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Author Topic: Vacuum Tube T-R Switches  (Read 34655 times)
KC5LY
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« on: June 07, 2014, 05:04:17 AM »

There was an article in QST Jan 1961 issue that described a tube type T-R switch using a 6HA6 tube. That particular tube was superseded by the 6HB6. Both tubes were intended for television vertical amplifiers and were similar to a 6BQ5 except for pin-outs. Did anyone ever try a T-R switch using one of the low plate voltage tubes used in late 1950s or early 1960s automobile radios?

Joe
KC5LY
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G3RZP
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« Reply #1 on: June 07, 2014, 06:26:18 AM »

The problem would be the voltage on the transmission line. In a 50 ohm system, anything over around 3 watts would be more volts than the tube is rated for.
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KH6AQ
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« Reply #2 on: June 07, 2014, 07:47:40 AM »

I looked at the article and it's an interesting circuit. The author says that with a 6AH6 it can T-R 300 watts. The transmitter RF voltage creates grid-leak bias and shuts the tube off. During receive the tube acts as a cathode follower. Its impedance as a follower is 1/gm or about 100 ohms. Into a 50 ohm receive path gain is -9 dB.

Checking the 12DZ6 space charge tube it looks like it would work but at what power level I don't know. It depends on the breakdown voltage from G1 to the cathode or G2. If it does break down to the cathode the TX is connected to the RX through 47 pF. The gm is 3800 uMHO and the receive path gain is -16 dB.
« Last Edit: June 07, 2014, 07:53:18 AM by WX7G » Logged
AD5X
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« Reply #3 on: June 07, 2014, 09:00:28 AM »

I actually built and used that TR switch when I was a kid in 1964.  I remember that it did work, but I kept having to replace the tube.  I was using a Knight T60 transmitter (and R100A receiver), so transmit power wasn't that high.

Phil - AD5X
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KD0REQ
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« Reply #4 on: June 07, 2014, 02:29:45 PM »

the knock on the EF Johnson one back in the 60s was that in transmit, it generated hash, lots of it.
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WB2WIK
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« Reply #5 on: June 07, 2014, 03:42:55 PM »

the knock on the EF Johnson one back in the 60s was that in transmit, it generated hash, lots of it.

I used the E.F. Johnson electronic T/R switch (1950s vintage, it was an accessory to their 50s vintage transmitters) and never had any problems with hash, or really anything else.  Theirs was rated for quite high power (maybe a kilowatt, I forgot) and it worked very well for me.  As I recall I used it on 80-40-20-15 meters and don't think I ever used it on 10m, so don't know if it degraded receive performance up there (it might have).  Used it with a variety of transmitters from Rangers to Valiants to Vikings, a DX-100 and a few other boat anchors, along with an NC-183D, an NC-300, SX-101A and 75A-3 which were my main station receivers over those years.
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KC5LY
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« Reply #6 on: June 08, 2014, 06:13:45 AM »

Phil;

I am not surprised that you built and tried that circuit when you were young. I took a closer look at the parameters on the low plate voltage tube I asked about and see that it is only rated up to a plate voltage of 16VDC, so it would likely only be useful at very low power levels. The older design obviously does create some loss since there is an impedance mismatch to begin with. Cathode followers always cause some loss compared to the input signal level - typically run about 90% of the input signal.

Joe
KC5LY
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W1BR
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« Reply #7 on: June 11, 2014, 08:51:51 AM »

the knock on the EF Johnson one back in the 60s was that in transmit, it generated hash, lots of it.

Hash was only a problem with certain rigs, such as the HT-32--that design kept the finals biased for linear operation in standby.
The designed used the PA tank for the input tuning on the TR switch. Hence the need for short coax cables, etc.

Whether one had a problem or not is a moot point; and just points to dumb luck rather than an understanding of the Johnson manual, which clearly points this out.

Pete
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N3QE
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« Reply #8 on: June 11, 2014, 09:58:46 AM »

Yeah see, a vertical deflection tube will be designed and rated to handle ridiculous element voltages (in case of 6AH6 it is rated to 2500V on the plate and 300V on the grid). Deflection waveforms are mighty wicked.

The automotive-radio tubes with 12V plates were designed for much lower voltages.

Note that transmitter RF voltage is applied to the TR tubes grid in the 6AH6 classic designs: http://www.arrl.org/files/file/Technology/pdf/QST_Jan_1961_p20-21.pdf

Tim.
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N2EY
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« Reply #9 on: June 13, 2014, 07:59:23 AM »

There was an article in QST Jan 1961 issue that described a tube type T-R switch using a 6HA6 tube.

The tube is a 6AH6. It's a high gain pentode; miniature equivalent of the 6AC7.

 
That particular tube was superseded by the 6HB6.

The 6HA6 was indeed superseded by the 6HB6. But neither tube was used in that TR switch.

Compare the specs of the 6AH6 and the 6HA6. They are totally different bottles.
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G3RZP
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« Reply #10 on: June 14, 2014, 03:21:29 AM »

Of the two, the 6HA6 might have been a better choice because of its voltage ratings. But the 6AH6 T/R switch is in the 1958 ARRL handbook, as is one using a 6C4. Presumably, the 6HA6 wasn't around then.
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KE0ZU
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« Reply #11 on: June 15, 2014, 11:29:30 PM »

I've used a Viking T/R switch for many years with lots of different transmitters and receivers.   Never had any "hash" problems.  Here is a pic of my Gonset GSB-100 driving my National linear to 100W AM carrier. Note the 4KW PEP notation.





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Regards, Mike
https://mikeharrison.smugmug.com/
Pics and bold print are usually links.
KB1WSY
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« Reply #12 on: June 17, 2014, 06:45:47 PM »

"Understanding Amateur Radio" (1963 edition) reproduces the project for the T/R switch using a 6AH6. "The principal point to observe is that the antenna and transmitter connectors should be close together, with the tube mounted alongside them so that the shortest possible grid lead can be used. This helps prevent stray coupling between the transmitter and receiver."

73 de Martin, KB1WSY
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