40M Transmitter help

[VK7ZL]

After completing and testing the power supply and modulator section I have started on the transmitter. I am going to use a pair of 1625's in parallel because I have them on hand along with a pair of new ceramic sockets.
One would imagine that due to the popularity of 807's/1625's there would be plenty of transmitter circuits available. I hunted the internet, a number of old ARRL Handbooks that I have and this Technical Forum and found nothing, so with the help of a local Ham I put together the circuit below.

I found some information by Pat, N4LTA, on this Forum regarding the Bias supply.
http://amfone.net/Amforum/index.php?topic=22678.0

Now to the next problem. Can someone point me to a source of information for winding the coils in the link section? I have found plenty of theory on how it works but I cannot find anything on the actual coil winding details.
Any comments on the circuit would also be appreciated.

Bob  VK7ZL

[K1JJ]

Bob,



Are the 100 ohm resistors for grid leak? Instead, you can place ONE resistor below the tuned circuit right after the current meter, just before the .01 bypass.   Or maybe you have the 100 ohms there for stability? That wud be OK too.

Anyway, I would make the grid leak resistor more than 100 ohms. Make the grid leak =  e/i  so that the grid leak voltage is at least 1/2 of the fixed protective voltage. Or enough so that the tube stays at a safe current if drive is lost. R= Desired grid leak voltage /  operating grid current.   Also, when in operation check to make sure the fixed bias is not creeping up due to RF rectification. There are ways to cure this if so.  I would stick an RF choke in there too, to keep the RF out of the fixed bias supply.  Some grid leak bias will help modulate the finals better than just fixed.

No neutralization OR swamping resistor?  I would do one or the other for 40M or it may take off.... maybe, maybe not depending upon layout.  A swamp could be a .001 disc cap in series with a non-inductive 4K resistor to ground at both  final grid pins. (no wire wounds) Adjust the resistors for higherst resistance value that still keeps the stage stable for RF operation.

T

[KA2DZT]

It would be better to modulate the screens of the 1625s through a dropping resistor from the modulated 750 volts.  If you lose the plate voltage while still supplying screen voltage from the 375V supply you will quickly exceed the screen dissapation of the 1625s.

The .001 1KV plate blocking cap does not have a high enough voltage rating.  Use a 5-6KV cap.  Same thing for the plate choke bypass cap.

You will need some way to adjust the drive current to the 1625s.  You can add a adjustable resistor in line with the screen voltage to the 5763.

Add a 2.5mH RFC from the output of the pie network to ground.  This is used to short the B+ to ground in case the plate blocking cap shorts.

The 100 ohm resistors on the grids of the two 1625s are for balancing the currents but you don't need them.

As Tom said, you do not have any grid bias resistor which gives you grid bias voltage (self bias).  With AM you need self bias.  The bias supply is something that you really don't need and can be more trouble than it's worth.  If you want to use the bias supply, it should be no more than 1/3-1/2 the total bias voltage.  Check a tube manual to find what the (negative) grid voltage should be for the 1625s with 750 volts on the plate for AM service.  Also 750 volts on the plates may be a little too high for the tubes, 650 volts would be better.  Then find what the grid current should be for the two tubes.  The tube manual has this value for one tube,  you'll need to supply twice the current for the two 1625s.  Divide the total grid current into the required grid voltage.  This will give you the value of the grid resistor.  If you add some bias, subtract that voltage from the total grid voltage and recalculate the grid resistor.

You may also need some RF feedback to keep the 1625s stable and a RFC in the grid circuit.  Do that in another post.

Fred

[KC2ZFA]

Can someone point me to a source of information for winding the coils in the link section? I have found plenty of theory on how it works but I cannot find anything on the actual coil winding details.

I think for best harmonic rejection you should have high L/C ratio (low-C) at resonance. Once you have the required L use an online calculator to find the turns and spacing given the diameter(s) of the coil forms you will be using. The link is typically wound on top of the rf-cold end of the coil with insulated wire, I have found 5 turns are good for 40M. If you put the links on top of the coils then you should have a rheostat on the screen of the 5763 to allow for setting the grid drive current. If you do not want a rheostat then you need to make one of the links variable (swinging) by placing the link as a continuation of the coil but on a separate form...then you can move the link (to achieve less/more overlap with the cross-section of the coil) in order to set the grid drive current.

