833A's "Low grid drive";any suggestions????

[N8ECR]

Gentalmen:  Having a problem getting enough grid drive to a pair of 833A's.  My driver tube (813) is producing about 175 to 200 watts input, however unable to get the required drive(22.5 watts or so) to the 833A's(by almost 50%)I desire about 150Ma the most I can get is 125ma pushing it the hardest, The tank curcuit of the 813 is shared with the grid input tank for the 833A's.  The 813 is coupled thru a .002 cap to this tank. The resonant dip is good, and the 813 nuetralized. A simular design is found in the Gates BC-1 transmitters, with the exception that they use a pair of 807's.  I have tried all ideas I can think of to no avail.  There is no unusal heating of componets.  So the 813 is running inefficient, or the power is being radiated.  I have disconnected the Plate leads from the 833A's with little change, eliminating the possiblity of the power feed thru to the dummie load.

Any one ever had simular problems, and what was your cure?
Or any suggestions to what I should check or do next.
Any Crack-pot engineering suggestions appreciated.
Cause this thing is beating me up!!!! :evil:

[K1JJ]

Hi Mike,

Yes, these problems can be hair pullers...

Ques: If the 813 is flowing 175-200W through itself  with only a small fraction of the power going to the output, is the 813 plate getting real red?  It is a 125W tube, so it should be if its not transferring it somewhere else..  Are the 813 plate current and voltage meters accurate?  Try another current meter or VOM to verify this.

The first thing to do is determine where that power is going, assuming it really IS 175W.

The 833A's are good, right?

What would happen if you disconnected the 833 grid input and substituted it with a 5K non inductive power resistor load? Can you verify the heat/power there?

Use your scope to measure the voltage at the 833 grids and calculate the power based upon current through the grid leak resistor. Recheck the accuracy of you 833A grid current meter too.

Let us know what these tests do...

73,
Tom, K1JJ

[W2VW]

While testing next time also check to see if you are actually resonant on the frequency of interest and not a harmonic. Light a receiver with little or no antenna and tune around.

[K1JJ]

While testing next time also check to see if you are actually resonant on the frequency of interest and not a harmonic. Light a receiver with little or no antenna and tune around.

Now that you mention it, that would be my #1 place to look first.

Lord knows how many times I've tried everything only to find it was a second harmonic.  A scope probe might show it up on the 833 grid as possibly a waveform having a double hump or most certainly seen as a X2 sine compared to the 813 fundamental input.

T

[W2VW]

While testing next time also check to see if you are actually resonant on the frequency of interest and not a harmonic. Light a receiver with little or no antenna and tune around.

Now that you mention it, that would be my #1 place to look first.

Lord knows how many times I've tried everything only to find it was a second harmonic.  A scope probe might show it up on the 833 grid as possibly a waveform having a double hump or most certainly seen as a X2 sine compared to the 813 fundamental input.

T

I heard some strange stuff on top of an FM broadcast signal on 100.5 yesterday. Was that you?

[WA1HZK]

http://www.criticalradio.com/833%20Rig%20Project/Web%20Pages/Default%20WA1HZK%20Page.htm

I tried everything else & ended up using a 4-400 to drive the thing. It's the only way I could make the 200 mA for a pair of 833's.
Keith

[2ZE]

It's the only way I could make the 200 mA for a pair of 833's.

200 mA's!
Wow, the grid dissapation is 80 mA max for each. 150 mA is plenty. I use an 813 and is more than enough drive.
I would definetely check to make sure you're not tuning to a harmonic first, then check the 833a's. Soft finals can do this.

2ZE

[WA1HZK]

I run 150 mA on a pair.
The book says 100 mA per tube in Class C AM.
Without that kinda drive, 833's will make no peaks.
Keith
WA1HZK

[k4kyv]

Try disconnecting the grid leads to the 833A's or simply removing them from the circuit.  Then wind a pickup link on the cold end of the 813 plate/833A grid tank coil.  Connect a dummy load through a wattmeter to the pickup link.  You may have to improvise a matching network to get the tank circuit to feed power to the dummy load.  A series variable cap may be enough, or a "transmatch" circuit may be needed... just whatever it takes to couple the 813 to the load.  Measure the maximum power you can suck out of the 813 while staying within its ratings.  That will give you an idea if you the 813 is  making enough power to drive the final.  Of course, make sure you are not loading up a harmonic.

I have a flaky problem with my BC1T driver circuit.  I have to dip the cathode  current of the 807 drivers precisely.  They draw between 200 and 225 ma, to give me 150 ma grid drive on the 833A's.  If the 807 cathode current is not precisely dipped, the rf output from the 833A's drops and the 833A plates start to glow orange, even though there is only a slight change in 833A grid current.  In other words, the efficiency of the 833A final drops tremendously if the 807 driver stage is even slightly off dip.

