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Author Topic: Henry K-2000 amplifier  (Read 20766 times)
N0SQ
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« Reply #30 on: April 06, 2017, 11:16:00 AM »

Hmm, if I do modify this amp for the 3CX800A7, I'll need to change the transformer since the 3CX800A7 requires 12 volts for the filament. The 3CX400A7 has a 6 volt filament but it looks like 2 tubes will provide about 1.2 kw. A pair of 3CX800A7 tubes would do legal limit though.

I guess I should have this thread moved to the Mods forum or start a new thread there?
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N3QE
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« Reply #31 on: April 06, 2017, 11:33:16 AM »

Hmm, if I do modify this amp for the 3CX800A7, I'll need to change the transformer since the 3CX800A7 requires 12 volts for the filament. The 3CX400A7 has a 6 volt filament but it looks like 2 tubes will provide about 1.2 kw. A pair of 3CX800A7 tubes would do legal limit though.

I guess I should have this thread moved to the Mods forum or start a new thread there?

Are the existing 8873 tubes bad? The case is low profile and you will have to do something really ingenious to get two air-cooled tubes in the case with sockets and air plenum all in there. Putting 1600 Watts of dissipation in the box along with cooling will require a lot of ingenuity because the 8873's were only rated 200Watts dissipation each.

The K-2000 has only a "SSB-duty" transformer and HV power supply so you aren't going to get more than a few hundred watts continuous out of this chassis no matter what you do.
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N0SQ
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« Reply #32 on: April 06, 2017, 12:03:50 PM »

Hmm, if I do modify this amp for the 3CX800A7, I'll need to change the transformer since the 3CX800A7 requires 12 volts for the filament. The 3CX400A7 has a 6 volt filament but it looks like 2 tubes will provide about 1.2 kw. A pair of 3CX800A7 tubes would do legal limit though.

I guess I should have this thread moved to the Mods forum or start a new thread there?

Are the existing 8873 tubes bad? The case is low profile and you will have to do something really ingenious to get two air-cooled tubes in the case with sockets and air plenum all in there. Putting 1600 Watts of dissipation in the box along with cooling will require a lot of ingenuity because the 8873's were only rated 200Watts dissipation each.

The K-2000 has only a "SSB-duty" transformer and HV power supply so you aren't going to get more than a few hundred watts continuous out of this chassis no matter what you do.

The tubes are still good. I'm just considering options in case the 8873's do fail. I got this amp third hand so I don't know how many hours have been put on these tubes. Anyway, I looked at the specs on both the 8873 and the 3CX400A7 and the sizes are the same. I've seen some modifications where the back of the amp was extended with some metal work or with a BUD box to give more room for a fan. I don't know if the chimney will be an issue but I will have about 1.5 inches between the top of the anode to the screen. The specs on the 3CX800A7 indicates that I'd  be able to put only one tube, only, in the amplifier. Anyway, one of the HV capacitors went bad while using the amplifier so I have replacement HV capacitors on order along with a replacement power switch and a couple of LED indicator lights. Anyway, it's sounding like this isn't the amplifier that I want/need but I think I'll either repair and keep it for awhile or sell it.
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N3QE
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« Reply #33 on: April 06, 2017, 12:25:07 PM »

Anyway, it's sounding like this isn't the amplifier that I want/need but I think I'll either repair and keep it for awhile or sell it.

I think a guy who does SSB only would appreciate this amp.

And thanks for all the 5 RTTY QSO's in past 4 years :-)
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N0SQ
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« Reply #34 on: April 06, 2017, 06:44:36 PM »

Anyway, it's sounding like this isn't the amplifier that I want/need but I think I'll either repair and keep it for awhile or sell it.

I think a guy who does SSB only would appreciate this amp.

And thanks for all the 5 RTTY QSO's in past 4 years :-)

The amp has a switch to select between CW and SSB. The CW position simply changes the tap on the transformer to provide a lower plate voltage.

