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[KD0VE]

This seems like a simple question but I get conflicting answers when I research it.  I understand the relay in some tube amps can require more than the current rating of the keying output on a newer radio.  Since a mistake could result in damage to the radio I want to make sure I do this correctly BUT I also don't want to add unnecessary hardware.

what is a "failsafe" procedure for determining the correct approach?

thx to anyone taking the time to offer advice.

[K0XM]

Simple. If it is tube amp, and of older vintage, the spec should show wht is the keying line voltage. In my activites, if it is not a newer solid state amp, I run a relay...no biggie.

[K6YE]

There are some amplifiers that have high voltage. Consulting each via its corresponding manual will yield the answer. A lot of hams take one of three approaches if buffering is required:

1. Keying buffer (such as Ameritron ARB-704)
2. Foot Switch (such as Heil FS-2)
3. Modify amplifier by installing a "Soft-Key" modification (such as Harbach).

I use a foot switch for my Kenwood TL-922A and Heathkit SB-220.

Be careful and good luck.

Semper Fi,

Tommy - K6YE
DX IS and CW RULES

[WA2EIO]

The specs for your radio will have the limitations of the amp key circuit.  (Probably in the paragraphs about how to connect an amp, not in the general specs listing.)   They  should include maximum voltage and maximum current that the rig can handle safely.   Compare the capabilities of the rig's circuit to the specs of your amp's keying circuit.   As mentioned, with older tube amps that keying voltage can be up over 100V (+ or-) so yes, you can cause damage  to your rig. 
Consider K6YE's suggestions.   
You might also try to get in touch with the rig's manufacturer and let them know what the amp's keying specs are and if they would suggest a buffer.

[AE9DX]

My theory is: Tube amp + solid state rig = buffer/cheap insurance.

[K7JQ]

According to your qrz.com page, if you're running the same radio/amp combinations, the IC-7300/ALS-600 doesn't require a buffer. The Heath SB-1000's keying circuit is 12VDC @ 100mA, so the TS-590S/SB-1000 also doesn't require a buffer. Of course, if you're talking about other, older amps, follow the other suggestions on this thread.

[KD6VXI]

How do you know?  Experience.

You use a voltmeter.  Find what the open circuit voltage is.

Then look at the schematic for your radio..... Or most manuals.  Find out what the spec is for the amplifier keying circuit.

Some radios, like the older kenwood rigs, had a relay.  Those would trip 110VDC amps, like the SB220, etc., just fine.  Other rigs have a transistor switch.  Those don't.

I built an IRF640 switch inside my SB220.  It now requires milliamps to switch, and no harmful voltages are on the keying line from amp to radio.

It's really a common sense thing.  A DVM and a manual will tell you all you need to do to ensure safe keying.


As an aside, I do NOT use the relay output of the kenwood radios.  That adds additional latency to the TR switching time and can cause hot switching.  I use my IRF640 based line to a different grounded on TX output.  Works fine, and switching time is limited by the time it takes the SB220 to switch (which is still too slow).


--Shane
WP2ASS / ex KD6VXI

[K8AC]

The idea of using a relay or buffer as "insurance" is simply hogwash.  Do what Shane suggests and measure the keying line voltage with a DVM.  Then, using the DVM again but in current measuring mode this time, use the DVM leads to connect the amp's keying line to ground.  That measures the current drawn by the keying circuit in the amp.  Then compare both the voltage and current measured to the transceiver keying specs.  Adding a device as insurance against an event that will NEVER happen is just a waste of time and money.  And - can introduce a delay in the keying line that may cause you other problems depending upon mode of operation. 

73, K8AC

[N2SR]

My theory is: Tube amp + solid state rig = buffer/cheap insurance.

I have a Acom 2000A.  Tube amp.  KEY OUT of my K3S to KEY IN of the amp.  Works just fine, even in the rare times I use QSK.

[K8AXW]

One other factor that determines if it's necessary to use a "keying buffer" if the operator's ability to wade through pages of technical information on two pieces of gear and or do simple voltage/current measurements.

Sometimes it's simply better to  just use a buffer while becoming more familiar with the equipment  at a more comfortable pace and eliminating doubt and anxiety. 

