Back to Articles List

We don't buy or sell products at - We bring together those that do.



Oak box.


Earlier Project

Power Supply below is for amplifiers utilizing the voltage doubler rectifier / filter circuit. By switching primaries it produces AC voltages of 900, 1100, 1200, 1330 and 1440 and the resulting no-load B+ with 2.8 X RMS multiplier.

It utilizes an extra DPDT relay so one of two pre-selected voltages can be selected from the little remote controller box.


Bertha box.

Means of pre-selecting two of the five available voltages available at he little remote control box.

Outlet for 234 or 117 VAC protected by soft start,



Just in case you might need to move it later.            
by Bob Hutchinson, N5CNN

This is about a method of dramatically reducing the weight of the linear amplifier by removing the plate transformer and placing it in a box on the floor or a separate location. This can result in a 50% weight reduction of the amplifier.

                      Click any picture larger.>

Projects in this article:
Make a box
QRO HF-2000
Ten Tec Centurion 422

1936 Chevy pickup?

The manufacturer does a good job of educating the Ham about the dangerous and lethal voltage available from a tube type linear amplifier. Those dangers and more are present when you take on a project like this - SO - DON'T DO IT. Don't even think about taking on any Lethal High Voltage project unless you have received proper training and have practical experience with High Voltage circuits and the dangers. The High Voltages and High Stored Energy in these rectifier/capacitor filter power supplies can
KILL YOU DEAD in an instant.

So don't read any further. Don't attempt this project. If your amplifier is too heavy, get a younger big strong man to move it. Leave it as it is. Don't remove the cover. This project is not for you. Well, you might click in the projects menu to see now it's done, anyway.

When HF linear amplifiers were comprised of two boxes, amplifier deck or box and separate power supply box, the power supply box contained the rectifier and filtering circuits. It also usually had mains circuit breakers, any voltage selection feature circuits and perhaps a soft start feature. Today it's all in the big box and plate voltage selection for legal reasons is a thing of the past. We will leave as much as possible in the original amplifier box and move the plate transformer.

Subtract and add?
The manufacturer can save some pennies by ordering the big transformer with multiple secondaries to provide power for ancillary functions like filament or heater voltage, bias voltage, grid supply, relay power, etc. The better manufacturers will provide a schematic with the amplifier showing these ancillary outputs from the transformer. If your amplifier has this arrangement you will have the option of routing these voltages back to the box from the remote transformer or providing these low power AC voltages from added small auxiliary transformers. In some amplifiers this is not necessary as the low power AC voltages are provided by separate transformers.

Project transformer
I ordered this Hypersil 46 lb. transformer from Peter Dahl with primary at 240 VAC, secondaries of 1900, 2100, 2600 and 2
800 VAC. Amperage is 1.5 ICAS. I suppose it is rated at 4 or 5 KW. This selection of secondary voltages will work for most amplifiers today with full wave bridge rectifiers/filters. It provides B+ voltages of 2660 and 2940 VDC for indirectly heated cathode type tubes and 3640 and 3920 VDC for the popular directly heated, warm glowing type tubes.

Home Depot, Lowe's
I have built lotsa power supplies in the last 30 years. Some really nice ones to show-off and some I wouldn't let anybody see. Many are still in use that I made over 25 years ago. For this conversion application, with the transformer most likely out of the way and unseen, inexpensive stuff can be used. No sheared 6061-T6 aluminum plate or expensive stainless steel. We can use Home Depot or Lowe's quite a bit. For the Ten Tec Centurion project I used some scrap or left over pieces of red oak planks. The bottom is red oak plywood.