73 de Peter

[VK7ZL]

Thanks for the comments and suggestions.

Tom, I had intended to install the finals sockets about 1" below the chassis to hopefully avoid the need for neutralization. As suggested by Fred and yourself I will replace the grid resistors with RFC's and make it self biasing.

Fred, I will supply the screens from the modulated B+ to be on the safe side. The .001 blocking cap that I intended to use is rated at 5Kv and I will install an RFC on the output of the PI network.

Peter, I found some more information that suggests for 40M to use about 20 turns for the coils with 100pF across them and between 2 & 5 turns for the link on the cold end of the coils. I thought the grid drive would be adjustable via the position of the link but I can install a variable resistor if needed.

I think I have enough information to get started so I will get a chassis folded up and start drilling some holes.

Bob

[K1JJ]

Thanks for the comments and suggestions.

Tom, I had intended to install the finals sockets about 1" below the chassis to hopefully avoid the need for neutralization. As suggested by Fred and yourself I will replace the grid resistors with RFC's and make it self biasing.

Bob

Bob,

Just to be sure, the RFC is not replacing your 100 ohm resistors. The RFC is going in the grid bias line below the tuned circuit to block RF from the fixed bias supply.  If you use all grid leak bias, then you don't need an RFC - the circuit can terminate to ground after the GL resistor and meter.  Maybe you already intend that, but not sure by the reply above.  RFC's in place of the 100 ohm resistors will not work... :-)

Here's a good example of using both fixed and grid leak bias together with a tetrode - AND neutralization if you later need it:

http://www.amwindow.org/tech/htm/813/813.htm

T

[VK7ZL]

Bob,

Just to be sure, the RFC is not replacing your 100 ohm resistors.

Tom
I was going on the comment by Fred "The 100 ohm resistors on the grids of the two 1625s are for balancing the currents but you don't need them."
and referencing the circuit below.

I now see what you are saying and have corrected my circuit.

Bob

[K1JJ]

OK, Bob -

What are the values of L1 and L2?  They are in the RF path and not needed.  If they are too big they will make the circuit unstable. Unless you mean them to be small parasitic suppressors or some kind of a neutralizing scheme?   I would just eliminate them and add them later if you have stability problems.  The grid circuit path should be noninductive and short, especially if the stage is not neutralized.

The bias circuit looks OK.  BTW, is that the jack for the meter?  If you are plugging it in when running, add a 100K to ground so that the grid is not floating for an instant when the jack is open.

For plate modulation, the screen still needs a dropping power resistor taken off the plate or a choke in series with the supply to modulate the screen, as mentioned in a previous post.


Did you get rid of the L/C tank in the grid and maybe now using the preceeding stage's tank instead?

You're getting there, OM...

T

[VK7ZL]

Tom

That piece of circuitry was from a 1955 QST magazine. L1 & L2 are parasitic chokes (which is what I thought you were refering to in a previous post). I understand now you meant an RFC in the grid bias line.

This is the modified circuit.

Bob

[KA2DZT]

Bob,

Just to be sure, the RFC is not replacing your 100 ohm resistors.

Tom
I was going on the comment by Fred "The 100 ohm resistors on the grids of the two 1625s are for balancing the currents but you don't need them."
and referencing the circuit below.

I now see what you are saying and have corrected my circuit.

Bob

Bob,

You don't need L1, L2.  I would just tie the two grids together.  The rest of your grid circuit looks OK if you're using the fixed bias.  Maybe 67 volts might be too much fixed and not enough grid bias.  807s require about 4ma grid current each.  The grid voltage should be at least -85 volts.  Your 3.9K grid resistor looks about right (.008x3900=31 volts) 31 volts + 67 volts = 98 volts neg.

As Tom mentioned, you're not showing a tuned grid circuit in your schematic.  If you add a tuned parallel circuit, it would be inserted between the two tied-together grids and the one 2.5mH grid RFC.  You would also need a bypass cap on the bottom end of the grid tank coil.   You can then link couple the grid tank coil to the previous stage tank circuit.  As has been mentioned by Peter,  link coils are a few turns of insulated wire over each of the two coils you are linking together.  These link coils are usually afixed at the bottom or cold end of the tank coils.

