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SB220 on 11 meter AM. Damage control.

nomadradio

Analog Retentive
Apr 3, 2005
6,935
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Louisville, KY
www.nomadradio.com
The Heath SB220 does a fine job on AM if you read the manual and select "CW", or low side.

It's built for sideband, and the tubes throw a few hundred Watts of heat with no drive in "SSB", or high side. This is called "Idle" or "Zero-Signal" current, and you need that to get a clean signal on sideband.

But each tube is dumping 200 to 300 Watts before the radio's drive power is applied. The added heat from only a 300-Watt AM carrier will overheat the tubes. This is why the instruction manual says to use only low side for AM. That will bring the heat down to a safe level. AND cut your power nearly in half. But that's the manufacturer's advice.

Never have met a AM operator who would stay on "Low" side. The solution is to change the bias on the tubes. Rather than the 100 mA of idle current on each tube, better to hold it down to about 50 Watts per tube. With a 300-Watt carrier, the tubes will be tossing only about half the heat they're rated for. Leaves you the headroom you need for 1500 Watt modulation peaks.

The original 5-Volt 10-Watt zener diode sets this idle current. Taking the two wires loose from it and substituting a series string of rectifier diodes will boost the bias voltage from 5 Volts to about 20 Volts. This will nearly, but not quite cut off the tubes' idle current.

homebrewbiaszenerpz7.jpg


You can just build your own. No secret sauce to wiring thirty 3-Amp rectifier diodes in series. The good news is the voltage rating. That number tells you how many Volts of reverse (turned-off) voltage a diode can handle before it breaks down. You can use the ultra-cheap 50-Volt version, the 1N5400. The diodes will ALWAYS be forward-biased, showing about 2/3 of a Volt per diode. Reverse-voltage rating is irrelevant for diodes that will never have that reverse voltage applied to them. These won't. The regulation of the bias voltage is not quite as tight as the zener was, but plenty close enough to a 20-Volt zener diode. But with about a 20-Watt effective rating. And zeners are famously sensitive to surge damage. Rectifier diodes are a lot more robust in this department.


Or, you could just buy a commercially-made bias board that comes with brackets to mount it behind the meters.


AMBias_v5_sm.jpg

The 10-meter input circuit in the SB220 is pretty flimsy. Okay for 100-Watt peak sideband, but a problem for AM. You can't easily reach the tuning slug to re-peak for 11 meters. And the drive level people try to use on AM just roaches the stock 10-meter coil and capacitors. The easy solution most folks go for it to just cut the coax from the input side of the relay where it goes to the input side of the band selector. The coax gets run directly from the relay to the tubes' cathode circuit.


This eliminates the wimpy-coil problem, but creates a new one. Your driver is now feeding into a 3-to-1 SWR. The amplifier's input impedance with direct drive will be about 150 ohms. This reduces the power you can get into the tubes. Just the same as a high SWR reduces the power you can pump into an antenna.
Most books about grounded-grid amplifier design will mention that a tuned-input circuit works best the closer it is to the tube sockets. Heathkit chose to locate their input circuits on the other end of a 12-inch long piece of coax so they could mount it in a convenient spot. A perfectly-decent compromise, but a compromise.


1nEsOa.jpg

This simple setup does require a couple of one-terminal tie strips like the one already found on the inboard tube socket. This setup has a 68pf disc cap to gorund fom where the coax feeds from the relay, into a 7-turn coil wound on a half-inch form. The other end of the coil connects directly to on side of the tubes' cathodes. A 47pf disc cap goes to ground from there, also. The coil gets fine-tuned by squeezing or stretching the turns for lowest input-side SWR with carrier drive. Should get it down to 1.3 or better.


To make room for this mess, the original fat .01uf disc cap C32 gets moved. It was strung from the single tie point where the coax feeds the tubes, to the tube cathodes.

It gets mounted on the other 1-lug tie strip, between the relay and the input coax.

hISakR.jpg

So long as you use disc caps rated for 2000 Volts or (better) more, this input-matching circuit will take whatever you throw at it. Won't make the amplifier one bit bigger, but just moves the weak link in the chain elsewhere.


This post has already gotten way out of hand. There are other measures to adapt the SB220 to high-power AM, but those are the highlights.

Maybe it's time for a more-detailed "how-to" post adapting the SB220 for heavy-duty AM service.
But it can be done. Just don't expect the factory-stock SSB-oriented setup to withstand 11-meter AM service. Things will break.

