$49.99 Buy It Now or Best Offer
free,30-Day Returns
Seller Store originalnomadradio
(3141) 99.5%,
Location: Louisville, Kentucky
Ships to: US,
Item: 116143889400
All returns accepted:ReturnsNotAccepted
Heathkit:Heathkit
Type:Linear Amplifier
Heathkit SB-220:Heathkit SB-220
This product probably deserves to have the safety-hazard disclaimer placed UP FRONT where nobody can miss it. Yes, the Heathkit SB220/221 has deadly-hazardous voltages inside, possibly even after the plug is pulled from the wall. Proceed only if you possess proper safety training and high-voltage experience. By reading this sale description you hereby agree not to kill yourself attempting to install our product. If this is not acceptable, DON’T READ ANY FURTHER! Besides, if you do end up dead fooling with the inside of your SB220 or 221, and your heirs sue me for wrongful death, I will immediately file a countersuit for three times the damages you seek, claiming breach of contract. You did agree not to, after all. So here is a replacement for the 10-Watt zener diode used to provide a fixed bias voltage in the Heathkit SB220/221 amplifier. It is suitable for most any other amplifier with both a pair of 3-500Z tubes *AND* a tuned input circuit. The 10-Watt zener diode in the Heathkit SB220 will last for decades if you treat it nicely. And if there are any surges or hard mileage the bias zener frequently fails. Usually as a dead short. Usually after a user selects “SSB” side to operate AM mode. Heat dissipation of the tubes goes through the roof after the zener shorts. Melted solder in the tubes’ filament pins is what usually comes next. Just bear in mind that when a tube is abused to this extent, that tube socket must also be replaced along with the tube that overheated and ruined it. If you try to buy a replacement for the 10-Watt bias zener you will find that this kind of component has gotten expensive since 1975. This circuit board replaces that zener with a series string of common rectifier diodes in series. The voltage drop across a forward-biased diode remains fairly constant. Each diode is only good for about 2/3 of a Volt by itself. This board has thirty of them in series for a max bias voltage of just over 20 Volts. The diodes are rated for 3 Amps, and should survive any surge that the HV power supply can withstand. The factory-original zener diode placed just under 6 Volts of bias on the tubes. This puts a heat load around one-half the tubes’ max ratings with no drive power at all. That’s perfect for sideband, more or less. There is no carrier when you key the mike, and only your voice audio adds to the tubes’ heat load. But the continuous added tube current from a carrier in AM mode makes the tubes run too hot unless you select “CW/Tune”. This is how the Heathkit instructions tell you to operate this model for AM. Cuts the power down somewhere between one half and two-thirds what it does on the “high” SSB side. And it’s guaranteed that people will ignore the instructions and select “SSB” mode on the amplifier to use it for AM. This overheats way too many things. Don’t really need a list. But hey, that’s what they want to see on the wattmeter. Go figure. Increasing the fixed bias voltage on the tubes will reduce the heat load created by the AM carrier. This board will allow you to operate AM mode with the amplifier selected for “SSB” side. Just one problem. You can’t just bypass the amplifier’s tuned input circuit. Never mind why. Not my fault. The tuned input circuit just needs to be there. Bottom line, this board replaces the factory zener diode. Should be less fragile if a surge occurs. The board has optional hookups where you can use less than the full thirty diodes if desired. It comes with two “L” brackets that fasten between the solder side of the amplifier’s meter/HV circuit board and the threaded posts at the upper two corners of the board. Hookup is simple. Unsolder both wires from the meter/HV board that lead to the zener diode AND REMOVE THEM BOTH. There will be a fat black wire soldered to the lower pad where the zener wires were removed. This wire leads to the relay underneath. Unsolder this wire from point “E” on the meter/HV board. Insert/solder this wire to the outboard hole on the bias board marked “SB220 RELAY or cathode”. Insert/solder the white wire from the bias board into the upper one of the two holes you just removed the zener wires from. It’s usually labeled “F”. No, there is no YT video or web page showing how to hook this thing up. You just saw the whole process. Pull the two zener wires from “E” and “F”, remove one fat black wire from “E” and move it to “Relay” on the new bias board. Now insert the wire from the new bias board into the hole just above where the black wire came out. No video needed. There are two pads at the front edge of the board, one marked “Alt. Relay 20” and “Alt. Relay 26”. The “26” hole is for the Drake L4 amplifier and others that used 2600 Volts for the anode supply, instead of the 3000 Volts used in the SB220. This board is based on a rule of thumb, one diode for every 100 Volts of anode supply. The “Alt. Relay 20” will place only twenty of the thirty diodes in the circuit. This supplies about 12 Volts of fixed bias, for those who want more than the minimum idle current. If you want to bias an amplifier with more than two tubes, you will need bigger diodes than these. This is an accessory for an amplifier that **WILL KILL YOU** if you fail to exercise safety protocols. If you don’t have those skills, hire someone who does. A bigger signal on the air ain’t worth dyin’ for. Oh, and about the Drake L4 amplifiers. Takes a different-style bracket and slightly-different hookup to the relay. With any luck we’ll get that version posted for sale before long. Yeah, we always say that. There are NO RETURNS AT ALL FOR ANY REASON OF ANY KIND!No matter how good they sound. We ship no later than the second business day following receipt of cleared payment. I prefer USPS First Class. It’s been VERY reliable and comes with tracking. I will insure it for the final sale price. IF YOU PREFER A DIFFERENT **USA ONLY** SHIPPING METHOD, CONTACT ME **BEFORE** YOU PAY! I will prepare a quote for the method you specify and send you an invoice for the corrected amount. This option is available to USA destinations ONLY! !!!! NOTE !!!!Shipping to destinations outside the US are by the Ebay GSP or Global Shipping Program. How much that will cost you is anybody’s guess. Ask Ebay if you want to know. Maybe they’ll tell you. They won’t tell me. Maybe they should rename the “GSP” the “Global Secret Program”. GSP ONLY!! GSP ONLY!! GSP ONLY!! No, I will NOT ship direct if you are outside the USA and its colonies. GSP ONLY!! GSP ONLY!! GSP ONLY!! If you know what it is and what it’s for, you can decide if you want or need it. And if you don’t know what it is, you probably don’t need it. Trust me on this. All routine Ebay precautions apply. Scammers take your nonsense elsewhere. Please. Thanks for looking and happy bidding!
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