[Heath] SB-110A

[Kenneth G. Gordon]

On 23 Apr 2013 at 11:18, Dave Edwards wrote:

> Some of the old Heathkit HP23's had a toggle switch on it to switch
> bias voltage. ...Not sure if it was the 23 or 23A.

It was the 23A.

I added such a switch to my first 23. It's easy to add to the models without 
one.

One thing you should watch is that the first model, the HP-23's +HV DC 
output was noticably lower than in subsequent models. Something like 40 V 
or more lower under load. 

The reason is that the secondary AC output voltage was 266 VAC in the first 
model, and 282 VAC in all subsequent models. But you have to find this out 
either the hard way (like I did) or look closely at the schematics.

This 16 volt difference translates to 45 volts difference (16 * 2.828) after 
getting through the voltage doubler supply, thereby making the +HV under 
load low enough to effect the ease with which the parallel 6146s can reach 
full output.

However, all models are rated the same, 820 VDC no load, on the HV. This 
rating, is however, not true for the first model. Instead of 820 VAC, the output 
would be 775 VAC no-load at best.

Also, two of the later models, B, and C, did NOT include a 6.3 VAC output 
via a center-tapped 12.6 VAC winding. .

The "best" model is the HP-23A. That one has the higher +HV and does 
include 6.3 VAC, the +LV switch, and adjustable bias, and is wire-able for 
both 110 and 220 VAC input, therefore making it the most versatile model. 

The "B" model did away with the center-tapped filament transformer, and the 
adjustable bias, and the "C" model did away with both of those, plus the low 
voltage switch.

The first model also had a pilot light which was done away with in all 
subsequent models, but was 110 VAC input only.

Ken W7EKB