Titan 425 P Hybrid Conversion
Real power from the Titan
with an extra KV of B+
and an extra KW of PO

Bob Hutchinson, N5CNN

Upgrade to the 3CPX800A7
Tubes & 5 KW +
 power supply






           Click any picture larger

What? Add a kilovolt to the B+?
There are several newly manufactured amplifiers on the market that utilize the Eimac 3CX800A7 tubes. Many older amplifier also use this fine tube which is well suited to amateur radio in single, double or triple configuration. However, the commercial popularity of this tube is falling off in favor of the more versatile 3CPX800A7 tube which is made in much larger volume for medical and industrial use. Let's take a look at this tube and see if there's some benefit for those that yammer, yammer, yammer on SSB. If these tubes are going to be plentiful and beneficial I would like to convert a Ten Tec Titan 425 to utilize a pair. Let's see if it's doable.


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 not powerful enough, buy a more powerful unit. Leave it as it is. Don't remove the cover. This project is not for you. Well, you might take a look at how it's done, but, do not attempt this project.


          From Eimac

3CX800A7 Maximum Ratings Typical Operation
Type of Service Plate Voltage (Volts) Plate current (Amps) Plate Voltage (Volts) Screen Voltage (Volts) Plate Current (Amps) Drive Power (Watts) Output Power (kiloWatts)
AB2 Cathode Driven RF Linear Amplifier 2,250 0.6 2,200 --- 0.5 23 0.750
--- Pulse Modulator or Regulator 3,500 8.0 --- --- --- --- ---

The P in the part number signifies "pulse" use, which covers a lot of territory. Experimental communications, spook works, metallurgy, techno-medical equipment, focused energy projects, etc. use the CPX type of tube for short duration high power RF energy applications. While SSB service is not a pulse operation, it is a low duty cycle operation or ICAS (Intermittent Commercial or Amateur Service) rated by technicians and systems that measure such things at 15% for Amateur SSB, yammer, yammer, yammer service.

Taller insulator
The P tube establishes the higher maximum plate voltage rating with slightly taller ceramic insulation internally. Overall height and other specifications are identical to the 3CX type. After examining these Eimac specification you'll find the operational plate voltage, in a light duty cycle situation, to be some 1300 VDC higher than the 3CX800A7. Well, I believe we all know that with more voltage we can transfer more power with the same wire size or the same power with smaller wire. Did I get that right? So, we don't really want to rewire with smaller wire, do we now?

          From Eimac

 3CPX800A7 Maximum Ratings Typical Operation
Type of Service Plate Voltage (Volts) Plate current (Amps) Plate Voltage (Volts) Screen Voltage (Volts) Plate Current (Amps) Drive Power (Watts) Output Power (kiloWatts)
AB2 Cathode driven RF Linear Amplifier, pulsed 3,500 8.0 3,500 --- 2.5 320 6.0
--- Pulse Modulator or Regulator 4,500 8.0 4,500 --- 5.0 25 20.0
AB Grounded Grid, FM Broadcast 3,500 0.6 3,200 --- 0.43 23 0.9

Is it true that Americans have come to love four wheel drive, horsepower, guns, and bombing other countries? I believe so. We buy autos with hundreds of horses and use forty or fifty of em on the freeways. Hams do that, don't we? We buy mo power, lotsa watts amplifiers and use just a few of em? Right?

During my experiences with amplifiers equipped with a pair of 3CX800A7 tubes, I have tuned them on several occasions exceeding 1.5 amps. Of course the voltage drop was considerable but the power was right up to 2,500 watts. My organic calculator tells me that operating the P tube at 3,500 VDC B+ with a healthy plate transformer will provide a 20% to 25% increase in available power or normal power at substantially reduced grid current and drive. The direct benefit can be longer tube life, less strain on the transceiver, reduced amperage through the wires, less idling heat. etc. How's that for rationalization?

I'm certain the popularity of the CPX800A7 tube in industry will make it much more available to the Ham Radio community in the future. Along those lines - I have two Ten Tec Titan 425 amplifiers that could use new P type tubes cause the old ones are slightly discolored from age, so let's take a look-see at the feasibility of converting both of these Titans to the P tubes.

