Re: Symptom(s) for a fading 8877 tube?
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Yes,? when shop/shack was constructed, I specified the wiring as #10 and multiple 240 outlets each on separate breakers (according to code) and over saw the construction and wiring.? I know if it was constructed properly, I don¡¯t know if
I have a current problem, but I checked the wall socket before connecting the Alpha earlier this year. ??I also have a 240 volt two wire motor switch for each 240 VAC outlet and I checked the voltage there also when I installed 20 amp fused in the switch box.
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I¡¯ll open the switch box and check later when it is under load.
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Thanks for asking.
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Dave, w6de
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From: [email protected] < [email protected]>
On Behalf Of Alek Petkovic via groups.io
Sent: 15 October, 2023 21:54
To: [email protected]
Cc: aa6yq@...
Subject: Re: [ham-amplifiers] Symptom(s) for a fading 8877 tube?
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Hi Dave.
My question is about your AC mains supply. Is it up to the task?
Section 3.2 and table 3-1 talk about what you can expect from different mains supplies.
Also, I looked in the manual to see if they mentioned the anode voltage but found no mention of it. Is 3200V the correct voltage and what does it sag to on key down? Excessive sag will also affect your heater voltage.
The Eimac datasheet says 1085W out at 2700V anode voltage.
Look at all the above before you go changing the tube.
73 and good luck, Alek VK6APK.
On 15/10/2023 7:21 am, Dave w6de wrote:
Good point, if the cathode emission is down, there aren¡¯t enough electrons being ejected from the cathode to make the higher power levels.? Time to switch tubes.? Since this amplifier case is already full and I have a spare tube, ?I¡¯m not
going to hack it up to make the filament voltage adjustable.?
But I going to wait until after the bands quiet down. ?Right now, I don¡¯t want to side-line the Alpha 9500 for my first time (with this amplifier) tube replacement and de-gassing cook in.
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73,
Dave, w6de
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If the tube is half in the grave, Increasing Filament voltage and current to increase emission from the remaining cathode carbon may help until the end ?
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73
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vk4tux
On 14/10/23 14:21, Dave w6de wrote:
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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