It is one of those "I should know this but ..." questions.

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For the Johnson 250-39 TR switch, Table I presents the resistance at pins 2 and 5 of the 6BL7 as 100K ohms.?

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For the life of me, looking at the schematic I don't see how that could be.? Assuming the DC resistance of the RF chokes and toroid to be negligible, where would 100K be coming from? My VOM readings basically show open circuit.

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Thanks,

Jeff

(callsign hidden out of embarrassment)

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Thanks for the reply, Mike.

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Both sections of C12 were horrible. I had restuffed the can with new caps which explains the OPEN and supports your theory.

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Jeff

W8KZW

Work to be done ...

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? About 10 watts into the TX jack and then on to a 50Ω dummy load from the Antenna jack

? Yields about 9.8 Vpp / 3.5 Vrms? across 50Ω into my scope (thankfully not my receiver)

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Any suggestions? The filter caps (C12) are new. I guess I could live with the waveform, but not the amplitude.

Jeff

W8KZW

Update:

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The output of the TR switch to the receiver is a bit more manageable driving it with 30 watts (see photo) than with the 10 watts I ran in my previous post:

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Assuming the maximum safe input to my tube-type receiver is +10 dBm, now -13 dBm should be OK.

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But I was hoping to use my TR switch with my homebrew 10W rig. Am I wrong to think that increasing R1 would move the bias to drive V1 to turn off output to the receiver at a lower transmit power level?

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Jeff

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The capacitors will not be perfect, they will have leakage. Usually it is a slow-charge leakage.

Some years ago, probably the 1980s when I was doing a QSK switch for Heathkit, I looked at the Johnson TR switch. One thing is it puts significant harmonics on the feedline. Anything coupled to the line that clips, unless through a low pass or bandpass filter or properly buffered, will back feed harmonics into the line.?

Do not expect this switch to cut off signal to the receiver into fractional volt levels. It cannot do that unless pass through good signals are limited in level, and that will cause increased IMD.? ? ?

You said:
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3.5V is only 1/4 watt.? That is perfectly fine on old tube radios. Many solid state radios will take that.? It is unlikely you can change a few simple parts and make this work much different. It has to rectify in the first grid to develop cut off bias.? You might try a hard fixed cathode bias on the second cathode instead of? the cathode resistor. The cathode resistor tends to keep the second stage in conduction. As the cathode current decreases so does the cut off bias on that stage. If you used a resistor from HV and a Zener and held the cathode at a steady positive voltage the rectified negative grid voltage from the first stage would have an easier time getting the second stage into hard cut off.?

Looking at the characteristic curves of the 6BL7 and guessing you have 250V anode, you need a net grid bias of -30V to cut the tube off.? You aren't going to get that without some circuitry assistance.? ? If the cathode bias was +10 V fixed you would need an additional -20 on the grid to cut the tube off. This is all just wild guesses in numbers but it is the general idea. Effective tube bias would be negative grid voltage differential to positive cathode fixed bias (even when cut off), so if you had +15 on the cathode and -15 on the grid you would have an effective bias of 30 volts.?

The problem using a fixed cathode resistor in both stages is as the plate current drops from adding negative grid bias, the cathode positive goes away.? ?

Thanks, Tom!

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I am encouraged to know I should be OK with the 3.5V to the RX. As I can increase my TX output to 20 watts, that should make the RX even happier by cutting off a bit more signal than 10 watts.

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As I'm not overly excited about making the bias changes you mentioned, I think I'll wrap it all back up and enjoy what's left of Novice Rig Roundup!

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Also, I do run a Drake low pass filter in line after the TR switch.? I probably have the world's biggest collection of low pass filters, so it is good to get some use out of at least one of them.

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I am so happy you took the time to comment here, Tom.? Thanks again.

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73,

Jeff

W8KZW

The harmonics are not just VHF, they are HF. If you are on 7 MHz the TR switch will generate 14, 21, 28, and so on.?

I'm not saying that is a problem that will "bite" everyone, but it will certainly be there. I forgot exactly what I measured but this is even a problem when solid state diodes are used to generate voltage for relay transfers and other things.

When the grid is rectifying RF the harmonics from the clipped waveform couple directly back via C2 into the transmission line. Whether someone notices a problem depends on how well the antenna radiates a bothersome harmonic.

As for the output voltage, every tube radio I have seen (and many solid states) would take that okay. It could be reduced if necessary.? ?

73 Tom

Happy to report the TR switch is in-line and did an awesome job between my homebrew TX and Drake 2B / Lafayette HA-350 during Novice Rig Roundup!

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Not too happy with my handiwork restuffing the can, but it was my first attempt - better job next time I promise.

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I just flipped the RX to Standby and used my old keyer to monitor my sending.

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Now I just have to put my old Hallicrafters S-107 in the lineup to complete my WN2SCH station.

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73,

Jeff

W8KZW

Thanks, Don.

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I used wide metallic aluminum tape for the cover.? It would have worked perfectly had I done a better job of cutting the can.

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I used a tubing cutter and I anticipated the ratty result - no surprise here.

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Next time I will chuck the cap in my lathe using something like this ...

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WEN LA4444 4-Inch 4-Jaw Self-Centering Chuck Set with 1-Inch x 8TPI Thread