No doubt harmonics, the pi net should be connected as closly as possible to the final stage or shielded...

How would I go about shielding the pi net without affecting its operation?

Also if I insert a coil in series with the output of the xtal oscillator would that help reduce the harmonics?

It isn't so much that the pi net be shielded but the lead to it should defiantly be shielded... I build the pi net in the chassis on mine, with a antenna connector on the chassis...

Tom

I had always thought about mounting the pi net in my cabinet, but wasn't sure how the coil would like being so close to the grounded metal cabinet. Plus I have wiring inside that would be close to the coil as well. Now if I could find the inductance of the coil I could maybe get a variable inductor and set it to the inductance of the current coil I am using which would make the pi net much smaller. Another thing is how well will the pi net work if I move the components inside the chassis since it is metal? There would probably be unwanted capacitance formed by that short coil of wire (in series with the second variable cap) and ground. Also there could be a ground loop created unless I can get a small trimmer capacitor the same value of the tuning cap that I am using which came from a Heathkit FM-3A tuner. I would more than likely leave things as is for now.

I can say that I was pretty surprised when the pi net actually worked as it was my first time building one. Tried to build it with a short wire between the output of the coil and the second variable cap, but it didn't work until I added a longer length of wire between the two and coiled it. I guess that longer wire provided some inductance?

Would one of those tall CB radio antennas that used to be seen quite frequently on trucks work for a good antenna? If so how should I redesign the pi net for that longer antenna?

"Would one of those tall CB radio antennas that used to be seen quite frequently on trucks work for a good antenna? If so how should I redesign the pi net for that longer antenna?"

I don't think that you would gain much with the CB antenna for this application. A 10 ft conductor (with one end folded over a curtain rod) is all that you will need.

Having the coil within a inch or so of any metal isn't a issue...

If you want the absolute best, build one of Bob Weavers tapped toroids... It will match about any antenna you can come up with, at most any frequency above 750Kc... With a few added loops of wire, it would match anything at the bottom of the BCB as well...

Here's mine(Bob built), mounted on a six position switch avail at R/S, value ranges from around 250uh to 850uh... The AA5 tuning cap is mounted on top of the chassis...




Tom

Cool. I will have to look into that.

I first need to get the power transformer situation straightened out.

I need I suppose somewhere under 250 Vdc B+ (filter caps rated 250 Vdc) from the rectifier. I also need at least one 6.3 Vac winding and another 6.3 Vac winding or I can build a voltage doubler and use just one 6.3 Vac winding.
I do plan on using a 1K multi turn trimpot for the 6GY6 cathode resistor. That way I can adjust for the highest signal with the lowest audio distortion. I hopefully can find a small panel mount meter to allow me to read cathode voltage and cathode current.

I like the current VU meter I have installed, but one from an AKAI M7/Roberts 770 reel to reel would look better as it looks like what one might have seen in the old mono consoles used in radio studios. I added the VU meter to make it easier to know when I am over modulating the transmitter.

The transmitter sounds most excellent when I listen to it with my R-390 receiver through a vintage 15" Jensen MI speaker with 5" Magnavox tweeter mounted on the baffle in front of the Jensen speaker.

I finally found a power transformer that will give me all the voltages I need.

http://www.edcorusa.com/products/691-xpwr094_120.aspx

Will that one work good?

2.5 amps on the 12 Volt CT secondary should be enough for the two tubes plus oscillator power supply and the vu meter driver power supply, right?

I assume that the .3 volt drop in heater voltage (6 Vac instead of 6.3 Vac) will not affect the operation of the tubes?

Unless you are using a lot of extra circuitry, you could power six 6888 xmitters with that and B+ will still be high(around 230/240v) using 1N4007 rectifiers... It would be good with a tube rectifier what has some voltage drop...


Tom

Cool. Maybe I could add a 6X4 rectifier. Given the chassis is totally enclosed will the added heat of the rectifier be too much?

It's not a good idea to place tubes inside a totally sealed enclosure unless you provide some means of ventilation. In the case of the 6X4, that tube will dissipate mounds of heat----one could fry eggs on something that hot.

Ok. Looks like I will need to cut a couple holes so that I can use a small fan to remove heat. I am pretty sure that I have enough current capability from the 12 volt op amp power supply to power a small 12 volt fan on reduced voltage. The fan itself wouldn't have to be that large.

Of course if the transmitter will be just fine running at the higher B+ then I will not need a tube rectifier.

What would be the effect of running the transmitter on that high of B+?

I do plan on adding a 1K variable resistor to the cathode circuit of the 6GY6 so I can adjust for a good balance betwen modulation linearity and output power.

I haven't seen the enclosure, but I don't know if I would go to that extreme in reference to the small fan. It's just that you would have to provide some means of allowing the heat generated by the tubes to escape.

"What would be the effect of running the transmitter on that high of B+?" I'll leave that question to the person who designed it.

"I do plan on adding a 1K variable resistor to the cathode circuit of the 6GY6 so I can adjust for a good balance betwen modulation linearity and output power." As far as I know, that's a proven circuit that has been qualified so I do not think that you would gain much with the 1K rheostat.

Only reason I would want to have an adjustible resistor is as B+ increases the cathode voltage needs to be adjusted as well in order to provide best modulation linearity and power output.

Yes, that's why I deferred your second question to the person who designed the device.

Yep. I suppose I would need an oscilloscope in order to adjust the cathode voltage properly.

When I get the correct value of resistance I can simply use a fixed resistor of the same value as what was selected by the 1K variable. That way I will not have to worry about the pot being adjusted accidently.

I do plan on adding an audio compressor as well. I would be able to mount it inside the chassis of the transmitter which was an old Sears automotive type meter.

Hmmm........I'm not sure that an audio compressor is going to help you here.

It will help as I will get more average modulation and less distortion.

With the small current draw of these Tx, I doubt a 6X4 will run all that hot, most of the heat dissipation would be from the heater... Also due to the low current draw, voltage drop may not be all that great... Would be interesting to see just how much there is, must still be a fair amount as I had a UHF converter that used a 6X4 and the xformer secondary was in the 170v range... Those generally have B+ in the 150v range so must still be some drop...

The 12v computer P/S fans generally draw something like .05ma, that transformer wouldn't even know it was there...

A 6GY6 would no doubt operate on 240v B+ but the audio amp plate resistor, 6GY6 screen and cathode resistor would need to be increased in value... By how much I dunno, as I've never ran one at much over 200v...

Ok I think I may stick with the diode then see where B+ is and adjust VIA resistor for 200 Vdc. I think I might have a resistor that may work. What I may do is install the resistor right after the diode to drop the voltage some then use a smaller value capacitor directly after the rectifier which will bring B+ up to between 180-200 Vdc. That way I don't have to change any resistor values and I also get better power supply filtering.