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<p class="MsoNormal"><span style="color:black">So careful with Heathkit SB-200 or SB-220 they were not designed for AM. If you reduce drive power to 20% of normal (15-20W max) then run AM, FM or RTTY you will still have to watch your amp for over heating.
HK’s are class B not class C amps which are designed for AM and FM transmission modes. HK did not put in CCS transformers and fans do not cool well when operated in those modes.
<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="color:black">73’s<o:p></o:p></span></p>
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<p class="MsoNormal"><b>From:</b> Heath <heath-bounces@puck.nether.net> <b>On Behalf Of
</b>RAY FRIESS<br>
<b>Sent:</b> Tuesday, May 15, 2018 3:32 PM<br>
<b>To:</b> Kenneth G. Gordon <kgordon2006@frontier.com>; Bill <hony53@aol.com><br>
<b>Cc:</b> Heath@puck.nether.net<br>
<b>Subject:</b> Re: [Heath] DX40<o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:10.0pt">I would be interested in tune up procedure for the SB 200 in AM mode.<o:p></o:p></span></p>
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<p class="MsoNormal"><i><span style="font-size:10.0pt">Sent from my Verizon LG Smartphone</span></i><span style="font-size:10.0pt"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.0pt">------ Original message------<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt">From: </span></b><span style="font-size:10.0pt">Kenneth G. Gordon<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt">Date: </span></b><span style="font-size:10.0pt">Tue, May 15, 2018 3:02 PM<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt">To: </span></b><span style="font-size:10.0pt">Bill;<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt">Cc: </span></b><span style="font-size:10.0pt"><a href="mailto:Heath@puck.nether.net">Heath@puck.nether.net</a>;<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt">Subject:</span></b><span style="font-size:10.0pt">Re: [Heath] DX40<o:p></o:p></span></p>
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<p class="MsoNormal">On 15 May 2018 at 1:18, Bill wrote:<br>
<br>
> Hi Mike and Ken.<br>
> <br>
> Yes, I will be replacing the 6146, but should I get a 6146 OR a 6146A?<br>
<br>
Doesn't matter. Any of the 6146 family will work fine in the DX-40. Even the 8298, and 6293
<br>
(if you can find one) are quite suitable. The DX-40 is not picky as far as final amp tubes are
<br>
concerned as some of the later rigs are. You should avoid the 12.6 V filamented versions,
<br>
though, unless you change the filament voltage (!)<br>
<br>
> I have a copy of the original schematic and the build manual. They both show a 0.1uf cap.
<br>
> Physically it is across from the 2 power supply electrolytic. However, on the DX 40 I acquired that
<br>
> capacitor has been replaced with a 30uf cap.<br>
<br>
Hmmmm....I am still at a loss as to where this is. Your description doesn't help me much.<br>
<br>
On my schematics, I find only one 0.1 uFd 600 V capacitor, labeled C-3, which is in the
<br>
modulator circuit. One end is connected to the junction of three 470 K resistors, and the
<br>
other end goes to ground. If that one was replaced by a 30 uFd capacitor, I would think that
<br>
it would, essentially, badly effect the audio coming from the 12AX7 speech amp by severely
<br>
reducing "lows". I suspect that who ever changed it, had some hum in the transmitted audio
<br>
and thought that by increasing that capacitor to 30 uFd, the hum would be reduced.<br>
<br>
If that was my rig, I'd change that back to 0.1 uFd 600 V.<br>
<br>
Usually, hum in the audio is caused by poor shielding of the mic cables.<br>
<br>
In any case, that capacitor is not going to effect the transmitter when in CW so unless you
<br>
intend to use the DX-40 on AM, you can safely ignore it.<br>
<br>
BTW, such controlled-carrier AM rigs as the DX-40 make excellent drivers for a <br>
grounded-grid linear amp for AM, such as the SB-200 or SB-220.<br>
<br>
Ken W7EKB<br>
<br>
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