A YC 156 Ten Meter Amplifier with PI-L output circuit

A TEN METER MONOBAND AMPLIFIER

USING THE YC 156

FOR A COOL 1500 WATTS OUT.

built in March 2010 by K1WHS

The Eimac YC156 is a triode that was used extensively in MRI machines. It is rated for pulse operation and is a cross between the innards of a 3CX15000A7 and the anode structure of a 3CX5000A7. It utilizes an indirectly heated cathode, and requires a very long warmup period. Six minutes can seem like an eternity sometimes! The triode is a very high gain design and behaves like a very high power 8877 with higher voltages being acceptable. The grid dissipation is very low and is similar to the 8877 at 25 watts. This tube should be run with a grid overload protection circuit, especially in a contest with unfamiliar operators. High voltage can be in the range of 5 to 6 KV for best gain and output,but for amateur power levels, 3500 volts works just fine.

The plate compartment houses a PI-L circuit. The gizmo on the left in the top photo, is a directional coupler. Plate coil is 1/2" silver plated copper tubing. The two ceramic blocking caps are each 200 PF. Later on they were replaced with 2 Centralab 859 15 KV 500 pf capacitors. The first capacitors worked quite fine in the circuit. I just replaced them because I had come across a few of the larger caps and could not resist installing them. Compare this picture with later views shown below.

Bottom view showing the input circuit during construction. It is tunable from the front panel. The right angle drives are for the vacuum variables. 50 watt Bias diode is on the heatsink on the right. I later modified it with a bigger heatsink and two 14 volt 50 watt zeners in series to limit my idle current a bit. The meter switch in the lower left corner is wired so I could monitor all internal voltages from the front panel. The final design eliminated the zeners altogether and has a FET shunt regulator on a different heatsink.

Another view of the underside of the 10 Meter amplifier. The large reostat is part of a soft start filament heating circuit, along with the large power resistor on the far left. These are installed in the primary of the filament transformer and serve to limit inrush current to the indirectly heated cathode structure. The reostat is adjustable on the front panel and you can adjust filament voltage to exactly 15 volts by looking at the multi meter. You can also see a vane air switch at lower left. This keeps track of cooling air.

A view of the completed amplifier front panel. The white button on the left is the power on switch. The red button comes on after six minutes warmup, signalling it is ready to transmit. The blue button is RF OFF. (to power the amplifier down) The yellow one is the reset button, and will light up when the protection circuit goes in. That circuit senses high voltage and excess grid current. If there is no high voltage, you cannot transmit. Too much grid current, and the amp will trip off. The reset light will light up yellow. You must push it to transmit again. All pretty straightforward. There is a column of colored LEDs in the middle of the panel. They indicate quick status of conditions.

Power On	Green LED
HV Ready	Green LED
Amp On Line	Yellow LED
Transmit	Red LED
Grid Fault	Yellow LED
Air Fault	Red LED

The little tuning dial to the left of the lower multimeter is the vernier input tuning control. I have not put any markings on the vernier dial, but it will tune a wide range of input frequencies. I tried tuning the amplifier to 12 meters and managed to get 1500 watts out, but with poor efficiency. The output tank is not tuned for that band, however the input circuit works at 24 MHz. The small meter between the big grid and plate meters is for reading incident power. All meter faces have new scales made on my Laserjet printer. The two handles were homemade with 1/2" aluminum bar stock. They came out very well and snazz up the front as well as provide protection to meters and buttons.

A good shot thru the rear removable panel, showing the input circuit. It is on metal standoffs, so the coil can extend over the tube socket for short direct connections. The bias circuit is mounted on the heatsink visible in the photo. It gets cooling air that goes around the tube. I built the input circuit outside of the amp initially, and terminated it with a 22 ohm resistor in parallel with 92 pf to simulate the tube. I then fudged with the values for best match. When I had it working, I dropped it in the amp. The input VSWR is 1:1. That worked out sweet! There is a 1.85 uh tunable input coil, a variable cap to ground, and another fixed 1.1 uh coil (air dux) to a .001 mfd 5KV ceramic cap and then the tube cathode.

The front panel wiring was a mess at first. I re-used some of the existing push button wiring that was in the plasma generator carcass already. I filled the open area shown behind the panel, with a 24 vdc switcher supply for controlling the vacuum relays, the keying relay, the six minute timer, plus the AC input relay and the protection board which will go down on the bottom. After I got all the wiring done, and laced it up, the thing looked great! This picture was taken while I was installing the protection circuit board. Some of the plug in relays have been temporarily removed. You can see the new board hanging in mid air before being bolted down.