I suspect some kind of parasitic.  Right now I just make sure it is carefully tuned up, but I plan to eventually get a decent oscilloscope and look at the waveform of the rf as it appears at the 833A grids to see if I can find what is going on.

I think the trouble lies in using a common tank circuit between the two stages.  I have never had such problems with my homebrew rigs, using a separate driver plate tank, link coupled to the final grid tank.

[W2VW]

I suspect some kind of parasitic.  Right now I just make sure it is carefully tuned up, but I plan to eventually get a decent oscilloscope and look at the waveform of the rf as it appears at the 833A grids to see if I can find what is going on.

I think the trouble lies in using a common tank circuit between the two stages.  I have never had such problems with my homebrew rigs, using a separate driver plate tank, link coupled to the final grid tank.

Exactly what you would see from a spur except that the spur can also be close in frequency and you will not get the extra red in the face look from the toob. I ran into the same thing with the 833 final. It always went away when running the grid current up to 100 mills. I'll bet you'd probably see a slight increase in final plate current when you detune the drive. Gremlins. Maybe the ghosts of departed slopbuckets.

[N8ECR]

Well guys:  I don't know!!!  Yes the 813 is on the intended frequency(3880) I have confirmed this with a freq. counter inside the cabinet.  Also the grid tank is a coil #20 wire about 2"(and center tapped), long or so, and 24 turns, the cap is a 150pf unit, and is 3/4 messed.  The 813 is not showing any color, last evening a stuck a tin plate 813 in the transmitter, just to check this.  The 813 works like this:  Plate volts  1100vdc, grid 20 mills, screen is at 200vdc and the grid leak is 8 k ohms.  screen voltage is a 50k resistor off the 1100 volt supply, with a 25k resistor as a voltage divider to ground.  My plate current on it is about 175 right now, and grid drive is just shy of 100 ma.  I got a long way to go!!!! to get my 150 ma's or so. I beleive I need about 200ma before I load it down when the plate is applied to the 833A's??

I think I may have to install a nother 813, but holy cow!!! I might as well just modulate them, and pull the 833's out.

As you can see this is rediculous!!!!

I just might try what K4KYV has suggested and find out what the 813 is really putting out.

And to answer the tube questions, all tubes are new and tested, or only slightly used in the case of the 813's  I have, but tested in another transmitter.

This GE trnasmitter used a 8005 to drive the 833A's before I did the mod to it.

[WA1HZK]

How about the really stupid stuff. How's the cap sending the RF from the 813 to the 833's? I used a 500 pf. doorknob here. Make up a HV probe with a few resistors & look at the 813 plate then go through everything else until you get to the 833 grids. Like I said before, after going through all of this grief I ended up using a bigger driver. I had the advantage of using a 50 ohm DX-100 chassis as a driver though. Can you publish the schematic? I bought up a dozen manuals from various manufacturers of 1KW class commercial rigs while I was trying to get enough drive to make mine work and I remember one design used a pair of 813's to do the driving. Yup, it's true, your driver will be bigger than most rigs.
Keith

[K1JJ]

Yes, as many of us have found, plate modulated low-medium mu triodes in the final need lots of drive.  It's well worth solving the problem on your 833's.

A great way to verify this idea experimentally on a working transmitter is to load it up to full power into a dummy load and put in an audio tone that can produce up to 140% positive modulation.  Then slowly increase and decrease drive to the final as you increase and decrease the audio tone.

You will see that there is a sweet spot where more drive does not help produce higher peaks. But more importantly, you will also see the value of needing lots of drive to let that triode climb into the reaches of outer space with positive peaks. When the drive is reduced below a certain threshold, the triode will flat top prematurely.

This same thing happens with my triode [FET] class E rig. I drive it into saturation and then the audio peaks are as high as the final will permit based upon it's ability to produce/transfer power.

T

[WA1GFZ]

Tom,
I also found the tank component values have a big effect on peak power.
This applies to tubes and 3 leg fuses.  I contend peak power in a solid state rig is when you have a clean gate waveform, clean drain waveform
with correct tank values. You can push it until the RDS on of the FETS
hurts efficiency. I worked my way up from 500 watts out to 1200 watts out
all with the same 50 watts of drive and 47 volts on the drains. Each step required more C and less L in the tank with some extra drain shunt C as the operating Z decreased.
I noticed the same effect in tube linears.  fc

[N8ECR]

Ok:  The 813 is fed to the grid tank curcuit thru a .002 cap.  Keith: you might be familar with a Gates BC-1 g or h.  I copied the grid circuit from them., only I got a 813 instead of two 807.

As for BC transmitters that used 813's as drivers for 833a's there was a Collins transmitter that used one 813 to drive a pair 833's, I think it was a 20K*  and Gates made a 500gy that used a 813 to drive a single 833A The Raytheon RA-1000 used two 813's in push pull, and the 833A's were also in push pull.

