833 Loading

[KA3EKH]

I have been slowly moving forward on my RCA BTA-1MX in converting it for 160 use. Getting the oscillator and driver to 1.85 has not been an issue, the driver tank needed to be removed and rewound so no issues with that so now I have progressed to the PA output tank.
I did not want to keep this as a 1 kW transmitter so I did remove one of the 833 PA tubes so now there is only one PA tube and in some ways this makes it a BTA-500 I did the course reconstruction of the PA tank by applying the twenty or so watts of drive to the PA with no plate voltage and connecting a spectrum analyzer to the output of the transmitter and building a tank for maximum output with minimum harmonics. The transmitter was originally on 1.360 so I kept the original plate tuning capacitor (C21) being that it was a nice 40 kV 100pf glass vacuum unit and reduced the tap on the PA tank coil. L7 has been reduced to being just an autotransformer for feeding the sample port and mod monitor. The real question is what should be the value of my loading capacitor on the output of the PA tank (L6)? Originally there was a mess of 10 kV 0.002 transmitting capacitors in series parallel combination for C23. Assuming that I was going up in frequency and down in power I figured that I would need less so removed all the output load caps and just installed two in series to build a 0.001 output loading capacitor.  If you look at the attached drawing you can see my current tank configuration. Before I start using the original HV plate transformer I decided to just stick in a little 400 volt transformer in the plate supply and run the transmitter up on that, being how much damage can you do at 400 volts? And using that much reduced plate voltage on the 833 I am developing about fifty to sixty watts of output with a plate current of around 180 mills. That’s an indirect power of around seventy fife watts or so with an efficiency of almost eighty percent. My concern is that having that high of a plate current at that low a plate voltage when I apply the full two to three kV the plate current will be excessive. So I wanted to get opinions and response from others who have used 833 in transmitters and maybe an idea of what type loading capacitors they are using. Yea, I know there are formulas and math and things like that where you can determine the exact value of things but its much more fun to just build up stuff and tune for the "sweet" spot.
Ray Fantini KA3EKH

[Bill, KD0HG]

For sure you don't need a 20 KV rated loading capacitor. At 1 KW into 50 ohms, there's only about 200 volts of RF across that cap, they oversized that cap for the sake of dealing with lightning strikes. A 1000 pf value is a good start, you might want to temporarily install a variable cap in its place to nail down good L/C tank circuit values at your full rated power. Then you can replace it with a fixed cap, if you want.

Another thing that you can do..If that plate transformer has a 220 volt primary, disconnect one of its leads from the hot side of the line and ground it instead. That will give you 120 volts on the winding and half plate voltage. Gates did that in the BC-1 to allow switching between 1 KW days and 250 watts at night while using two 833s.

[KA3EKH]

Will try the 120 volt primary idea, although RCA did everything possible to make cretin you use 220 for everything, not just the plate transformer but the mid voltage supply and the filament transformers are all wired for 220 only the blower and all the contactors are 220 also. Neutral dose not enter the transmitter except for a glow bar lighting circuit. So will just remove feed to one side of Plate transformer and run other side to ground. Isn't there something about not returning a leg to ground on a sub feed panel? The same sub feed panel that feeds the transmitter drops a 120 volt circuit for the equipment racks in the shack and I did install a isolated neutral from the ground bar but both are tied back in the main panel.
If you want to see what all this looks like I have just updated the web page I have for this project at:
http://staff.salisbury.edu/~rafantini/RCABTA.htm

Ray F.

[WA1GFZ]

plate tune cap at 100 pf for 160 sounds like a very low Q. A low Q will limit your output power and reduce efficiency.  I bet a higher value will work better after you reduce the L to resonate. Try hanging more C in parallel and reresonating the tank.

[W7IXZ]

Be sure to change the neutralizing cap (C-16 in your drawing) to about 1/2 the original value.  RCA used fixed caps and in the 1 KW transmitter it was selected for two 833's.  If you removed one of them you will need to change it or install a variable cap in its place like a Johnson Neutralizing cap.  I have not completed my RCA BTA-1M yet, but on my Gates BC-1T I just unloaded the two 833's to about 400 watts output.  Triodes can be unloaded and you don't have to worry about the screen current going nuts.  I fiddled around with all of the taps on the primary to get the lowest voltage on 220 so the plate resistance was close to what it would be with full volltage and current for 1000 watts output.  It works great that way.

