Thank you everyone for your replies! I’m still working on debugging this transmitter. For a 1965 vintage design, this transmitter is making me feel quite dumb! Following are some findings to date. I think I’ve responded to all the queries? (I think I may be experiencing multiple issues here.)
G8HQP – The loading seems to be working from the following observation .. I can “peak” the loading capacitor C4. The plate current peaks in the middle of C4’s travel but I can only get “close” to 3.4 mA plate reading .. Sometimes I can get 3.0 mA or so (representing about 150 mA of plate current v.s. 170 mA suggested.)
N3QE – When I connect the 6AG7 cathode to ground, the transmitter “fires” every time I key down the 6146B stage. So, I agree that the oscillator is at least part of my problem. (Of course, my receiver/monitor can "hear" the oscillator running perpetually.)
G3RZP – I get the same effect with other crystals, but the transmitter seems better behaved at higher frequency (i.e., 40m as compared to 80m). I don’t have another 6AG7 to try right now. Yes, grounding the 6AG7 cathode “fixes” the oscillator problem. BUT .. I have discovered another issue by doing this. Now, the output section appears to break into some sort of secondary oscillation .. maybe a parasitic when sweeping the Plate and Load capacitors. ALSO .. I have discovered that the amplifier section (6146B) can be made to oscillate WITH THE 6CA7 completely removed! This must be input/output circuit feedback or some neutralization failure?
K8AXW – I think the metering is fairly close. I’ve replaced the 10 Ohm and 470 Ohm resistors for the plate circuit with 1% resistors. If I set the Plate current to 2.3 mA indicated by the Weston 301 panel meter and measure across the 470 Ohm resistor with a Fluke 8010A, I read 1.105 VDC. ( 1.105/470) = 2.34 mA calculated and the meter displays 2.3 mA which is close. If I put my Fluke 8010A in series with the Weston 301 meter, the current measurements as close: Weston 301 = 2.5mA, Fluke 8010A = 2.59mA. Likewise, if I measure across the 27K Ohm resistor in the Grid circuit (26.5K Ohms measured) I read 71.9 VDC. (71.9/26.5) = 2.71 calculated, Weston 301 2.5mA displayed.
KC2ZFA – I have not tried this yet. But I see what you mean. They’ve also added a 100 Ohm resistor in series with the 6146B grid.
N5RMS – I’ve measured these voltages in 3 modes: TUNE, OP, and OP/DOWN KEY
- Junction of 10K & RFC2: Tune = 405VDC, OP = 412VDC, Downkey = 379VDC
- Screen Pin-6: Tune = 115VDC, OP = 137VDC, Downkey = 108VDC
- Fill pin-5: Tune = 6.93VAC, OP = 6.95VAC, Downkey = 6.82VAC
Note – I have found that the DRIVE measured in TUNE mode needs to be bumped up then in OP/DOWNKEY mode. The above measurements were made without re-adjusting the DRIVE control.
N2EY – 1.) I think the meter is fairly close now (see comments to K8AXW). 2.) When in 80m range, peaking C1 will drive the meter off range as the ARRL article says it “probably” will. They suggest de-tuning C1 to bring the drive down to 2.5mA. That is what I have to do. 3.) I am using 0B3 and 0C3 as indicated in the ARRL schematic. Voltages are as follows:
B+ 832VDC Key Up, 750VDC OP/KEYDOWN
Screen 197VDC Key Up, 193VDC OP/KEYDOWN (in CW/125 Watt mode)
Screen 109VDC Key Up, 108VDC OP/KEYDOWN (in PH/75 Watt mode)
I'm including some links to some photographs. Maybe my layout is bad? Maybe the oscillator is too close to the 6146 and some shielding
is needed? I'm using a "temporary" front panel.
Thanks again .. I am in need of some Elmering!http://www.bignick.net/6146B_Xmitter/back.jpghttp://www.bignick.net/6146B_Xmitter/back2.jpghttp://www.bignick.net/6146B_Xmitter/bottom.jpghttp://www.bignick.net/6146B_Xmitter/bottom2.jpghttp://www.bignick.net/6146B_Xmitter/bottom3.jpghttp://www.bignick.net/6146B_Xmitter/bottom4.jpghttp://www.bignick.net/6146B_Xmitter/bottom5.jpghttp://www.bignick.net/6146B_Xmitter/front.jpghttp://www.bignick.net/6146B_Xmitter/top.jpghttp://www.bignick.net/6146B_Xmitter/top2.jpghttp://www.bignick.net/6146B_Xmitter/top3.jpg