If you use a dropping resistor for the screens of the two 807s, it should be about 20K ohms 15-20W.  This value is for both screens, one 20K resistor for the two tie-together screens. (one resistor for both screens, NOT one 20K for each screen).  This 20K value is for a plate voltage of 600-700 volts.

You need to connect your screen bypass caps at the socket pin.  You can use one .001 disc ceramic cap for each tube.  You can leave the 18ohm resistors in the screen circuit if you like.  Do Not add anymore screen bypass caps in your screen circuit.  Connect the 20K screen resistor to the 600-700volt modulated B+.  You don't need shielded wire for the screen circuit.

Fred

[KA2DZT]

Tom

That piece of circuitry was from a 1955 QST magazine. L1 & L2 are parasitic chokes (which is what I thought you were refering to in a previous post). I understand now you meant an RFC in the grid bias line.

This is the modified circuit.

Bob

Bob,

I see you answered while I was typing my last post.

I looked at the updated schematic, it's looking better but a few mistakes.  The 20K screen resistor gets connected to the other end of the plate choke where the bypass cap is.  The .01 bypass is too large use only one .001 2-5KV bypass on the plate choke.

I see the grid tank circuit is back in the schematic.  You need a bypass cap .001 at the junction of the grid RFC and the grid tank coil.  Although, if you add RF feedback from the plate to the grid circuit, the bypass cap must reduced to about 250-300pfd. The feedback gets connected to that junction of the grid tank coil and the grid RFC. Also, you didn't include the grid bias resistor that we already discussed, but I guess that is just an oversite.  You can leave in the two 100ohm resistors on the grids, they don't hurt anything.

Fred

[VK7ZL]

Fred

Hopefully I have it correct this time.

Bob

[KC9LKE]

I believe that some time back the topic of interstage coupling was beat to death on this board but I didn’t check.

(ooops try this "Coil construction for link coupled grid input")


Your interstage coupling circuit is for the driver and amplifier being separated a distance from each other coupled by coax. It will still work fine but you could choose a less complex interstage lashup if they are on the same chassis. Just My $0.02.


If you know the theory then I assume that you can find XL based on the 1625 data sheets, grid driving power, grid current, desired Q, etc. Remember the tubes are in parallel. So you are asking how do I determine the physical size of the coils? You can do a search for “coil form factor” and read up on it. Yes you can just pick the form size from other builds but its some good reading anyway and more importantly you will learn why coils have certain physical properties for their application. At 40M the capacitance between the windings will probably be negligible so you can close wind the big coils. You could use a 1” or ¾” diameter form for each “ transformer”, and the length of the form will be determined by the required number of turns for your desired XL and chosen wire diameter. An online calculator or coil table will do to determine the number of turns. I like the online calculator so you can play around with form diameter, wire size etc. #22 or #24 AWG for the big coils and #18 for the small windings should work fine. Or try what you have on hand. The goal is low resistance, little or no heating. BTW there is a fairly good explanation in the 1957 ARRL handbook on how these windings interact.

I have a 1625 x 2 transmitter schematic with coil data. If you would like a copy PM me.

Looks like You are getting some good direction from the board on your project. Stick with it, looks like a FB project.

Best Regards
Ted / KC9LKE

[K1JJ]

Gooood going. Your latest circuit looks FB, Bob.   I might have missed a detail, but you are very close now. As Ted suggested, now physical layout will become important.

Just add the grid leak resistor right after the fixed bias (before the grid tank) and you should be ready to build it.  When testing, you can always add and adjust things, like the grid leak vs: fixed bias  value, add parasitic chokes, swamping, etc in case it's needed. We can advise you if you have problems.. and you may. Very few rigs that are newly designed and "hambrew" work perfectly when first built.

I realize with so many cooks in the kitchen it may get confusing here, but all of the advice has been good. You've probably learned a lot so far.

Please take some pics and post when it gets built, OM.

T

[KA2DZT]

Fred

Hopefully I have it correct this time.

