4-65A Modulator

[W9ZSL]

I have to buff the slight corrosion off the plate iron core and re-paint.  Here is my array. No doubt I'm serious about this...and REALLY "anal retentive" when it comes to waiting until I can find all the right parts! LOL.  I'm not a novice when it comes to building gear.  My build project in Electronics in my Senior year in High School (Class of '64) was a CW rig as a final exam.  Aced the course.  I'm not really a designer, but construction is no sweat.  I've already bread-boarded a high voltage supply based around these components. Don't worry about my safety around HV.  I'm totally "grounded".  That's a pun.  I always keep one hand behind my back because putting it in my pocket would be too much of a temptation!  

If I don't use these choice pieces, a Wouff Hong is my only future.

[N2DTS]

For bleeder resistors, you can get the dale type ones that mount to the chassis for a heat sink, they are small and cheap and you can run them in series and parallel to get plenty of wattage.

[W9ZSL]

Here's my concern about bleeders.  If the chokes were in series, then one bleeder would suffice.  However if the output of the rectifier were to be split to two chokes in parallel with one feeding the RF and the other feeding the modulator, would you need two bleeders and if so, they would be in parallel  raising the question of what value and wattage would they have to be to maintain 2KV on each leg?

[KA2DZT]

If you use the two chokes separately coming off the rectifiers you would need bleeders on both caps to be safe.  This would increase the bleeder load on the power supply.

For an AM xmtr the bleeders are there to drain off any charge in the caps.  Don't worry about the voltage because the bleeders have little affect on the plate voltage you end up with.  The 813 will be drawing upwards of 200 ma.  The bleeders may only be drawing 20 ma.  Same in true for the modulator to a lesser extent, since the idle current may only be 50-80 ma.  If you leave the HV on during receive then you need to watch how high the HV will rise when in stand-by.  This is when you need enough bleeder current to keep the HV from rising too much.  This is true for both filter circuits (bleeders on both legs).  If you key off the HV supply on receive then the bleeders are not doing much since the PA and modulator keep a large load on the HV supply.

Fred

[N2DTS]

That makes it easier, just double the resistance of each bleeder section, and go light on the bleeder, it will be mostly for safety.

I once got a fist full of 50 K 25 watt dales and have used them in most of my supplies, series parallel, been working for 40 years so far, up to 3KV.
You can get them new at Mouser and other places really cheap, I got some 50 watt jobs (10K) for my screen modulators, under $5.00 each I think.

[W9ZSL]

Dividing the rectifier output into two separate legs sounds like the way to go considering the entire current would not have to go through the first choke in a two-stage filter.  One little detail just so I can be sure: two resistors of equal value in parallel doubles the wattage of a single resistor?  I know that the total resistance would be half that of one resistor.  To come up with 100K total bleeder resistance, I'd need 200K on each leg?  That is based on ending up with 2250 VDC with only the bleeder for load.  I found that 60K for a bleeder gives me just 2050 VDC.  Under load, that would be closer to 1850 VDC so the 100K would be a logical choice to get me to about 2KV on each leg.

[N2DTS]

You can do series and parallel, to get the resistance and wattage you want.
Since I had a bunch of 50K ohm resistors I used a bunch.
Here is one supply that used four 50K 25 watt jobs for a total of 50K 100 watts.

http://n2dts.smugmug.com/Ham-radio/i-vcDSc2b/A


And here is a nice simple RF deck that will do 300 to 500 watts carrier easy at 1500 to 1700 volts.

http://n2dts.smugmug.com/Ham-radio/i-k3PMKRC/A

[W9ZSL]

I looked into some power supplies and bleeders in the '64 Handbook and according to that, putting resistors in series doubles the wattage.  Now I'm confused.  If that's the case 2 x 100K at 100 watts each in series would give me 200K at 200 watts?  Nice station!

[KA2DZT]

I looked into some power supplies and bleeders in the '64 Handbook and according to that, putting resistors in series doubles the wattage.  Now I'm confused.  If that's the case 2 x 100K at 100 watts each in series would give me 200K at 200 watts?  Nice station!

Correct,  two 100K 100W in series is 200K 200W,  two 100K 100W in parallel is 50K 200W.

Don't over think the power supply as to exactly what voltage you end up with.  You should build the best supply using the iron you have on hand.  Use the amount of bleeder current that is correct for the way you plan to key the xmtr on and off.

If you plan to leave the HV supply on and put the xmtr in stand-by using some method of cathode bias then you need a little more bleeder current to keep the HV from rising too high.  HV too high may exceed the voltage rating of the filter caps.

