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.
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THIS CAN KILL YOU DEAD, DEAD, DEAD 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.
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?
transformer Home Depot,
THIS CAN KILL YOU DEAD, DEAD, DEAD
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?
Once you have the transformer on the bench, plan your components, sizes and dimensions based on your desires. Some options are:
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.
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.
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.
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
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.
continues to serve
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.
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.
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.
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
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
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
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
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.
117 volt accessories
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.
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.
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 - -
If you would like to publish an article here contact Bob Hutchinson, N5CNN.
Bob Hutchinson, N5CNN