Transmitter Project, more early stage musings

[W7SOE]

I just received my plate transformer from Bill, KD0HG.  Thanks again Bill!

What a monster!  It is easily 75 lbs.  3560/4580 VAC and 1440 VA.

I will modulate a pair of 813s with 813s.  I am reconsidering the power output that I desire.  I am also going to go without a heising reactor (BC-610 mod xfmr), at least initially.

I have never built a transmitter before and I have a lot of learning to do.  I want to scale back this project to minimize the troubles that I will run into but leave the possibility to "scale up" later.  For instance, do I really want to go for a 500W carrier with my close in QTH and simple inverted V antenna?  Perhaps I should go for 200W?  Perhaps modulate a single 813?  I could always add a second right?  I could always add a Heising reactor later right?  

I know y'all have tons of experience with this stuff but what would be the "sane" path for a slob like me?

At least I have more plate transformer than I will ever need.  This thing scares me just sitting here on my bench.  ;-)

Rich

[W1RKW]

Headroom! Do 500w. 2(813) x 2(813). Be done with it. The shake down will be the same whether big or small.

[WA1GFZ]

going 1 to 2 will cut the plate Z in half so pi network will need to change.
Better off with 2 tubes and run reduced voltage. Start with plate transformer on 120 volt line to cut voltage in half. Work your way up from 120 volt primary fed into 240 volt winding and low tap on secondary.

[KL7OF]

I agree with GFZ.....Go with 2 tubes and adjust plate voltage and loading for the desired out put....

[KA2DZT]

going 1 to 2 will cut the plate Z in half so pi network will need to change.
Better off with 2 tubes and run reduced voltage. Start with plate transformer on 120 volt line to cut voltage in half. Work your way up from 120 volt primary fed into 240 volt winding and low tap on secondary.

I agree with GFZ also.  Adding a second 813 afterward may be more work than building the PA with the two 813's to start with.  Too many factors change from 1 tube to 2 tubes.  Better to lay out the chassis for the 2 tubes  than to try to fit a second tube into what is already built.

That Thermador xfmr is not too big, in fact, it might just be enough to run the PA and modulator.  I have some Thermadors, they're good iron.

I guess the 3560/4580 vac is center tapped.  1780-0-1780, 2290-0-2290.  1440VA, the xfmr is probably good for about 500ma to 600ma

The xfmr will make about 1500VDC or about 2000VDC with a choke input filter.  I would only use a choke input filter.  You'll need a good 500ma choke.  Using only a capacitor filter or cap input and choke filter is not a good idea for an AM xmtr.

You can run the xmtr on the low voltage taps to start.  Two 813's should easily make about 300 watts output with 1500 volts.  with about 180ma plate and screen current for each tube.

You won't be able to use the xfmr on 115vac, the voltage output with only half the primary voltage will not be high enough.

Good luck with the project,

Fred, KA2DZT

[WBear2GCR]

Say... wait a minute!!

Aren't you that famous East Coast AM'er, "Fifty-One Watt Fred"??

How come you know so much about HIGH POWER AM transmitters?
Do you have one, and are being sneaky??


                    _-_-bear

[K1JJ]

You said:

"I would only use a choke input filter.  Using only a capacitor filter or cap input and choke filter is not a good idea for an AM xmtr."


Fred,

I'm curious of your feelings on this?  For capacitor input: Assuming the transformer is rugged, enough filter capacitance and it is a common final/modulator power supply, the class C final should provide a heavy, constant load, providing good voltage regulation for the modulators. And if enough capacitance is used (at least 40 uf) there should be no noticable hum at this high level stage.  

I often switch between choke input and capacitor input to get various voltage levels for the 4X1 rig and have no problems. Maybe there is another reason I missed?

T


BTW, Rich - I would certainly use two tubes right from the start. The only reason to go with one tube is if the mod transformer is better suited to one final (ratio) or the mod iron you have will never be able to handle the power of two finals without blowing up. You can always adjust the plate current of two tubes to better match the mod xfmr, however . But because we often run into larger iron as time goes by, go with two tubes anyway. You never know.

[W7SOE]

Thanks to all for your input.  I will stick with the two tube final.  Fred, thanks for pointing out the limitations of that transformer on 115V, it looks like I will be running 220 to the shack.

I got concerned when I saw the size of the transformer I think.  I will install a variac so I can choose the plate voltage.  I was planning on using a choke input filter on the PS.

Onward, upward.

Rich

[KA2DZT]

You said:

"I would only use a choke input filter.  Using only a capacitor filter or cap input and choke filter is not a good idea for an AM xmtr."


Fred,

I'm curious of your feelings on this?  Assuming the transformer is rugged, enough filter capacitance and it is a common final/modulator power supply, the class C final will provide a heavy, constant load, providing good voltage regulation for the modulators. And if enough capacitance is used (at least 40 uf) there should be no noticable hum at this high level stage.  

