[Central-Electronics] 20A VFO drive requirements

[Jim Candela]

Bob,

You raise a good question..It would be useful to know how you are driving that VFO input. Do you have a sine wave at 8-10v peak? With the BC-458 CE Mod'd VFO, 40m is a bad band because the VFO has to be tripled to make 7Mhz when mixed with 9 Mhz. The result is often low VFO drive.
If you are using a wide range DDS and amplifier to get a nice sine wave drive, then you are not hand cuffed to make a 5-5.6 Mhz VFO get you on all bands. You can mix either side of 9Mhz to get the desired band. One side might be better than the other.
Also keep in mind that the 6BA7 mixer has one or more tuned traps (can't remember how many). Your injection frequency might need to keep those in mind.
The 12BH7 cathode follower to the finals is a weak link in the RF chain. In my QRO 20A experiments, I replace the 12BH7 with a 6S4 and have a 30mm CCS LM317 in the DC  cathode return. This works very well for 160-40m, but in the higher bands I have reduced drive to the finals which begs me to neutralize the final tubes (I use 7591A's).

If you have a good scope, and compensated 10X probe, perhaps add a 3 pf capacitor on the 6AG7 G1 such that one side is a test point hanging up in the air. This way the loading of the probe will be just a few PF. If the probe has 10pf to ground, you have a broad band capacitive voltage divider. Then compare the peak drive to the 6AG7 grids going band to band.
You can do the same trick with a 3 pf capacitor to monitor the input and output of the cathode follower.
Keep us informed with your progress.
JimWd5JKO 

    On Saturday, March 4, 2017 5:14 PM, Robert Nickels <ranickel at comcast.net> wrote:
 

 A question for anyone who may have played with using a non-458 VFO to 
drive the 20A:

I'm seeing considerably reduced output for the same VFO input level on 
40, 15 and 10 meters compared with the other bands, and wonder if this 
is normal?    Everything tunes as it should and the trimmers are all peaked.

Thanks and 73,

Bob W9RAN

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