[Heath] SB-100 TX audio gain question

[Kenneth G. Gordon]

On 27 Dec 2015 at 15:02, Alexander N. Gerli wrote:

> 
> Hi, Folks!
> 
> My SB-100 is working fine, with one little exception: to have any TX 
> audio out, I need to have the MIC/CW Level set virtually max. 
> Checked the output waveform on my SB-610 scope, and it appears 
> clean. The tubes in the mic audio chain are good, so I'm not certain 
> what may be up. Seems as if the level should be optimal around 50% 
> or a little greater, but not maxed out.
> 
> Anyone have a suggestion where I might start looking?

Sandy:

First of all, the mic amp tube in that rig is V-1A/B, a 6EA8. 

If you measure the plate voltage on that tube, pin 6, you may find that it is 
extremely low....like 40 VDC or so. Check R-1, 100 K. If that is OK, check 
C-2 which may be either shorted or open. 

Usually, in most circuits, if plate voltage is very low, the tube may be drawing 
excessive current, but as I remember it, Heathkit PURPOSELY made that 
circuit so that the plate voltage to the 6EA8 was very low. I don't know why.

You can "adjust" the value of R-1 to give you higher plate voltage at that 
point. You could also "adjust" the value of R-2, the 150 K screen voltage 
dropping resistor, to increase the gain of the stage a bit.

What I did with all of my SB transceivers when I had this problem was to 
simply replace the 6EA8 with a 6GH8A, which has a lot more gain in the 
pentode section. It plugs right in with no mods necessary.

However, the Heathkit rigs were originally designed for one particular mic 
which has a lot of gain, the Shure 444, I believe. Most other mics have much 
lower gain than the Shure.

After the change to the 6GH8A, I was able to set my mic gain at about 50% 
with any mic I chose to use.

Ken W7EKB