I often switch between choke input and capacitor input to get various voltage levels for the 4X1 rig and have no problems. Maybe there is another reason I missed?

T

Tom,

A lot of what you say is right.  The class C final does provide a constant load keeping the peak no load voltage down.  Most common plate xfmrs are current rated to be used with a choke input filter.  If you use a cap input filter you have to reduce your current drain by 20-30% to stay within the VA rating.

Other issues,  starting up a cap input supply is more difficult, requiring a step start (something that should be used in any case).  If the builder designs the final by biasing the 813's off and leaving the plate supply on,  the supply voltage will soar to near the peak voltage, not good.  You do end up with better regulation overall with a choke input filter (if the choke you use has a low DC resistance).  Chokes with higher DC resistance cause regulation problems with changing loads (as with class B modulators).

More things I considered,  the xfmr the OP is using.  The current capacity of that Thermador will probably be just enough for a pair by a pair.  The OP stated that he was thinking about keeping the RF output down to about 250 watts to start.  Also he stated his lack of experience.  He is less likely to make dangerous mistakes with a choke input filter.

Tom, you have much more experience with screwups, explosions, crapouts, flameouts, arcs and sparks, etc.

Tom,  the xfmrs that you're using (pole pigs) probably have a VA rating way above anything you're drawing from them.

I think, a supply with no chokes will always have some ripple left.  Question is, can you hear it in the audio?  Probably not if, like you say, you use enough capacitance.  High voltage caps with high capacitance are hard to find.  Easier to use a choke and less capacitance.  Especially, if you do not need the higher plate voltage a cap input filter will provide.

Tom, you have had great success with your 4X1 rig, so I will not be questioning anything that you're doing.  I'm still at 51 watts.

Fred, KA2DZT

[KA2DZT]

Rich,

One thing I should have clarified,  you will not get enough voltage running 115vac across the whole 230vac primary.  You can run that xfmr on 115vac with full output by connecting 115vac across the two primary windings connected in parallel.  Look at the drawing on the xfmr,  should be, connect 1 to 3 and 2 to 4,  I can't see the numbers that well to be sure.

Does the xfmr state a current rating on it or just the VA rating??  Using the xfmr on the lower voltage taps will give you more current capacity since the voltage is less.  Less current on the higher voltage taps.  That's the whole idea behind a VOLT AMP rating on the xfmr.  There are a number of other factors involved in understanding a VA rating.  But you can do what everyone else does.  If the xfmr doesn't get too hot, you're probably OK.  If it gets too hot, you're probably over loading it a bit.  The amount of time the xfmr is under load (transmit and receive) helps the overall picture.  Some modern xfmrs are designed to run hot (they have a temperature rise rating on them).  Your early Thermador was not designed that way.  Just give it the finger-on-the-core test after its been running under load.

One last thing,  if you see smoke, you made a mistake

Hope some of this helps,

Fred, KA2DZT

[ke7trp]

build it big and run it down.  It will run cooler and likely be more reliable.  I have a couple of old chokes here if you need them. I believe they are 500 ma.  They where used in a supply to put 3Kv on two 4-400s in class C.

C

[K1JJ]

OK Fred -

Good point about lessening the requirement for a soft-start circuit when using a choke. All in all, it appears a choke is a good addition to the supply for the three reasons we discussed.

My point is that it's not the end of the whirl if we don't have a choke. A cap input supply will still function OK, but just needs a little more precautions. I personally use chokes for both my big and small HV supplies here for most operations and recommend their use. Though I use vacuum relays across the chokes to give me that extra voltage boost when needed. I don't use Variacs for HV supplies, so need all the tricks I can get.

Later-

T

[ke7trp]

Lets talk about why Variacs are a bad idea on an HV supply with decent current. That sounds infomative.

C

[W7SOE]

Lets talk about why Variacs are a bad idea on an HV supply with decent current. That sounds infomative.

C

Variacs are on my mind as well.  I like the idea of being able to bring up B+ from zero.  But now I am looking at running this thing on 220V..  I have found these options:

run one 110V variac on one side of the 220v (as described in another thread by K1JJ), that gives me an adjustment range of 110V to 220V

Run an external 220V variac on the whole thing during development only

Run no variac  (smoke test)

Opinions?

Thanks again for all the help.

Rich

[WBear2GCR]

If you variac the HV I think ur ok, but if you variac the whole PS, then the fils don't come up and that is a problem... the variac needs ample capability.

Fred, you don't think anyone is buying your story, do you??

                         _-_-bear

[W7SOE]

If you variac the HV I think ur ok, but if you variac the whole PS, then the fils don't come up and that is a problem... the variac needs ample capability.