This wooden box is designed as a utility power supply, usable for about any outboard transformer project. It utilizes two double pole relays for remote switching and soft start, has an auxiliary single phase outlet for the amplifier and has four HV AC tap arrangements. See pictures.
                                Click any picture larger.>

Once you have the transformer on the bench, plan your components, sizes and dimensions based on your desires. Some options are:

  • 1.  Route primary mains power and secondary connections from the amplifier to the floor transformer with an extension to utilize the existing circuit breakers, switching and soft start facility. This "extension cord" method is the practical way if the amplifier has a single purpose HV transformer. Usually requires Molex or Amp type connector work but it's inexpensive.
  • 2.  Amplifier has multi-purpose transformer, providing HV AC and ancillary power for bias, control, filament, etc. Ancillary power, with the exception of cathode heater power, can be routed into the amplifier, no problem. Heater voltage may become too low if three feet or so are added to wiring. Adding small transformers to the space left vacant by the big transformer may be required. See QRO conversion below for ideas.

Without the rectifier/filter and their circuits in the box, cooling considerations are almost non existent, but I left some gaps or vents in the box anyway.

A table saw or 12 inch miter saw is handy or you can get Home Depot or Lowe's to cut boards to your length. Use some sort of clamping method to hold things while you drill pilot holes for screws. Then drill clearance holes in outer board and chamfer proper side of outer boards for deck screws. I love this drilling and screwing with myself Christmas presents.


  • About 6 ft. of 1x10 boards or plywood.
  • Lotsa 1 5/8" deck screws (not supposed to rust).
  • 2 20A. DPST or DPDT 117 VAC. coil relays, open or covered.
     A 20 A. DPST 240 V. domestic switch from Home Depot or Lowe's will work fine for the switch.
  • 1 1K, 1 watt pot or Dayton 2A560 solid state delay timer for soft start circuit.
  • 2 10 to 20 ohm 10 watt resistors or 10 to 20 ohm thermisters, any steady state, for the soft start circuit. RF Parts.
  • HV 30 KV wire. RF Parts.
  • 1 #6 x 8 or 10 terminal strip.
  • 3 fuse holders. 1 for relays, 1 amp, and 2 for mains.
  • Screws, fasteners, aluminum sheet, etc.
  • Casters from Home Depot or Lowe's

If you use the extension cord method you can skip the info below.

I laid out the relay components, fuse holders and terminal strip on a scrap piece of 1/4" X 9" X 10" red oak veneer and used short #6 sheet metal screws to secure the items. I wired up the first power relay as if it were the on-off switch, which it is. The bat handle switch on the outside panel or the remote connection from the amplifier provides 117 VAC to the relay coil to activate. The thermisters are arranged in series to the transformers primary.

Short job
The thermisters or resistors are only needed in the circuit for a fraction of a second until downstream amplifier components such as filter capacitors, filaments transformers and cathode heaters are softly powered up. After the soft start job is done the thermisters are removed from the circuit by shorting around them with another relay that is activated by the Dayton .05 to 1.0 second adjustable delay timer. The timer starts when the first relay is activated. About .5 seconds delay works well.

You can use a 1 K, 1/2 watt variable resistor here from the downstream side of one of the legs to the relay coil to adjust the coil activate voltage threshold based on startup load. If the load changes the timing will change too. I like the timer because it is not dependent on startup load. Use a separate fuse holder with 1 amp fuse for the relay control circuits.



QRO HF-2000 Conversion
The QRO HF-2000 has a multiple voltage, multiple purpose transformer that provides B+ plate power and the ancillary AC power for filaments, bias, relays, meter lamps, etc. The ancillary power requirements are:

  • 5 VAC @ 30 amps for filaments.
  • 12 VAC at 1 to 2 amps for metering
  • 110 VAC at low current for bias voltage.

Plus the filament secondary winding center tap is used for ALC and Fan 117 VAC power.

This amplifier is already legal limit and extra plate power is not needed but I will use an existing HV power source. A good, detailed schematic is furnished in the manual. An extension of about 3 feet can be made between the male and female Molex plugs to the transformer for outboard benefits. However, this extension method is not a solution for tube filament power unless the extension is extremely short, like one to two feet. Reason, too much voltage drop. See text below.