73
 

Just don't expect the factory-stock SSB-oriented setup to withstand 11-meter AM service. Things will break.

Agree!...but in most cases Break and or MELT...
But Alas the masses don't believe this in most cases. I get a big kick out of those listing the SB220/221 as a "Legal Limit+" amplifier... or 2KW amplifier...Yea right!
She is all original..."Works Great"
NFL!...( not for long):rolleyes::ROFLMAO:
All the Best
Gary
 
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One important item to remember when using an Amateur amplifier is the switching control.
These amplifiers are set-up to use an auxiliary relay control from the PTT circuit in the transceiver. They are NOT carrier or RF controlled relays. This is why most need to use a foot switch type set-up that controls Both the radio an amplifier simultaneously otherwise you can damage both the amp and radio.
Hence this type of switching device: https://heilsound.com/products/fs-2-footswitch/
So one must remember these are not plug and play units, if the transceiver does not have this provision, as most Amateur transceivers have this provision.
One must also remember if they do not have a soft key module installed the older amps can have in the case of the SB220 which uses 120v switching which will fry even the most protected transceiver relays. Hence so many companies sell an isolation type relay control that separate the transceiver and amplifier circuits so damage does not occur.
Worth noting when buying one of the older Heathkit/Drake or older B&W amplifiers etc..if they have not been upgraded with newer caps and diodes in the last few years this needs to be done, otherwise failure will be eminent...so just a few points worth noting when making your purchase.
All the Best
Gary
 
One important item to remember when using an Amateur amplifier is the switching control.
These amplifiers are set-up to use an auxiliary relay control from the PTT circuit in the transceiver. They are NOT carrier or RF controlled relays. This is why most need to use a foot switch type set-up that controls Both the radio an amplifier simultaneously otherwise you can damage both the amp and radio.
Hence this type of switching device: https://heilsound.com/products/fs-2-footswitch/
So one must remember these are not plug and play units, if the transceiver does not have this provision, as most Amateur transceivers have this provision.
One must also remember if they do not have a soft key module installed the older amps can have in the case of the SB220 which uses 120v switching which will fry even the most protected transceiver relays. Hence so many companies sell an isolation type relay control that separate the transceiver and amplifier circuits so damage does not occur.
Worth noting when buying one of the older Heathkit/Drake or older B&W amplifiers etc..if they have not been upgraded with newer caps and diodes in the last few years this needs to be done, otherwise failure will be eminent...so just a few points worth noting when making your purchase.
All the Best
Gary
Excellent advise! Thank you!
 
Good info on keying the amplifier. Some modern amplifiers use 12vdc for the keying circuit and can be safely connected to the rig without a buffer. If you aren't sure check the voltage on the relay jack. You can also use your multimeter to measure the amount of current drawn by the keying circuit and see if it falls under the max spec for your radio. Some radios will not tolerate much current and even if the voltage is 12 vdc a buffer relay may still be needed.

Some guys like to put an rf sensing keying circuit in these amps like you find in cb amps. Some consider that an upgrade but I don't believe in it. You can key it with a foot switch or make a pigtail for your microphone that triggers a relay and keys the amp when you press the ptt switch. No more crummy ssb delay. The rf sensing circuit allows the relay to hot switch and arc across the contacts. Its even worse if you have a ssb delay. Ideally you'd key the amp, then the radio and unkey in reverse order. Since there is no rf going through the relay as it opens and closes it never arcs.
 
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Some guys like to put an rf sensing keying circuit in these amps like you find in cb amps. Some consider that an upgrade but I don't believe in it. You can key it with a foot switch or make a pigtail for your microphone that triggers a relay and keys the amp when you press the ptt switch. No more crummy ssb delay. The rf sensing circuit allows the relay to hot switch and arc across the contacts. Its even worse if you have a ssb delay. Ideally you'd key the amp, then the radio and unkey in reverse order. Since there is no rf going through the relay as it opens and closes it never arcs.

Absolutely AGREE! The amp must be connected to the antenna BEFORE drive is applied...and then drive removed before amp switches back....otherwise the "HOT" switching of the amp relays occurs.
I won't even use RF switching even if the amp has that provision...I use a direct connection from PTT circuit...Screw the SSB/CW delay...ASKING for FAILURE...
All the Best
Gary
 
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Good info on keying the amplifier. Some modern amplifiers use 12vdc for the keying circuit and can be safely connected to the rig without a buffer. If you aren't sure check the voltage on the relay jack. You can also use your multimeter to measure the amount of current drawn by the keying circuit and see if it falls under the max spec for your radio. Some radios will not tolerate much current and even if the voltage is 12 vdc a buffer relay may still be needed.

