Notes on Parts Selection and Procurement

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 link my vendors the first time I mention them, and again 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.
        The use of two power switches for separate switching of the tube heaters and the high-voltage circuits is known in DIY audio as a Human Powered Time Delay; and is a feature made neccessary by the use of sand (silicon, solid state) diodes for rectification.   The tube heaters take a rated 11 seconds to bring the cathodes to operating temperature, while the diodes will apply power instantly to the HV main circuit, which will bring the tubes' plate-cathode circuit up to voltage in a second or two.   Two things will then happen.   Since there's no current draw in this circuit yet (the cathodes not being up to temperature, and unable to emit electrons), the voltage can rise rather higher than its steady-state value in the operating circuit, stressing components.   Also, the application of high voltage to a cold tube can shorten tube life substantially.   There are a few methods of addressing this problem; separate switching of the heaters and the HV circuits is the simplest.   The appearance of Switch 1 in two places is an artifact of the drawing.   This is a 10A 120V SPST miniature toggle, also from RatShack.   AC live (black wire) from the fuse goes to one side, and the main and bias transformer primaries to the other.   Switch 2 is a 6A 120V (4A 240V) DPDT miniature toggle, also from RatShack.   One pole of this switch switches the line AC to the primary of the HV negative transformer, and the other pole switches the 150Vac that supplies the HV main voltage doubler.   There is a code issue here, which will become more plain when I explain the HV main circuit.   Most of the old orphan gear I've seen like this used an HV winding of 150 - 170Vac, and selenium diode voltage doubler rectification.   If you can't find a power tranny like this, and are obliged to buy new, you may be using one with an HV secondary of 250-CT-250Vac, 275-CT-275Vac, or 300-CT-300Vac.   In these cases, you are outside the voltage rating of a 240Vac switch, and will have to find a high-voltage switch to stay legal.   I have used 240Vac switches in these situations without problem, but you must understand that I can not recommend or suggest that you do so.   Rules is rules, and to understand how switches are rated for voltage, and the consequences of operating outside that rating, you should be an EE with specialization in that specific subject; and I'm not.

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.
        The main high voltage winding and its circuit can be of several topologies, depending on the transformer sourced.   Mine is not exactly as drawn, actually.   Since the 150v trans from the receiver was in my tube tester chassis, I used a very similar Fisher unit, which has a 155Vac secondary, and put a 100ohm 10W cement block resistor in series with the choke to bring the voltage down from 325Vdc, which was occasionally eating one of my 6V6's.   As far as I know, transformers like this are not available commercially, so if you are going to buy from a vendor, you'll have to choose from other options.   I'll continue with the description of the circuit, since most of the components are common to all; I'll explain where they differ.
        The tube heaters will require 6.3Vac at 3A.   If you use some sort of dedicated tube power tranny (two of the three options presented here), there will be such a winding on the main transformer.   If not, you'll need to source such a transformer. This is a very common value, available from any of the vendors except, oddly, RatShack; theirs are all 12.6V center-tapped units, from which you can make a 6.3Vdc supply, but why?
        The HV negative supply transformer is a little 6VA flat-pack 120Vac that I got from RatShack on order; but there is a mounting issue with this unit, since it's meant to be PCB mounted.   A few of the vendors have chassis-mount transformers like this; look around.   This unit needs to supply 10-20mA at -140Vdc, so 6VA is fine; but a 12VA unit would be good too, if you have the room.
        The bias transformer in my amp is a tiny chassis-mount 12.6Vac unit that dear old RatShack keeps on the wall, part #273-1385, for about six or seven bucks; amazing.   I like to buy stuff from them like this whenever I can to try and keep them stocking.   Where I live, they're the only game in town, and they're slowly backing out of carrying these things in favor of cellphones and computers; I'm trying to delay this process as much as possible.

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.
        The first cap, you will notice, is a small-value (.01µF to .25µF is shown) film-foil or NPO ceramic capacitor between the diode stack and the transformer, and between the diodes and the PS circuit, in all these schems.   This cap shunts the sand diode turn-off transient burst to ground, so it doesn't exite high frequency resonances in the tranny windings or appear in the power supply.   I recently started using these, and noticed a slight but definite blackening of the background in the amp.   They're optional - the first place I put them was right after the diodes, before the choke.   The rest help, too; and they're cheap medicine.   The guys on the Bottlehead list like 1KV 10,000pF (.01µF) ceramics in this application.   Angela has the Sprague 716P's up to .47µF (the ceramics only come up to .01µF), and I'm always ordering from them; they work fine.   Again, take yer pick.
        For the larger-value caps, those marked (with a '+') as being polarized, there are several alternatives.   The first is a common electrolytic, available from the bulk vendors, and sometimes even from Radio Shack, in their assortment pack (these vary in their contents; look inside and see what you can see).   The (22µF 450v) voltage doubler caps on the HV main supply are this kind.   If they're 'visible' to the audio circuit, these can sound grainy; but here, with that small-value film-foil and the choke between them and the circuit, they don't seem to make audible trouble.   The final big caps in my power supplies have been Elna Cerafines up to now, available from Angela. These are no longer being made, and the price is gradually going up.   If you have plenty of dough, the acknowledged best of this type of cap is the Black Gate, from Angela or Michael Percy.   An economical alternative that many people like a lot is the Panasonic TSHA / TSHB electrolytic, available from Digi-Key.   I haven't tried them, they're about six bucks.   For applications of up to 100v, a very good cap is the Nichicon, also economical, and also available from Michael Percy.   I use these in the bias supply.

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.
        One is a dedicated power transformer; Hammond is the most common, and Angela has a very complete stock of them, including the 270HX and 370HX which have 275VCT secondaries at 200mA, and the 272FX and 372FX which have 300VCT at 150mA (or the 272HX at 300VCT, 200mA).   As I said, these will all have the 6.3Vac filament winding on them as well, and the 300 series will have a bias tap on the HV winding, as well.   In the case that you use one of these, the HV main circuit will look like this:


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  1.414Vac 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 (this is a .pdf page; you'll need Acrobat) 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.

  • Angela Instruments   Tubes, sockets, resistors, capacitors, transformers, chokes, other stuff. Nice folks.
  • Triode Electronics   Tubes, sockets, transformers, Other stuff. Good links to tube data and schematics.
  • Michael Percy   Wire, solder, switches and pots, connectors, capacitors, resistors, the fancier stuff.
  • Antique Electronic Supply   Tubes, sockets, interesting transformers, capacitors, guitar amp and old radio parts.

  • Mouser Electronics   Dale resistors, switches, cat5plenum wire, UF diodes. A general vendor, lots of interesting stuff.
  • Digi-Key   Much like Mouser, some different brands.
  • Allied Electronics   The biggest of the general vendors. Higher voltage transformers, Nichicon caps.
  • Audio Note Great Britain   Nice transformers, also tube kits and other parts.
  • Sowter Transformers   Very good transformers. Specializes in custom winding, if you get that far.

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

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