To use Bertha
This conversion will utilize the 5 KW power supply I made in the summer of 2001, pictures on green background to the left. This Bertha box is for use with amplifiers that use voltage doubler rectifier/capacitor filter/regulator circuits. More below about HV wiring and connection.

The major effect of three feet or so added to the wiring will be on the filament voltage. The voltage for Eimac tubes that provides rated performance and longest tube life is 4.85 volts at the tube socket. Between the nine conductor Amp or Molex type plug and the filament pin at the tube socket a voltage drop of .78 volts was measured. This is about .31 volts per foot. Adding three or four feet would cause a voltage drop of more than one volt, perhaps 1.2 volts. This won't work unless the filament voltage at the plug, under load, is around 6.83 volts, which it isn't. So I'll install ancillary AC power in the space vacated by the big Peter Dahl transformer.

Heathkit continues to serve
Just for such purpose I had saved a filament transformer from an SB-220 Heathkit amplifier that I blew up. It is rated 5 volts @ 30 amps, has the 110 VAC secondary for bias circuitry and center tap for 117 VAC required by the fan and ALC circuits. From Radio Shack I bought a 12 VAC, 1.2 A. center tap type transformer for the 12 V. requirements. Center tap not used. See pictures.

When the transformer is removed from the HF-2000 amplifier circuit the soft-start circuit needs to be disconnected as it will not operate without the rectifier/filter circuit for voltage rise timing. The Bertha box power supply provides soft-start for the HV rectifier/filter and anything plugged into the auxiliary 234 VAC outlet.

I used a piece of scrap 1/8" aluminum to mount the SB220 filament and 12 VAC transformers along with a fuse holder and a solder terminal strip. I bought some Amps connector pins at the surplus electronics store for making the connections. The schematic in the HF-2000 manual gives all the voltages and wire color to make the wiring job easy. I wish other amplifier makers would do the same.

Solder job
Not too bad with right tools, like a 20-25 watt soldering tip and some way to hold things. I clamp a clothes pin in my little swivel vice thingy to hold while I solder.

The filament voltage produced by the Peter Dahl custom Hypersil in the HF-2000 is around six volts at the connector, under filament load of .1667 Ohms (150 watts). About two feet of #12 wire (green) and a long filament choke sags the voltage to about 5.2 VAC at the tube, according to Ray at QRO. The SB220 filament transformer has a lower voltage of about 5.65 VAC because in the SB220 the transformer #6 secondary wire has only a 10 inch run to the stubby filament choke which is connected directly to the tube socket. Minimum voltage drop.

Capture volts?
After taking some voltages with the warm glowing tubes glowing, I found the filament voltage to be 4.65 VAC at the tube socket. Bummer, not enough, .2 volts too low.

I attached #12 soldering stubs to the #6 Heathkit transformer leads, which had been previously cut to half their original length, bound the #12 leads direct and tight to the choke leads on the terminal strip with #22 copper wire and soldered. This bypassed the connector wiring altogether providing a sizable reduction of resistance. Warm glowing tube filament voltage at the socket now at the recommended 4.85 VAC under load. If this could not be done I would need a slightly higher voltage transformer or a small, 1 to 2 amp Variac or transformer to raise the primary voltage by five or six percent to 6 volts.

Lights up
The 12 VAC transformer provided no surprises but I did provide a one amp fuse circuit breaker for it and the remote start circuit. I put power to the amplifier - - -  no, not the zapping voltage, just the 234 volt stuff. Tubes and meters light up properly. Nothing lit up that was not supposed to.

No sawing
I proceeded with the drilling of four holes in the back of the amplifier. Two 1/2" for the Millen HV bulkhead connectors, 1/4" for a 200 Ohm, 12 watt Rheostat for the fan, which this amplifier does not need, it's very quiet. And a 1/4" hole for a 117 VAC RCA plug outlet for automatic remote start of the HV power supply. Holes were dressed and holes were drilled and tapped for the #4-40 screws to secure the Millen HV connectors. Fan speed rheostat was installed and 117 VAC connector finished up.

