The AM Forum

A few folks have blown up the Johnson TR switch by accident on KW-input transmitters. What might accidentally happen to it on a 3CX3000 rig?

The 6BL7 used in the Johnson 250-39 T-R switch is apparently one of the beefier indirectly heated receiving dual triodes. Notably it has a 10W plate dissipation, high current cathode, and high negative grid voltage ratings. There might be beam tubes that are tougher in the grid department, but whether or not they would work covering the HF band is a good question.

What small indirectly heated transmitting tubes would have tougher grids and be an upgrade for the 6BL7?
In the Johnson, the first 6BL7 section is running about half its rated cathode current.

There is the 2C39, and they seem to be cheap enough, and in a TR switch there's no need to run anywhere near full ratings, so little or no cooling may be needed. They also have more published grid ratings. I came up with this special diagram and wonder what yall think of it? Is there a more easily socketed alternative?

Interesting, given any thought about how it might perform with the stock 170V  B+?   Are you going to give it a real world try?  

Stock those are rated for 4KW PEP.

(https://photos.smugmug.com/EF-JOHNSON/250-39-TR-switch/i-9jNrrT5/0/f9dd70c1/XL/closeup-XL.jpg) (https://mikeharrison.smugmug.com/EF-JOHNSON)

hi Pat and mike ... this is an interesting problem because the tr switch does 2 functions automatically ... small signals are processed normally by the cascode amplifier pair .... it adds a bit of noise during reception ... as the transmitter provides rf the large signal swing overdrives the first stage and by grid rectification provides a large negative voltage to cut off the second amplifier.

bringing in a power tube pair to this circuit will bring increased noise and the possibility of too much rf being fed to the receiver causing a crap out.... ??? :P :-[

I used the ARRL handbook 6C4 tr switch as a novice with reasonably good results... I think you might keep the 6BL7 circuit or even a smaller triode pair and use a trick that was used in the Command set rigs ... use a NE2 neon to ground to fire at 60 Volts of rf and by limiting its current keep everything copasetic

           rf -----| | -------resistor ----.-------||------> to !st grid
                    c2                            |
                                                   ----- NE2------> gnd

I don't think I want to use it with that carrier-operated bias circuit, but the rest of it might be OK.

The Johnson circuit has a potential issue in that the second tube's cutoff is late due to RC constant R4-C8. I saw an explanation of this here: https://www.w8ji.com/johnson_tr_switch.htm
The person did an spice simulation of a 700W carrier applied, using a 6SN7 because he did not have the 6BL7, and it showed a 580V p-p RF waveform starting at T=0, decaying to 0V at T=1.35milliseconds.

I do not know all of this person's work, but I found many of their contributions to be very useful and have no cause to doubt the results shown. This would be a reason for anecdotal evidence that sometimes solid state stuff will be damaged when the Johnson unit or design is used.

The top goal is to have an electronic switching device that can operate along with the other devices in the overall system to allow 'break in' operation. The big obstacle I always run into is the time taken for the relay on the backside of the transmitter to actuate. The Johnson switch is OK, except for the delay in receiver blocking.

My goal in the schematic was to investigate the operating point for a bigger  triode and how the bias values might have to be changed.

My overall goal was to find something that might handle the 3CX3000 amplifier in case full RF power was accidentally applied. It had to be:
1.) easily available,
2.) have a tough grid and cathode, and,
3.) be able to run at a small fraction of the rated current and,
4.) be able to run at 1/3 to 1/2 of the rated plate voltage.
There is a box of used 2C39 tubes here, so..

The 6BL7 in the Johnson runs about half its 'normal' plate current and about 2/3 its normal plate voltage. I do not know how tough its grid is, but it has a high rating for peak negative grid voltage, even if it is for when the tube is used as a vertical deflection osc/amp.

In contrast the 2C39 has a higher peak grid voltage, and a good grid current rating for continuous RF use.

What I have learned so far there is that a carrier operated bias system is not acceptable if one wants to avoid the RF spike to the receiver. I wanted to think about an electronic cutoff bias for at least the second tube that would have a faster operating time and eliminate the RF spike.

It should be reasonable to find a way to cut off the second triode first, then very quickly remove cutoff bias from the power amplifier, then key the transmitter. If done fast enough then break-in should be transparent, with the transmitter's keying and un-keying time being the largest interruption.