And, you're welcome.
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KM1H
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« Reply #35 on: April 07, 2017, 01:11:48 PM »

The grid survivability of the 3CX800 is no different than the 8873, etc; both are rated at only 5W dissipation. A grid protection board is highly recommended for either.

If there is enough physical separation between the 800's two can be used but one will do the job. In the SSB position that amp is PS limited anyway with either tube and the only benefit of two tubes is an ease of cooling and if they are lower cost than replacement 8873's.

Id also suggest taking photos of the power of the 8873's once the amp is working properly. They then can be sold on Fleabay for a nice profit once the 800's are up and running.

With the 800 place a muffin fan on the top of the cabinet puling air out. A chimney can be made from the red silicon rubber sheet found at Grainger, Fleabay, etc.

Carl
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N0SQ
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« Reply #36 on: April 07, 2017, 03:42:47 PM »

The grid survivability of the 3CX800 is no different than the 8873, etc; both are rated at only 5W dissipation. A grid protection board is highly recommended for either.

The only commercially available grid protection circuit that I've found is the Ameritron GOP-100. There have been some suggestions that say to fuse the grid circuit or use a resistor in the B+ HV power supply. W8IJ had developed a board but he's not selling it anymore - I think it was called a TOF.

Quote
If there is enough physical separation between the 800's two can be used but one will do the job. In the SSB position that amp is PS limited anyway with either tube and the only benefit of two tubes is an ease of cooling and if they are lower cost than replacement 8873's.

The 3CX800A7 and even the 3CX400A7 are a bit expensive also. I've noted that the 3CX800A7 and the 3CX400A7  requires 2000 volts while the 8873 requires 2200 volts. I'm not sure if the 200 volt difference is enough to justify a different transformer for the 3CX400A7 but I'll need a different transformer for the 3CX800A7 since it requires a 12.6 volt filament voltage. If I were to modify the amplifier I would choose to go with the 3CX400A7 - wouldn't give me 1500 watts but I should get 1200 watts with a pair of them.

Quote
Id also suggest taking photos of the power of the 8873's once the amp is working properly. They then can be sold on Fleabay for a nice profit once the 800's are up and running.

I wonder if I could find anyone willing to fork out $800+ for used 8873's. I think most people would rather modify their amplifier to use a less expensive tube(s).

Quote
With the 800 place a muffin fan on the top of the cabinet puling air out. A chimney can be made from the red silicon rubber sheet found at Grainger, Fleabay, etc.

Sounds like a simpler solution than removing the heatsink and replacing it with some metal work and a blower. I'd probably get rid of the heatsink and BeO anyway. Thanks for the suggestions. 73
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KM1H
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« Reply #37 on: April 07, 2017, 06:02:10 PM »

Quote
The 3CX800A7 and even the 3CX400A7 are a bit expensive also. I've noted that the 3CX800A7 and the 3CX400A7  requires 2000 volts while the 8873 requires 2200 volts

All those tubes are specified at 2200V or less if you read the spec sheets and the 3CX/3CPX800A7's are in current production in 3 countries and near full power pulls are available. The others are produced in small quantities for replacement use, they are obsolete for new designs.
Start calling the 3CX400 the 8874 which it is best known as.

https://frank.pocnet.net/sheets/140/8/8873.pdf
https://frank.pocnet.net/sheets/140/3/3CX800A7.pdf

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I wonder if I could find anyone willing to fork out $800+ for used 8873's. I think most people would rather modify their amplifier to use a less expensive tube(s).

Ive seen them sell for $250-300 each many times. Most hams havent a clue how to modify anything.
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N0SQ
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« Reply #38 on: April 07, 2017, 06:29:46 PM »

Quote
The 3CX800A7 and even the 3CX400A7 are a bit expensive also. I've noted that the 3CX800A7 and the 3CX400A7  requires 2000 volts while the 8873 requires 2200 volts

All those tubes are specified at 2200V or less if you read the spec sheets and the 3CX/3CPX800A7's are in current production in 3 countries and near full power pulls are available. The others are produced in small quantities for replacement use, they are obsolete for new designs.