Here is the almost finished protection circuit board. I got this board from FAR Circuits. It protects against loss of high voltage and excessive grid current. I also modified it to trip when air pressure is lost. If you look closely you can see a little resistor hanging in midair above the board. I can use this to simulate high voltage present when working on the protection circuits. The six minute timer is located just above the circuit board. The smaller realy just to the left is the grid trip relay. On the far left is the power on relay that turns everything on.

The heavy wires are the filament leads that temporarily ran outside the chassis to a remote DC supply. Note the new doorknob caps! They are CRL 859s and rated at 15 KV and 500 pf. Here is another picture or two. showing the new large xmitting caps.

To get a feel for the difference in transmitting capacitors, here is a photo below, with a CRL 850 (5KV) 857 (7.5 KV) and the big 859 at 15 KV. The 859 is so big that I had to weigh the two of them together before I installed them in the amp. Would you believe 3 lbs? They are huge!

This amplifier chassis used to be part of a plasma generator. It utilized a 5CX1500B power pentode. When I got a hold of it, the old tube and socket were gone, but the chassis was in great shape, saving me a bunch of metalwork. All chassis parts are made from 1/8" thick aluminum sheet, making for a very rugged unit. The only serious metal work needed was to cut a huge hole for the YC156.

The finished amp fitted with the larger Centralab 859 transmit caps. They almost look as big as the Eimac triode! I ran some extensive testing on the amplifier into a dummy load, while looking for overheating components etc. Nothing gets warm.The coil remains cool even at high powers indicating reasonable Q with no excessive circulating current. The TUNE vacuum variable capacitor has very low residual capacitance, a must in a good ten meter PI network utilizing triodes. The large silver plated coil has an inductance of about 0.7 uH which is smaller than the modern ARRL handbook specifies. Interestingly, the L portion of the PI-L has more inductance than the ARRL bible predicts. I fudged the values while looking into the output connector and adjusting for minimum return loss. I tweaked the coils for maximum return loss using plate load resistors from 2.2K up to 3.7 Kohms. I saw -70 dB nulls with these two coils. I think my procedure was good, because this amplifier will provide very high gain, and very good efficiency numbers with a wide range of plate voltages. It is happy with both low voltage and high voltage on the anode. All that is required is a different tweaking of the tune and load vacuum variables.

The performance of this amplifier is absolutely great. I am surprised at the high gain exhibited at even the lower voltages this tube is designed for. At 3500 volts it is possible to get 1500 watts out with about 40 watts of drive. At 5 KV I can get 1500 watts output with only 17 watts of 28 MHz energy. This is a gain of over 19 dB. This is a triode, so why is the gain so high? The YC156 is a very hot tube! I tried to see how much power I could coax out of the amplifier, but, alas, my house wiring is not that good, and I have 20 and 30 amp breakers in line that keep tripping at the higher powers! I did see 4000 watts output with 93 watts of drive. If I try to tune it, the circuit breakers pop before I can get a peak! I guess I should put pennies in the fuse box! For use on the band, I hooked the YC156 amplifier up to my old 144 MHz EME power supply. It can put out an amp or so at 3700 volts. At amateur power levels the 3700 volts is a good B+ value. The gain is not as high, but it is possible to get 1500 watts out. with very low drive. I recorded some tuning variables for the CW and phone sections of the band. The rig must be retuned for excursions of 400-500 kHz. Power drops by about 1dB if you do not retune. With judicious heavy loading, I can get full power at 28.0 as well as 28.450, so QSY is simple.

Having a stiff or well regulated high voltage supply is good for linearity, and I have been working on a serious high voltage power supply for use with this amplifier. My remote ham shack is powered by a 20 KW diesel power system that provides 3 phase 120/208 power. I decided to try building up a 3 phase HV supply and was very happy and pleased with the outcome. I am seeing about 2.6% regulation from no load to full load. At 5410 volts no load, the supply will deliver 5265 volts at full load! It helps to have robust components, but the regulation comes mostly from the 3 phase full wave rectification connections. There is some voltage drop in the primary lines, and I suspect most of my loss is due to primary voltage changes before the power supply! Thank you Nikola Tesla for developing 3 phase power and providing such fantastic performance in a high voltage power supply. For the record, the supply used a choke input fullwave design. The choke is 1 henry. The filter capacitor is 40 MFD. The choke resistance is 3.4 ohms. There are six diode rectifier stacks for converting the 3 phase AC to DC. The transformer secondary winding resistance is a mere 10 ohms! A large power resistor limits capacitor charging current, while another resistor in the HV lead protects against arcs within the tube. I will publish a few pictures of the supply and hope to post output and efficiency numbers as soon as I get the YC-156 wired into the new supply. My 3 phase power panel has a 15 amp circuit breaker for the amplifier. 15 amps is more than adequate for ham use, but not good for maximum power out tests! I am afraid that the YC156 is capable of using all of the power available from the 20 KW diesel generator! In any event, I will post some more numbers as time permits.

