Homebrew 40M AM Tx Output Power Problem

[DMOD]

Awesome Joel. What kind of cable/wire was it?

A secondary load impedance of 4200 is correct, but I question the Plate to Plate primary impedance.

Are you going for Class B or  AB2?


If you are targeting Class AB1 then see the last schematic.

Phil - AC0OB

[K1JJ]

So it looks like the coax (or audio cable?) had its shield grounded?  Did we have a 50 ohm cable in a high impedance RF plate circuit with a massive swr?

Did the cable act like a padding capacitor with a lossy dielectric that heated up and caused the power loss?  The 10 watts of heat had to go somewhere.  The center conductor was big enough to handle the current.

Yes, a single wire or strap is all you need to connect any of the pi network components together, including the plate  circuit.

That was a fine piece of troubleshooting and helpful suggestions, guys!  In hindsight, I'll bet it would have gone quicker with a few close up pictures of the rig...

T

[KA2DZT]

Tell me about it! And I had been fighting this thing for weeks before finally giving up and asking for help. 

The wire is about 7" long and it is reading 6.5 ohms of resistance and 35pf of capacitance. Really?!?

I have one more question regarding the secondary impedance of the Mod transformer. I read that to calculate the modulating impedance you take the PA DC pate voltage and divide by the DC plate current. That's 370/.090=4100. Is this correct? If so then I'm leaving the secondary at 4200 ohms.

Thanks
Joel

What is the wire, a piece of shielded wire??  A seven inch long wire in the PA plate circuit is way too long.  Better placement of parts is important in the PA.  Leads should be short.

As for the mod xfmr impedance, that is correct  PAv/PAa and 4200 ohms is close enough.

Oh No,  I forgot about the modulator.

Fred

[KA2DZT]

So it looks like the coax (or audio cable?) had its shield grounded?  Did we have a 50 ohm cable in a high impedance RF plate circuit with a massive swr?

Did the cable act like a padding capacitor with a lossy dielectric that heated up and caused the power loss?  The 10 watts of heat had to go somewhere.  The center conductor was big enough to handle the current.

Yes, a single wire or strap is all you need to connect any of the pi network components together, including the plate  circuit.

That was a fine piece of troubleshooting and helpful suggestions, guys!  In hindsight, I'll bet it would have gone quicker with a few close up pictures of the rig...

T

Yes a few pictures, just what I was thinking.  Probably would have taken us two minutes to spot the problem.  Piece of shielded wire in the PA plate circuit.  I always told him that the problem is a part breaking down under load. Who would think that there would be a piece of shielded wire in the plate circuit.

[K1JJ]

And so we shift over to autopsy mode:

Hmmm... I wonder why that shielded cable had a 6.5 ohm center conductor?  Strip it and find out.

Actually, normal shielded wire cable would have worked OK IF the shield was left ungrounded. It would act like just another piece of wire. But grounding the shield made it a padding cap and a lossy transmission line at huge swr, I suppose.  

Usually there is not a big demand put on the path between the tube plate and the first tuning capacitor of the pi-network. The big current is inside the pi-net resonant circuit. If that wire was there instead, it may have caught fire and made it easy to find.

Still, it showed up when the tuning had to be "chased" as the heat increased showing something was happening AFTER the tube plate.  I wonder if the shielded wire actually got warm?  Touchy-feely troubleshooting.

T

[DMOD]

And so we shift over to autopsy mode:

Hmmm... I wonder why that shielded cable had a 6.5 ohm center conductor?  Strip it and find out.

Actually, normal shielded wire cable would have worked OK IF the shield was left ungrounded. It would act like just another piece of wire. But grounding the shield made it a padding cap and a lossy transmission line at huge swr, I suppose...  

...Still, it showed up when the tuning had to be "chased" as the heat increased showing something was happening AFTER the tube plate.  I wonder if the shielded wire actually got warm?  Touchy-feely troubleshooting.

T

It appeared to be a shielded, single conductor audio cable (could be wrong) and I too would like to know exactly what it was.

If it was, I suspect (as you) the dielectric heated up, melted, and partially shorted the shield to the inner conductor.

