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Title: Parasitic Suppressors Cooking on 10m Post by: W7SOE on October 04, 2015, 04:29:12 PM Thanks all for the help on my 2x813 ssb linear. Thank you Dan for the meter! I have managed to get it to tune on all five bands. The latest issue is on 10m. When tuning my suppressor resistors went up in smoke. They were of arbritrary construction, 7 turns over 50 ohm 2w carbon resistors.
I made another set, 4 turns over two 100 ohm metal oxide resistors in parallel. These start to smoke as well after a few seconds of tuning. Perhaps there is still to much L, I have seen them constructed of a single turn of silver strap..... Rich Title: Re: Parasitic Suppressors Cooking on 10m Post by: W7SOE on October 04, 2015, 06:46:23 PM Single turn on 75 ohms. No smoke yet...
Rich Title: Re: Parasitic Suppressors Cooking on 10m Post by: K1JJ on October 04, 2015, 09:35:54 PM Hi Rich,
You should be OK now. Your present modification uses a big diameter coil - and that's actually 1 1/2 turns. It's possible that there weren't any parasitics in the first place with the 7 turns. It may just be that the turns and tube plate capacitance and stray were capturing enough parallel resonate energy at 10M to heat up the resistor. Those resistors are small compared to say, Glow-bars. It doesn't take much energy to heat them up. I have built several 160 -10M amplifiers that did not need any parasitic chokes at all. It all depends on what you have created - IE, whether it needs suppression or none at all. IIRC, that's a GG amplifier, right? It should be pretty stable if you used copper strap at the grid and screen pins to ground.. or at least super short buss wire connections. BTW, I would replace that thin wire from the 813 plate caps to the C1 tuning cap with some 1/2" copper strap. Hopefully you used strap throughout the pi-network connections, especially since it covers 10M. T Title: Re: Parasitic Suppressors Cooking on 10m Post by: w7fox on October 05, 2015, 12:34:07 AM Rich,
I have had the same trouble on a 100 watt rig, and had to reduce the coil until it no longer smoked. However, I built my pair of 813s without suppressors and it works fine, no parasitics. It is a typical gg linear with 2800 volts on the plates. Title: Re: Parasitic Suppressors Cooking on 10m Post by: W7SOE on October 05, 2015, 12:26:10 PM Thanks guys.
I will replace that wire with copper strap, I found some flat copper from a LV transformer. Rich Title: Re: Parasitic Suppressors Cooking on 10m Post by: KB2WIG on October 05, 2015, 02:30:58 PM I wonder if anyone has used motor brushes ?? Ima gonna try 'em for the plate caps. ( the Cu, not the actual brush) klc Title: Re: Parasitic Suppressors Cooking on 10m Post by: N8ETQ on October 05, 2015, 03:25:37 PM Tin up some solder wick... /Dan |