|This is a case where it wasn't broke yet some corporation just had to fix it. Transistor dippers simply just don't have the sensitivity of a good tube GDO.|
The Heathkit GD-1 series work perfectly. Output power? Surprisingly stout - I set my GDO on 2 meters, not coupled to anything. I then tuned in the dipper on my FT-51Rh handheld and went for a walk. The dipper was on my desk, in a house where the inner walls are chicken wire and stucco and the outer walls have aluminum siding. Still, I got 400 yards away before the carrier could not be heard on my HT (with the HT's OEM duck antenna).
As strong as they are, they are remarkably sensitive to fields near the coil. I took a air variable cap and soldered a 1/3" dia x 8t coil to it and I could detect it's resonant freq on the dipper 8" away from the coil.
I got lucky with mine - I got a near mint 1955 GD-1B with 2 sets of coils plus the extra hard to find low band coils (350-700 and 700-1900 khz) for about $25..... The part number for the extra two low band coils is #341-A.
If you see the extra coil set - grab it! They are getting hard to find, as is a GD-1 in good condition. I saw one, battered and beaten with MELTED coils go for $44 on eBay!