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In high resolution audio circuits, the power supply is of great importance. I've been learning this by painful experience for several years. Here is, approximately, what I've figured out; starting from the AC in (left), and working to the DC outs (on the right). I'll list my vendors at the end. Line Cord I've tried cheesy zip, great big AC - clothes dryer zip, boutique hi-end; and the biggest bang for the buck is 16ga 'appliance cord'; the round black one you can get in Home Depot. Zip, large or small, is terrible. Some of the boutique cords are a little better than the appliance cord, some substantially worse. So far, I've hardwired my power cord and used the appliance cord. In the future, I'll mount an IEC connector on the gear, so I can tweak at will.
Switches and fuses The fuse and fuseholder are standard 3AG (.25" x 1.25") parts from Radio Shack. The use of a slo-blo fuse allows the fuse to be rated more closely to the draw of the circuit for better protection, and still withstand the current surge at turn-on into all those empty capacitors. Take the live (black) wire of the line cord to the end terminal of the fuseblock, as this keeps it away from fingers when the cap is off. If there's a green (safety ground) wire in the line cord, take this to the single point on the chassis where you bring your ground bus or star; this will keep hum and noise down.
Transformers This will be sort of optional, depending on what you, as an amateur constructor, can source. As you can see, the PS consists of 4 secondaries and their circuits. The first supplies the HV main; 300 to 315Vdc, and the driver HV - about 10Vdc lower; at a total of about 150mA. The second supplies 6.3Vac at 3A to all the tube heaters. The third is the 140-150Vdc at 12 - 15mA 'longtail' negative supply for the driver diff-amp. The fourth is the other negative supply, 25 - 35Vdc at a few milliamps, for biasing the output tubes. How many of these will be on your 'main' transformer, and how many will require supplementary trannies, depends on what you can find around, and how you choose to do it. My main trans has the high-voltage winding on it, and the filament winding as well . Many dedicated power transformers will have these, and often a bias winding (actually a tap on the main winding, usually) as well. Rectifier Diodes The ones I've been using are regular 3A 1KV (1N5408) types from Radio Shack, #900-2882; they were the most mongo high-voltage diodes in the place, and were about a buck or so for three. I notice that RS now no longer stocks this diode on the wall, but they do have a 2.5A 1KV diode that will work fine. A better deal is the General Semiconductor UF4007 from Mouser. These are a quarter each; I have a hundred of them coming for $19. They're quieter and have a more easily filtered turn-off transient (the wart on the sand diode's nose) than regular 1NXX-type diodes. The RatShack parts have worked okay for me for three years; take yer pick. A prime reason that I use sand diodes in my circuits (besides that they're cheap, don't require heater supplies, and sound good), is that they lend themselves easily to the circuit illustrated. This is called voltage doubler rectification. There are shortcomings associated with it, primarily poor load regulation; but in these days of limited availability of high-voltage transformers, it's a very useful technique. It takes the 150Vac from the trans secondary and produces approximately 300Vdc or so, depending on the filter circuit and load.
Capacitors Capacitors are one of the main items of discussion in Rec Audio. You ask a hundred experts, you're going to get ninety nine different opinions; but most of them will agree, approximately, with mine. Chokes (Inductors) Having a choke in the power supply (supplies) really blackens the background and increases the effectiveness of the little RF-shunt capacitors. The one shown is from my fixed-to-death Dyna ST-70. Angela sells the equivalent modern Hammond part. A larger value choke will definitely not hurt anything, and Steve also has the 2H similar part. In the future, I'll be putting inductors in all my supplies. In this amp, the negative and bias supplies would also benefit from them; they would replace the resistors shown in the similar location, and the 1K resistor in the main, as well. Hammond has a nice 20H 25mA one that would work for the negative supply, and a keen 150H 8mA for the bias. Get as mongo here as you like, can afford, and have room for. Assorted Stuff Resistors, pots, wire are thankfully not as critical, quality-wise, in the power supply circuit as in the audio signal chain. In fact, very high-bandwidth series parts may be considered detrimental in this application. Low noise is, however, a consideration. I use regular PVC hookup wire from wherever I find it, Radio Shack pots, and regular-grade metal-film resistors. Don't use metal oxides for that last resistor before the power takeoff; they're a little noisy. I have used reclaimed wirewounds from old gear to good effect; it's probably the only thing other than the transformers (and big air adjustable capacitors) that I look for when salvaging.
