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Author Topic: 813 tube biasing in grounded grid?  (Read 596 times)
K1ZJH
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Posts: 3327




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« on: December 05, 2017, 06:17:12 AM »

Has there ever been any data published regarding running 813 tubes in grounded grid?  I am working on bias switch and I am not sure how much any voltage drops in the cathode DC return path will affect operation.  Also trying to figure a bias resistor for ABS idle that just puts the tube into AB1. 

Pete
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KM1H
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Posts: 2633




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« Reply #1 on: December 05, 2017, 11:53:31 AM »

About 5V for SSB linear up to about 2500V; figure on 20 ma idle per tube.

I ran a regulated adjustable bias from an old lab supply to a quad of them for 160-10 years ago at 3200V and could select from Class C CW (just to the point of no plate current) to low IMD SSB.
AM was in between to reduce heat with minimal effect on the spectrum analyzer. A small muffin fan did the cooling.

The graphite plate tubes can take a lot of beating and no color which leads me to believe the Pd is really in to 200-225W region and not the 125W in the books. Compare the size of the 160W Cetron 572B anode to the 813. DO NOT try this with the later sheet metal 813's, they act like 811A's Grin

At 1500W out with the quad there was just some dim color on voice peaks and no loss of power in close to 15 years.

Ive also run a pair of 803's at about the same power; that monster block of graphite is still rated at only 125W!

Carl

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HAMHOCK75
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Posts: 396




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« Reply #2 on: December 05, 2017, 01:05:37 PM »

I have been restoring a grounded grid amplifier. I found the plate currents under the same bias for four tubes varied 2 to 1 so not sure how helpful published data would be with such a variance.
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K1ZJH
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« Reply #3 on: December 05, 2017, 02:26:53 PM »

There is no published data for the 813, unless you run cathode driven with normal screen operating voltages.

Many of the overseas hams run 400 to 700 volts on the screen, cathode driven, and the RCA data for biasing follows to form.

But, when you ground the screen and control grids and reverse bias both the literature goes out the window for grounded grids. I guess I'll try using a diode string to find the sweet spot for just going into AB-1 on quiescent automatic bias operation.

Pete
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HAMHOCK75
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Posts: 396




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« Reply #4 on: December 05, 2017, 04:50:45 PM »

Here is a grounded grid design with the 813 screen grid grounded.

http://faculty.frostburg.edu/phys/latta/ee/wing813amp/813schematic.html
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JS6TMW
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« Reply #5 on: December 06, 2017, 07:16:56 AM »

Here is a grounded grid design with the 813 screen grid grounded.

http://faculty.frostburg.edu/phys/latta/ee/wing813amp/813schematic.html
What a great websiye/pages
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G3RZP
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« Reply #6 on: December 06, 2017, 07:30:56 AM »

The 1969 RSGB Radio Communication Handbook contained an article on a two 813 GG amplifier. That ran at 2500 volts on load, with the screen, beam plates and control grid grounded: in receive, it used a 25 k ohm 15 watt wire wound resistor in the filament transformer centre tap return. Produced a clean 400 watts.
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KM1H
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« Reply #7 on: December 06, 2017, 03:21:24 PM »

Quote
I have been restoring a grounded grid amplifier. I found the plate currents under the same bias for four tubes varied 2 to 1 so not sure how helpful published data would be with such a variance.

Those sweep tubes are all unknown emission to start with and mixing brands adds to the mix.

Carl
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K1ZJH
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Posts: 3327




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« Reply #8 on: December 06, 2017, 07:46:51 PM »

Here is a grounded grid design with the 813 screen grid grounded.

http://faculty.frostburg.edu/phys/latta/ee/wing813amp/813schematic.html

I like this one much better.  A professional job, start to finish.

http://www.kg7tr.com/813-linear-amplifier.html
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