W8IK@Subich.com>
       <www.qsl.net/W8IK>
 


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