The HV Main Transformer Problem You may not want to go cruising swap meets for old gear; and even if you do, you may not come up with a 150Vac or 155Vac power tranny. They're not sold, as far as I know, commercially from the vendors; but, fortunately, there are other alternatives available.  The voltage-doubler rectification has been changed to a full-wave rectifier circuit, and the ground now goes to the CT of the winding, with the B+ switch in the ground leg. The next capacitor, labeled '.01µF to 1µF' is the voltage adjust capacitor. Here's how it works: If this cap is absent (has a value of zero), the filter is a choke-input type, and will develop a DC voltage of .9Vac of the secondary. For a 300-0-300Vac secondary, this would be about 270Vdc. At some value of this capacitor usually between 1µF and 2µF, the filter becomes a capacitor-input type, and will develop a DC voltage of  Vac of the secondary. Again for the 300-0-300Vac secondary, this would be about 425Vdc. So, the output voltage may be adjusted over a range of 270Vdc to 425Vdc by changing the value of this cap from, say, .001µF to 2µF. If you have either the 275v or 300v transformers, a stash of .01µF, .1µF, and .25µF caps, and an experimental soul, you're in business at 315Vdc. Actually, anything from 300V to 315V is fine.Using this type of transformer secondary will bring up a couple of considerations, besides the power switch problem mentioned earlier. The first is that, in a full-wave type rectifier circuit, each rectifier sees the entire secondary voltage as its reverse voltage. With a 300-0-300v secondary, this will be 600Vac, or about 850Vpeak. This is within the spec of the 1KV diodes, but not a lot; if you're going to use this transformer, you should put the amp on a good line conditioner / surge protector (a good idea anyway). Sand diodes can be considerably overamped for short periods of time, but overvolting them kills them in a big hurry. How do I know this, hm-m? You will also notice that in this case I have up-specced the filter choke. This circuit will operate at least partially as a choke-input filter, and so the choke will have to deal with a lot more ripple on its input than in a cap-input filter. I would consider the 2H 200mA Hammond as a minimum part in this circumstance, and Allied has a very nice looking 6H 200mA part, #967-1006, for very reasonable money that I bet would work really well. Another transformer that I see a lot of in the catalogs has a single 230-240V secondary. It's, like, a line step-up, and you can theoretically get 325Vac from this voltage with a cap-input filter. With losses, it should come out to just about the 300-315Vac we're looking for. These also come in toroidal types from Hammond, Plitron, and Amveco, which are simple (although bulky) to mount, and have very low strayfield, which helps keep hum out of your audio circuits. If you do use a toroidal power tranny, be sure and use all the little RF-shunt caps shown, since toroids have very wide bandwidth, and will couple RF from the primary to the secondary (and back) very well. Providing 300Vdc at 150mA will take 45VA of capacity; get a tranny of 80VA rating, minimum. The HV main supply will now look like this:  The diode stack is now configured as a full-wave bridge, the rest of the circuit is pretty similar to the original at the top of the page. This is firmly a cap-input filter, so the 1.5H choke will be sufficient; but, as before, more won't hurt. Using an HV main transformer like this, or any other type that doesn't have any of the filament windings on it as well, allows all the power switching to take place at the 120v line side of everything, as you see here. This simplifies switch selection, and usage as well. Identical SPST 6-10A switches can be used for both the filaments and the high voltage circuits. The List Again, here are the guys I use; in no particular order of preference, but approximate frequency of use. <
That's all I can think of. If I've left anything out, or you have other questions, drop me a post at poinxie@yahoo.com. As I said, I'm working on making this amp buildable from the page, and I welcome suggestions. Poinz |