?

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 ?

?

73

?

vk4tux

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 ≈ 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

No need to "side-line" your amp... just throw that spare 8877 in and run it!

No need to "side-line" your amp... just throw that spare 8877 in and
run it!

?

No need to "side-line" your amp... just throw that spare 8877 in and run it!

Also, being that the 8877 (3CX1500A7) is a ceramic tube, there's no need to "cook" it in or do any sort of "degassing", or anything else.? They are not prone to leakage, as a lot of old glass tubes are (like the 3-500Z's).

Tube swap is very easy, only takes about 30 minutes...? Removing (and reinstalling) all of the cover screws takes longer than the actual tube replacement. LOL!? Pull that old one out, plug the new one in, hook up the anode lead, put on the cover... and you're set.

I swap the tube in my big Henry 3K-Classic fairly often (I go between an 8877 and a 3CPX1500A7, pulse rated tube).? In addition, I just recently swapped-in a spare Eimac YU-157 into my Alpha 9500 (pulled the OEM 8877 out) just to exercise it.? I'll run it for a while, then pull it out and go back to the 8877.? No problems whatsoever.

As long as your no-drive idling plate current is good (which it should be, if your spare tube is good), you shouldn't need to do anything else.

73,
~Alan
W5ARM

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.

?

73,

Dave, w6de

?

?

From: [email protected] <[email protected]> On Behalf Of Adrian Fewster via groups.io
Sent: 14 October, 2023 04:36
To: [email protected]
Subject: Re: [ham-amplifiers] Symptom(s) for a fading 8877 tube?

?

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 ?

?

73

?

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 ≈ 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

?

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.

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.

?

73,

Dave, w6de

?

?

From: [email protected] <[email protected]> On Behalf Of Adrian Fewster via groups.io
Sent: 14 October, 2023 04:36
To: [email protected]
Subject: Re: [ham-amplifiers] Symptom(s) for a fading 8877 tube?

?

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 ?

?

73

?

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 ≈ 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

?

Hey Dave,

If your amp has been operating "normally" for a length of time (i.e., with apparently normal/full output) and only recently started showing signs of decreased power, then it's probably a bad tube.

However, it would be prudent to check your filament voltage, to ensure that it is within spec. If anything will kill an 8877 tube, it's LOW filament voltage.? An under-temp/cold cathode will cause striping of the coating off the cathode, dramatically shortening its life. The filament voltage must be checked at the tube base, at both static (no load) and loaded (max RF) conditions.

Tom Rauch, W8JI, has documented this... it is worth the read. HERE:?? and HERE:

On the 9500, you can access the tube base by setting the amp over on its right side (viewing from the front), and removing the cover on the amp bottom, below the tube. Obviously, for safety: Do this with the amp unplugged, and be certain the B+ is fully discharged (although no B+ voltage on the bottom). Make the connections to your meter, then plug-in the amp and power up. No harm in running the amp on its side for the short period of testing.

Connect a DVM to the filament pins, and then fire-up the amp. Allow the tube to come up to temperature (3 minutes), then make note the filament voltage. Then apply some drive and get RF output (preferably near legal-limit) and again make note of the filament voltage.? If the filament voltage is NOT within specs, then there's an issue.

It's better to have slightly high voltage unloaded, and at or near specification (or slightly high) under load, than it is to have low voltage, especially under load.

While replacing your existing tube will likely cure the low RF output problem, continuing to run with low filament voltage will kill the new tube also, although it may take a while.

Just as a point of interest, my 9500 runs at closer to 3450-3500 volts on the plate, and my input line-voltage is always very close to 250v (247-249).

Good luck. Let us know what you find...

73,
~Alan
W5ARM

Measured Voltage results of Alpha 9500 key down testing.

AC Mains Voltage at Amplifier switch box.

142 VAC Amp on at Standby.

239.6 VAC Amp at 1.3 KW into Dummy load

?

Filament Voltage

5.3 VAC at initial power on.

5.45 VAC after 180 Second warm up remains at 5.45 during subsequent Standby (not Transmitting).

5.16 VAC on Key Down 1.3 KW.

?

