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Ten Tec Centurion 422
The big warm glowing tubes still perform here too.  
by Bob Hutchinson, N5CNN

I may not give up until I have personally reviewed, tested and fondled all of the amplifiers that use warm glowing tubes. From the Review of the QRO HF-2000 I reproduce this: "The 3-500 tube style amplifiers are still offered by many amplifier manufacturers to satisfy the undying demand of those afflicted with OCTD, known in the medical community as Obsessive-Compulsive Tube Disorder."

Well, I still have the disorder and to feed it I bought a ten year old Centurion. Oh yes, Ten Tec still makes this fine dual 3-500Z amplifier today. Click Here to see the info at their web. I bought a used one to save some budget bucks. I received this Centurion in excellent cosmetic condition, but the excellent electronic condition stated by the previous owner was a big over- statement. So, I ordered a new Centurion box from Ten Tec, packed up the amplifier and shipped it to them to be refurbished to like new condition.
I got it back with lotsa new parts, boards and stuff and it appears to be in like new condition. The saving of budget bucks theory didn't really pan out, I should have just bought a new one. Now, come along, let's examine this sweetie up real close, light it up for the Olympus digital camera, light it up for real and run it. We'll see if the warm glowing tubes still perform.

All I had to do to prepare it was remove the numerous #6 machine screw and cover, install the original Eimac 3-500Z tubes, which I did not send along with the amplifier to Ten Tec.

OK, it's tent duty
Routine pictures. Some with tubes lit up. Some pictures were taken a little later after installing new RF Parts Chi-Com knock-off tubes, the graphite anode type.

RF section
Ten Tec has always used the quality stuff in the RF section. These components are of the same style and quality as the Titan II Biggie I reviewed a few weeks back. Many would say the band switch is too small, but we have to remember that this is not a legal limit ++ amplifier. It is rated by Ten Tec at 1,000 watts CW, 1,300 watts PEP and 650 watts for heavy key down power requirements. It is in the same power class as the classic Heathkit SB-220.

                                                     Click any picture larger. >>

Vernier drives for the Load and Tune air-variable capacitors are 6:1 ratio type and provide the velvet smooth feel that many like. Repeatability to the numbers is excellent.

Centrifugal squirrel cage type noise blower?
Oh no! Not this sweetie! The big, warm glowing tubes don't need centrifugal blowers. Since they cool mainly by radiation, as in radiating heat to surrounding components and air, moving air through all the components works just fine. For high output operation with glass tubes, the metal glass seal area around the pins needs special attention. Like the classic SB-220 design, the Centurion tube-axial fan is mounted with a portion of the lower fan blade arc below the tube socket/metal plane so plenty of air circulates through the pins and under side hardware.

This can be seen in the picture up above. Fan mounted slightly below the socket/metal plane so 10% to 15% of the air circulates under the tubes, through the pins and pin clamps and out through the exhaust holes provided. Without this feature to provide bottom end cooling, the wonderful, warm glowing tubes would fail under heavy use.

Some amplifiers use a plenum chamber, glass chimneys, a centrifugal blower, and the associated noise, for the 3-500Z tubes. This provides good cooling for the pins and under side hardware, but chimneys do not provide much efficiency for over all cooling capacity because the tubes radiate heat to the surround components through the glass chimneys. That heat still has to be removed by conduction and convection which requires air flow. The method used by Ten Tec, and most makers today is best. If memory serves me, I believe a maker, no longer in business, actually switched from chimneys to the fan method used by makers today to reduce cost and noise.

Noise, fan noise
Well, let's see. I'm afraid I would have to give this amplifier a low noise rating, very low. Matter of fact, it makes less noise than any tube amplifier this writer has ever used, operated or tested. The tube axial fan is of the 9 watt variety, which is right for quiet operation. However, some later Centurions were shipped with fans of 20+ watts capacity, which was a mistake as the noise level is much too high. It becomes a howl with a fan of such capacity. A series 350 ohm, 12 watt rheostat on the back panel is called for in such case so the owner can eliminate the excess air volume and associated noise.

The QSK capable T/R design is non-noisy also.  I suppose this really means a high noise rating because of the low noise, huh?  As fan noise ratings would go, this is the quietest amplifier, the QRO HF-2000 would be next, then the Ameritron AL-572. All three are quieter than what I consider the standard, the SB-220.

Good choking
Nice big plate choke, same size as used in the more powerful Ten Tec amplifiers. It has front row seat to the fan.

Conventional with single purpose left meter always indicating plate current up to 1000MA. Right meter is combo unit, indicating B+ plate voltage, grid current, RF power out or RF reflected power, depending on meter switch position. And, believe it or not, real, accessible, replaceable light bulbs, available from any Radio Shack. >>>

QSK on board.
Ten Ten has always been an innovator with fast and quiet T/R circuits that are full break-in QSK capable. This Centurion had a small vacuum relay circuit when I received it from the previous owner, but Ten Ten replaced it with the current T/R circuit that utilizes an open frame relay on the circuit board. A vinyl cover on the relay keeps it amazingly quiet.

The manual
Ten Tec has always produced fine manuals and this is another example.

