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Amplifier Tubes

Mar 5th 2015, 15:34

W5RAA

Joined: Feb 22nd 2015, 21:35
Total Topics: 0
Total Posts: 0
Other than obvious physical appearance of defective or inoperable power amp tubes, how can one tell when its time to replace tubes which are going soft?
Mar 5th 2015, 15:40

W5RAA

Joined: Feb 22nd 2015, 21:35
Total Topics: 0
Total Posts: 0
I have a several year old AL-811 amp. Using a dummy load and 30 watt CW input, the maximum power output I'm getting is anywhere from 200 watts on 10 and 12 meters, up to 400 watts on 160 through 20 meters. If I try to increase the exciter level, the grid current goes into the red region and I don't see much of an increase in output power. Does this mean that the 811A tubes may be getting soft? I believe that the MFG tuner power meter is fairly accurate because it shows my exciter output accurately.
Mar 5th 2015, 18:49

W5RAA

Joined: Feb 22nd 2015, 21:35
Total Topics: 0
Total Posts: 0
One more thought...in this three 811A tube amp, if it's outputting roughly 2/3 of its rated power, is it safe to the amp to remove one tubes at a time to isolate if one of the tubes is weak, keeping in mind all of the safety considerations!
Mar 6th 2015, 04:47

WA9WVX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I would suggest read this article written by W8JI

http://www.w8ji.com/al811h_schematic.htm

And then replace all the 811 / JAN using matched tubes from Surplus Sales http://www.surplussales.com/Tubes-Sock-Acc/TubeSpecials.html Scroll down to the 811 / JAN Special Prices. The 811 / JAN Tubes were built for our military during WW II as a Joint Army Navy Contract which made them heavy duty at a low price. If I were in the market for these type of tubes, I purchase two sets of them for future use.

Dan
WA9WVX
Mar 6th 2015, 11:28

W5RAA

Joined: Feb 22nd 2015, 21:35
Total Topics: 0
Total Posts: 0
Thanks Dan, I'm familiar with Jan qualified devices and this is good advise. I'm still wondering if pulling one tube at a time in the Al-811 amp would cause any damage to it. For example, if I pull one tube and see no effect on output power, then I would know that the tube I pulled is defective.

Rick
W5RAA
Mar 6th 2015, 20:50

WA9WVX

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Hi Rick,

I use to work on Commercial Two-Way Radio FM amplifiers that ran a pair of 8560AS Ceramic tubes, the company switched tube suppliers to ones Made in China, the Quality on these were terrible as it took almost 30 minutes or longer to insert a pair into the sockets (the U.S. version took 10 to 15 minutes to change) and when I started tune the Power Amp, you could hear a metal Pling and watch the RF Power Output drop down to 200 W or so as the Plate Connection popped off the tube. It didn't affect the Tank circuit or the Power Supply ... just more time spend on replacing another tube. Pulling out one or two tubes shouldn't affect your amplifier just as long as you isolate the Plate Connections away from the metal chassis so they don't short out.

Dan
WA9WVX
Mar 7th 2015, 12:08

W5RAA

Joined: Feb 22nd 2015, 21:35
Total Topics: 0
Total Posts: 0
Thanks Dan, I'll pull them one at a time and see how power output is affected.

Rick
W5RAA
Apr 5th 2015, 22:16

WA4MIT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Rick I have been working on old Yaesu FL2000B amp it had low output and I purchased a AL-811H amp the Yaesu uses 2 572B tubes which are a HD version of the 811A. I removed 2 of the 811A and tried them in the Yaesu and amp and it had good output. Not much else can go wrong in amps that would not result in something you would see or smell due to the voltage and such used.

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