Using the 30 KV (60 KV breakdown) wire, I patched the HV AC from the Millen connectors to the proper pins in the Amp connector and checked everything, twice and again. Wherever necessary or prudent I slipped heavy clear plastic tubing over the HV wire and checked everything again.

Roll out bertha
This aluminum and red oak power supply was made by this writer in summer of 2001 to supply HV AC to amplifiers that utilize voltage doubler rectifier/capacitor filter/regulator circuits such as Heathkit SB200, SB201, SB220, SB221, HL2200, SB1000 and of course, this fine QRO. It has five primary taps that provide secondary voltages for B+ selections -  2400, 3100, 3400, 3700 and 4100 VDC. It has a remote control for on-off and voltage selection. The Peter Dahl Hypersil transformer weights 64 lbs. This power supply can provide as much HV AC power as the HF-2000 can handle.

The high voltage wire, rated at 30KV with breakdown voltage of 60KV, should provide a huge safety factor for this less than 1500 volt HV AC. However I take the extra precaution of pulling the red wires through a heavy clear vinyl tube and slip smaller clear tubes over the separated ends next to the connectors, all secured with cable ties.

No sparks, no smoke.
I put the HF-2000 back together, moved it to the test bench, plugged it into the auxiliary receptacle in Bertha and touched "ON" on Bertha's remote control. Halleluiah! I love Ham Radio. No sparks, no smoke. Further testing provides excellent results. This conversion cuts the amplifier weight in half and Bertha provides a nice increase in available output power. And, by turning in 50 to 75 Ohms on the fan speed control this amplifier is noiseless. I hear no noise, no hum, no fan. Halleluiah! I love Ham Radio.

Hams have asked me about slowing down the fan and how it affects cooling. Well, being human and having only a 15% duty cycle label on SSB, cooling is no problem. I monitor cooling on all of my amplifiers with a Radio Shack digital thermometer sitting on the exhaust of the amplifier. Nominal for this HF-2000 when doing the yammer, yammer, yammer thing is 95 degrees. See the article about quieting the centrifugal blowers on amplifiers.



Ten Tec Centurion 422
The Ten Tec Centurion 422 model I just reviewed has a single voltage, single purpose plate transformer. The ancillary AC power for filaments, bias, relays, etc. is provided by separate means. (See the Centurion Review.) This, and the possibility of providing additional plate power that this amp really needs, makes the Centurion an excellent candidate for a Lighten Up conversion. Little detail is furnished in the manual about supply voltages but inspection reveals that the plate transformer has but one secondary. Other AC voltages are supplied by what appears to be a standard dual 3-500Z type filament transformer with bias voltage and 12 volts for other circuits.

I measured the no load B+ on the Centurion at 3250 VDC with 245 VAC mains. The power is only 1,200 watts on 75 meters with the Eimac tubes that came in the amplifier. After I replaced the old Eimac tubes with the RF Parts Chi-Com graphite tubes the power output increased by about 125 watts.

A 3 KW transformer with 2800 VAC secondary or similar will make the Centurion a legal limit + amplifier. The Peter Dahl I used in this Lighten Up feature is fine for this purpose. It is capable of B+ voltages up to 4,200 at about 1.0 amps. The 4,000 watt Not-A-Box power supply drives the Centurion to near legal limit in CW mode. It needs more voltage to utilize all of its capacity.

Modify it, whack it.
I believe I'll just take the original Ten Tec putt-putt plate transformer out for good to lighten up the amplifier and install Millen jacks on the back so I can put some real AC power to the bridge rectifier/filter circuit. I'll then have a Centurion of half the original weight and capable of legal limit + power.