Some guys like to put an rf sensing keying circuit in these amps like you find in cb amps. Some consider that an upgrade but I don't believe in it. You can key it with a foot switch or make a pigtail for your microphone that triggers a relay and keys the amp when you press the ptt switch. No more crummy ssb delay. The rf sensing circuit allows the relay to hot switch and arc across the contacts. Its even worse if you have a ssb delay. Ideally you'd key the amp, then the radio and unkey in reverse order. Since there is no rf going through the relay as it opens and closes it never arcs.

Taking notes. So the amp and transceiver can be keyed at the same time with the same switch? Or if using the same switch they still need to be done in this order? Thanks, just trying to understand.
 
A legal ham radio will usually include a socket on the back panel that closes a circuit when the mike is keyed. It's typically labeled "accessory" and may have multiple pins with other features connected to it. A cable from this socket to the keying jack on the amplifier activates it whenever the radio is transmitting. A foot switch is just a way around this setup when using a radio that lacks this feature. A CB would not be legal to sell with this feature in it, since you are not permitted to use a CB with an amplifier. We routinely install a small circuit board with a relay in radios that will be used with a ham-type linear, and connect it to a RCA jack on the rear panel of the radio. A patch cord between that jack and the "relay" socket on the amplifier is all that's needed.

73
 
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Taking notes. So the amp and transceiver can be keyed at the same time with the same switch? Or if using the same switch they still need to be done in this order? Thanks, just trying to understand.

Sequencing is ideal but you can key them at the same time. Just don't connect them to the same switch unless it is a double pole switch. The wiring for the amp relay and mic should not be tried together. I know a guy that does this with an ameritron amp and nothing bad happened but I wouldn't chance it.

What nomad described is easy to do. Get a mini relay and put it inside the radio. connect the positive side of the relay coil to the power jack or find some switched power at the on/off switch. The negative side of the relay coil will go to the TX pin on the mic jack. Now the relay will engage when you key the radio. Then all you have to do is wire the relay to the amp however you like and you've got a keying circuit in your radio that will work on any amp without needing a buffer.

My usual setup uses a Linemaster 636-s foot switch. It has 2 separate switches inside and is dual stage. When you press it half way the first switch keys the amplifier and when fully pressed the second switch keys the transmitter I only do it like this because I use a studio mic and it's convenient. Heil makes a similar 2 stage switch but I found the linemaster cheaper on ebay.
 
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Ahhh, don't try the "mike jack" trick on any radio that has a computer-controlled LCD display.

All of the RCI 2950/70/85/95 radios have a computer that runs the display and tuning knob. This includes the Galaxy Saturn Turbo, Eagle 5000, Galaxy DX22B and 2527, among others.

The problem with hooking a 12-Volt DC relay coil directly to the mike jack has to do with how these radios are keyed. The mike socket "keys" an input pin on the microcontroller (CPU) and only then does the computer turn around and key the radio itself by an output pin from the CPU.

This means that any voltage over 5 Volts feeding into the mike jack's transmit pin will

**** ASSASSINATE ***

the rare and expensive computer that runs the radio. The last remaining new replacement for that board was sold out many years ago. The controller/display board for a "DX" type or newer radio is not compatible. Not even partly.

A typical symptom of this kind of sabotage is a radio that keys up as soon as the power is applied, with the small "TX" indicator on the display always visible, whether the mike is plugged in or not.

The reason we use the radio's internal transmit/receive switching voltages is to avoid this hazard. The 5-Volt CPU chip will NOT tolerate input voltages higher than 5 Volts. 12 (or more) Volts feeding through the coil of a relay will permanently short that input pin on the CPU. From then on, the radio will be stuck in transmit until the blown computer board is changed for a good one.

Best of luck finding that board for the 1995-to-Y2k radios of this family. That computer/display board is made from unobtanium.

Here is what we use. Only takes three resistors a diode and a transistor besides the relay. It's powered by the radio's main 13.8 Volt supply, and draws just under two milliamps from the radio's internal transmit-only voltage. Not enough current draw to cause trouble later. Simply powering the coil of a 9-Volt relay from it could cause trouble later.

gp9joq.jpg



More than one way to skin a cat. But if the cat has a blown CPU, it's already been skinned.

So to speak.

73
 
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