Fred, you don't think anyone is buying your story, do you??

                         _-_-bear

Ugh, yes, dumb idea. :-)

Rich

[Opcom]

Watch out for supplying the 813 screens when the plate volts are zero.
Why not use a variac just for the plates and also link it to the screen grid supply?
-Alternately use a high voltage diode to supply the plates slightly via the line-connected G2 supply when the plate variac is at zero. The G2 supply may need to be beefier but it ought to be heavily bled anyway.

An oversized variac is best, like a 20A one, so the regulation is good.

The cap input C-L-C filter is fine only if the plate transformer has a quite low impedance or is oversized, else the on audio peaks it will suck down the HV and compress your modulated signal. Pole pigs rule! The input capacitor will cause much higher peak currents in the transformer and rectifier and this is where the regulation will become poor.

Since you are building it, you can JS it and see which way it works best before finalizing all of it.

[KA2DZT]

Lets talk about why Variacs are a bad idea on an HV supply with decent current. That sounds infomative.

C

Variacs are on my mind as well.  I like the idea of being able to bring up B+ from zero.  But now I am looking at running this thing on 220V..  I have found these options:

run one 110V variac on one side of the 220v (as described in another thread by K1JJ), that gives me an adjustment range of 110V to 220V

Run an external 220V variac on the whole thing during development only

Run no variac  (smoke test)

Opinions?

Thanks again for all the help.

Rich

Rich,

You can use a 115v or 120v variac to adjust from 230 down to 115, but, you will need to run a 230 line with a neutral (three wire with ground).  The 115v variac gets connected to one hot leg and the neutral,  the wiper going to the xfmr.  The other hot leg goes to the other side of the 230v primary.  A 15amp variac will work OK.  20amp variac is better, 10amp will probably be a little light.

Two drawbacks in using variacs on the plate supply.  One, they affect the overall regulation of the supply.  Two, they draw an amount of idling current which helps to run up the electric bill.

Unless you are going to use the HV supply for more than one xmtr (that may require different levels of plate voltage),  you could probably not use one.

One reason to use a variac is, in the summer when line voltages are down due to heavy drain on the grid, you can crank up the line voltage.

The variac will serve no purpose in keying up the xmtr (PTT),  for that you will need to design a step-start relay circuit (not difficult).  As in most AM xmtrs, you will be turning on the HV supply on transmit and turning it off on receive.

Fred,  KA2DZT

[KA2DZT]

Say... wait a minute!!

Aren't you that famous East Coast AM'er, "Fifty-One Watt Fred"??

How come you know so much about HIGH POWER AM transmitters?
Do you have one, and are being sneaky??


                    _-_-bear

Bear,

What makes you think I'm that famous East Coast AM'er

[ke7trp]

How does the reduced input to the plate trans effect the transformer itself? 

C

[KA2DZT]

Watch out for supplying the 813 screens when the plate volts are zero.
Why not use a variac just for the plates and also link it to the screen grid supply?
-Alternately use a high voltage diode to supply the plates slightly via the line-connected G2 supply when the plate variac is at zero. The G2 supply may need to be beefier but it ought to be heavily bled anyways.

The most common way to supply screen voltage in an AM xmtr is through a dropping resistor from the plate supply.  This way the screen voltage drops to zero when the plate supply is turned off (on standby).  The screens will be modulated from the modulated plate supply.  Using a separate screen supply is another good way to supply screen voltage but it gets somewhat more complicated.  Correctly keying up the screen supply with the plate supply and needing a screen choke to self modulate the screens.  You will also need some sort of safety feature to prevent the screen supply from turning on if the plate supply fails to turn on.

Opcom does bring up an important point,  never run any screen grid tube with screen voltage and no plate voltage.  Unless you have a large supply of tubes, you can wipe out the screens within seconds.

Fred,  KA2DZT

[Ralph W3GL]

    Hey Freddy, you gots some FM on your carrier...

     

[KA2DZT]

How does the reduced input to the plate trans effect the transformer itself? 

C

No ill effects that I can think of.  Transformers are passive devices,  the  ac voltage that appears on the secondary depends on the ac voltage applied to the primary.  Can not say the same for electric motors,  low voltage can damage a motor.

Fred

[The Slab Bacon]

Say... wait a minute!!
Aren't you that famous East Coast AM'er, "Fifty-One Watt Fred"??
How come you know so much about HIGH POWER AM transmitters?
Do you have one, and are being sneaky??
                    _-_-bear

Bear,
        He has been telling us on the air about his "Imaginary 813 Rig"  that he is building for years now but we are yet to hear it. I think he's building it in his imagination.    

51w = "Bang, lookie here, squashed just like a bug!"        

[WD8BIL]

What happened??? Last year Fred upgraded to 52 watts!!