[Heath] SB 100 and SB 102

[Kenneth G. Gordon]

On 19 Apr 2012 at 8:12, Glen Zook wrote:

> The SB-100 and SB-101 use a 6AU6 as the receiver "front end" and the
> SB-102 uses a 6HS6 which gives a slight improvement.

More than "slight", in my experience and testing...

>  Unfortunately,
> the 6HS6 tubes are scarce these days and most have gone back to the
> 6AU6 when that tube goes bad...

...which they do very quickly since they are running beyond their maximum 
ratings...

I have found that the 6AH6 will replace either the 6AU6 or the 6HS6 and will 
work better than either of the other two tubes.

First of all, the 6AH6 is far more robust, far more easily available, and much 
cheaper than the 6HS6, and it has identical filament current requirements 
and almost identical transconductance.

It provides almost identical enhancement of RF gain, and the much lower 
noise that the 6HS6 does.

I use 6AH6s routinely in all SB/HW all-banders that I restore.

Be aware that since the filament string in the SB-100/101/102/HW-
100/101/101W are connected in series-parallel, that BOTH the receiver RF 
amp and the 1st receiver mixer must be exchanged together at once, 
otherwise the filament string will become unbalanced.

The 6AU6 draws less filament current than either of the other two tubes 
mentioned, and if you use a 6AU6 as the 1st receiver mixer, and either the 
6HS6 or the 6AH6 as the RF amp, or vice versa, the filament string will 
become unbalanced.

Since the receiver RF amp and the 1st receiver mixer are on opposite sides 
of that series-parallel filament string, replacing both of those two tubes with 
tubes with higher filament current requirements will enable the filament string 
to remain balanced.

Also, don't forget that in the SB-102 one of the tubes was changed from a 
single-section tube to a dual-section tube, and that other section was used 
as an optional crystal oscillator for crystal control of the transceiver 
frequency.

Ken W7EKB