Pump it up, way up
Plate voltage on the Titan 425 is about 2700 VDC even though the meter usually indicates about 2400 VDC. If I arrange for a no load 3600 VDC B+, which will sag to maybe 3200 VDC under load, there may be some complaint from components that were not prepared for such. This is a kilovolt higher than Eimac's 2200 VDC operational voltage recommendation for the 3CX800A7.

Weak links? Porsche components
I once bolted a 140 horsepower Chevy Corvair engine up to a Volkswagen transaxle of the 36 horsepower variety for a mo-horsepower project. Oh yea, my mo-horsepower desires exceeded the 36 horsepower Volkswagen transmission capabilities, finding the weak links just as the Volkswagen dune buggy specialist guy predicted. Also as predicted, I bought higher capacity, wallet smashing Porsche drive train components to better serve my mo-power desires.


When I started this project to build robust power supplies for the Titan P Hybrid, I planned to use as many of the original components as possible. But, as I got further into the project I decided to scrap almost all of the original components and use all new and better stuff. I kept the 20 A. on-off relay with 117 VAC coil with the several resistors to drop 234 VAC from the deck on-off switch to proper relay coil voltage to energize the power supply. I utilized the old circuit board for the bridge rectifier/filter for the 28 VDC supply. Now though, I suggest a little 4 A. bridge rectifier puck and 1000uf capacitor be gooped to the case of the 20 VAC transformer for the purpose and the old board pitched.

I built two of these power supplies at essentially the same time, each just a bit different depending on component availability. You may find slightly different components in the pictures such as two different types of bleeder resistors on the WD7S circuit boards.

Extra kilovolt in the power supply? Early Titans utilized, what I believe to be, four 3 A. or 6 A. 1000 volt diodes in each of the four strings of the bridge rectifier, providing a peak inverse voltage of 4000? OK for 2700 B+ but marginal for 3600 B+. Some Hams added the extra diode in each string like the picture and Ten Tec provided five diodes per string in some Titans. The eight filter capacitors in the Titan are rated at 3600 volts working voltage, 4200 volts max surge. This will, - - -  maybe, could be, probably be, OK? No, it's not - just go for the new, higher capacity components and build the WD7S rectifier/filter circuit. This circuit board can be completely assembled in an hour.

Power Supply Voltages
The original transformer has two secondary windings that are normally in series for maximum B+ voltage. The smaller winding is removed from series when low voltage is selected. This feature is left over from the old days. The design utilizes a small 277 volt relay on the circuit board. Yes, 277 volt working AC voltage relay for 1850 HV AC. This hybrid project will not use this relay at all as we will use better replacement components, including small, inexpensive new surplus high voltage vacuum relays of the latching variety with 28 volt coils.

Any Muscle?
A Peter Dahl Muscle 46 lb. transformer is utilized to provide AC voltage selections of 2250 and 2600 for 3132 and 3832 VDC B+. By the way, this transformer is much bigger and heavier than the original and just barely fits into the case. Peter Dahl mentioned that it has to be carefully smashed in a hydraulic press to a more rectangular form just to get it in the case.

With 245 VAC mains-

Primary 1 Sec. 2125 Low = 3132 VDC B+
Primary 1 Sec. 2600 High = 3832 VDC B+
Primary 2 Sec. 2125 Low =  2886 VDC B+
Primary 2 Sec. 2600 High =  3532 VDC B+

Eimac specs. the 3CPX800A7 maximum voltage at 4500 VDC. At the normal light ICAS (Intermittent Commercial or Amateur Service) duty cycle this power supply is capable of 5.5 kilowatts. At 65% efficiency that's mo power (to the dummy load).

