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Author Topic: Henry K-2000 amplifier  (Read 19772 times)
N0SQ
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« Reply #15 on: April 03, 2017, 11:46:41 AM »

I just got donated this amplifier and have one question, for now. That is: do I need a relay buffer between my Kenwood TS2000 and this amplifier? The manual that I have isn't clear on that.

Lee, I think another question is, do you really want to use that amp (8873 tubes) on RTTY ?  Watch the grid current.

Glenn AE0Q

Good question. The guy who gave it to me also gave me a muffin fan that he used to keep the amplifier cool. I guess anything to help keep the tubes "cool" is worth the effort. Anyway, according to my versa-tuner wattmeter terminated into a "cantenna", I am getting 1050 watts with 95 watts of drive with 25 mA of grid current. According to the manual, the grid current and plate current are within specs. When I turn the drive down to get 600 watts output, the grid current is almost zero (plate current 460 mA). Kinda difficult to read the grid current since the meter floats down from a high current reading until it settles - it's possible that I would fry the tubes waiting for the meter reading to settle. Anyway, there are modifications that can be done to change this amplifier from convection cooling to air cooling - which means a different tube (such as the 3CX400A7 ($390) and 3CX800A7 ($440)) - still much cheaper than an AL-82 ($3000) or even an AL-80B ($1600).
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N0SQ
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« Reply #16 on: April 03, 2017, 07:42:55 PM »

How does one check the HV diode in this beast? A normal diode check with my Fluke 77-4 doesn't work. The diode is actually 2 diodes in one and the Fluke shows both diode sections with very high resistance both forward and reverse. Heh, I really need to get HV probes if I'm gonna work on amplifiers. For some reason, my HV dropped from 2200 volts (according to the amplifiers meter) to 1400 V. And the HV substantially drops with drive applied. The HV probably did drop because the grid current is higher than the plate current now.
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AE0Q
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« Reply #17 on: April 03, 2017, 08:36:55 PM »

I really need to get HV probes if I'm gonna work on amplifiers.

We use the Fluke 80K-6 High Voltage Probe every day at work, you can usually find them on ebay for about $50 or less.

Glenn AE0Q
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Glenn and V-NATCH Katie,
HP-O, MXB, MJB, XF, TKI
http://www.hoopsandjumps.com/
http://funagility.webs.com/
N0SQ
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« Reply #18 on: April 04, 2017, 07:12:46 AM »

I really need to get HV probes if I'm gonna work on amplifiers.

We use the Fluke 80K-6 High Voltage Probe every day at work, you can usually find them on ebay for about $50 or less.

Glenn AE0Q

Thanks Glenn. I was looking at buying an 80K-40 until I saw your post. I think either one will work with my multimeter but it looks like the 80K-40 is more expensive. Soon, my ham shack is gonna look like an electronics repair shop (again).
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W1QJ
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« Reply #19 on: April 04, 2017, 07:42:29 AM »

How does one check the HV diode in this beast? A normal diode check with my Fluke 77-4 doesn't work. The diode is actually 2 diodes in one and the Fluke shows both diode sections with very high resistance both forward and reverse. Heh, I really need to get HV probes if I'm gonna work on amplifiers. For some reason, my HV dropped from 2200 volts (according to the amplifiers meter) to 1400 V. And the HV substantially drops with drive applied. The HV probably did drop because the grid current is higher than the plate current now.

Bad filter caps
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N0SQ
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« Reply #20 on: April 04, 2017, 08:14:20 PM »

How does one check the HV diode in this beast? A normal diode check with my Fluke 77-4 doesn't work. The diode is actually 2 diodes in one and the Fluke shows both diode sections with very high resistance both forward and reverse. Heh, I really need to get HV probes if I'm gonna work on amplifiers. For some reason, my HV dropped from 2200 volts (according to the amplifiers meter) to 1400 V. And the HV substantially drops with drive applied. The HV probably did drop because the grid current is higher than the plate current now.

Bad filter caps

That's what I was thinking but I was too lazy to check until tonight. These are 180 mfd capacitors. According to my Fluke capacitor function, they range from 200 mfd to 220 mfd except for one which did not have a capacitance value - obviously bad. The resistor shunts all check OK. I don't know how old these capacitors are so I guess I'll spend the money to replace all of them - especially since the type that's in the amplifier are no longer available. Looks like the capacitor string can handle up to 2700 volts (6 x 450v). I've followed a thread where the hams were saying that the capacitors should be matched sets but I guess they have the money to buy capacitors in bulk quantities. I found a suitable capacitor replacement at digikey (Cornell ERWE551LGN181MA80M) but it's a 550 VDC capacitor- the cost for 6 capacitors isn't too bad ($55 + shipping). It is my understanding that there is a type of capacitor that snaps into place and has tighter tolerances but I don't think I'll go that route.

