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STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by WD6DBM on September 27, 2009
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My Henry 2002A is still acting up. I now cannot tune it at any loading and tuning combination with any drive level between 8 and 20 watts that does not produce excessive grid current on the order of 40ma+ for outpuit of only 400 watts. When it was new I could tune it to 900 w out at 35ma grid current with 10W drive.
Here's the suspicious part--as the amplifier heats up, the grid current goes DOWN for the same output. When it cools off a bit and I transmit again, grid current is high. I have checked the actual grid current with an external meter.
I'm hoping someone familiar with this ampliifer can advise me on where to look. I spent $400 on sending it back to Henry, they did nothing to improve it and no longer respond (the mighty fall).
Could it be the bias Zener? The bank of huge 50W resistors? Yes, I have replaced the tube--no change and another $500 down the can. If there is someone in the SF Bay Area who can fix it I will pay for it.
Thanks and 73, Eric WD6DBM
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by W8JI on September 27, 2009
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Changing grid current with tube internal temperature is a very common problem with 3CX800 tubes. It might not be anything inside the amplifier components other than the tubes.
Good luck on finding a 3CX800 tube that does not drift grid current with internal heat. That will be a real chore.
The best fix is to load the amp extra heavy by increasing the antenna coupling, so the variation does not cause excessive grid current.
New Eimac 3CX800's are now 800 dollars each *wholesale*. Tubes are on the way out.
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by HFRF on September 27, 2009
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I had your amp at one time and I thought it was a very nice amp although I didn't use it much which is why I sold it.
My last quote from Richardson for a 3cx800a7 a month or two ago was below $800. Pound for pound this tube costs more than gold, I think.
The bottom line is that tubes are nearing the end of their commercial usefulness. But tubes will be around for quite awhile. A few tubes like the 8877 will still be around from Eimac for years to come and there are literally thousands and thousands of tubes sitting in basements, commercial inventories, service stock, etc.. Also, there are tubes that still must be made for spares required by government contracts that will eventually get dumped. Sweep tubes haven't been made in 30 years and you can still get them.
If you have to buy a new tube (a tube guaranteed from an official distributor), is it worth it to keep your amp instead of dumping it and buying something new and chezzy.
Buying new amps at present prices is getting stupid. Ameritron is getting closer to pricing their high end cheapo amps that are not so cheapo anymore out of the market. The AL82,1200,1500 prices are closing in on Alpha prices!
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by WD6DBM on September 27, 2009
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Perhaps I wasn't clear. I have owned this amp for 10 years, and I am now on my third tube. The amp is UNUSABLE because of excessive grid current--this is not an internal tube heating problem. When it was functioning normally, the grid current might change 5 ma in 3 hours of operation, but still be in usuable range. Now the grid current meter is pinned at 40ma at only 400w out at any condition of loading and drive. Max grid current for this tube is 60ma per Eimac specs.
If anyone can help me with this problem PLEASE?
73, Eric WD6DBM
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by HFRF on September 27, 2009
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Maybe you have a heat problem you did not have before. From what I remember, the whole series of vhf amps from Henry were built on a similar chassis cooling system. And I also remember that all these amps ran hot. I heard that many owners of this amp made changes to add more air to keep the internal temp down. They also cleaned everything frequently. Manufacturers tend to starve amps of good airflow to keep noise complains down. Also, owners of the amps tended to want to slow the fans and blowers down to keep noise lower.
If I were you, I would take the covers off the amp, clean the dirt and dust from all air moving devices and the tube fins, get a small house fan and dump lots of air into the amp and see if that changes anything. And go from there.
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by HFRF on September 27, 2009
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I forgot to add that keeping the amp cool longer from start up may allow you to see whats happening before the grid current goes down. Also, since Henry puts a lot of ventilation holes in the cabinets, hit some bias parts with freeze spray and see if you get a big change in grid current. You may find a bad part that way. Henry could have designed the amp to have some part that has a way to compensate for the grid current change while heating up but never documented it. That part may not be working.
If it is not the tube you have to get creative and start measuring stuff to get clues. If you don't know how to do that get some local help.
