The AM Forum

(https://s20.postimg.org/g2s61dqbh/500_K_in_place.jpg)

Last summer I acquired a GATES BC-500K, 500 watt AM tx using a single 833A in the output.
The output network is kinda complicated and really wants to stay on one frequency.  I can see no reason why I cannot borrow the design appearing in the 1980 ARRL handbook which features a linear using variables and an appropriate coil in a typical pi network. I figure vacuum variables would be even better. Even the values for various freq's are given so it looks like 0-300 and 0-1000 pf will put me the neighborhood for operation at 3800 to 3900 KHz. Opinions welcome.

The old girl should easily slip to 160m with no problem. But you mentioned 75m. Could be more work than it's worth......I'm no a big fan of 75m. Sorry.

Same case and setup as my Gates BC-1J except for the single 833. They're good looking transmitters.

One of these days I'll get started on mine. :)

Greg

Good Looking transmitter  ;D

Don R   W4DNR

Thanks Don R, and thanks for helping me "tote" this and the 1-T from the hinter-lands.

Fred, it actually will be easier, because everything on the top shelf will clear out except got the Final, the oscillator, IPA stage (small box) and the two vacuum variables and the coil.

The audio driver chassis with the 4 5881's was very problematic.

Be ready for a rework, or a better-designed system.

73DG

That audio driver is unique but I kind of like it, although I can see that it may take some tinkering.

On the other hand I like odd caliber guns and antique railroad motorcars so odd audio drivers are par for the course.

Greg

Is the audio driver circuit the same basic circuit as the driver in the gates BC-5P? I think it used 6550's however.

I don't know much about BC transmitters, but with an 833 Final tube on 80m, that 833 will need to be neutralized. A output Pi-Net won't allow you to do plate side neutralization like a BC-610 did with a 250TH. Do you plan to have a split coil and split variable cap on the grid side where one side feeds the Neutralizing Capacitor going to the plate?

Jim
Wd5JKO

(https://s20.postimg.org/ydmnuc1a5/833_A_Schematic.png)

Here is the schematic of the 833A amplifier says it is good for 40 to 160 meters. This is what I planned to borrow the tank circuit from.

That 50 Watt dummy load on the input should certainly keep it stable.   National used the same scheme on their NCL2000.

Is that design your amplifier?   I have a gates 500GY I'd like to do the same type of thing to.

Mike   

I would refer to this thread where some AM Fone heavyweights weigh in on the topic of a single ended triode plate modulated class C amplifier using a Pi network output along with grid side neutralization:

http://amfone.net/Amforum/index.php?topic=29178.0

Jim
Wd5JKO

The schematic you show is for a linear amplifier and not a class C final.  Saying that, the tank components should get you close.  I did not look up the original design of the 500T tank circuit but you should be able to use the original tank coil.  Some of those transmitters used a pi L type of output network.  Basically for what your doing, just use the topology for a pi network.  You will need an input variable cap, tank coil and a variable loading cap and a RF choke to ground to protect things if the plate blocking cap shorts.

Joe-W3GMS     

Do you have a schematic of the 500K?

I worked on many a BC-500GY for mom n pop AM stations and converted a few for other hams (didn't have the room in my own QTH for one).

The only problems I had were the bias electrolytics in the grid circuit of the modulator stage.

I would keep the PI-L network because of it's wide tuning range, but change L and C values as necessary.

Phil - AC0OB

Whoa! I'm trying to follow this thread and it seems that the PA stage is going to be modified to be a linear amplifier? Or you just want to copy the tank circuit.

I got a McMartin BA-1K, using a pair of 4-500's in the final to be happy from 160-40, but did it by keeping the original tank coil, in a pi configuration with 2 vacuum variables. I knew the resistance of the tubes. I disconnected both tubes and connected a resistor from one of the plate leads to ground, to be the tube resistance. I connected my MFJ 259 into the RF OUTPUT of the transmitter. Without turning the TX on and no HV, I was able to mark tank coil taps and variable vac cap settings to get the tank into resonance, showing 50 ohms and no reactance.
Removing the MFJ analyzer and re-connecting the tubes. I was able to key up the TX, feeding the finals with RF from my Ham transceiver, into an antenna tuner, into the grids of the finals, and was able to get 1KW pout of the TX, 160-40M without straining or stressing the transmitter circuits or tubes. Just a simple PI-network. Now a triode may be another story for neutralizing...........not sure.

Jim 5JKO might have a good link for your class C single output PA.

You'll have to use ingenuity to design a nice audio driver. Always seems to be the weak link in the audio chain to drive the modulator tubes. Yikes!!! I'm looking at the PDF for the schematic of your TX..A quad of 5881's driving two 833's ? 833's modulating a single 833? Interesting project you have there. Don't go the route for a quad of 807's, audio driver. That circuit sux too. The solid state audio driver from a Gates BC-1H was the best!!!

Here's the PDF
http://r.search.yahoo.com/_ylt=A0LEVu6U2VJYr14AwsknnIlQ;_ylu=X3oDMTEycDBhOTJiBGNvbG8DYmYxBHBvcwMxBHZ0aWQDQjI3OTVfMQRzZWMDc3I-/RV=2/RE=1481853461/RO=10/RU=http%3a%2f%2fwww.americanradiohistory.com%2fArchive-Catalogs%2fGates-Harris%2fGates-BC-1J-Transmitter-Manual-1955.pdf/RK=0/RS=ftloa09WheGtBU_c4.RGGspZ06k-

From what I read,  it's very simple.   He wants to bulldoze everything past Cblock all the way to the antenna connector.