So far, I've found a bad on-off switch (I'll have to fabricate something to get a new switch to fit since everyone I've looked at is either too big or too small for the mounting hole), 2 burned out incandescent indicators (which I plan to replace with LEDs), a bad power supply capacitor, and a difficult to use push button switch. When the tubes fail I'll consider modifying the amplifier and put in a cooling fan and a pair of 3CX800A7 tubes.
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KM1H
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« Reply #21 on: April 05, 2017, 07:21:39 AM »

There is nothing wrong with the 8873, just the lack of grid protection in the amp. It is the same as the 8874 and 8875 internally.
https://frank.pocnet.net/sheets/140/8/8873.pdf

I own a pair of Heathkit SB-230's (one I converted to 6M) and added a muffin fan to the heat sink to improve the miserly 200W rated plate disipation. The 6M one has been running a steady 600W for about 30 years of CW and SSB, the other one has been here only 3 years and used in the BR with a tiny TS-130.

Carl
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N0SQ
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« Reply #22 on: April 05, 2017, 08:25:24 AM »

There is nothing wrong with the 8873, just the lack of grid protection in the amp. It is the same as the 8874 and 8875 internally.

I suppose I could spend the money for the grid overload protection board that Ameritron sells (http://www.ameritron.com/Product.php?productid=GOP-100). They claim that it'll work with any amplifier. And it's only $40.00.
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N0SQ
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« Reply #23 on: April 05, 2017, 08:37:18 AM »

I get the impression that capacitor manufacturers are moving away from the screw terminal capacitors in favor of the "snap in" type. The snap in types appear to be cheaper and with tighter tolerances. Not to mention that these snap-ins are physically much smaller than the equivalent screw types. So, I'm wondering if I should stick with the old screw type caps or convert the amplifier to use the snap-in types. I suppose I could use the existing mounting board to install the snap-ins but I would have to drill some holes unless there is some other option to keep the existing board or maybe fabricate a new board with printed circuit traces. Plus I'm wondering about how secure a snap in would be. Oh well, it's only money.

I know, I know, this is a lot of work but it's fun. Besides, I haven't done this kind of work in almost 2 decades since everything became "black box" maintenance - that's why my job got boring and I couldn't wait to retire (not mention I became "rusty").
« Last Edit: April 05, 2017, 08:40:37 AM by N0SQ » Logged
W9IQ
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Posts: 2968




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« Reply #24 on: April 05, 2017, 08:52:03 AM »

You can certainly use snap in caps. Just pay attention to the hours and temp. of operating life, tolerance, WVDC, ESR, ripple current, and leakage current when making your selection. They are all important for maximizing the life you will get out of them.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
AC2RY
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Posts: 681




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« Reply #25 on: April 05, 2017, 10:42:44 AM »

You can certainly use snap in caps. Just pay attention to the hours and temp. of operating life, tolerance, WVDC, ESR, ripple current, and leakage current when making your selection. They are all important for maximizing the life you will get out of them.

- Glenn W9IQ

ESR and ripple current are two most important parameters for filter use. Also I would NOT even think about using 85C rated capacitors anywhere near tubes.
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W9IQ
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Posts: 2968




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« Reply #26 on: April 05, 2017, 11:22:38 AM »

Alex,

The ripple current rating is directly related to ESR and the thermal resistance of the capacitor. While they play a role in the design it isn't to safe that they are the "most" important - for example, pick a capacitor with too low of a voltage rating or too high of leakage current and your ESR/ripple current prioritization won't be meaningful for very long.

The design issues related to putting capacitors in series to create a high voltage capacitor bank are not trivial. The factors I listed earlier all come into play and I wouldn't discount any one of them when choosing capacitors. There are certainly trade-offs that can be successfully made but not without a full appreciation of the design issues.

Temperature ratings of electrolytic caps are one of the most misunderstood and misapplied parameters. Everything else being equal, choosing high temperature caps is a safe bet - ratings of 125°C are commonly available. But if you run an 85°C 2000 hour cap at 45°C you will increase its life rating to 32,000 operating hours. With reasonable forced air cooling, a 113°F operating temperature is attainable even in a tube environment.

- Glenn W9IQ
« Last Edit: April 05, 2017, 11:24:47 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N0SQ
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« Reply #27 on: April 05, 2017, 01:04:13 PM »

Since I haven't been able to find screw terminal caps rated at 180mfd/450vdc without having to buy bulk quantities of 100, I'll have to look into alternatives.  Has anyone heard of RUBYCON aluminum electrolytic capacitors? A web search can't find them but they're available through Amazon.
« Last Edit: April 05, 2017, 01:21:01 PM by N0SQ » Logged
W9IQ
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Posts: 2968




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« Reply #28 on: April 05, 2017, 02:07:24 PM »

Rubycon is a major brand on the low end of the price spectrum. you can get 105C / 5000 hour versions with reasonable ESR from them in snap pin format. Digikey stocks them.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
N0SQ
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Posts: 1




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« Reply #29 on: April 05, 2017, 06:30:52 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.
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