I ordered the transformer primaries for 234 VAC and 255 VAC. This can be a single purpose or multi-purpose  transformer. Three or four ancillary voltages for bias and control, filament and fan can be provided by small transformers or built into the custom Peter Dahl. The smaller voltages are: 13.5 VAC @ 3 A. for the filaments, 28 VDC @ 1.5 A. for bias and control, 117 VAC @ about .250 A. for the deck cooling fan and 117 VAC relays and 12 to 14 VDC for an added small power supply fan. I prefer to provide the smaller voltages not from the big transformer so I can effect closer control of the voltages.

I found the needed small transformer items at Ace Electronics here in Houston: a 14 VAC @ 4 amp circuit board transformer with 234 volt primary for the 13.5 VAC @ 3 amps filaments requirements,  a conventional 20 VAC @ 4 amps center tap transformer with 234 volt primary for 28 VDC @ 1.5 A. for control and bias and 12 to 14 VDC for PS cooling fan. The readily available 24 or 28 VAC transformers provide more than needed voltage.

UL Rating
In the old days, really old to you children, 234 VAC appliances utilized 117 volt components such as fans with half of the 234 VAC circuit and the neutral. Not today. Can't have current in the green neutral wire during normal operation. Oh, it's doable but not good design and not legal.

Here, the 117 AC can be obtained from half of the 234 VAC primary winding of the 20 volt transformer, which is lightly loaded or, a 234/117 @.5 A. VAC step down transformer. A European/American voltage converter of 50 watts or more capacity will also work for the 117 VAC power source.

If you look at the original 20 A. Ten Tec primary input on-off relay you will see several two or three watt resistors attenuating 234 volts to 117 for the coil. Although this is OK, and avoids current in the neutral, utilizing a 234 volt relay would be a better design. Leave the original as is unless it needs to be replaced. I use half of the 20 VAC transformer secondary for 10 VAC, rectified and filtered with a small bridge, a capacitor and 12 watt speed control rheostat to power the small DC power supply cooling fan. 

Stuff it
Notice in the picture the big difference in size of the two filter capacitor sets. The small one has 56 uF capacity and the large one 50 uF. The small blank board with instructions is available from WD7S. Working voltage for the caps used on the WD7S board is 4000 VDC, surge about 4900 VDC and it has the 5000 PIV rectifier arrangement on board if you want to populate it with 3 A. or 6 A. diodes. Yes, the large one is out of the Titan, small one utilizes much later technology in the snap-in style HV capacitors, which are available from All Electronics. These are the same capacitors used in new amplifiers made today.

By mounting the smaller filter unit where the original was, plenty of room is available for the small transformers under. If you order a Peter Dahl with the additional secondaries of 13.5 @ 6A., 20 VAC @ 1.5 A. and 117 VAC @ 2 A., you won't need the smaller transformers. If you need 12-15 VDC for a DC fan or other appliance, have peter Dahl provide a 10 VAC tap also.

Wire it up
I added a more robust soft start circuit and relay using the big 20 ohm, 32mm thermister surge limiter. A resistor of the 15 to 25 watt size can be used here instead. Big strong soft start components on both legs of the AC mains with a long timing circuit are not really necessary. An inrush start up protection resistance of 10 or 20 ohms for the first few AC half cycles, about 1/30 second, even on one leg, is all that is necessary. But, like most that have made their own soft start circuits, I like to watch the high voltage meter come up slowly for about one second until the relay closes and the panel lamps pop to normal brightness. It must be self appreciation of one's own handy work, huh?

In the pictures you will notice that I use a Dayton solid state adjustable delay relay for the soft start timer. It is adjustable from .05 seconds to 1 second. The relay is also available in a .1 second to 10 second type. Later you can see how I use this delay relay timing as a hard-start circuit for the deck centrifugal blower and the PS cooling fan.

Your original Titan wiring diagram will come in handy for identifying the cabling in the umbilical. Become familiar with the colors so you can hook things up proper. I have not provided a diagram or a "solder-here" set of instructions. I already know you are familiar with circuits or you wouldn't be reading this. Sketch out for yourself the components and how you are going to drill & screw, route wiring and hook shit up. You will work your mind day and night to plan and finish this advanced project. There is one wire in the umbilical that is not used, except it may have been used in the very first model of the Titan to operate an indicator lamp for the hi-low B+ feature. It's the green wire, on the left in the picture (HV  SV). By the way, you may want to re-read everything about lethal voltages. This is not a practice or novice project.