Sorry to make such a long explanation!



I had not thought about the noise so much but hoped that a small UHF planar power triode would not be too noisy at HF frequencies. There is also the much smaller 416A/B/D tube, that has been used as a grounded grid 1st RF stage in military receivers and others where it was wanted to protect the front end. The TMC GPR-90 used a 6AB4 that way. The 416B might be usable and has a low noise figure and a 15mA grid, but its grid voltage ratings are small, +1.5 and -15V. Close spacings probably working against higher voltages.

Is the added noise related to the generally larger size of the tube elements? This is very interesting and I'm not sure how to understand it.

Whatever is used might see potential transmitter output voltages:
1000W = 223V RMS, 315V peak
4000W = 447V RMS, 630V peak
8000W = 632V RMS, 894V peak
These are pretty onminous figures and get higher when the match/SWR isn't good in the direction of higher impedance.

What might the 6C4 tolerate? -I think I saw that article and because it was for the Novice class the tubes were lower powered. The 6C4 is rated -50V on the grid.

The use of the NE2 is a great idea as an extra protection. The GE glow lamp manual states 25-300us for ionization time depending on external illumination and how much voltage above the ionization potential is applied. The manual does not give ionization times for each lamp, just has a couple of paragraphs on it.

I should mention I looked at PIN diodes. Right now I want to look at tube solutions because tubes are all over the place here and expensive PIN diodes are not.

Pat asked ...

Is the added noise related to the generally larger size of the tube elements? This is very interesting and I'm not sure how to understand it.

AIRC there was a noise related equation with Boltzman's name attached .... I think it related generated noise with current flow

that was from a long time ago  :D

This handles antenna switching but what handles receiver muting to prevent feedback? At least in my case, the receiver muting circuit cuts the audio amp and drops the AVC gain, so that recovery time is a factor for break-in.

A few years ago built up an antenna distribution amplifier and BCB reject filter, to follow my switch and feed several receivers.

With tube input receivers the output during the initial millisecond or so really doesn't matter, but with SS inputs not so much.

The output power from the T/R switch is such that some lowly 1N914s and 1N60s have easily handled the problem for several years now.

I don't have any idea what might happen at 4KW but I don't run that much power, however, at around 1200 watts Peak, there have been no problems.

Here is what I've done (https://mikeharrison.smugmug.com/Antenna-Stuff/Antenna/i-4mPcpjc/A)

   

You might also look at the Dow-Key circuit.
Similar power rating, but different tube.

Someone else marketed this sort of thing...B&W?

I was also concerned about W8JI's report on the uSec pass through transient.

I've been told that the back-to-back protection diodes screw up the receiver IMD figures, but I have no
first hand information on that.

Also contact K1JJ, I believe he told me that he had built such things into his amplifiers.
Think he said something about Mosfets too, not sure.

Let me know if you come to any conclusions on this?

I looked at what Mike did. Looks pretty good to me. Rather than dislike those old-design TR switches for their behavioral artifacts which are seen as flaws today, it seems good to try and do something about it. I mean come up with a new circuit based on what is learned from studying the old ones and considering modern receivers.

Protection diodes could cause IMD for the receiver, but wouldn't they have to conduct to cause that problem or do I misunderstand?

Dipoles and really any large antenna can pick up a lot of undesired stuff. Modern receivers don't have so much tuning up front, not like the older tube gear. I think I've seen >1 volt of random stuff from my own antenna.

Mike's setup there's using a BCB filter, but assuming the TR switch he's using keeps the amplifier plate tank on the antenna at all times, so that its tuning helps narrow the receiver's view of the rest of the spectrum, there should be a large reduction in off-frequency noises. If a 'tuner' is being used, the attenuation should be even more.


The Dow-Key circuit is interesting. How does it work? Is the first tube is cut off by grid current charging the 13pF coupling cap? Or is it just limiting the signal by clipping action?  One thing that jumps out at me is the second tube does not look like it is cut off. The 12AZ7 has a max neg grid voltage rating of 55V. Curves the same as the 12AT7. -but the switch rated 1KW instead of 4KW.

When I was using the Johnson TR switch  I never had it cause harmonics or wreck tube receivers.. One note on that setup is that every single thing was as well-bonded to racks-ground as possible, and the racks were as directly earth-grounded as possible.