Plate Voltage

3.5 KV during any and all standby conditions.

3.2 KV during key down 1.3KW.

?

Looks like normal Voltages to me.? Strongly points to a weak tube.

?

Thank you, Alan and Alex for urging the Voltage tests.

Alan, the filament voltage check was really simple through the bottom plate.

Having two Fluke VOMs made this testing easy and less dangerous.—no having to move probe tips after set-up.

?

73,

Dave, w6de

?

From: [email protected] <[email protected]> On Behalf Of Alan - W5ARM via groups.io
Sent: 16 October, 2023 17:02
To: [email protected]
Subject: Re: [ham-amplifiers] Symptom(s) for a fading 8877 tube?

?

Hey Dave,

If your amp has been operating "normally" for a length of time (i.e., with apparently normal/full output) and only recently started showing signs of decreased power, then it's probably a bad tube.

However, it would be prudent to check your filament voltage, to ensure that it is within spec. If anything will kill an 8877 tube, it's LOW filament voltage.? An under-temp/cold cathode will cause striping of the coating off the cathode, dramatically shortening its life. The filament voltage must be checked at the tube base, at both static (no load) and loaded (max RF) conditions.

Tom Rauch, W8JI, has documented this... it is worth the read. HERE:?? and HERE:

On the 9500, you can access the tube base by setting the amp over on its right side (viewing from the front), and removing the cover on the amp bottom, below the tube. Obviously, for safety: Do this with the amp unplugged, and be certain the B+ is fully discharged (although no B+ voltage on the bottom). Make the connections to your meter, then plug-in the amp and power up. No harm in running the amp on its side for the short period of testing.

Connect a DVM to the filament pins, and then fire-up the amp. Allow the tube to come up to temperature (3 minutes), then make note the filament voltage. Then apply some drive and get RF output (preferably near legal-limit) and again make note of the filament voltage.? If the filament voltage is NOT within specs, then there's an issue.

It's better to have slightly high voltage unloaded, and at or near specification (or slightly high) under load, than it is to have low voltage, especially under load.

While replacing your existing tube will likely cure the low RF output problem, continuing to run with low filament voltage will kill the new tube also, although it may take a while.

Just as a point of interest, my 9500 runs at closer to 3450-3500 volts on the plate, and my input line-voltage is always very close to 250v (247-249).

Good luck. Let us know what you find...

73,
~Alan
W5ARM

?

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 ≈ 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

On 20 Oct 2023, at 6:45 am, Adrian Fewster <vk4tux@...> wrote:

?On Mon, Oct 16, 2023 at 10:01 AM, Alan - W5ARM wrote:

If anything will kill an 8877 tube, it's LOW filament voltage.? An under-temp/cold cathode will cause striping of the coating off the cathode, dramatically shortening its life

This is not true, 'strip off' of the cathode coating is caused by cathode overdrive. Low 'heater' voltage in the oxide indirectly heated cathode increases cathode poisoning.

also see ;

Oxide Cathode Life: Investigations into the Causes of Loss of Emission

A paper* read recently at the Institution of Electrical Engineers described the results of work by the Electronics Division of the Post Office Research Station (with its associated small valve factory) on the development of long-lived valves. The Post Office is interested in such valves mainly because of the need for them in submarine repeaters and other inaccessible apparatus; but their development is of considerable moment to others as well.

Apart from structural failures, with which the paper was not concerned, the life of a valve depends mainly upon the rate at which the emitting qualities of its cathode deteriorate with use. Loss of emission is the disease; if its causes are completely understood, we are more than half-way on the road towards finding a cure. There are two main accepted causes of oxide cathode deterioration: poisoning by attacks from residual gases, and the development and growth of inter-face resistance; on both of these the paper throws much new light.

Interface resistance was the subject of a recent article in?Wireless World.** The authors of the paper agree with Eaglesfield in accepting Eisenstein’s view that interface resistance is due to the growth of a film between the cathode body and the oxide matrix, and that its production is due to deliberately introduced impurities, chiefly silicon.

