NEED TUNED GRID INPUT CIRCUIT FOR 4-400's CLASS C

[KL7OF]

I have been working on a Mcmartian BA1K ..4-400s X 4-400s....Single band 75 meters...RF driver is an AF-67....I have it working very well and it is giving me rated output ...I am using an antenna tuner between the AF-67 and the grids of the PA tubes....It tunes very well and only requires about 8 watts of drive from the Elmac ...I want to put a tuned grid circuit in there so I can have my antenna tuner back....I tried a 200 pf variable in parallel with a 50 uh coil ....cold end to ground, other end to grid input cap....not working...Got it from the 50's handbook....
   Can anyone suggest a circuit for me to try ?  This transmitter will be complete once I get this tuned input going....  Thanks...Steve

[KA2DZT]

Did you use link coupling from the AF-67 to the 50uh coil??  Put the link down near the cold end.  50uh coil may be a bit large for 75M but it should tune with the 200pfd cap.  Maybe try shorting some of the coil.

Does the tube require some bias or a grid resistor in series with the coil??  Grid needs to go negative to drive the tube into class C.

For testing, you can figure a grid resistor from what the negative grid voltage should be and the total grid current for the two 4-400s.  Put the grid resistor in series with the coil at the cold end, by-pass the resistor to ground at the coil-grid resistor connection.  You can add a grid current meter in series with the resistor to ground.  Should work.

Fred

[K3YA]

The 50uh inductance sounds too high.  This lowers the Q and reduces the coupling, assuming you are using link coupling to the grid circuit.  I didn't see any reference to a link in your post.  4-6 turns insulated wire wrapped around the cold end of the tuned circuit is about right.  You may need to ad or subtract turns on the link to make the AF67 happy. 

Here is the one I wound for my Collins 30K4 on an old pill bottle.

[KA2DZT]

A coil too large does lower the Q of the circuit and reduces the level of RF voltage developed across the coil.

Fred

[WQ9E]

Steve,

You could also look at the manuals for the Johnson Desk KW, Johnson Thunderbolt, and Heathkit Chippewa to get idea from what these manufacturers used.  The Johnson manuals are on ebay and I believe I have a PDF scan of the KL-1 Chippewa manual in addition to the physical copy that came with mine.

[KL7OF]

Thanks everyone.....I made a mind typo on the coil...It is 25 uh...and I direct coupled it ...no link...I saw it in an older handbook and thought "this looks simple"....But it won't work, so link coupled it is.....Ill let you know when I get it working.....

[KL7OF]

OK....This is what I have come up with..I'm bringing the bias into the center tap of the 25uh coil...Is that correct?  Do I need the "cap to gnd" (see schematic)  if so, what value..orange drop OK? 
   All comments welcome

[W3GMS]

Steve,

Here is a 4-400 amp out of the 64 ARRL handbook which show the driving scheme to a single 4-400.  It should work well for you. 

See attachments.

73,
Joe GMS 

[KL7OF]

Steve,

Here is a 4-400 amp out of the 64 ARRL handbook which show the driving scheme to a single 4-400.  It should work well for you. 

See attachments.

73,
Joe GMS 

Thanks Joe..looks almost like what I have cooked up.....The bias doesn't necessarily need to go to the center tap of the coil.....according to this...and no 1000 pf coupling cap on the grid?

[W3GMS]

Although not a great picture, this is the mechanical implementation.  

Joe GMS

[W3GMS]

Steve,

Here is a 4-400 amp out of the 64 ARRL handbook which show the driving scheme to a single 4-400.  It should work well for you. 

See attachments.

73,
Joe GMS 

Thanks Joe..looks almost like what I have cooked up.....The bias doesn't necessarily need to go to the center tap of the coil.....according to this...and no 1000 pf coupling cap on the grid?

That's right Steve.  You do not want the coupling cap to the grid since the bias voltage needs to get to the grid of the tube.  Just use the entire coil shown on the schematic since your only interested in 75M.

Joe   

[N2DTS]

Can not have that cap in the grid line if the tube is to get any bias.
Do not need center tapped coil unless you RF ground the center tap and use the lower coil for neutralizing.