And the power supplies?
The single purpose plate transformer is a Hypersil type held together with the steel bands. Based on weight and performance, it appears to be a 2,000 watt capacity intermittent duty  unit of about 2,250 VAC, no load. Weight is stated as 32 pounds. The HV bridge rectifier/filter arrangement produces 3,250 VDC B+. The rectifier is rated at 5,000 VDC with 3 amp, 1,000 PIV rectifier diodes and high quality computer grade capacitors. These components provide a wide safety margin, which we can take advantage of later, huh?

Many claim the bridge type circuit provides better performance but the modern voltage doubler circuit is becoming more popular with amateur radio amplifier designers because the performance is there as well as the weight and cost savings. I wouldn't be sticking my neck out if I said 99% of the microwave oven makers use HV voltage doubler rectifier circuits for all the same reasons.

Our test bench is equipped with a 2000 watt fan cooled dummy load, 3,500 watt low pass filter, Bird 43 and Coaxial Dynamics watt meters cabled in series, each with 2,500 watt slug. The bench has a superb 7,500 w. antenna switch utilizing Jennings vacuum relays, designed and built by this writer, for access to 75M & 40M dipoles, 20M, 17M and 15M verticals and the dummy load. Various putt-putt wattmeters are used between exciter and amplifier to measure drive power and SWR.

Accuracy of measurement
We use the average of the two wattmeters mentioned above. Both manufacturer's spec. an available inaccuracy of + or - 5% of slug rating at a mid-scale reading. This works out to be an error factor of + or - 125 watts at mid-scale. The maker's don't even mention accuracy at close to full meter right deflection. Remember, these wattmeters are the bottom of their line, most inexpensive of the manufacturer's offerings.

Will this warm glowing tube sweetie perform? Does it run?
There is more than one test. One with the original Eimac tubes and a second test with the Chinese made Graphite anode tubes, which provided an increase in power output. And a third test, huh?

 Data below is grouped:
Grid Current MA / Plate Current MA / CW Power Out Watts
Original Eimac Tubes
75W CW Drive
100W CW Drive
1.90 Mhz 240/650/950W 250/775/1,150W
3.90  Mhz 240/600/1,075W 250/675/1,200W
7.200 Mhz 200/500/900W 240/600/1,125W
14.225 Mhz 250/700/1,250W 280/750/1,275W
18.140 Mhz 300/800/1100W 300/900/1,150W
 21.350Mhz 240/700/1,100W 300/800/1.250W
28.400 250/800/900W 280/800/1,050W


Second test
After installing the new RF Parts Chi-Com 3-500Z graphite anode tubes the below results were recorded for 75, 40 and 20 meters

 Data below is grouped:
Grid Current MA / Plate Current MA / CW Power Out Watts

Chi-Com Tubes
100W CW Drive
1.90 Mhz 260/800/1,275 W
3.90  Mhz 260/775/1,325W
7.200 Mhz 250/700/1,225W
14.225 Mhz 280/800/1375W

Well, the Chinese tubes work fine. I've used them for several years in various 3-500Z amplifiers and have always had power output improvement over the replaced tubes.

This nice amplifier is the quietest amplifier I have ever used. When using VOX on sideband the amplifier never makes itself known. I notice neither the fan or the T/R relay. It is silky smooth. However, part of the quietness is the result some modifications. Yes, - - - I'm afraid I, I, - - - just couldn't resist, I needed a drilling, screwing, sawing and filing hit so I improved things a bit. I installed my favorite fan quieting modification, a 200 ohm, 12 watt rheostat in the fan circuit so I could establish the right fan speed and noise for my yammer, yammer, yammer SSB duty cycle.

But, I couldn't stop there. I removed the heavy and under achieving Ten Tec plate transformer so I could power this Centurion with a real transformer, a 46 pound Peter Dahl Hypersil. See the Centurion modifications at Lighten Up.

Then I took to whacking out the right side air exhaust slits to create a big rectangular hole so I could see the big warm glowing tubes. Kinda like I did with the QRO HF-2000, I covered the hole on the inside with a piece of 1/2" expanded stainless steel. I can see those babes now.

Other Conclusion
As mentioned in this article, the Centurion is an excellent candidate for an addition to my Lighten Up feature. See how I lightened this sweetie up and created a legal limit + amplifier. Hallelujah! I love Ham Radio.

Here is a glimpse of the performance improvement utilizing the Lighten Up power supply from the Lighten Up article. The B+, no load, is 3,800 VDC and about 3,340 VDC at the indicated power level.


Band 100W CW Drive
3.90  Mhz 300/810/1,750W
7.200 Mhz 280/800/1,700W
14.225 Mhz 290/975/1,800W

The power supply is capable of more so I set it up at 4,100 VDC B+ and made short test on 40 and 75 meters at 2,000 watts output. This is the maximum for the 3-500Z tubes.

Added: As I finish this project up I have moved this sweetie to the left position at my U shaped office work area. I run it at 3,800 VDC B+ with 75 watts drive for legal limit SSB. It is without noticeable noise during use. Best of all, I can watch the anodes in those nice, warm glowing tubes turn dull cherry as I yammer, yammer, yammer.

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

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