Re: Symptom(s) for a fading 8877 tube?
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I replaced the tube in my Alpha 9500 with an Eimac 8877.? This tube produces 1500 Watts with less drive than the previous tube required to produce 1300 Watts.
Turns out the original tube that was in the amplifier when I bought (February 2023) was not a Thompson tube it was a Penta Labs tube, made in China, with a 1013 date code on it.? It is not labeled as either an 8877 or 3CX1500A7, it is labeled
VTX-X101.? It is now put away as a just in case spare.
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Thank all of you for your advice.
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Best Regards,
Dave, w6de
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From: [email protected] < [email protected]>
On Behalf Of Dave w6de via groups.io
Sent: 14 October, 2023 04:21
To: [email protected]
Subject: [ham-amplifiers] Symptom(s) for a fading 8877 tube?
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I have an Alpha 9500 and I find it difficult to get 1500 Watts out of the current Thompson 8877--age unknown.
The symptom is: I can easily get 1,000 to 1,100 Watts out of the amplifier but continuing to increase the drive level (to get power above 1.3KW) power output does not continue its somewhat linear increase in power output.?
I reach a level where I am concerned about increasing the drive because I don't see increase in output power.? And the grid current is reaching 90 Milliamps where I stop trying.
There is 3,200 Volts on the Plate and the plate current won't increase above ¡Ö 0.7 amps without exceeding grid current limits. Running class AB2 a linear is about 60% efficiency I should be getting about 1,344 Watts and I'm seeing 1370 Watts on the built-in
power meter which matches my Bird power meter pretty closely.? But When I try to increase the drive, the amplifier will momentarily indicate 1500 watts but immediately drop back to 1300 watts.
All testing into a large surplus 50 Ohm dummy load.
Side Note: the Alpha 9500 has automatic tuning of the plate/output circuit.? I leave it on as I do several band and mode changes during an operating session.? And I and use a Steppir antenna and the Steppir doesn't always go to the same resonant frequency as
I change bands and modes within those bands. However, that automatic plate circuit tuning goes nuts when I try to push the power to 1500 Watts.? Automatic tuning doesn't like to go above 1100 to 1200 watts (depends on the band) and stay still.
Any thoughts?? I do have a supposed NOS 8877 in the original Eimac box.? I just want to know if this is what to expect from an aging Thompson 8877 or any aging 8877.
73,
Dave, w6de
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