Trying to fix an amp over the internet isn't easy. And every problem doesn't have a single unique cause relationship. A general kind of problem like high grid current could be caused by a lot of possible failures.
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by W8JI on September 28, 2009
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Eric,
Some 3CX800 amps were so poorly designed they had no grid current overload protection. You need a very fast grid current trip that knocks the amp off line if current is too high. You have to be very careful of grid current or you can seriously hurt tube life in MOX cathode amps that have no automatic grid trip circuit. The TenTec 3CX800 is a good example. I'm not sure about your Henry amp, but be careful of the extra current because extra grid current can start to eat the gold plating off the grids.
If the grid current in your amp is initially high, assuming the meters are correct, then you need to load the amp heavier. Grid bias itself has very little effect on grid current. The largest effect on grid current comes from anode load impedance in the amplifier. This means how tightly the amp is coupled to the load is critical.
A grid metering circuit reads the current between the negative PS rail and the chassis. The negative PS rail floats and goes to the cathode through the shunt for the plate current. If the grid current is not reading correctly, then it has to be some kind of short in the negative lead of the power supply to the chassis making the grid meter partially read anode current. It could also be a resistor off value in the metering circuit.
Sometimes there are HV fault protection diodes between the negative rail of the supply and the chassis. These diodes keep the negative rail of the PS clamped within a few volts of the main chassis in the event of a HV arc or HV fault to chassis ground. If one of the diodes fails, or any other component between the PS negative rail near the power supply and the chassis fails, the grid meter will partially read anode current. You can spot this by keying the amp with NO DRIVE and watching the grid current. If the meter shows grid current with no drive power, you probably have a metering issue.
Tom
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by K6AER on October 1, 2009
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I had this amplifier for five years and I normally never ran much more then 30 mA of grid current with 1000 watts SSB duty. You did not mention what the type of service you are using the amplifier in, that is FM, SSB, AM etc.
The cooling on the amplifier was fine and I never had heat problems. I will say the 3CX800A7 will not take any grid abuse and even though the tube specs say it can handle 60 mA of grid current. If you need much more than 30 mA for 1000 watts SSB output the tube is going bad. It does not take much grid current very long to start to degrade tube performance. You will see higher and higher grid current to produce the same output and the tubes start to avalanche to failure pretty fast.
The Henry amp did not have grid current trip and I suspect the conventional wisdom of the time was tubes were cheap. I added a grid trip circuit on my amplifier and it was well worth the effort.
You might also check the metering circuit to check if the meter is accurate. Henry used a lot of different components in their manufacturing and your meter might be off by quite a bit. Conventional wisdom was if it looked good ship it.
By the way if the Eimac 3CX800A7 was made of gold it would cost $10377.00. The tube weighs 10.48 troy ounces.
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by VK3MH on November 2, 2009
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Hi Eric, I am wondering if you got to the bottom of the problem with your 2002A amp as I am having the same trouble with my 2004A 70cm amp.
Thanks Brendan
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RE: STILL NEED HELP WITH HENRY 2002A AMPLIFIER
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by VK3MH on November 14, 2009
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Hi Eric, not sure if you have been reading this post. I have resolved the problem in my 2004A 70cm amp. The heater voltage was low, around 11.9 volts so I moved the tap from 240v to the 220v tap and it bumped the voltage up to 13.6v and now the amp is going like a rocket, I was getting as little as 60w maximum on 70cm with 20w drive now I am getting 800w CW easily with 70w drive. Have done the same change to my 2002A and it does 1KW CW on a soft tube that was doing 350w so the 3CX800A7 seems to be very sensitive to heater voltage, the plate is bumped up to 2600 volts when you do this which exceeds the tube spec however the FM position will be at 2100v and it works to spec on that voltage too. Also the unregulated relay and lamp supply jumps up to 18v so I put the transformer primary across the 100v tap which now has 107 volts on it and that dropped the output voltage to 16v DC which is a little high but will only reduce the lamp life in the standby/operate lamps, I plan on clamping it with a 12 volt zenor to regulate it. My main supply voltage is 235 volts at 50 Hz.
Regards Brendan
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