[Amps] Alpha 77Dx Filament

[N7SC]

Hi Paul...
One benefit of isolating the filament circuit and adding the choke is 
that you can connect the cathode to the filament and use  most tubes 
that have shorts between those two elements. Also, there is less of a 
possibility  of damaging the filament windings with that type of short.
That much voltage on the filament is real trouble and really shortens 
tube life. If your original issue turns out to be a filament/cathode 
short...that would likely be the cause. I run my 8877's a bit under 
5vac....I just adjust it a little above the voltage where tube emission 
starts to drop. Most of my 8877's are at least 20yrs old and still have 
full output...even on 144mhz.
Steve

Paul Christensen wrote:
Just thought about another possibility...

If I add an RF Parts bifilar filament choke to the 77Dx, there may be 
just enough series IR loss in the choke to get the 8877 back into the 
specified filament operating range of 5.0V (+/- 0.25V).

Paul Christensen wrote:
> Just thought about another possibility...
>
> If I add an RF Parts bifilar filament choke to the 77Dx, there may be just 
> enough series IR loss in the choke to get the 8877 back into the specified 
> filament operating range of 5.0V (+/- 0.25V).
>
> The 77Dx grounds one side of the 8877 filament to the chassis.  If I add the 
> choke (and use balanced bypass caps to ground), I would like to lift the 
> ground (like done on the Ameritron AL-1500) and run a dedicated wire to the 
> other side of the tube's filament connection.  Is there any benefit to 
> grounding one side of the 8877 to the chassis in GG configuration?  I always 
> thought that the AL-1500 was designed better than the 77Dx in the filament 
> area.
>
> Paul, W9AC
>
>
> ----- Original Message ----- 
> From: "Paul Christensen" <w9ac at arrl.net>
> To: "Amps Amps" <amps at contesting.com>
> Sent: Thursday, January 22, 2009 1:45 PM
> Subject: Alpha 77Dx Filament
>
>
>   
>> Looking for opinions on the subject of 8877 filament voltage...
>>
>> Due to high AC line voltage here (~ 127/254 VAC), my 8877 filament sees 
>> approximately 5.75 Vrms (as measured on a Fluke 189 True RMS DVM).   The 
>> 8877 is rated by Eimac to run at 5.0v (+/- 0.25V).  Eimac specs filament 
>> current at 10A.
>>
>> Since the filament current is significantly less than it would be on a 
>> directly-heated cathode, I am considering bringing the filament voltage 
>> down 0.75V by using a series 0.075-ohm @ 20W wirewound resistor with 
>> integral chassis-mount heatsink on the secondary side .   The resistor 
>> would bring the filament down to 5.0V and dissipate about 7.5 watts of 
>> heat in the process.  However, if I can find a nearby mounting hole to add 
>> an angled aluminum mounting bracket, I could use a 0.1-ohm @ 25W rheostat.
>>
>> I suppose yet another option involves series limiting the common 
>> HV/filament primary side to bring the line voltage back down to 235VAC or 
>> so.  Whatever I use, I need to make the mod reversible without hacking 
>> things up and affecting the re-sale value of the 77Dx amp.
>>
>> Paul, W9AC 
>>     
>
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