They find, though, that the film does not increase in thickness as the valve ages. It appears to build rapidly up to its maximum thickness, after which a steady change in its nature sets in; it is to this change that the increase in resistance is due. They consider that it is caused, in part at any rate, by deactivation of the interface by gas poisoning, much as the matrix itself is deactivated; hence the complete elimination of residual gas would be likely to check the growth of interface resistance, in addition to ensuring longer life for the emitting surface of the cathode.

A method has been developed of making a triode or pentode measure its own residual gas pressure. As electrons flow from cathode to anode some collide with gas molecules. The positive ions, so formed, travel to the negatively biased control-grid and set up reverse grid current ?rg, from which the residual pressure can be derived. It has, however, been found more convenient to use a “vacuum factor”:

?rg?(??A)
k?= ————
?a?(mA)

Investigation of the behaviour of the residual gas under working conditions showed that when?k?was plotted against time there was always a sharp initial rise, followed by a slow, roughly exponential fall until a value?k0?(the “residual vacuum factor”) was reached; the value of?k0?is constant. The area enclosed by the?k=f(t) and?k0?curves is a measure of the residual gas driven into the cathode and has been named the “gas integral.” It was found that valves with a high gas-integral were short-lived and that a low gas integral was an indication that long life might be expected.

Here, then, was one method of forecasting valve life after a test of comparatively brief duration; reliable tests of this kind are clearly needed for dealing with valves whose working lives may range up to 60,000 hours—say seven years of continuous running! Another of great value that has been evolved is the low-temperature total-emission test. Under working conditions emission is limited by the space charge; to measure the total emission there must be no such limiting factor. It is also desirable that the cathode temperature should be low enough for ionic equilibrium to be maintained within it. The method developed is to use the control grid as collector, making its potential +5V and earthing the other electrodes; for 6.3-V valves the heater is at about 2.6V.

In practice, valves are taken from the life-test rack, where they are running under working condition, and put through the total-emission test. The total emission, after a given number of working hours, can thus be plotted as a percentage of its original value.

The authors have no doubt that the amount of barium used for gettering the average valve is amply sufficient to absorb all residual gas if (and that is one of the big problems) physical association of gas and getter can be established. They have given much attention to the preparation of electrodes and supports and to pumping, gettering and ageing valves. It has been found that when residual gas is reduced (by methods suited so far to the laboratory rather than to the mass-production factory) to amounts far smaller than those in commercial valves, no deterioration in the emission occurs after thousands of hours of use. The authors’ estimates are always conservative; they see no reason why valves, with assured lives of 40,000 hours or more, should not be produced.

Poisoning of the cathode by occluded gases released from metal parts produces non-emitting patches on the surface of the cathode, which may be small and evenly distributed, or large and irregular. The authors in their investigations have found a new and unexpected source of cathode-poisoning gas. “A gas derived either directly or indirectly from the heated glass envelope,” they state, “is more destructive in action than any of the normal gas so far examined.” This gas is believed to be water vapour, which has been shown to have dire effects on a cathode at 1,000? K.

A second possibility is that the water vapour reacts with metallic carbides in the valve to produce unsaturated hydrocarbons of the acetylene type, which dissociate to form non-emitting carbon patches on the cathode.

R. W. H.
WIRELESS WORLD FEBRUARY 1952

* “The Life of Oxide Cathodes in Modern Receiving Valves.” G. H. Metson, S. Wagener, M. F. Holmes
and M. R. Child.
** “Valve Cathode Life.” C, C. Eaglesfield.?Wireless World, December, 1951

"? src >?

73

vk4tux

On 21 Oct 2023, at 12:24 pm, Adrian Fewster <vk4tux@...> wrote:

?

Some good material here ;

73

vk4tux

On 21/10/23 14:19, Adrian Fewster via groups.io wrote:

On Thu, Oct 19, 2023 at 08:12 PM, John Sparkes wrote:

Thanks gents - the below was awesome. If you have any other scientific papers/references on tubes, please post the links.

Many thanks.

John VK6JX?

Glad to help with the post John. Tube amps are a dying breed in VK, with solid state taking over, as pricing and devices improve.
It is good to save all the tube amp info one can find.

73

vk4tux

?