807 Tube Audio Amplifier Schematic High Quality

At the very bottom of the page, you will find the schematic and parts list for the 807 CW transmitter. I suggest you read the following text carefully first, before you spend any time with the schematic. There's some good information here, and some important safety information that you must not skip.Notes:- Plate current at maximum output power should be in the vicinity of 110 to 120 mA with a high B+ supply of about +750 VDC.- You can use anything from +500 VDC to +800 VDC for the high B+ supply. Power output, of course, will be a function of plate voltage.- You can use anything from +200 VDC to +300 VDC for the low B+ supply, but don't exceed +300 VDC because the maximum rating of the 6AG7 is +300 VDC.- You can either build your own power supply or buy a commercially-made supply.- If you want to buy a power supply, two great candidates are the Heathkit HP-23 and HP-23A. Use the 6.3 VAC output for the tube filaments, not the 12.6 VAC output! I recommend that you set the Low B+ selector to the 250 VDC position because the +300 VDC setting will probably run higher than +300 VDC at today's higher AC line voltages. You can also use the HP-23B, HP-23C, or PS-23, but you'll have to wire the tube filaments in series and use a 12 Volt pilot lamp bulb because those later models don't have 6.3 VAC filament outputs. The older Heathkit HP-20 will also work, as long as you wire it to put out 6.3 VAC for the filament voltage. The much older Heathkit UT-1 will work, although the High B+ is only +600 VDC, so your transmitter output power will be substantially reduced. As with the PS-20, be sure to wire the filament output for 6.3 VAC. The Heathkit HP-23 and HP-23A supplies are quite common, easy to repair, and don't cost an arm and a leg, so they're really a good choice. Other vintage power supplies should work fine, too, as long as they meet the voltage and current requirements for this project.- If you want to build a power supply, refer to older editions of the ARRL Handbook from back in the 'vacuum tube days'. Make sure that your chosen power transformer can supply enough current to power the oscillator and power amp. Look for one that can put out at least 150 mA. If you have a large enough chassis and cabinet, don't be afraid to build the power supply into the transmitter.- You will find the tube pinout information in any older edition of the ARRL Handbook, GE or RCA Vacuum Tube Manual, etc. If you don't have an old Handbook, GET ONE. They're readily available on eBay and at hamfests. Until yours arrives, you can pay a visit to your public library. They should have one.- Trimmer capacitor C1 should be adjusted for the least amount of crystal 'chirp'. Although a value of 8 - 50 pF is shown in the parts list, anything in that general range will work fine.- Feel free to install several different types of crystal sockets, wired in parallel, so that you can use different holder types. The most common ones will be the ol' faithful FT-243 and the more modern HC-33/U, and you might want to include one of those big ol' sockets for BC-610 crystals. Alternatively, you can make up some adapters, which is what I did. Having more than one socket type means that you'll have more chances of finding crystals that you can use. Don't bother with those tiny little HC-18/U and HC-25/U crystals (the kind used in CB radios, scanners, and so on). They aren't designed for the kinds of voltages that this transmitter will present to them.- SW1 is the 'XTAL / VFO' switch. Setting the switch to the 'VFO' position converts the 6AG7 oscillator into a buffer stage. The 0.01 uF capacitor that is switched into the circuit kills the tendency for the 6AG7 to oscillate.- SW2 is the 'Spot / Send' switch. In the 'Spot' position, it allows you to key just the oscillator so that you can find your signal with your station receiver. In the 'Send' position, the PA becomes active as well.- The oscillator stage is the classic 'Grid-Plate' oscillator from the ARRL Handbook. For best performance, the ARRL recommends that you stick close to the component values shown in the parts list for the oscillator stage. I've found their recommendations to be correct.- When you choose your fixed-value capacitors, be mindful of the DC and RF voltages which could be present in the circuit! I suggest you use caps with at least a 1 KV rating for those parts whose ratings aren't otherwise specified.- I recommend that you make the 80m and 40m coils interchangeable so that the rig will work on both bands. There are several ways you can go with this. You can use vintage plug-in coil forms, which require a matching socket (you'll find 4-, 5-, and 6-pin styles out there). You can use octal tube sockets and tube bases (salvaged from some old 'dud' tubes). You might get lucky and find some vintage ceramic James Millen jack-bars which will allow you to use B&W or Air-Dux coil stock. Just use whatever you can find, but be sure you can find enough to handle the number of coils you intend to use (two per band!).- If you find coil forms that are some diameter other than 1-1/4', that's no problem. You'll just have to re-calculate the coil parameters. You'll find the formulas for calculating coil parameters in any ARRL Handbook (how's your high school algebra these days?). No, I will NOT help you do this. If you don't already know how, then you need to learn how. I'm not trying to be a 'meanie'. If you plan to build equipment like this, you'll have to know this stuff anyway.- If plug-in coils aren't your thing, you can always come up with some kind of band switch arrangement. You'll need to be able to switch both the oscillator and PA tank coils. You could use separate switches or a multi-deck switch for this.- No, I don't know where you can buy these parts, so please don't ask. You'll have to scrounge for them, just like I did. Where should you look? Hamfests, surplus stores, electronic parts catalogs, other hams' junkboxes, eBay auctions, For Sale listings on QRZ.com, QTH.net, and the Usenet newsgroups, and so on. I often buy other hams' old homebrew projects and old military surplus equipment and strip them for parts. Hunting for the components for a project like this is a big part of the fun! Besides, nothing truly rewarding ever comes easily. The harder you work for something, the more it will mean to you in the long run.- You'll need some means of switching your antenna between your receiver and the transmitter. Go figure it out.- Don't try to use your typical electronic keyer with this cathode-keyed circuit unless your keyer has a keying relay or a really stout keying transistor that's rated for high voltages and high currents.- Tuneup is simple. Set C7, C14, and C15 to mid-range. Connect the transmitter to a suitable antenna or 50 Ohm dummy load. Plug your Morse key into J3. Apply power to the transmitter and let the tube filaments warm up for a minute or two. Set SW2 to 'Spot' and hit the key. Tune in your oscillator signal on your station receiver. While listening to the receiver, adjust C7 for the loudest signal. Let off the key. Set SW2 to 'Send' and hit the key. Watch the plate current meter and adjust C14 for the LOWEST plate current (known as the 'dip'). Adjust C15 to bring up the plate current by 10 to 20 mA, then dip again using C14. Repeat this alternating process until you're loaded to 110 to 120 mA output. You can watch your Wattmeter also. When bringing up C15 no longer increases your power output, you're fully loaded and there ain't no more power gonna come. In any case, C14 will always be your last adjustment of the PA stage. If your keying sounds chirpy, you may need to touch up a bith with C7.

