The AM Forum

Gentlemen:
     I picked up an old Heath DX-20 at a hamfest. Only getting about 15 watts out CW. Someone replaced the original power transformer with an oversized, old tv type  unit. My key-up voltage is 500 volts on the 6DQ6B plate ; have not checked the key down voltage yet. The key-down screen voltage on the 6DQ6B is only 115 volts. That seems a bit low. Does anyone know what the key-down voltages should be when the rig is fully-loaded, say on 80 or 40 meters?
     The rig keys ok and otherwise sounds good on the air. I'd like to get 30 watts out, which I suspect is about what it should put out. Thanks for checking this out.
                                      ---Marty, KK4RF---

Hi,

Here's a Skizmatic with the Tranny Voltages..:

If you have plenty of grid drive, try replacing the final tube.

Skee-matic shows a 600-0-600 xfmr running FWCT, so you should have about 800v no load, right? The replacement xfmr. seems to be a little light on the potential it can produce. I think max recommended screen voltage on the 6DQ6's is 150v.


73, Phil   

The schematic shows a choke input type filter so the voltage will be around 480 or so when loaded.   

Joe, W3GMS 

Gentlemen,
     Thanks for all the info, ideas and the schematic. I need  to work on getting the screen voltage up a bit and maybe put silicon rectifiers in in place of the 5U4 tube  to see if I can't beef up the plate voltage as well. I've been working in the yard all day around here and it's time to get back to ham radio. Thanks again to all.
   ---Marty, KK4RF---

Gentlemen,
     I took out the 5U4GB and put in a pair of silicon rectifiers and the plate voltage went from 380 up to 455 key down and loaded and the screen voltage improved from 115 up to 140 volts on the 6DQ6B. I'mk now getting 25-30 watts out presently and it seems to key OK. I was surprised at how much difference the silicon rectifiers made.
     Also, I checked the AC voltage on the secondary of the substituted power transformer (likely from an old TV set)  and it was only about 380 volts each side of center tap. This was with no load on the transformer and the rectifiers not hooked up.  Another change I made last fall when I bought this thing was to change the choke-input filter to a capacitor-input filter.
     Anyway, it is putting out more power and no ovious chirp when keying.
     Thanks again to all.     ---Marty, KK4RF---
     

Ah, that's why your getting higher voltage, since you changed from a choke input filter to a capacitor input filter.  I think your pretty much close to the original DC voltage that the stock transformer provided.  The output power sounds right where it should be.  Enjoy a classic! 

Joe, W3GMS         

That big old TV xfmr is probably a much better xfmr than the original.  I use two of them in my 6146 xmtr.  I use one for the 300 volt supply and filaments.  The second one is used with a FWB rectifier and delivers over 650 volts with a LCLC filter at 300ma (6146 and two 6550's modulators).

Post the part number on the TV xfmr and I'll try to find the ratings for it.  Most of the TV type xfmrs were rated for about 250-300ma and filament current around 8-10amps.

Fred

Gentlemen:
     Thanks again to all who've replied.
     Fred, the transformer number on the top of the tranny is PC-1098A239. I'm not certain who the manufacturer of it was. Any info on it would be appreciated.
                   ---Marty---