I guess it is off to a different design here with this project:  I am a little disgussed it did not work out.  I fiquered I would have so much drive to deal with, and this idea fell flat on its face.

Thanks alot for the words of wisdom.....N8ECR see ya on 75

[WA1HZK]

Don't tune me out on the idea of a 4-400. Cheap & dirty.
Keith

[K1JJ]

Tom,
I also found the tank component values have a big effect on peak power.
This applies to tubes and 3 leg fuses.  I contend peak power in a solid state rig is when you have a clean gate waveform, clean drain waveform
with correct tank values. You can push it until the RDS on of the FETS
hurts efficiency. I worked my way up from 500 watts out to 1200 watts out
all with the same 50 watts of drive and 47 volts on the drains. Each step required more C and less L in the tank with some extra drain shunt C as the operating Z decreased.
I noticed the same effect in tube linears.  fc

Frank: Yes, it really is a juggling act when it comes to optimizing values throughout a final in the testing phase. They interact, which makes it even more sporting.  Just like a computer optimization program, you try many values of many parts and eventually you get some truly outstanding parameters/performance in the end.

Mike/ECR: As Mike/ZE said, it IS possible to get the 813 to put out enuff power to drive a pair of 833's.   Other designs do it, as you said too. Heck with 3kv on a pair I could get 800W of carrier on AM to be 100%+ modulated....  I wouldn't give up yet. Keep eliminating possiblities and dividing it down to eventually conquer the problem, OM.  Putting a bigger tube in there might just mask the original problem.  I'm still curious as to WHERE the power is going when you are drawing 200W thru the 813 and the plate is not getting red

T

[2ZE]

Mike,
Do what I did...
connect the813 grounded grid. connect the beam forming plate, grid and screen to ground. I need about 20 watts of drive to get 160 mA of grid drive to the 833a's. Its link coupled to the grids of the 833a's and its just a simple single ended tuned circuit on the 813 stage. I use a 2K drooping resistor off the plate supply to get my plate voltage for the 813. very simple, and works pissa'

2ZE

[WA1HZK]

That's what I have been talking about.
Use a 4-400, 3-500, etc. It runs on the same voltage the 833's need and just connect it up grounded grid. No dropping resistors, no neutrialization. (Boy, killed the spelling on that one!) now the rig can be driven by anything making 20-30 watts.
Keith

[K1JJ]

Mike,

Another idea....

Are you sure that the 833A bias supply (fixed supply) is not charging up with RF drive causing more bias than when it is idling?

I had that problem on many class C rigs with combo fixed and grid leak biases. The solution was to put a small iron core power supply type choke and 1N4007 diode in the circuit to keep the RF out of ther supply and make it one direction only. [It was a Tron circuit and I have it somewhere]

Bottom line is to measure the grid bias just below the RF choke to make sure it is stable under RF drive and not making it harder to drive the tube.  ie, the tube may be getting biased way past normal class C operation.

Just a another thought...

T

[W2VW]

One way to tell if you are making lots of rectified grid Voltage is to wait until the grid safety bias supply filter cap explodes.

[WA1GFZ]

a 1N4007 conducts RF very well with a little bias on it. They make great pin doides at HF. The Choke was going all the work.

[N8ECR]

Well this transmitter is a old GE broadcast unit, and uses the best of power supply componets.  All supplies are built with dual chokes and capacitors.  I had a RF choke in the grid curcuit of the 833A's so no RF should have been getting back to the protective bias supply.

But last night I ripped all that stuff out, and will try another method of driving the 833A's instead of direct as it was, I will try a capacitance coupling method simular to what was originally in it, except I will try a simpler method of neutralizing that the transmitters original curcuit.

You can see a simlifiied diagram here on the amfone net just go to the header page and click on the AM Radio Network from there click on the topic of converted transmitter, the XT-1A and scroll to the bottom of this mans web page.  His transmitter was once owned by Dale KW1I  I have one just like it see the Febuary issue of ER mag.

73's Mike/N8ECR

[K1JJ]

a 1N4007 conducts RF very well with a little bias on it. They make great pin doides at HF. The Choke was going all the work.

Frank:

The 1N4007 was not in the RF path - it was befrore the RF choke, DC only.  I must dig up the circuit again. It was a good solution.

Mike:
When you get the new config running, still check for rectified grid bias from the final.  Quality components in the grid bias supply will not help this problem.  

I think you will be keeping the same grid circuit, tho the new neutralizing circuit may change it, dunno. I understand your need to move on to another config.

T

[WA1GFZ]

I still don't understand what is is doing for you. If you reverse bias the diode the voltage will fly up. There needs to be rf isolation with the choke but a shunt path to absorb the extra bias due to drive.