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Author Topic: 3cx800a  (Read 3525 times)
NORTHCOUNTRY
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« on: October 24, 2006, 05:53:42 AM »

I considering buying an amp that uses the one 3cx800a tube, the AL-800.  Initially I would want to use the amp at 300 to 800 watts SSB output.  I was thinking of skipping the less than 1kw amps so I could grow into the amp over a couple of years to run it at the higher output above 1kw.

My question is does that tube operate effectively at the 300 to 800 W range or must it be loaded up to 1KW in order to operate efficently?  

Other than the additional $1000 cost of the 3cx800a amp would I be better off just getting an 811 amp?

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KB9CRY
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« Reply #1 on: October 24, 2006, 06:44:04 AM »

I think that's a lot of money for that amp where you could do the same thing (a kilowatt is a kilowatt regardless of where is comes from ) for a lot less by getting a nice used SB-220 or SB-1000 or AL-80B.  Very proven design and ruggeded.  Tubes are available at a decent cost.

No it's not inefficient to drive the amp a lower than max power provided you tune it up correctly, meaning you tune  and increase power in a step-wise manner until it's properly tuned at max power and then you back off the drive to the desired output power.

Phil  KB9CRY
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AA8LL
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« Reply #2 on: October 24, 2006, 07:06:04 AM »

One other thing to consider about this tube.  It is of a class that requires 3 full minutes to warm up before you can transmit.  I don't know what you like to do in ham radio, but for me, three minutes is way too long.  I like my amp to be cool and quiet and then be ready to go in less than 10 seconds when I want it.  If you are a QRO contester or a DXpedition and are going to run the amp all day, it doesn't matter.  I use an Ameritron AL-1200 which uses the 3cx1200A7 and is ready to go as fast as I can dial in my tuning.
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WB2WIK
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« Reply #3 on: October 24, 2006, 08:00:42 AM »

The 3CX800A7's been around a long time and it's a good tube.  As pointed out, it takes a warm-up period and is not "instant on."  I think two minutes (120 seconds) does it, though.

No problem running reduced power with any amplifier.  Efficiency can suffer a bit, but that only relates to your electric bill and really nothing else.  Your electric bill running an amp or not running it will change so little you won't notice the difference.

Having said all that, in your shoes I'd go with an AL-80B instead.  Cheaper, lighter, much cheaper tube, "instant on," and runs ~1 kW PEP, which evidently is all you really want anyway.  Easy to "upgrade" any time in the future, and these things hold their value quite well so you wouldn't lose much upgrading in the future.

WB2WIK/6
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AA8LL
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« Reply #4 on: October 24, 2006, 08:11:54 AM »

I guess warm up depends on the amp.  The Ameritron AL-800 manual says on page 5, line 2:  A time-delay circuit provides a 180-second warm up to eliminate potential damage to the tube cathode.  If there's one thing I've learned, I'd better check my tech details before I post on this site! And 180-seconds is still 3-minutes by my calculator.
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WB2WIK
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« Reply #5 on: October 24, 2006, 09:43:55 AM »

AA8LL, you're right, the recommended warm-up time is 3 minutes.  Ouch!

Another good reason for 3-500Z's and 3CX1200's in HF amps.

I still have some long warm-up amps in house, but they're for VHF-UHF with the instant-on tubes peter out and don't work well, or don't work at all.

On HF, where there are lots of choices, I like "instant on."

WB2WIK/6
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K3GM
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« Reply #6 on: October 26, 2006, 05:47:16 AM »

I'm going to stick my neck out by saying that I don't care for this tube.  This is a bad tube! The number one problem is that it's prone to arcing, and this is due to the design of the tube.  The 3CX800 is a combination of several designs.  It's really a 3CX400 base with a 3CX1000 style plate. Problem is, the grid design isn't robust enough for the 800W style plate.  What happens is the high plate voltage causes excessive bombardment of electrons from the plate to the grid. This eventually dislodges enough grid material to cloud the tube and create a path for the arc to form.  Amps may have built-in arc detection, but that only protects the power supply.  But one arc, and the tube is hosed. If you ever hear the tell-tale ping you'll know what happened.  The amp may still work, but it's time to shell out for a new tube(s).  I would avoid any amp which uses this tube.
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K3GM
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« Reply #7 on: October 26, 2006, 08:00:47 AM »

Because of the lower gain of the 3-500G(is it G now?), you have to run your exciter, in this case your rig at full smoke to achieve the rated output of an amp running this tube.  I don't know about you, but if I could back the drive off a little and still get the rated output of the amp, I'd prefer to do that.  You could go with the 3CX1200, and run it the lower output you desire.  The downside of this tube is its replaement cost. Wow!  Of course running it at a lower output wil probably extend its life to where replacement costs are not a factor. But you never know.  I like the 4CX800 (GU-74B) tube; reasonable gain, rugged, cheap.
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K6AER
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« Reply #8 on: October 31, 2006, 04:06:00 PM »

The 3CX800A7 is only prone to arcing if you are loading the amplifier into a very high impedance. The AL-800 won’t let you go there. Protection circuitry will fault. These tubes are very high gain and you will be able to produce about 1300 watts PEP with about 25 watts drive. The tubes are very robust and Alpha has been using them in their models 86, 87A, 89 for over 20 years. Also the tube is very linear in grounded grid operation, has very good IMD numbers and is very efficient. Yes you have to wait 180 seconds for the tube to warm up but I just turn on the amplifier first thing when I come into the shack. Grid current for full output is about 20 mA per tube.

My Alpha 86 has been on the original 3CX800A7s since it was built in 1988 and still will put out 2600 watts PEP. I just ran it for 48 hours on the CQ WW at 1500 watts. With two 3CX800a7’s the amplifier is loafing along. The down side of the tube is the replacement cost is about $500 per tube. The AL-1200 uses a 3CX1200A7 which has a replacement cost of $270. The tube has less gain but to a new amplifier owner you have a less sensitive tube to learn operation on. The AL-1200 will put out about 2300 watts PEP so you have lots of head room at the legal speed limit of 1500 watts.

Just a note; today’s amplifiers have to put out more than the FCC allows in order to meet the IMD numbers at their rated output. It is up to the operator to operate them legally.
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NORTHCOUNTRY
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« Reply #9 on: November 01, 2006, 06:06:40 AM »

Thanks for your responses.  The comments here have been very helpful.  Yesterday I picked up 100' of 3Cond #12 at Home Depot($70 ouch) so one of these amps is in my future.

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