tubes in the HW101

[Kenneth G. Gordon]

On 27 Feb 2009 at 17:30, Dick KF4NS wrote:

> While I was doing some tube inventory, I came across what appears to
> be a sub for the 6HS6. I have several 6AH6 tubes which, depending on
> the cross ref direction, can be used but have never heard of anyone
> trying it.

Well, I have been suggesting using the 6AH6 instead of the 
6HS6 for a number of years, and I always use those in my 
HW-101 and SB-102.

Also in my VF-1s.

There are two important points to be considered in the 
substitution: 1) the transconductance (sometimes 
inaccurately called the mu), and 2) the filament current.

The 6AH6 is identical with the 6HS6 in the filament current 
department. 

However, since, in the SB/HW transceivers all the filaments 
of all the tubes in those rigs are in a series-parallel 
arrangement in order that the rig can use 12.6 VAC (for 
mobile use), instead of 6.3 VAC as the filament voltage 
source, you MUST be careful which tubes you use where.

In the case of the HW/SB all-band transceivers, the 6AU6, 
which were originally used by Heathkit as the receiver RF 
amp and 1st mixer, has a lower filament current 
requirement than either the 6HS6 or the 6AH6.

Therefore, if you replace ONLY either the RF amp, OR the 
1st receiver mixer, the filament system will become 
unbalanced.

This means that if you wish to use either the 6HS6 or the 
6AH6 in your rig in place of the 6AU6, you MUST replace 
BOTH the RF and Mixer, since those two tubes are on 
opposite sides of the filament system and thus balance is 
retained.

Secondly, the transconductance issue: the 6HS6 has the 
highest transconductance rating of any tube with the same 
base diagram and (close) interelectrode capacitances as 
the 6AU6. 

The 6HS6 was chosen by Heathkit to replace the 6AU6 
soley due to its high transconductance. 

High tranconductance in the RF and 1st mixer stages 
results in much reduced internally generated noise, and 
higher gain, at the cost of reduced dynamic range.

However, the reduced dynamic range does not appear to 
be a significant problem in the use of our transceivers, but 
the higher gain and reduced noise makes our rigs more 
"transparent" feeling and significantly more enjoyable to 
use.

Now the difference in transconductance ratings between 
the 6HS6 and the 6AH6 is very slight, the 6AH6's being 
slightly lower.

However, the 6AH6 is both a far more rugged tube (the 
6HS6 is running at the ragged edge of its maximum 
ratings), FAR less expensive, and far more easily obtained.

I routinely measure a sensitivity of 0.1 microvolt for a 
completely readable signal with an HP-8640B signal 
generator with my proper aligned HW/SB rigs.

> 
> The main differences appear to be the Mu, gain and interelectrode
> capacitance. Not sure if any would dramatically affect the operation
> in the assigned purpose for the 6HS6s.

They do not cause any measurable differences, in my 
experience.

> Any opinions?

See above.

> Oh, and the 6AH6
> is much cheaper today.

Tell me about it! :-)

Ken Gordon W7EKB

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