6AH6 vs 6AU6

[Kenneth G. Gordon]

On 20 Feb 2008 at 13:53, L.G. Robertson wrote:

> Sorry this was delayed, Carlos, but the ListServ started enforcing an
> email rule it hadn't enforced before, and it's caused a lot of delay.
> 
> Umm...the filament current IS different, according to RCA Tube Manual
> RC-20; ...the 6AU6 is 300 ma, the 6AH6 is 450 ma.  Having said this,
> most filament transformers/windings can handle the extra 150 mils, but
> you might want to check the schematic to make sure.
> 
> Replaced the 6AU6 with a 6AH6 in my TX-1 for some more VFO stability,
> and the filament transformer's been fine for two years now.

150 extra mA really shouldn't make much difference in the 
case of the TX-1, IMHO.

However, you bring up a very important point.

When these are used in such rigs as the HW/SB 
transceivers we have to be careful: in those rigs, the 
filaments of all the tubes are in a fairly complex series 
parallel arrangement, and if the filament current in any one 
tube is very far off specifications, it can badly unbalance the 
entire filament circuit, causing some tubes to have low 
filament voltage while others are too high.

This is why I replace BOTH the RF amp and the mixer with 
the 6AH6. These two tubes, (V-10 and V-11 in the HW-101) 
are on different "sides" of the filament circuit, and therefore 
compensate for one another so that their additional current 
drain does not effect the filament circuit balance.

One is between the 12 VAC source and the "center-tap" 
while the other is connected between the "center-tap" and 
ground.

Ken Gordon W7EKB 

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