807s were used in audio power amplifiers, both for public address and Hi-Fi application, usually being run in push-pull pairs in class AB1 or AB2 giving up to 120 watts of usable power. The plate voltage limit is 750 volts and the screen grid limited to 300 volts. Because of the 300 volt screen grid voltage limit, the 807 cannot be triode connected for high power applications. Failure to observe this precaution will cause screen grid failure. Less commonly a single 807 was used in a pure class-A, single-ended audio output stage delivering about 10 watts.

The 807 is fully rated to 60 MHz, derated to 55% at 125 MHz in Class C, Plate-modulated operation, thus they were popular with amateur radio operators (radio hams). In this application a single 807 could be run in class-C as an oscillator or amplifier which could be keyed on and off to transmit Morse Code in CW mode. For voice transmission on AM a final amplifier with one or more 807s, up to about four, could be connected in parallel running class-C. Connecting multiple 807s in parallel produced more power to feed to the antenna. Often the modulator stage (simply a transformer-coupled audio amplifier for A.M., with the secondary of its output transformer in series with the anode supply of the final amplifier), was also constructed using 807s. Many hams found multiple paralleled 807s a cheaper alternative to a single larger valve, such as a single 813, as many military surplus 807s became available cheaply after World War II. In Australia 807s are affectionately referred to as 'stubbies' because they are almost as ubiquitous as that common Australian beer container.The class C operational values in the info box at the right are for anode modulated A.M. operation; for CW operation a maximum anode voltage of 600 is permissible, whereby the anode current increases to 100 mA and the anode/plate dissipation rises to 25 watts. The screen voltage is the same, at 300, but its dissipation rises to 3.5 watts.37 watts of R.F. power is produced from 220 mW of drive but only a 50% duty cycle is allowed. The maximum allowable negative control grid, g1 excursion allowable is -200 volts and average control grid current is 5mA in both A.M. and CW modes.[1]Later versions could be used on CW with a supply voltage up to 750 V and a current of 100 mA to produce 50-55 watts of output power. 2b1af7f3a8