A shielded cable will only possess a measured, specific impedance when terminated in it's characteristic impedance:

http://www.standard-wire.com/coax_cable_theory_and_application.html

And usually a shielded wire is shown as thus on a schematic and nothing was shown as one:

[KA2DZT]

Same thing I thought, the shielded cable wasn't shown on the schematics

[AB2EZ]

A short* length of coaxial cable, having a characteristic impedance of Z₀ ohms, can be represented by the following, simple, lumped element circuit:

1. A capacitor between the center conductor and the shield, having value C (farads) = C₁ x D... where C₁ is the capacitance per unit length specified for the coaxial cable, and D is the length of the piece of coaxial cable

2, An inductor in series with the center conductor, having inductance, L, equal to Z₀ x Z₀ x C (henrys)

For example:

If Z₀ = 50 ohms, D = 0.25m, and C₁ = 80pF per meter

Then C= 20pF, and L= 50 x 50 x 20 picohenrys =  0.05uH

Stu

*I.e. where 2pi x f x C x Z₀ <0.3


 

[AB2EZ]

Duplicate deleted

[w4gon]

Phil, I'm using Class AB1 on the modulators so will study your schematic. I went with 9k p-p impedance on the primary as I tried to follow the values on the datasheet for AB1 (attached). If this is not correct, I can change the primary impedance easy enough since I'm using a UTC S-19 transformer. I don't see a secondary of 4K though so might be a bit off there if I change.

Yes, the wire was a piece of shielded audio wire from the old VHF transmitter. I had it grounded on both sides. I'm also using the same type of cable in the audio stage. Let me try to find a shielded cable symbol so that I can add to the schematic. I'll take it apart later to see if it was breaking down or melting. When I was checking for warm components I didn't think to check the wires also so not sure if that one was getting hot, although I'm sure it was. The wire was between the plate coupling cap and the Pi tuning capacitor. It was that long because I have the choke, parasitic suppressor and coupling cap under the chassis and the Pi network on top. Since the radio is wide open on top I didn't want to have those high voltages so exposed... I'll work on the placement of the parts in that stage more when I start working on the enclosure. As it stands now the piece of copper wire in use now is about 6" long still, but everything is working fine now...

In my defense I did make the statement that I had a feeling that it was something I did during the build process, just didn't know what it was. 

Joel 

[DMOD]

Phil, I'm using Class AB1 on the modulators so will study your schematic. I went with 9k p-p impedance on the primary as I tried to follow the values on the datasheet for AB1 (attached). If this is not correct, I can change the primary impedance easy enough since I'm using a UTC S-19 transformer. I don't see a secondary of 4K though so might be a bit off there if I change...


...In my defense I did make the statement that I had a feeling that it was something I did during the build process, just didn't know what it was.  

Joel  

For class AB1 approx. 9k plate-to-Plate on the primary and approx. 4k on the secondary should work.

Hey, we all learn by doing.  

One other note with ref. to schematic 1. Depending on the voltage of the Keybias line, you may have to experiment with R7 in order to get the voltage up around 50V in Keyup in order to make sure the Mod. and RF stages are shut off during Keyup.

In my view, there is no need to control the speech amp or phase splitter in the audio section since their power dissipation is minimal.

Glad to see it is operating as expected.


Phil - AC0OB

[KA2DZT]

The PA plate RFC should be on top of the chassis.  Coupling cap should be on top near the plate tuning cap and RFC.  The parasitic choke and resistor should be on a short lead right off the plate terminal ceramic cap to the RFC.  The by-pass cap on the RFC should also be on top of the chassis.  The RFC should be standing upright bringing the top end closer the tube plate terminal and other connections etc.  Only thing that should be coming through the chassis from below is the modulated B+.  The screen resistor for your low power rig can be below the chassis.

Fred

[w4gon]

Thanks Phil. I debated as to not bothering to key the phase inverter. I compared my design with a Lettine 240, among others, as I was building and that one had it being keyed so that's why mine is like that. I think I'll just ground the cathode for that stage as well. So R7 is used to keep the keyup voltages from soaring correct? That's a concern I have in my current design...

Thanks for the placement details Fred. I'll be moving those parts above the chassis once I make this rig safer to operate.

Joel

[KA2DZT]

Easiest way to key the modulator is to key the 6L6 screen voltage.  The 6L6s will shut off with no screen voltage.

[DMOD]

...So R7 is used to keep the keyup voltages from soaring correct? That's a concern I have in my current design...



Joel

Correct. It has been my experience that you need at least 50 volts on Keyup in order to completely cut off those stages.

I use a 25k 5W WW pot. between the Keybias line and ground to set the resistance needed for the cut off voltage.    

In some cases I have had to use a pullup resistor of about 220-270k from B+ to that line in order to get around 50V. It just depends on how much leakage current is available through the tubes .


Phil

[DMOD]

Thanks Phil. I debated as to not bothering to key the phase inverter. I compared my design with a Lettine 240, among others, as I was building and that one had it being keyed so that's why mine is like that.

Joel

Actually. the Lettine 240 only switched the B+ to the phase inverter, not the cathode circuit.

