New project: Buzzardly 6 meter AM Amp.

[KL7OF]

Bill,

It seems to me the 11M big tube builders added a few turns between the coupling cap and the input to C1 but I'm am not sure - I've never used the technique.  But some of the guys know what I am talking about and will chime in...

T

  Now that I know to look, I have seen a small ferrite bead being used as the inductance between the blocking cap and C1 on Palomar amps.

[KL7OF]

The 71 arrl handbook has a 6 meter amp project with a tuned coil.... interesting.

[KD1SH]

Is this the one with the three 4CX250's? I don't have a 1971 Handbook, but I do have a 1968, and they often repeat.
If it's the same amp, I see that he appears to have run into the same minimum tune cap issue that I did, and wound up building himself a low-minimum cap of his own. Calculating his tank coil based on his description, the inductance is nearly identical to mine.

The 71 arrl handbook has a 6 meter amp project with a tuned coil.... interesting.

[KL7OF]

Is this the one with the three 4CX250's? I don't have a 1971 Handbook, but I do have a 1968, and they often repeat.
If it's the same amp, I see that he appears to have run into the same minimum tune cap issue that I did, and wound up building himself a low-minimum cap of his own. Calculating his tank coil based on his description, the inductance is nearly identical to mine.

The 71 arrl handbook has a 6 meter amp project with a tuned coil.... interesting.

It's not the amp with 3 tubes.... this one uses a single 3-500 but the output circuit  is  a loop inside a coil.....

[KD1SH]

Interesting. I may have a 1972 HB around here; maybe it's in that one. In any event, the values, and the issues the builder of the 3X 4CX250 amp had, seem to follow pretty correlate closely with mine, which gives me confidence that I'm on the right path.

Is this the one with the three 4CX250's? I don't have a 1971 Handbook, but I do have a 1968, and they often repeat.
If it's the same amp, I see that he appears to have run into the same minimum tune cap issue that I did, and wound up building himself a low-minimum cap of his own. Calculating his tank coil based on his description, the inductance is nearly identical to mine.

The 71 arrl handbook has a 6 meter amp project with a tuned coil.... interesting.

It's not the amp with 3 tubes.... this one uses a single 3-500 but the output circuit  is  a loop inside a coil.....

[KD1SH]

Alright, some notable progress today: I discovered that I have several other air-variables at hand, exactly the same as the one I've got in there now for C1, which made hacking one up for experimentation more attractive. So, I used my Dremel to remove one rotor and two stator plates, which brought the minimum capacitance down to approximately 6.0pf as opposed to its original 10, and a maximum of 15pf as opposed to its original 30. Now, I get a nearly perfect transformation庸rom the 50 ohm output side to the 2200 ohm input side謡ith SWR indicating 1.0:1 and X=0 on my RigExpert analyzer, and this with both C1 and C2 roughly at their midpoints; an almost ideal scenario. Now, if this holds up under actual operating conditions, it'll be fine business indeed. Time to turn this thing belly-up and start wiring up the cathode side.

[KD6VXI]

A trick I've used involves moving Ctune.

Install Cblock as normal.

Wire L1 of the tank circuit to Cblock.

Wire Ctune after the first turn of L1.

This also slightly lowers the total voltage Ctune will have to handle, but not by much.

Usually this will help.  If not, L1 needs to be looked at.

When running three or four 4cx15s in parallel, you have to get creative.


--Shane
WP2ASS / ex KD6VXI

[KD1SH]

Thanks葉hat sounds like an approach I'd like to experiment with. Since I've got everything all dialed in right now, at least as far as the "cold" tuning, I'll leave it as is, but if it looks like it could use some improvement when I actually test it under operating conditions, that's the first thing I'll try.

A trick I've used involves moving Ctune.

Install Cblock as normal.

Wire L1 of the tank circuit to Cblock.

Wire Ctune after the first turn of L1.

This also slightly lowers the total voltage Ctune will have to handle, but not by much.

Usually this will help.  If not, L1 needs to be looked at.

When running three or four 4cx15s in parallel, you have to get creative.


--Shane
WP2ASS / ex KD6VXI