Usually, a buffer is an install and forget item. 

[AE0Q]

My theory is: Tube amp + solid state rig = buffer/cheap insurance.

That is  NOT a good rule.

Almost all modern tube amps (made in the last 30 years) use keying circuits that are solid-state, voltages usually around 12vdc or possibly 24vdc in some.  Some do directly ground a 24 vdc relay via the keying jack.  Using an unnecessary keying interface just slows down the amp relays going into transmit, not a desirable situation.

Just stick a volt-meter in the amp keying jack with amp in Operate to measure the voltage, then put the meter in mA current measuring mode to see what the keyed current is when the key jack is shorted via the meter.  Compare to the specs of the radio being used.

If it is really older, using a 120 vac relay with 120 vac on the keying line, then of course an interface is needed.
Glenn AE0Q

[KD6VXI]

One other factor that determines if it's necessary to use a "keying buffer" if the operator's ability to wade through pages of technical information on two pieces of gear and or do simple voltage/current measurements.

Sometimes it's simply better to  just use a buffer while becoming more familiar with the equipment  at a more comfortable pace and eliminating doubt and anxiety. 

Usually, a buffer is an install and forget item.

I'd rather teach people the proper way, even if it's harder.  This is a technical hobby, after all.

Dumbing it down to Baofeng and a Battery doesn't help.

--Shane
WP2ASS / ex KD6VXI

[KM4AH]

I doubt the transistors in an ARB 704 slow anything down much.
 I'm thinking the relay in an IC751 was only good for like 50 mA .  I know I had to replace one and don't remember it being so much fun.
Anyhow, I have used a buffer ever since. Ya'll can do whatever blows your dress up.

[VE7RF]

The idea of using a relay or buffer as "insurance" is simply hogwash.  Do what Shane suggests and measure the keying line voltage with a DVM.  Then, using the DVM again but in current measuring mode this time, use the DVM leads to connect the amp's keying line to ground.  That measures the current drawn by the keying circuit in the amp.  Then compare both the voltage and current measured to the transceiver keying specs.  Adding a device as insurance against an event that will NEVER happen is just a waste of time and money.  And - can introduce a delay in the keying line that may cause you other problems depending upon mode of operation. 

73, K8AC

U nailed it.  Just measure the ocv (open circuit voltage) of the amp's keyline input.   Then measure with dvm in series  with the amp's keyline.. this time measuring the current.

I would not use a mech relay in the xcvr to key the amp, that adds a ton of delay...= a disaster if using VOX.   In some cases the SS key output of the xcvr  won't handle the current /voltage of the amps keying input..so a SS buffer is used.

You can still easily have an issue when using VOX. On SSB, a lot of xcvr's  output power in just a few msecs.

My yaesu MK-V puts out 50% of full power, after just 12 msecs. Arrl lab measures that by inserting a tone on the mic jack, then hitting the PTT.  On the MK-V, on CW mode only, it has an adjustable 0-30 msecs digital delay, to give the relays in the amp a head start..... without truncating any dots or dashes... on the 1st word sent.

On VOX  SSB, I don't want ANY power coming out of the xcvr, until the relays in the amp have operated. To fix this problem, I use the xcvr's SS key output to key the opto-isolator in the amp, which keys some sped up vac relay used for tr swicthing.  But even they take time.  For VOX  ssb, to get around this, I use the handy dandy TX inhibit function in the MK-V.   I also use the yaeus's small  spst mech relay ( 5 msec operate, 1-2 msec rls)  to actually grnd the TX inhibit lead....dead last. That's gives the vac relays in the amp, a full 5 msec head start. ( that same spst relay in my 1000D is 15 msecs) 

That small spst mech relay in the yaesu was originally intended to be used to key an amp, in cases where the SS keying output of the yaesu was exceeded.    I just put it to a better use.

[HAMHOCK75]

Probably the most dangerous situation I have run across is with the Ameritron AL-84 amplilfier. It has a 12V relay easily driven by most solid state rigs.

The problem is shown below,



The relay has no snubbing diode to prevent a 100 volt spike from occurring when it disconnects. That voltage will exceed the ratings of many transistors.

The solution is a diode across the relay coil.



This is the result with the diode in place.