Install a vacuum variable,  from Cblock to ground.   Install a switched inductor from Junction of Cblock and Ctune to another vac variable going from opposite end of Cblock on the inductor to ground.

In other words,  he's installing a new pi network on a transmitter and removing what's in it.

I wouldn't use the published values.   Download Ian's spreadsheet or Tonne's Calc and design for the pep level you think you're going to run at.   Better efficiency,  and use a Q of about 8.  Makes tuning up easier and you won't have the need to return as you make small QSY within the segment you're tuned for.   Iowa,  tune for 3885, and run from 70 to 95 or so.

--Shane
KD6VXI

I have a question about the audio driver. It's a quad of 5881's. What makes it a problematic circuit? The complexities of the 4 output tubes and balancing? Phasing problems? Impedance match to the modulator tubes? poor freq response? Excessive distortion? Or just bad design?

I guess I didn't make my plan simple enough.  All I want to do is eliminate the numerous fixed inductors in the original circuit, and replace them with quality components that can be tuned to different freq's quickly. The schematic I have shown is indeed from a linear, published in the ARRL Handbook 1980. But, getting an 833A tuned up for Class C, or Linear service should involve the same output network components should it not ? I mean, resonant is resonant AM, FM or SSB!

My plan is to use 10KV vacuum variables and a healthy coil. For 80 meters, with 2500 VDC on the plate (which Gates designed it to use), it should need 157 pf on the input, 1080 pf on the output and a coil of 14.19 uH.

Just see to the lengths of conductors between all parts of the output circuit that they are short. I found a 20" piece of wire from the tank to the plate choke made an excellent tuned circuit around 110MHz.

This sounds like a great project - however (oops, those darn howevers) the circuit you are proposing has a number of issues that will cause you problems.

First, for class C operation (according to the 833A tube data) you are going to need around 300V (more or less) of DC grid bias, preferably created by a grid leak resistor.  Developing this sort of voltage across a 50 ohm resistor will dissipate a WHOLE lot of power so you'd have to use a significantly higher resistor value, and that would negate any possible swamping value.  The peak RF grid voltage should be around 450V or so.  You need the high bias and high drive to operate the tube in class C, which is the only mode which will create a square law curve - because the tube is operating in saturation when conducting.

Also, for modulated operation, neutralization is highly recommended when using triodes.  This is EASILY accomplished in the grid using a balanced grid circuit. I used big triodes for YEARS and always neutralized in this way, and used a PI section output.  Stable and easy.

A PI section in the output is good - nice and simple and easy to adjust.

Again, looks like a good winter project... so when can we expect to hear you on with the rig  ;D ;D ?

Regards,  Steve

I built that amp with the 833.  The swamping resistor idea never did work right and I finally put in a tuned grid circuit.  It took about a 100 watts to drive it but she did
walk the dog as someone once said. 
My first trip to Dayton, all the parts for that amp were on my shopping list.  The very first table I came to was a guy who was going to build that amp and decided
not to.  He had all the parts in a box for sale. I came away with it all.

Okay, one more comment from me.  ALL I am going to change is the transmitter output circuit.  The transmitter will currently run in the ham bands and do 500 watts carrier AM with no problem.  
ALL I want to do is change the output network. I will be running only on 3,880 and 3,885.

Ah ha... well, you didn't say it worked on any ham bands  ;)  That is certainly good - puts you ahead of the game (depending, I suppose)...

Which ham bands does it currently support, and is the input circuit a balanced input or something else?

That will have some bearing on the configuration of the output circuit for sure.

Regards,  Steve

These items may help

https://frank.pocnet.net/sheets/049/8/833A.pdf

Phil - AC0OB

Phil,

Thanks for the 833 data sheets. Good stuff!

Frank WO4K

Another online calculator is:

http://www.ee.oulu.fi/~timor/javaa/matcher2.html

Use the following input parameters:

Source resistance = 4800
Source reactance = 4.8e3
Load Resistance = 50
Load Reactance = 0
Q = 12
Freq = 3.88e6

Results seen in the "Low Pass Pi Matching Network" is:
L ~ = 14.4 uH
Csource (Plate Tune) ~ = 128 pF
Cload (Ant. Load)  ~ = 782 pF

The point is, look at the various results and make your three elements Variable enough to work over the range of values shown.

Phil - AC0OB
 

We seem to be dancing around what the input circuit consists of, and whether or not the 833 will be neutralized.  As Steve QIX correctly pointed out, the 833 will need to be neutralized. A swamped grid like that linear circuit will not work if you plan to run the 833 as a class C plate modulated amplifier due to the high peak grid swing required, and deep negative bias.

Some napkin doodle shows the 833 plate grid capacitance, around 6 pf to have an Xc of ~ 4K at 40m. This will be too much to ignore unless you wish to run a modulated oscillator.

With a pi network output, that eliminates plate side neutralization like they do in a BC-610. My first post in this thread had a link to another post by W8ACR on how he tackled the issue using a big triode final.

Good luck with this project..

Jim
Wd5JKO

For neutralization, I would simply adapt the system used in the BC-1J:


Phil - AC0OB