I use a HQ-129X with a Viking I, similar to what I ran in the 50's. Back then a 717A in the RF hole woke up 10/15M a lot.
These days Ive gone a bit further with a 6SG7 RF and 6X8 mixer/oscillator that was written up in CQ maybe in the late 50's. I can dig it out if needed. Now its fine on all bands and the AGC isnt disturbed.
Carl
hi Carl ... I have a 129x and have the CQ article for the 6X8 .... what did the 6sg7 mod entail?
They are pin compatible but the ground must be removed from Pin 3 at the socket and any other component tied to it and brought to another ground if required, Ive seen different factory wiring in that area so look close.
The 6SG7 has the suppressor grid internally connected to the cathode and pins 3 & 5 are common. The 6SS7 has the suppressor to pin 3 and directly grounded.
To get the full AGC range the screen voltage should be 125V and R-30 180 Ohms but I dont think R-30 makes much of a difference, the stock value gives a bit more gain and you can even go to 60 Ohms according to the spec sheet. Experiment and pick a value that gives the best balance between sensitivity on 10M and overload on 75. I use Beverages on 160/80 so went for the 10M end. I used a 25K pot to set the screen voltage and then picked a fixed resistor, maybe 15K.
Realign that stage and it should be completely stable; if not get back to me for the usual cures. If youre feeling energetic you can try a 6AB7 with various component changes but it will probably require taming, it did in a HQ-120X I sometimes use but that required a grid wire change for the single ended 6AB7 (I didnt have an 1851)
Carl