What about heating the cathodes?
Well, I guess we do need to provide some indirect heat to the cathodes huh? The ancillary transformer provides a little more than 13.5 VAC to the heaters so I used a 12 watt 35 ohm rheostat from http://www.surplussales.com/SSIndex.html to slightly adjust the final voltage. You can see this addition gooped to the top of he transformer.

What about the two secondaries?
Why not? When ordering a transformer from Peter Dahl you can specify, within limits of physical space available in the case, any primary or secondary windings or taps you want. The two identical transformers I ordered were specified for 2125 and 2600 VAC @ 1.5 amps ICAS (Intermittent Commercial or Amateur Service) for SSB use with 234 VAC and 255 VAC primary taps. My AC mains single phase voltage varies from 234 to 245 VAC, so the second tap allows me to reduce the B+ some with a SPDT switch arrangement. The 2600 VAC secondary with a tap for 2125 provides additional flexibility resulting in four available B+ voltages. I designed and made a prototype controller that utilizes two Jennings RF1J 28 VDC SPDT latching type HV vacuum relays, rated 3.6 KV @ 10 A. These are available from MGS at around $9.50 ea. One to allow selection of the B+ voltages and one to control a 117 VAC indicator lamp.

Quieting the noisy deck blower
The original ?Titan power supply had two voltages from primary taps on the big transformer for a two speed blower arrangement. Because this project is generally for SSB yammer, we will slow down the blower a bit to reduce noise even though the Titan 425 is the quietest of all amplifiers in this class. The blower motor is a type that will run ok at reduced voltage from the 350 ohm, 12 watt rheostat. I found these at:> http://www.surplussales.com/SSIndex.html You can set the CFM to suit you. Cooling is not  problem with the Titan in SSB mode. However, the blower motor will not start at a low voltage. It will rotate but it will not come up to speed. See hard start below.

Cool it?
The original Titan power supply does not have a cooling fan and probably does not need one for SSB operation of about 15% duty cycle. I calculated the dissipation at about 8 watts for standby with most heat coming from the bleeders. Dissipation figure for really high power, high duty cycle operation, including heat from the Peter Dalh transformer would be about 50 watts. The little fan blows right across the circuit board and bleeders and exhausts through the openings at both ends of the case. The PS fan and the RF deck blower speeds are adjustable.

Hard Start, Kick Start?
I installed a small double pole, double throw 117 VAC relay that provides 117 VAC to the RF deck blower motor for one second at start up. This is actually a hard start circuit.  The timing is provided by the Dayton delay relay circuit so the blower motor gets a one second kick at normal voltage by shorting around the rheostat, then lower pre-selected voltage for running. The other poles on this same relay also provides a hard start kick for the small PS cooling fan in the same manner by shorting around the rheostat for one second.

Wad-a-ya mean, does it run?
At the beginning of this article I mentioned, Add a kilovolt B+? Well, let's see, an extra kilovolt or so, actually about 1,250 VDC more and, with much more muscle from the much bigger Peter Dalh plate transformer, Oh Yea!, it does run and there is one hell of a lot more power out available.

Band Loafing Column
20W. Drive
Grid Negligible
Driving Column
50W. Drive
Grid- 25ma
Overdrive Column
100W. Drive
Grid - 75ma to 100ma
1.950 Mhz Legal Limit 2,400W 3,200W
3.925  Mhz Legal Limit 2,500W 3,400W
7.245 Mhz Legal Limit 2,500W 3,400W
14.250 Mhz Legal Limit 2,400W 3,400W

This Titan is now a real race horse. With a few watts of drive it tunes and behaves like before but just a touch on the drive control and it's over legal limit. If your ready for an amplifier project that gives you the presence on the band, this may be for you. However, the Titan P Hybrid is not a non-destructive load tester, huh?

The four rubber feet were removed and a composite board attached to the remaining pem nuts. Casters were attached to the board.

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

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





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