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Multi Amplifier Keying Buffer

Michael S. Higgins (K6AER) on August 18, 2009
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Often I see request for some kind of keying buffer circuit to either key older amplifiers which used negative 125-150 VDC keying or newer amplifier keying which use a low positive key voltage. Each have their problems as they relate to interfacing old and newer amplifiers.

Keying circuits on newer transceivers are limited to maximum key voltage of generally under 20 volts DC and less then 50 mA of current. When you use diode steering to operate several amplifiers from a transceiver such as the Kenwood TS-2000 or newer radios, you may be keying multiple different amplifier key voltages. In the old days this was not a problem for you had plenty of voltage to exceed the voltage drop of the steering diodes and the transceiver had relay keying. With today's microprocessor amplifiers if the keying voltage is above 1.4 volts the amplifier will not key.

In the case of using older amplifier you need a keying mechanism that will handle any polarity and voltage which can be switched by the low voltage keying from the newer transceivers. Some these amplifiers can put quite a keying voltage spike on the key line. The quickest way to blow out your keying circuit in a new transceiver is hook it up to an older grounded grid amplifier such as the TL-922, SB-200 and SB-200 as well as others made before 1995. Many of these amplifiers have vicious spikes as the key voltage is pulled across the magnetic relay coil.

The enclosed schematic is a keying circuit I have used for many years. The component selection is strictly from the Radio Shack component bin. Many might have argued for an optic coupler design or transistor for switching but the read relay will handle 60 WPM code speed with out problem and polarity output switching is not a problem with a relay.

The Lithium 9volt transistor radio batteries will last about 2-3 years with everyday use. They are hooked up in series for the 18 volts. R1 and the LED is used for a keying indicator. D1-D4 is used to isolate various transceivers to key a single amplifier. If you only need to key one amplifier from one radio you can eliminate D1-D4. D5 is to prevent voltage spikes from the reed relay coil on the key line. Keying voltage is 18 volts with keying current being under 25 mA. R2 is to limit the keying current to make the batteries last longer. The 12 volt reed relay will pull in with as little as 9 volts.

The second schematic is how diode steering is constructed from a transceiver to key multiple amplifiers.

Construction is strictly Rad Shack components using dead bug soldering and construction. Total project time will be under 2 hours if your handy with a drill, solder iron and understand schematic diagrams.

I have attached some pictures but due to limited web page size you probably won't be able to tell much in detail.

This is a great beginner project and is much cheaper than ordering a keying interface from MFJ.

Dead Bug construction used between the various RCA Jacks

The project is placed in a Radio Shack plastic box with a metal lid

Member Comments:
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Multi Amplifier Keying Buffer  
by QRZDXR2 on August 18, 2009 Mail this to a friend!
Nice idea.. but the schematics didn't come through. As the hackers of microsoft said with NT (stands for-- Nice try)

hey no different than getting married.. some take two or three times to get it right. I am sure the wizard will wave his wand and make things right.

I am sure lots need the info for them old good amps to get keyed by these new radios. (I still am amazed that the radio is the size of the cigar box-- (for those of you who don't know a cigar box is what 6 inch cigars came in when it was politically OK to smoke)-- and the amp is a giant on the table. Technology has come a long way from the 60's-70's but the old tube amps still to this day can hold their own-- says a lot for Henry, heathkit collins and the others that built 'em for us...
RE: Multi Amplifier Keying Buffer  
by K0BG on August 18, 2009 Mail this to a friend!
It would be nice to know which relay you used. Fact is, I'd be leery in some cases keying a grid-block amp with some reed relays as the current and voltage are well over their ratings.

Alan, KōBG
RE: Multi Amplifier Keying Buffer  
by W9OY on August 18, 2009 Mail this to a friend!
I thought the article was about a new rig keying an old amp so I don't get the grid block keying comment
Multi Amplifier Keying Buffer  
by HC2AD on August 18, 2009 Mail this to a friend!
Nice article... i did something similar... I use an old cellphone charger inside the amp to provide clean 5v... I put it in serial with amp key connector and 5v relay... contact in relay connected to internal key contacts of the amp. To finish it a bright red led in paralell with 570 ohm resistor to the coil in order to turn on when the amp is keyed...
After that my heathkit sb-200 is ready for everything.
73, Allan/HC2A
RE: Multi Amplifier Keying Buffer  
by K6AER on August 18, 2009 Mail this to a friend!
The relay used is a 1 amp 12 volt reed relay. I have not incountered any keying demands on amplifiers that was above 400 mA at 150 volts. The circuit dipicted will handle 99.5% of the amplifiers made.
RE: Multi Amplifier Keying Buffer  
by K1DA on August 18, 2009 Mail this to a friend!
I use 2 dollar Radio Shack FETs.
RE: Multi Amplifier Keying Buffer  
by W9AC on August 18, 2009 Mail this to a friend!
Very useful information, Mike. Folks who like solid-state switching between their rigs and amps may want to consider using MOSFET devices instead of a relay. The Jackson Harbor "Keyall" unit safely switches a maximum of 500V (DC or AC) @ 2.5A.

That pretty much covers any amp of any vintage. Moreover, the device uses an optically-isolated input and a battery could still be used or one could activate the opto-isolator with a wall-wart supply or power from the transceiver.

