|[AMRadio] AMRadio Digest, Vol 73, Issue 11|
ranchorobbo at gmail.com
Fri Feb 12 13:55:10 EST 2010
Jim, where do you get a 4D32 these days?
> First off, I have an HT-20 on the air and it does well on AM, so the problem is not generic to the transmitter design. I have not used a light bulb for a dummy antenna, at least not recently, but my average-reading watt meter does not bobble during modulation. I would expect a light bulb to get brighter during modulation, not dimmer, which is something you also know.
> From the list of things that Larry suggested, above, I would not think that #1 would not be a cause of this particular problem. Human voice audio is not symmetric, so you can achieve more full modulation by selecting the better phasing connection, but neither phasing connection should cause "downward modulation." #5 is also not likely to be the problem if the rig has the original modulation transformer. The other suggestions are all likely causes. Another one might be a very flat (low gain, low emission) 4D32 final amplifier tube.
> Let me know how much final grid drive you are running and what the loaded plate current is for the final amp. The manual recommends 260 ma plate current for CW and 320 ma for phone, but I don't run any more than 200 ma for either phone or CW. 4D32's are too expensive! It also recommends 10 ma for grid current. The ARRL Handbook ratings for a 4D32 are a maximum of 240 ma for CW and 220 ma for phone, with 12 ma of grid current for the CW condition and 10 ma for phone. I can compare your operating parameters to the ones I'm running currently on my rig. Mine is tuned up on 14286 right now in anticipation of the Classic Exchange AM event on Sunday.
> Good luck,
> Jim Hanlon, W8KGI
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