You're right. That's the maximum voltage rating. I was looking at the typical class AB2 spec where it shows 2000 volts.

Quote
Most hams havent a clue how to modify anything.

I have to agree with that assessment. Not many hams are into building or modifying stuff. I don't know what the stats are but from my observations most of them aren't technically inclined.
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N0SQ
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« Reply #39 on: April 10, 2017, 11:01:12 AM »

I'm trying to make sense of the specs for cooling tubes. The 3CX400A7 spec says for 400 watt dissipation the air flow needs to be only 8.6 CFM? If so, I guess a fan that does 32 CFM is overkill?
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N3QE
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« Reply #40 on: April 10, 2017, 11:06:47 AM »

I'm trying to make sense of the specs for cooling tubes. The 3CX400A7 spec says for 400 watt dissipation the air flow needs to be only 8.6 CFM? If so, I guess a fan that does 32 CFM is overkill?

If you have a good sealed air plenum, correct chimney, and only a single tube, then 8.6CFM is enough.

If you aren't sealing everything up then you need more CFM depending on how much is leaking out and the pressure drop.

I kinda like using my HVAC skills when working on amplifiers :-)
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N0SQ
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« Reply #41 on: April 10, 2017, 11:31:44 AM »

I'm trying to make sense of the specs for cooling tubes. The 3CX400A7 spec says for 400 watt dissipation the air flow needs to be only 8.6 CFM? If so, I guess a fan that does 32 CFM is overkill?

If you have a good sealed air plenum, correct chimney, and only a single tube, then 8.6CFM is enough.

If you aren't sealing everything up then you need more CFM depending on how much is leaking out and the pressure drop.

I kinda like using my HVAC skills when working on amplifiers :-)

This amplifier has a cage screen and then a top screen to cover everything. Currently, there are a pair of 8873's in it right now that utilize convection cooling and I'm considering modification options in case they fail someday. . Anyway, since these screens have holes in them then I guess I need a higher CFM fan. How would I determine what the fan CFM should be for something like what I have? I would like to use something that gets the job done without ending up with "excessive" fan noise. I was also looking at re-using the 225 degree F temperature switch (currently used with a high temperature indicator) to alter the fan speed so that it would kick up to a higher CFM/RPM when needed. Otherwise it would loaf around a lower CFM/RPM. The original(?) owner of the amplifier was using a 105 CFM fan but I don't know why since he's SK.
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W9IQ
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« Reply #42 on: April 10, 2017, 11:41:02 AM »

Quote
...the air flow needs to be only 8.6 CFM? If so, I guess a fan that does 32 CFM is overkill?

Not exactly. I will repeat a post I just did in another forum to give you some insight.

When engineering a forced air cooling system, there are two first order variables placed into the equation:  the heat generated by the device and the maximum temperature rise allowed.

The required fan CFM at sea level can be estimated as:

   CFM = 3.16 * Watts of Heat Generated / ΔT

   where ΔT is the maximum desired temperature rise and CFM is the cubic feet per minute of the fan

Note that this is only an estimate. If the equipment is to be operated at higher elevation levels for example, then the airflow must be increased as cooling is not determined by airflow but by the mass of air. Thus the reduced air density at elevation requires an increased CFM.

Then the system impedance must be estimated or measured. To measure the system impedance, a manometer is typically used while the required CFM is flowing. A simple example of this is:



This can be thought of as "back pressure" on the fan. A particular fan can only produce a given CFM under a given system impedance. Thus the engineer must consult the charts of the fan manufacturer to select an appropriate fan for the job. In high system impedance situations, placing fans in series (push / pull) is often more effective than a single higher CFM fan. In low impedance systems, parallel fans are also a design option. The number of fans can also be affected by system reliability goals.