The only circuits keyed are the Osc. and the Final.


Phil - AC0OB

[w4gon]

Thanks Phil. That's right, the Lettine does switch the B+. I didn't want to switch such high voltages and only wanted the one switch to key all the tubes anyway. I'll work on implementing R7 as this is needed on my rig for sure.

BTW, I took the bad wire apart and didn't notice anything wrong with it. No melting or damage of any kind. Must have just been me grounding the shield that messed everything up! 

Thanks again guys!
Joel   

[K1JJ]

Thanks Phil. That's right, the Lettine does switch the B+. I didn't want to switch such high voltages and only wanted the one switch to key all the tubes anyway. I'll work on implementing R7 as this is needed on my rig for sure.

BTW, I took the bad wire apart and didn't notice anything wrong with it. No melting or damage of any kind. Must have just been me grounding the shield that messed everything up! 

Thanks again guys!
Joel   

Hi Joel,

Consider using a 12 volt multi-pole relay for the switching job. You will keep the voltages away from you and the switch. A tiny toggle switch can control the T/R. Better yet, a hand-held push button switch will make a better T/R control with the relay.

BTW, where did you measure 6.5 ohms the other day on the bad wire? Was it between the shield and center conductor OR from end to end of the center conductor?

T

[w4gon]

Thanks for the tip Tom. I measured the 6.5 ohms from end to end. After I took it apart I measured again and it was around 1 ohm. I personally never want to see that piece of wire again!

Joel

[DMOD]

Talked to Joel tonight on 40 meters (7.290) (> 505 miles) and although band conditions were poor, his signal came through 7-9, and his modulation was really good.

Good job on getting it on the air.


Phil

[WA4WAX]

Joel:

A few thoughts.  Some may have been mentioned.

1) I would not use a 2E26.  My first choice would be a 6AG7.  6V6 might be OK.  A 6L6 has too much soup for modern crystals....even if you put a pilot lamp in series with the crystal.

2) Parallel the rock with 50 or 100 K.

3) Clamp tube a good idea.  If used, the cathode resistor can go.

4) Screen resistor likely a bit high.  See discussions by Tom, W8JI.  Your best  result will likely be between 7500 and 20K.  It will take some experimenting.

5) Plate block cap between driver and 6146 would be fine in the range of 100 to 25O pF.  1000 pf a bit much.

6) HV ceramic disc only for bypassing.  No orange drops.

7)  Plate block cap between 6146 and tank should be a proper, transmit rated cap.  Think Centralab doorknob, or 2000 VDC minimum Mica Tx cap.

  Use old '50's, '60's FT-243 rock.  Better yet, an old Bliley from before World War Twice.  Those things can take lots of current.
Good luck!

[WA4WAX]

Joel:

One more thing:

PVC is an abomination for tank coil forms.

Use G10 or ceramic.

Even large pill bottles make decent coil forms as they are polystyrene.

Nice job with the rig!

[w4gon]

Thanks Phil! It was a pleasant surprise to be able to make the contact with you on this rig which you helped me get on the air. There ended up being a bit of an AM pileup there after our QSO. 

I appreciate the tips Matthew. Yep, I've been reading through W8JI's site. The PA screen resistor still needs to be adjusted some. I need to place an order for additional components for sure. I can now justify spending a bit on this rig since it looks like it's working. There are no PVC forms being used, only air coils.     

Joel

[K1JJ]

Joel:

One more thing:

PVC is an abomination for tank coil forms.

Use G10 or ceramic.

Even large pill bottles make decent coil forms as they are polystyrene.

Nice job with the rig!

Hi Matt -

Always curious, I've always wanted to run some tests on ABS, PVC and ceramic material - and did so tonight.

I used a 4" PVC pipe, a 4" ABS pipe and a ceramic standoff insulator.  Each item was tested separately in a 1500 watt microwave oven for 1 minute.

The results as follows -  a laser temperature gun was used for measurements:

The room temperature was 73.8F.

Ceramic insulator: 73.8F after one minute

Black ABS pipe:  92.3F after one minute

White PVC pipe: 102.6 after one minute.

The ceramic was stone cold to the touch. The ABS was hardly lukewarm.  The PVC pipe was warm to the touch.

My question is, this was a test at microwave freqs. Has anyone tested PVC and ABS at 160-10M, especially 75M?  There would be SOME degree of heating at HF as shown by the microwave test, of course, but just how much?  I wonder if the heating would decline linearly from microwave to HF and become insignificant?  For example, 2,450 MHz (microwave oven freq) is 600 times higher than 4 MHz. So would the heating be 1/600th on 4 MHz?

Opinions?

T