Bob

Bob,

Starting to look like a xmtr that will work.  Add another .001 bypass on the low side of the grid RFC and then your grid bias resistor along with whatever fixed bias voltage you chose to use.  Remember that with 8ma of total grid drive the voltage drop across the grid bias resistor and the fixed bias voltage should add up to at least 85 volts neg.  If you chose not to use any fixed bias, increase the grid bias resistor to a value that will drop the entire -85 volts (85/.008=10625ohms.  .008x.008x10625=.68W)  you can use 11K.  You can use any combination of resistors to get the 11K, at least 2-4W.

I'll be at our yearly Field Day outing this weekend, so you may not hear from me until Monday.

Fred

[KA2DZT]

Very few rigs that are newly designed and hombrew work perfectly when first built.

T

Except for the ones that I build

Fred

[The Slab Bacon]

Very few rigs that are newly designed and hombrew work perfectly when first built.
T

Except for the ones that I build
Fred

But we will all die of old age waiting for you to finish them.......................

After all, the single 813 rig took somewhere around 27 years to finish.   

[KC9LKE]

I realize with so many cooks in the kitchen it may get confusing here, but all of the advice has been good. You've probably learned a lot so far.

T

Yup, I was some what reluctant to post with all of the good direction already being offered. Just wanted to offer my $0.02 on the interstage coupling.

Ted / KC9LKE

[K1JJ]

Fred, maybe now is the time for you to teach Bob about your pounds/watt philosophy ..


Bob,

I just realized how easy it would be for you to add neutralization to your latest circuit.   Just connect a 0-10pf variable cap to one plate cap (two plates wide-spaced or a simple plate facing one tube)  and connect the other side to the junction of the .001 and RFC at the grid circuit. Then reduce the grid bypass cap from .001 to 500 pf. (.0005)  That's it.  You would adjust this negative feedback capacitor so that there was minimum RF feedthru from grid to plate of the 1625's.  Or adjust it for maximum power out while getting a coincident plate current dip.

You may not need it, but if the rig takes off on 40M, this could be the ace in the hole to fix it.  I neutralize all of my grid-driven rigs, triodes and tetrodes alike. It makes a huge difference when using the higher bands.


Later -

T

[KA2DZT]

I realize with so many cooks in the kitchen it may get confusing here, but all of the advice has been good. You've probably learned a lot so far.

T

Yup, I was some what reluctant to post with all of the good direction already being offered. Just wanted to offer my $0.02 on the interstage coupling.

Ted / KC9LKE

Ted,

Feel free to jump in anytime, I'm sure we can all learn something from each bit of info.

Nobody here knows everything about xmtr construction (except me of course)....... (OK, well maybe JJ does)

I got to get going to the Field Day site.  Don't talk about me while I'm gone.

Fred

[VK7ZL]

Gooood going. Your latest circuit looks FB, Bob.   I might have missed a detail, but you are very close now. As Ted suggested, now physical layout will become important.

Just add the grid leak resistor right after the fixed bias (before the grid tank) and you should be ready to build it.  When testing, you can always add and adjust things, like the grid leak vs: fixed bias  value, add parasitic chokes, swamping, etc in case it's needed. We can advise you if you have problems.. and you may. Very few rigs that are newly designed and "hambrew" work perfectly when first built.

I realize with so many cooks in the kitchen it may get confusing here, but all of the advice has been good. You've probably learned a lot so far.

Please take some pics and post when it gets built, OM.

T

First of all thanks to everyone for their input. As you would have gathered I am not an electronics technician. I am a mechanical fitter by trade and worked on heavy earth moving equipment & big stationary generating plants so this is a steep learning curve for me.
Unfortunately the Ham who was helping me is in hospital and probably wouldn't appreciated me visiting him with a circuit diagram in hand.

As far as chassis layout is concerned I have plenty of room. The original base station unit was mounted in a 19" x 19" x 12" (deep) rack cabinet. The PS / modulator slides in the bottom and the RF deck will go above that. I will be using my Hallicrafters SX24 as the reciever.

Fixed bias - The original fixed bias was taken from a 68V tap on the transformer so if my thinking is correct I can half wave rectify that and regulate it with a zener diode??

I think that I have addded the right components in the right place this time.

Bob

[KA2DZT]

Bob,

The 3.9K grid resistor should be in series with the grid circuit.  Connect it to the meter terminal instead of to ground.  Leave the other connection to the RFC as you have it, it is correct,  just the ground is wrong.

Fred