[W3SLK]

Why don't you just follow the drawings/schematics for the BTA-500MX? I think they both use the same schematic with the exception the extra 833 and associated changes with it are in dashed-lines.

[KA3EKH]

The more I work the less I understand. I played around with the taps on L6 this weekend, with 800 Ep and 0.38 Ip I have an input power of around 300 watts. According to my non calibrated wattmeter there is just about 225 watts being delivered to the load. I figure this is giving me an efficiency of around 75% or so. I obtained these numbers by playing around with the tap on L6 being that values for C21 (plate tune) and C23 (plate load) are fixed. I did try different values for C23 being .002, .001 and .00068 and have found that .001 appears to work best for C23.
C20 ( not shown on my drawing) sets the feedback for the PA stage and is still the same value as for a two tube transmitter(120pf) so I will change that to the value for a 500 MX (62pf) will also change the control grid resistor for the PA (R10) from 1K to 1.6K, the only other component changes I see are the value of C51 that decouples the modulation transformer from the to the PA from 20 uf to 10uf but being that I don’t have a spare 10 uf 10 kV cap laying around will leave that the same.
Issues: first problem, after I found the sweet spot for maximum output and minimum spurious from the PA I no longer see a plate current dip when changing the plate tuning control. I do see a difference in the output power of around fifty watts dependent to where the control is set but my plate current just sits there. In a different spot on L6 I would see a small plate dip and corresponding power peak but my PA efficiency was lower. Second problem: harmonics. At this time my second harmonic that falls in the 80 meter band is only attenuated 22 Db down from the carrier. The third harmonic is 34 Db down with all the others being attenuated at least 40 Db below the carrier. No matter where I go on L6 cannot get below these numbers also have noted that my fifth harmonic is very affected by plate tuning control. I can vary the fifth between 60 to 40 Db below the carrier with the control. I also have noticed some spurious products out around 46 MHz and 50 MHz but they are all 40 Db below the carrier.
Last Issue, Power supply inefficiencies: I am getting 800 volts by using the transmitter's plate transformer, with 240 volts on the primary set to the 240 volt tap. I removed one of the two rectifiers and assumed I would have a half wave system and half the voltage but instead I have quite a bit less than half, why? I do not have the original 8008 tubes and am using solid state plug in replacements. Think it may have something to do with the huge filter reactor in series with the center tap of the plate power transformer to ground (L13). What is the function of L13? It’s a 10 Hy choke with a spark gap on top. With just one tube in power supply instead of two I have seen that when I modulate the transmitter the PA voltage and RF output drop to about half under modulation with a 400 cycle sine wave. I have not put the second rectifier in yet because I only have a three hundred watt load so will be going to borrow a 1 kW load from somewhere this week and try full wave at that time.
Thanks for the use of the bandwidth, hope no one minds my long postings but it helps to write these things down and get others responses.
Ray Fantini

[W3FJJ]

In designing tank circuits the type tube doesn't matter, the only thing that matters
is what plate voltage and what plate current do plan on running? That determines the
output impedance (plate voltage divided by plate current).

For a Q of 12, and running 833 with 2500 volts and 500 watts input, I think your Pi network should
be
C21 = 250-500pf  L6=20uh  c2= 1800-2500pf   for 160meters.

That should get you in the ball park, have fun twiddling!

[WA1GFZ]

You can improve your second harmonic attenuation if you increase your plate tune cap value and raise the output tank Q.

[N0WEK]

I would just try using 120 vac on the plate transformer primary and go back to full wave rectification.

All this discussion is relevant to my upcoming conversion of my Gates BC-1J to 160/80 so I'll be saving it!

Thanks!

