SB-301/SB-401 repair - one more thing.

[Glen Zook]

The cathode resistors are 10 ohm and there is one from each of the 3 connections of the cathode of the 6146 tubes to ground.  Each 6146 has the cathode connected to 3 separate pins on the base.  This makes a total of 6 resistors.

Replace the original 1/2 watt 10% resistors with 1 watt 5% (or better) resistors.  The modern 1 watt resistors are not much larger in physical size than the original 1/2 watt resistors and by using 1 watt resistors the value is considerably less likely to change over time.

Remember that the SB-401 actually reads cathode current when the meter switch is in the "plate" position.  To get the actual plate current you have to subtract the screen current which averages around 12.5 mA per tube.  That means you have to subtract 25 mA from the reading to get the actual plate current.

I automatically replace these meter shunt resistors in both the Heath and Collins transmitters because in at least 90% of the transmitters the resistors have gone high in value.  This, in turn, causes the meter to read a higher value than what the actual current is.  The Collins S-Line and KWM-2 series use 12 ohm resistors instead of the 10 ohm resistors in the Heath SB-Line.  Remember, as the shunt resistors go higher in value more current goes through the meter instead of through the shunt.  Therefore, the meter will read higher than the actual calibration.

Glen, K9STH

Website:  http://k9sth.com


--- On Sat, 10/2/10, Kenneth G. Gordon <kgordon2006 at VERIZON.NET> wrote:

One other matter that might be considered here:
 
In the SB-401, I believe the final amp circuit is very close to the  same one that Heathkit used in all of their SB/HW 5-band tube- type transceivers which use a pair of 6146s. 
 
That circuit includes a bunch (6?) 1 ohm resistors in parallel in the  cathode to meter the plate current. In several of those rigs I have  worked on, those resistors have drifted very high, throwing the plate current indication, and thus the bias level, way off.
 
If you cannot achieve normal full-load plate current (~250 mA) with known good finals, check those resistors.
 
Be aware that if they ARE off, adjusting the bias to show 50 mA will instead really result in much higher plate current, and the stage will no longer be operating at class AB1, but instead much closer to class A.
 
Although this might be somewhat more linear, it is hard on the finals as they have to dissipate more heat.


      

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