I unsoldered the HV AC leads from the rectifier board, disconnected the four primary leads, removed the four 1/4" - 20 x 1/2" transformer bolts from the bottom, slipped a heavy one inch nylon strap through the center metal band on the Hypersil transformer and lifted it out. I put it away as parts.

I drilled two 1/2" holes for the HV Millen jacks, one 1/4" hole for the 200 ohm rheostat for fan speed control and one 1/4" hole for the RCA jack, which provides remote start to the power supply.

The 117 volt accessories
A ground jumper for the fan and a one amp fuse holder for a 117VAC RCA plug on the back to remotely start the power supply, which also provides the soft-start in-rush current protection for the rectifier/filter circuits. (See pictures.)

I spanned the empty transformer space with 30KV wire from the Millen jacks to the bridge rectifier board and soldered the wire feeding the remote start RCA jack. Since the mains circuit in the Centurion would no longer power the plate transformer, I removed the 15 amp fuses and replaced them with 4 amp fuses. I opened the wooden trap door on top of the power supply and connected the HV tap to the proper lug for 3,800 B+ and installed a RCA patch cord for the remote start. After plugging everything up I flipped the bench master switch on. WOW! It lights up!

See the bottom of the Ten Tec Centurion article to see how this sweet amplifier performs with the new power available.



1936 Chevy?
In learning to drive in a beat-up 1936 model Chevrolet pick-up truck with the crank-out windshield, I always noticed the little printed message in the speedometer under the 50 MPH mark. It read, "Safety First" and comes to mind every time I screw around with high voltages. On the left side of the test bench, where we test amplifiers for product reviews, there is a single phase 234 VAC, 30 ampere master switch with an amber pilot ON indicator light. Well, that's fine but I have another work bench a few feet away.

Anytime I am using this second work bench as in powering up an amp or HV power supply I use this ritual: I use a 234 VAC extension cord from the test bench, hang it from over head so the actual connection to the amplifier cord is between me and the amplifier and loosely connected. It is "in my face" so to speak, as a physical and visual warning of the potentially lethal situation. I always have the master switch off when plugging up the device so I have to take a few steps away from the work bench to turn on the power.

High Voltage measurement
Let's handle High Voltage

You sometimes may get the impression that I don't take high voltage lethality serious. Let me seriously assure that I don't screw around when it comes to lethal high voltages. It is deadly stuff and I don't "play" with it or demonstrate bravery to guest.

You will see a few references to high voltage measurements, such as AC plate transformer secondary volts and rectified/filtered B+ DC voltages. How do I do it? I do it the professional and safe way. First I don't demonstrate this to anyone, no distractions - my full attention and concentration.

Master switch off, I clean off the bench. Nothing but the amplifier or device and the actual equipment to be utilized. Usually just the 10 mega ohm input digital volt meter, Fluke 40 KV high voltage probe properly plugged into the meter and ground clip attached to equipment chassis ground. I then run through, in my mind, exactly where I am going to measure voltage with the probe and make a note on a pad off to my right. I note whether the voltage is AC or DC and what voltage or range to expect. After I am clear and focused on what I am about to do, I set the meter for voltage type, slip the fresh thick leather welding glove on to my right hand and with left hand in my pocket, I pick up the high voltage probe and carefully touch the probe tip to the connector, terminal or spot for measurement and read the voltage in decimal kilovolts on the meter.

I lay the probe over by the note pad, remove the glove and note the voltage. Then I repeat for next measurement. After last measurement, I turn the master switch off an go do something else while voltage and energy bleeds off.

Oh yea, a while back I got a grade five surprise. The scale, important upper numbers  - - 

5 getting the shock of your life and scared, almost to death, overcome with self stupidity .
6 as above and being holed in several places.
7 all of the above and knocked on your ass.
8 all of the above and you couldn't breathe and thought you were dead.
9 all of the above and hospitalized, people trying to keep you alive
10 - - - - - - - - - - - SK.


If you would like to publish an article here contact Bob Hutchinson, N5CNN.

Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
713 467-0077