But, what the Keyall circuit does not have -- and to one of K6AER's points, are steering diodes that should be used when one device is used to switch multiple amps with DC key lines. Amps with AC key lines can still benefit from the Keyall circuit, although the AC key line should be isolated such that one separate Keyall circuit feeds its own amp. While only one amp may be used at a time, it's still a good idea to keep the amp key lines isolated from one another.

One could purchase the Keyall or duplicate it with only four parts. For use with more than one amp in the shack with DC key lines, one steering diode should be added for each amp (e.g., three amps in the shack, use three diodes).

In place of the 1N4148 diodes, I would consider substituting 1N4007 types with the Keyall circuit, especially if a wide range of amp vintages and models will be used in the shack.

Paul, W9AC
RE: Multi Amplifier Keying Buffer  
by AC7NA on August 18, 2009 Mail this to a friend!
I use this:

Brian AC7NA
RE: Multi Amplifier Keying Buffer  
by W9AC on August 18, 2009 Mail this to a friend!
Forgot to add that input steering diodes should also be used (as shown by K6AER) when more than one rig will activate the Keyall circuit. On the input side, the 1N4148 diode types are fine.

Paul, W9AC
RE: Multi Amplifier Keying Buffer  
by W9AC on August 18, 2009 Mail this to a friend!

> "I use this:"

That's another good option when the DC key line is under 400mA. I built one for testing and it performs very well. For the "damn the expense" crowd, probably the ultimate solution may soon be available. See this link:

I'm anxiously awaiting Jim's limited run after the article appears in QST.

Paul, W9AC
Multi Amplifier Keying Buffer  
by VK6HZ on August 19, 2009 Mail this to a friend!
Great article. I could have used this about 2 months ago to switch a TL-922 with my Icom IC-735.

I searched the 'net for days, but couldnt find anything helpful. So with necessity being the mother of invention, I designed my own using a 12V relay and a diode. Very simple cct using the 13.8v that is supplied by the ACC ports on the back of the Icom.

The satisfaction of designing and building something myself was fantastic. Its been years since I have done anything like that...I think sometimes we forget about the experimental side of this great hobby!

Anyway, thanks again for the cct and info. I will now look at improving the design of my Icom buffer and ways of making it smaller!

72's John
de vk6hz
RE: Multi Amplifier Keying Buffer  
by W4VR on August 20, 2009 Mail this to a friend!
When I had tube-type amplifiers I used a MOSFET switch which worked quite well. I will admit though, using a MOSFET switch with the Alpha 89 using PIN diode switching caused some sequencing problems and as a result the amp would go into a soft fault from time to time. I had to resort to a reed relay to T/R this amp.
Multi Amplifier Keying Buffer  
by AB3CX on August 21, 2009 Mail this to a friend!
For us newbies, I wish the author would explain what connects to what, it's not so obvious. What connects to the relay? What connects to AMP Key jack and AMP KEY Plug? I presume the XCVR line goes to the relay jack.
Multi Amplifier Keying Buffer  
by N8DOD on August 21, 2009 Mail this to a friend!
Thanks Mike

Great info, love the skizos and photos. Simplicity at its best.

RE: Multi Amplifier Keying Buffer  
by K6AER on August 22, 2009 Mail this to a friend!
The schematic shows how to wire the circuit. At this point you need to buy the ARRL handbook and the handbook will show you what each component is and how they are in relation to the schematic symbol. Iím a bit surprised with an extra license you canít read schematics but no problem, ham radio is just one long learning curve. This is the fun of having a small project to learn about electronics.
RE: Multi Amplifier Keying Buffer  
by 5R8GQ on September 2, 2009 Mail this to a friend!
K6AER Said:
"Iím a bit surprised with an extra license you canít read schematics....." Amen, I was thinking the same thing.
RE: Multi Amplifier Keying Buffer  
by WA2JJH on September 10, 2009 Mail this to a friend!
It is to bsd the SB-220 uses 120V for keying. You can buy them cheap. Add the improved rectifier/bias board from Harbach. They also sell the HV cap bank. They have a higher erating than the original griefkit.

I find the amp is easier to tune and I can get 800W in the low power mode. My amp came wired for 110VAC. I cant get 220V here. (unless I pay off an electrician.

I went the simple way for the 110VDC to 5-12V.
I added a tiny 12V small 300ma transformer full wave bridge rect, and the LM-317.
The LM-317 is a 3 terminal voltage regulator. 2 fixed resistors(make one 10turn variable to dial up what ever you rigs external amp relay wants.
I used an opto isolator as well. Kept that nasty 110V line isolated from my rig.

To answer an earlier question, why all the work for simple TX/RX/bypass operation when using any EXTERNAL LINEAR AMP.
The FCC demands it. The illegal CB ams use a carrier activated relay. On SSB, these amps use ultra sensitive TX keyed amp. Still syllbles get chopped off.

The life of the tubes are greater when the amp is "soft keyed''

Safty note.....with the SB-220, you may want to design a better 150v power supply. I had a problem when the 150 V filter cap dried up.
The result was some grid bias That allowed full current floe in RX mode. I fried one of the 3-500's.
A cascaded follwed my filiment xfomer smoked extremly. Unfortunily the same transformer that provided bias is in the same frame as the 5V 20amp filiment tranformer.

I am doing a 4th rebuild/design of the amp. I will have a higher capacity supply that will be EXTERNAL.
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