The reduction of system impedance can be quite intuitive (e.g. straighter paths, larger exit vents, etc.) and can also become quite esoteric (shaped inlet and outlet ports, heat stacking, etc.).  By constructing a simple manometer, the hobbyist can experiment with techniques to lower the system impedance and directly confirm the effectiveness. Any meaningful reduction would allow the fan speed to be reduced or a lower CFM (quieter) fan to be used (see formulas below and note about pressurized systems).

Since a well designed product will consider the altitude variations of where the product might be used, it is often possible to reduce the CFM or speed of the fan when the product will be consistently operated closer to sea level. There are some relationships that can help estimate the effects of such changes:

    1.) CFM is directly related to speed ratio. Cut the speed in half and the CFM drops to one half.
    2.) Pressure is directly related to the square of the speed ratio.
    3.) Noise reduction of the fan is related to speed by approximately: ΔdB = 50 * log10(New speed / original speed)

So if you drop the fan speed in half, the CFM drops by 50%, the noise drops by 15 dB, and the temperature rise will double - assuming constant heat generation inside the device.

Some designs, particularly those with closely spaced cooling fins, require a minimum system impedance in order to properly cool the device. If the system impedance is too low, the air currents tend to flow around the closely spaced fins and thereby not adequately cool the device. By pressurizing the enclosure, the density of the air is increased thereby increasing the cooling capacity at the expense of a (typically) noisier fan.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N3QE
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« Reply #43 on: April 10, 2017, 11:44:17 AM »

The original(?) owner of the amplifier was using a 105 CFM fan but I don't know why since he's SK.

The least possible CFM will be when you are blowing air right through the anode fins with proper plenum and chimney and back-pressure.

As to 105 CFM, if you are just blowing air past a heat sink that is thermally coupled to a conduction-cooled tube, with thermal resistance at every junction, it actually takes more CFM to get the same amount of heat removed. I haven't seen a fan of the 105CFM retrofitted to heat sink but I doubt that the fan would make 105CFM with any back-pressure.

I know these amps were sold originally with low or no blower noise as a selling point. If you want to do that with a retrofitted air-cooled tube you will have much more to work than if you were building an air-cooled amp from scratch with quiet as a goal.

I personally don't think quiet should be a goal - I do RTTY too and should the blower ever stop turning I want to be able to hear that it stopped turning!
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N0SQ
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« Reply #44 on: April 10, 2017, 01:35:51 PM »

The original(?) owner of the amplifier was using a 105 CFM fan but I don't know why since he's SK.

The least possible CFM will be when you are blowing air right through the anode fins with proper plenum and chimney and back-pressure.

I'm considering pulling air instead of pushing air.

Quote
As to 105 CFM, if you are just blowing air past a heat sink that is thermally coupled to a conduction-cooled tube, with thermal resistance at every junction, it actually takes more CFM to get the same amount of heat removed. I haven't seen a fan of the 105CFM retrofitted to heat sink but I doubt that the fan would make 105CFM with any back-pressure.

I'm considering a 12VDC fan at 250CM/4800RPM. The bad thing is that I'll need a separate 12 VDC supply because this fan will pull 4 amps. I suppose I can use this fan to determine what I would need for a quieter fan - the reviews say this fan is very loud.

Quote
I know these amps were sold originally with low or no blower noise as a selling point. If you want to do that with a retrofitted air-cooled tube you will have much more to work than if you were building an air-cooled amp from scratch with quiet as a goal.

This amplifier didn't come with a fan.

Quote
I personally don't think quiet should be a goal - I do RTTY too and should the blower ever stop turning I want to be able to hear that it stopped turning!

Good point. But, there will be enough noise coming from the RTTY signals and band noise (even with the 200 Hz filter).  Shocked
« Last Edit: April 10, 2017, 01:38:56 PM by N0SQ » Logged
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