With any high voltage tubes I want a nice air gap between coils, if the coil is wound of a form, the link coil should go below the grid coil, not wound over top of it.
A big tube has some very high voltages floating around, tape will not cut it...

[WD8BIL]

Why not just scale down your tuner and use that circuit?

[W3GMS]

Steve,

Here is a 4-400 amp out of the 64 ARRL handbook which show the driving scheme to a single 4-400.  It should work well for you. 

See attachments.

73,
Joe GMS 

Thanks Joe..looks almost like what I have cooked up.....The bias doesn't necessarily need to go to the center tap of the coil.....according to this...and no 1000 pf coupling cap on the grid?

That's a very basic scheme and works extremely well.  You will be pleased with the results.   

Joe

[KA2DZT]

Steve,

Get rid of the 1000pfd cap at the grid.  Connect the bias to the cold end of the coil not the middle.  You can put another by-pass cap to ground at the lower end of the RFC.  The grid resistor has to drop the right negative grid voltage minus whatever negative fixed bias you use.  So, if the right grid voltage is -150 volts and you use -75 volts fixed bias, then the grid resistor has to drop the remaining -75 volts.  I just used these numbers as an example, I'm not sure what the correct grid voltage for a 4-400 is.  Use Ohm's Law to find the value of the grid resistor using the correct amount of grid current for the two 4-400s.

Fred

I took a look at the schematic that Joe suggested,  it's the same basic circuit that I described and very close to what you have drawn on your schematic.  You just need to remove the coulping cap, grid current won't flow through a cap and connect the RFC to the end of the coil.

You may need change the circuit slightly if you have to neutralize the 4-400s.

As someone else mentioned, your link can be over the the grid coil or right next to it at the bottom.

[KL7OF]

OK...got it hooked up and working.....Bud..My first thought was to duplicate the tuner circuit but the junk box couldn't come up with the right stuff....
   Took out the 1000 pf cap at the grid , put in a 500pf doorknob to ground using the schematic that I drew (same as the handbook schemo that GMS posted)  
   25 uh coil, 100 pf cap .....fired up the rig and it works better than the antenna tuner did...  Ran it at high power for a full 10 min into the dummy load....No heating of the coils.....Works very good...Thanks to all...By the way this transmitter belongs to W1LYD , Brian and is on the air in his shack in Lake Havasu City, AZ...Ithank you Brian thanks you..

[W3GMS]

OK...got it hooked up and working.....Bud..My first thought was to duplicate the tuner circuit but the junk box couldn't come up with the right stuff....
   Took out the 1000 pf cap at the grid , put in a 500pf doorknob to ground using the schematic that I drew (same as the handbook schemo that GMS posted)  
   25 uh coil, 100 pf cap .....fired up the rig and it works better than the antenna tuner did...  Ran it at high power for a full 10 min into the dummy load....No heating of the coils.....Works very good...Thanks to all...By the way this transmitter belongs to W1LYD , Brian and is on the air in his shack in Lake Havasu City, AZ...Ithank you Brian thanks you..

Glad it worked out Steve.  I kind of figured it would since the circuit is a very common and well proven approach. 

Joe, W3GMS

[KA2DZT]

Steve,

OK FB, I looked at the pic of the grid circuit.  The wiring is a little weird and usually we don't allow pink wire in AM xmtrs

Anyway, glad you have it working.

Fred

[KL7OF]

Pink wire is in abundnce around here....big roll of it...1kv insulation...Now that this TX is working, I will tie up some of the wiring...This thing has been a real challenge to get going...When I started on it it was disassembled and several important components missing....I am impressed with the iron in this rig...The mod TX and reactor look like 5KW stuff...Originally had 4-500s in it...i left the screens on the modulators at 700volts (AB2) from the onboard regulated supply but I put in a dropping resistor from the modulated B+ for the PA tubes...(class C) they are much happier....running at 500 volts on the screens

[N2DTS]

For grid and screens, its best to use short fat strap, not wires with any extra length.

[KL7OF]

pics.........

[flintstone mop]

I owned a BA1-K and I thought that the mod iron was a little wimpy. It was in an oil filled container and the lowest audio frequency was about 80hz. I tested a Collins 20V-2 and it easily went down to 40hz.
The McMarshun was able to make wonderful positive peaks with the original 4-500's.

Fred