[Heath] DX40

[Kenneth G. Gordon]

On 15 May 2018 at 1:18, Bill wrote:

> Hi Mike and Ken.
> 
> Yes, I will be replacing the 6146, but should I get a 6146 OR a 6146A?

Doesn't matter. Any of the 6146 family will work fine in the DX-40. Even the 8298, and 6293 
(if you can find one) are quite suitable. The DX-40 is not picky as far as final amp tubes are 
concerned as some of the later rigs are. You should avoid the 12.6 V filamented versions, 
though, unless you change the filament voltage (!)
 
> I have a copy of the original schematic and the build manual. They both show a 0.1uf cap. 
> Physically it is across from the 2 power supply electrolytic. However, on the DX 40 I acquired that 
> capacitor has been replaced with a 30uf cap.

Hmmmm....I am still at a loss as to where this is. Your description doesn't help me much.

On my schematics, I find only one 0.1 uFd 600 V capacitor, labeled C-3, which is in the 
modulator circuit. One end is connected to the junction of three 470 K resistors, and the 
other end goes to ground. If that one was replaced by a 30 uFd capacitor, I would think that 
it would, essentially, badly effect the audio coming from the 12AX7 speech amp by severely 
reducing "lows". I suspect that who ever changed it, had some hum in the transmitted audio 
and thought that by increasing that capacitor to 30 uFd, the hum would be reduced.

If that was my rig, I'd change that back to 0.1 uFd 600 V.

Usually, hum in the audio is caused by poor shielding of the mic cables.

In any case, that capacitor is not going to effect the transmitter when in CW so unless you 
intend to use the DX-40 on AM, you can safely ignore it.

BTW, such controlled-carrier AM rigs as the DX-40 make excellent drivers for a 
grounded-grid linear amp for AM, such as the SB-200 or SB-220.

Ken W7EKB

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