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Re: [Amps] Building 833 Amplifier. Hints and Suggestions?

To: Greg Weinfurtner <weinfurt@ohio.edu>
Subject: Re: [Amps] Building 833 Amplifier. Hints and Suggestions?
From: Roger <sub1@rogerhalstead.com>
Date: Tue, 16 Mar 2010 19:09:40 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>

Greg Weinfurtner wrote:
> Hi all,
>          I am new to the list so be kind! I went through all of the threads 
> on amps@contesting.com concerning an amplifier using 833's and pretty much 
> came up with very little usable information. Why 833's?  Why do I still 
> deer hunt with a muzzle loader with just iron sites? Why do people still 
> build tube regens?  Just 'cause I guess it is a challenge and the 833 is a 
> magnificent looking tube all aglow!
>   
Others may not share my views but:

I find the 833 intriguing but I've always thought it was ugly<:-)) OTOH 
I too have wanted to build up an amp using them.  
w5jgv.com/downloads/RCA%20*833A*.pdf
shows a pair of them can run the legal limit and a single one can get 
about 1200 PEP out, but this is with a plate dissipation of  only 350 
watts so a lot depends on the duty cycle. The filaments ae 10V @ 10A 
thoriated tungsten so unlike many of the big power tubes the filaments 
are not terribly power hungry.

With forced air cooling they are referring to 40 cfm out of a 2" nozzle 
directly over the tube with the flow directed between the plate and grid 
connections.  Using a "swamped grid" approach takes about  40-50 watts 
of drive for a pair of them to get over the legal limit out., but that 
can vary widely depending on plate voltage.

I think a pair of those with the ability to view them while running 
would be impressive.

Due to the physical geometry I'd think (but don't know) that the 
isolation of the input circuitry from the output would take a bit more 
care than with modern tubes.

I've been looking at the idea of building up a similar amp using a pair 
of the GU81M Russian tubes.  Those things are BIG!

more below.
>          I was an Electronic Design Specialist for Ohio University for over 
> 22+ years and have built a hybrid solid state 160 meter SSB/CW 100 watt 
> transceiver (Using a pair of 6146's) from the ground up, ( 
> http://oak.cats.ohiou.edu/~weinfurt/160mrindex.html )including a 500 hz CW 
> filter. Age is mid 50's and was in Tube/Solid State transition during 
> college years. Yep, slip sticks to calculators to SMD's. BSS (Bachelor of 
> Specialized Studies in Shortwave RF Engineering) is from 1994, so that and 
> being an EDS helped keep me in the loop until I recently retired.
>
>          I have all the parts to put together 160 meter single band 
> amplifier. I realize that it is a low mu tube of about 35, and would 
> probably not make a great grounded grid tube... but maybe at 160 meters a 
> pair of them could reach legal limit with less than 100 watts input?
>
>          I have 5 each of 833A pulls, with only 6 months of use. I got 
> these from the engineer in charge and he said that usual pull time is at 
> least over a year or unless some failure occurs.  But, the owner of the 
> station demanded that he do it every 6 months no matter what the condition 
> and he kept the 'like new' ones.  Yes they are old, 20 years or so.
>
>          I would prefer help from someone who has actually used the 833 in 
> some type of RF circuit.  But anyone with engineering experience with a 
> behemoth of this type knowledge would be great!  I plan to set up a web 
> page and with my progress to keep those helping informed of my advancement 
> into the past!
>
> Characteristics for the 833A are 
> at:  http://oak.cats.ohiou.edu/~weinfurt/833A.pdf
>
> ************************************************************************************
> Plan.
>
> 1 Two of them in parallel or Push-Pull?
>   
Single band you can take your pick. Use link coupling with PP. Parallel 
is a bit simpler.
> 2 Grid driven with a 55 ohm, 75 watt non-inductive resistor to ground.
>
> 3 Pi output network or Pi L ? Or Link coupled output?
>   
Single band, take your choice.
> 4 Operation is CW and occasionally SSB. Bias voltage on Cathode developed 
> across about 60v Zener to ground?
>   
According to the data sheet, you might want to run around -90 as a stand 
by bias to limit dissipation. 
> 5 Power supply is capable of 3000 VDC at 1 Amp.
>   
That "to me" appears to be a bit toward the low end for these.  But 
running higher voltage gives a plate load impedance of over 10K ohms.
> 6 Forced air cooling up from pressurized bottom chassis to box on top 
> housing tubes and output components.
>   
No.  Due to the rather unique physical geometry of these tubes the 
maximum cooling air is needed between the plate and grid seals keeping 
the temperature down to a max of 145C. 40 cfm from a 2" nozzle directed 
down and between the plate and grid terminals.
> 7 Tubes mounted in vertical position. Heat sinks for pin connections.
>   
That might be a tad unstable...BTW the only permissible mounting 
position according to the data sheet is vertical.

73 and good luck,

Roger (K8RI)
> Greg Weinfurtner AEE BSS
> NS8O
> 40192 State Route 689
> Meigs County
> Albany, Ohio 45710
> United States of America
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