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Testing 811A?




 
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wa2tak
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« on: May 28, 2023, 08:46:11 AM »

Picked up a vintage home brew 2 x 811A amp.
One had broken glass at the plate cap.

Would like to know if the other is OK.

The heater needs 4 amps...my PS does not go that high.

What to do, please?
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Patrick J. / KD5OEI
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« Reply #1 on: May 28, 2023, 11:06:55 AM »

Other than checking to see if it will stand the plate voltage to be used, not much, as you must have filament power.

Can you find an old style 'analog' car battery charger of more than 4 Amps?
It will have a transformer of anywhere from around 6 to 24 volts depending on whether it's a 6V or 12V unit, and if it has a full wave center tapped or bridge rectifier. Can you use a variac on the primary to adjust the secondary voltage to 6.3V?
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wa2tak
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« Reply #2 on: May 28, 2023, 11:59:23 AM »

I'll be darn!
One PS does have 6.3 @ 4amps
HV 400 v @ 100 ma.

Can I expect something 'out' with these to the one "good"? 811

Which should not 'pop' my Cantenna dummy load.

( Had a friend..now SK..blew up his cantenna! )

How many SSB watts in?

Any CAUTIONS???

PS...permit me show it...I LUV homebrew's


* Vintage 2 x 811 panel .JPG (298.38 KB, 1600x1200 - viewed 133 times.)

* Output tank.JPG (245.31 KB, 1600x1200 - viewed 289 times.)

* Vintage 2 x 811 panel .JPG (298.38 KB, 1600x1200 - viewed 268 times.)
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wa2tak
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« Reply #3 on: May 28, 2023, 02:06:03 PM »

Looked at the 811 specs.
Should expect 'something' at 400 on the plate.
The graph shows plate current vs grid voltage.

Alas!...my 811 grid pins have NO wires!!

Guess it's not a grounded grid amp ?? Smiley

Photo attached.


* Empty grid pins on tubes.JPG (261.17 KB, 1600x1200 - viewed 286 times.)
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W1RKW
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« Reply #4 on: May 28, 2023, 03:47:03 PM »

https://www.amwindow.org/tech/pdf/TransmitterTubeTesting.pdf
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Bob
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Home of GORT. A buddy of mine named the 813 rig GORT.
His fear was when I turned it on for the first time life on earth would come to a stand still.
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Patrick J. / KD5OEI
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« Reply #5 on: May 28, 2023, 04:53:10 PM »

Looked at the 811 specs.
Should expect 'something' at 400 on the plate.
The graph shows plate current vs grid voltage.

Alas!...my 811 grid pins have NO wires!!

Guess it's not a grounded grid amp ?? Smiley

Photo attached.

------------------------------------------
It is grounded grid. Notice the ceramic caps from grid pins to ground and the coaxial cable, you wil find, probably goes to the grid meter shunt. Notice the capacitor from the filament to somewhere? it probably goes to the amp input. There may not be a T/R relay inside the amp. I didn't see one, but the pictures do not show enough. I believe the empty pins are pin 2.

400 Volts will get you very little output. With a normal high voltage of at 750-1500 it will make output with one tube, but probably less than half of the total amount since the PI circuit tank will be made for two tubes and therefore present half the impedance to the one tube, which is an overload. Using a low-ish Hign Voltage may improve the match situation since the plate voltage swing will be lower with the same plate current, making the expected impedance of the tubes lower and therefore closer to using two tubes at a higher voltage and double current. The driving impedance of a pair of 811s is about 150 Ohms. It's not critical but a 1:2 step-up toroid may help, though you should try to use a broadly tuned L circuit for each band. These articles might help:

There is an article for one grounded grid 811s at 1500V that may help you see what's going on. It says 200 watt, but that is input, not output. Two 811s can make about 200W out, more on SSB's low duty cycle. Also refer the to tube manual and use the audio ratings for two tubes in push pull class B or AB2, which will give an idea of the acceptable operations.
http://www.qro.it/amp/schemi/pdf/68hb187.pdf

The heathkit SB-200 is a two-572 amp, but you can use 811As in it. The tubes are almost alike except the 572s have more plate dissipation. It would be a good amp to be the example for upgrading yours.
https://www.w6ze.org/Heathkit/Heathkit_033_SB200.pdf

There's a video on tuning up 811 amp, His has 4, but the ideas are the same.
https://www.youtube.com/watch?v=MTt6QtgsIFY

Be careful and watch the grid current, it is easy to burn up the grids on 811s, since they are smallish tubes. A 100W radio will destroy them. A rule of thumb is that the lower the plate voltage, the higher the drive wattage. but not always.

And indeed some people push 811s to 2000V. They will take it but be careful especially with any questionable tubes.

hope these items help. Please send full pics so everything can be seen.
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wa2tak
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« Reply #6 on: May 28, 2023, 05:02:57 PM »

I have only 1 tube.
Can I do anything with that?
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Patrick J. / KD5OEI
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« Reply #7 on: May 29, 2023, 06:12:20 PM »

Of course, one is fine. You can do 120-160W peak with it.
What are you wanting to drive it with? it might take only 8 to 20W.

The data sheet is here:
https://frank.pocnet.net/sheets/049/8/811A.pdf

See the linear power amplifier ratings on page 4.

The whole question isn't real simple. Yes you can run it with one tube, but performance and efficiency will be poorer than with two as it was designed.

One can make 160 Watts PEP (single tone/CW) with 1500V on the plate, and 8W drive to the grid, but your amp is grounded grid, so the only 'required' change to use just one tube might be the PI network coil taps moved so as to match the single tube's 'effective rf load resistance'. Somewhat more drive power will be required.

Please add a low-Q tuned input circuit between the input jack to the filament, so that you get the most power (and least distortion). That amp is old and back then the harmonics/distortion was not considered as important, so it was never built-in.

There are a few good articles to be found using a single 811A. You may have to wade through google-garbage to get past the ads and MFJ/Dentron sales material. LOL. One thing about MFJ though, they usually sell sub-assemblies. Could be helpful if you do not have the right slug-tuned coils etc.

A program, 'elsie' will even calculate the coils and capacitors changes for you, based on your available high voltage and using that to see what the tube data sheet says. The plate coil in that thing is nice and beefy and able to easily handle those tubes.
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wa2tak
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« Reply #8 on: May 30, 2023, 09:10:55 AM »

Hi Patrick:

Tnx for the continuing help!

Over these many years, had little interest in 'amps'.
Rather 'on the other side'...used a Millen grid dip meter as a transmitter... link coupled to my dipole...was heard 5 miles away.

This amp is going to be a "long-time" project as I have get all things needed to fire it up.

I'm not looking for a lot of output...any output would be fine with me.
Have various xmtr's for exciting it....would like to use my ( yet to prove working OK ) Central Electronics 20A with it.
Would Luv to operate AM with it!! ( I know AM & linears don't go well together )...what the heck.

Had put together pieces for a 1930's rig on my drafting table..2 tubes ( forget the # ) the size of a large thermos bottle...used Heising modulation with vintage 'stick'
telephone...worked darn fine.
Wonder if I do that with this amp???
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W7TFO
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« Reply #9 on: May 30, 2023, 10:45:58 AM »

I can help you with power supply parts if you want to build it.

73DG
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Just pacing the Farady cage...
wa2tak
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« Reply #10 on: May 30, 2023, 11:03:47 AM »

Hello Denis:

I NEED all the help I can get on this project!!!
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