[KA3EKH]

Progress, I have installed both solid state replacements for the 8008 rectifier tubes back in the transmitter and wired the plate power transformer with one leg of the 240 volt primary to the AC line and the other to ground. This runs the transformer at 120 volts input and provides around 1,200 volts of DC to the plate of the three 833 tubes ( two modulators and one RF). The plate supply no longer has any issues with regulation and stays within +/- 100 volts regardless of tuning, loading and modulation. What a difference full wave makes. Just have to figure out what to do with the neutral. I did several experiments with increasing and decreasing the size of the plate tuning capacitor 50pf and 100 puff at a time and found the more capacity I add to C21(plat tuning) the poorer the efficiency and the higher the plate current. Finally settled on a 25pf fixed capacitor for C21, I started with a nice 100pf 40Kv vacuum capacitor but it was just too big. The RCA manual tells you that as you move up the band in frequency you decrease the size of C21, and on a 500 MX there is no capacitor for C21 perhaps they get the necessary capacity for the tank circuit from the internal capacity of the 833? Using the microscopic 25pf 10Kv capacitor for C21, tap number22 from the top of L6 and a 0.001 capacitor for C23 I am getting 325 watts on 1.885 with an Ep of 1,200 VDC at an Ip of 0.380 Ma for an indirect power of 450 watts with an efficiency of around seventy percent. I still have an issue with my second harmonic being only 26 Db down from the carrier but my 3rd and forth are all below 40 to 50 Db below the carrier, and still have mysterious spurious products in the 40 to 55 MHz range. I am still experimenting with the grid resistor for the RF PA (R10) and the nute capacitor C20 and intend that to be this weekend's project. Things learned so far: The book is always right. 120 volts full wave way better than 240 half wave and you can suck over 800 Ma thru an 833, turn it cherry red and it will still work afterwards.
Ray Fantini

[WA1GFZ]

When you increase the plate tune cap you also need to increae the loading cap value to get efficiency back after reducing the L.

[W2PFY]

Wow, running 1200 on the plate, the tube should last a bazillion years

[KA3EKH]

I can easily develop 3 kV plus on the plate supply but what's the point? Looks like three to four hundred watts of carrier is the number one is supposed to run on 160.  May play around with the taps and run it up to 1.5 kV just to see what that dose, real question is what was the plate voltage for the RCA 500 watt transmitter?
Ray Fantini

[W2PFY]

I don't know the RCA voltage but I know of a Gates in operation everyday and it is run at 2700 volts.

[W3SLK]

Ray said:

May play around with the taps and run it up to 1.5 kV just to see what that dose, real question is what was the plate voltage for the RCA 500 watt transmitter?

Ray, it is identical to the two-holer. Like I said in a previous post, the BTA-500 & 1 MX share the same schematic and parts. They even share the same manual.

[KA3EKH]

Another update. Had to change C21 to 25 pf, and increase C23 to 0.001 also changed C20 the neutralizing capacitor from 120 pf to 50 pf and now with the tank coil taped at twenty turns down from C23 I am developing around 325 watts output with a input power of 390 watts. That’s a efficiency of over 80%, running 1,300 volts at 300 mills on the plates of the 833 Second harmonic is still kind of high only being 35 dB down from the carrier but all other harmonics are  50 dB below the carrier. May try building a second harmonic trap using L7 and a fixed capacitor. The plate transformer appears happy running from 120 volts and the old line selection taps on the plate transformer give you the ability to vary the plate voltage up or down about two hundred volts. Am currently using the 0 and 220 tap, still have the 190 and 240 to go up or down a little if needed. New problems are getting the old G/R 1931 modulation monitor up and running and doing the audio proof. That and a lack of good 807 tubes have seen thousands before at hamfest but never thought I would need any. Have several spare 833 tubes including couple brand new ones but no 807 tubes.
Ray Fantini

[flintstone mop]

IF you gotta MFJ259 it gets really simple to change the tank to 160M.

Make sure TX off and B+ discharged!!!! REMOVE plate cap from 833!!!
If you need to check the tank circuit, you can use an MFJ 259B if you have one.  Just run it into the RF deck, connected to the RF output of the tank. Use a resistor from plate cap to ground on the final. The value being the plate resistance for the 833.
And get the component values needed by the tuning chart from RCA for 1600kc and tweek the components for 1:1 match,,,, for example 1880kc and you're there. No stress to the final PA circuit. 


The manual should explain how to get the TX to the top of the AM band. That's all you need to do. Get the  TX to the top of the AM band 1600 and the rest is a piece of cake.
And Mik(y) has the rest of the story.

Fred