My first attempt at a filament supply involved a solid state switching power supply rigged to deliver 15 volts at 15 amps more or less. For initial testing I used a bench lab supply set to 15 volts, and it worked swell. The nice thing was that I could set the current limiting right at 15 amps and watch as the filament voltage slowly climbed to 15 volts. There was no potentially harmful startup surge that might shorten the life of the filament over time. Unfortunately the "permanent" switching supply I built into the amplifier produced an annoying S1 noise on 28 MHz that was impossible to get rid of. I could knock it down but could never eliminate it. I used the amp with the switching supply for a few weeks and lived with the slight added noise, but figured I wanted a better solution. I dug through my junk box of transformers and found one that would produce a bit over 15 volts, so rigged up a primary power resistor to lower the voltage and also provide for surge limiting. A big rheostat in the primary provided for exact adjustment of the filament voltage to the recomended 15 volts. There is a position on the multimeter that reads filament voltage so it is easily set right from the front panel.

The relay system in the amplifier utilizes vacuum relays on the output side, and small 24 vdc rf relay in a can available from MAX Gain Systems on the input switching point. Alan Bond at Max Gain has a good selection of both types of relays. The input relay is an Italian design with 4 millisecond switching and power handling of 200 watts cold switched, and 75 watts hot switched. I programmed in a 8 millisecond delay on my K3, and have been using the amplifier in semi break in operation with smooth results. I tried full break in, but the clacking vacuum relays were a bit distracting. I think it worked OK up to maybe 25 wpm.

So how does it work on the air? In the first evening of use at the end of April, 2010, I worked a station in Santiago, Chile on SSB for my first QSO. My first CW QSO was Sao Paulo, Brazil a few minutes later. Then I caught a little E skip to Florida and got some great reports on the audio. No evidence of over driving, flat topping, or splatter. All of a sudden, while Florida is rolling in on Es, I get a call from a KH6 on some form of back scatter propagation. He peaked at about 215 degrees. This is a nice feature. With this amplifier, the DX is calling me for a change!! That didn't happen much with my DX-40 and zip cord dipole. I actually see a fairly big difference between the old L4B amp on ten and this amp. I am running the new amp about 1.7 dB harder and with weak signals it makes a difference. The L4B would make 1000 watts out, but it is really designed for 1000 watts DC input and runs nicely at about 600 watts out. The trick of running CW with the HV switch set for SSB and 2 KW PEP, means that the amplifier runs very hot in a contest environment. I just worried that the power supply would fail. I could keep my mug of tea quite warm while storing it above the 3-500's. Really the amp was just too darn warm for continuous duty. A better fan would help, but I did not want to hack up the Drake L4B. The new single band amp on the other hand, makes for an effective pileup buster and will run all day and night with little heat rise! For example, at 1500 watts output key down steady carrier output for 60 seconds only raises the exhaust air from the tube anode by 20 degrees F. It is the perfect contest amplifier as everything is made with severe overkill both in the amplifier and the power supply. It could be run at well more than 1500 watts for days with nothing being overloaded or taxed heavily. At the amateur legal limit, it truly just coasts along! I cannot wait for good conditions on 10 to really see how it performs. The December 2010 ARRL 10 Meter Contest was a good baptism for the amplifier. I dragged it up the hill and hooked it to the much higher voltage power supply and immediately saw that my bias circuit was not good for the higher voltage. The idling current now soared to 500 ma with a plate voltage of 5300 volts. That is almost 3000 watts of hot air blowing out of the top of the anode! Needless to say, we did not need any heat in the shack that weekend. The amp ran fine even with the excessive idling current.and I pledged to make a new bias circuit that was adjustable for various voltages. I incorporated the new bias circuit into the amplifier shortly after the contest. Now it idles at a more repectable level at 5300 volts. The air out of the amp is quite cool again!