OTH, if you plan to key the HV on and off with xmit and receive. then you don't need that much bleeder current because the PA and the modulators will keep a heavy load on the HV.  The bleeders with this method are almost not even needed.

Fred

[W9ZSL]

The "Ah Ha" moment!  I'll be keying the HV on and off.  This rig will be used primarily for AM.

[W9ZSL]

Fred, I have a couple of 75K Ohmites @ 100 watt with taps here.  I'm thinking on getting a pair of 100K at 100 watts fixed.  That would be good for 175K @ 200 watts on each leg if I used a 100K fixed and 75K tapped in series on each leg with the variable on the grounded side of the bleeder.  Together I'd have 85.5K at 400 watts bleeder wattage once each leg is in parallel.

Given the variable taps, I COULD use the one on the modulator supply to provide screen voltage to the 4-65A's since the current draw is minimal.  The swinging choke would be in that leg. I wouldn't use the tap in the supply for the 813.  I can provide a separate supply, regulated for its screen in case I want to use the amp in a linear mode. I should end up with something in the neighborhood of 1900 volts under load on both the modulator and RF.

Though those Dale bugs are cheap, 2 x 100K Ohmites @ 100 watts in series with what I have makes sense and the price would be about the same as starting over with all Dales.

As for the modulator driver, I can loaf the 6V6 job through the UTC S-8 into the 4-65A grids.  Yeah, it makes for an extra transformer as a driver but it is more versatile for me because one speech amp / driver will work for everything and all the iron will be UTC for both modulators.  Further, at the higher voltage, Eimac actually recommends using a driver transformer, and recall that phase inverter resistance / capacitive coupled circuit was for a pair of 813's operating at 200 watts.

With close to 2KV on the 4-65A's and  280 volts on the screens, theoretically they can modulate an RF amp with 480 watts input.  I'm not looking for over 400 watts RF on the 813 and the UTC S-22 mod iron is rated at 250 watts.  1900 volts on both should rock.

One more question.  Filter caps.  With one plate transformer, how do the capacitor banks effect the system?  Standard procedure is load up the cap on the modulator to even the greater voltage swings relative to the PA.  I have a good variety of caps and can mix and match.  My original design with two supplies has somewhere around 18 MFD for the modulator and around 12 for the PA.

[KA2DZT]

I guess you have 75K 100W Ohmites with the sliding adjustment.  You can use one 100K 100W Ohmite on each leg of the filters.  Then connect each 100K to the ends of one 75K 100W Ohmite.  Put the slider in the center of the 75K and then ground the slider.  This will give you a total of 100K + 37.5K bleeder on each leg.  Just so you understand how power resistors work,  if you use 1/2 of a 100W power resistor (one with a sliding adjustment) and leave the other half unused you now only have a 50W resistor.  The power rating is based on the entire length of the resistor being used.  So, using the set-up I described above you have 100K 100W + 37.5K 50W on each leg.  This is OK, because the power dissipated is the current squared times the resistance (IxIxR).

If you have about 2KV with 137.5K bleeder, the bleeder current would be about 14.5ma. So, .0145 x .0145 x 100K = 21 watts dissipated in the 100K resistor.  Doing the same for the 37.5K, .0145 x .0145 x 37.5K = 7.88 watts.  You can see that the power dissipated in each resistor section is well under the power rating of the resistors.

Now if you have two the same bleeders (137.5K), one on each filter circuit, you'll have a total bleeder current of a about 29ma (2 x 14.5ma).  This should be just right for what you're doing.  Meaning, you're planning to key the HV supply on and off with xmit and receive.

In general, I don't like using the bleeder resistors for anything more than bleeder purposes.  I would built a separate regulated screen supply for the 4-65s.  But, on second thought, this will create a major problem.  You can't leave the screens on (with screen voltage) and remove the plate voltage.  This will damaged the screens in only a few seconds.  Pulling the 4-65 screen voltage from the bleeders is what you need to do.  This way when the HV goes off, so does the screen voltage.

Fred

[W9ZSL]

I don't really care if the 75K taps are used or not.  I have them on hand.  Here's the basic diagram of the PS.  I'm considering economy.  If I have 175K in each leg at 200 watts, the combined would be 85.5K which just "sounds right."    The 175 K would be a 100K and 75K in series.  Each would be 100 watts.  Any screen voltage will will not be applied before plate voltage is.  The only screen voltage derived from one of the 75K taps would be for the modulator alone if practical.  I can do a separate supply for the modulator screens keyed by the plate supply. 

The other one (variable) in the 813 bleeder is "just there" and would simply be treated as a fixed resistor for the 813 plate side of the supply.  I have more than enough iron to build separate supplies and tons of high-voltage diodes to build any power supply including bias.  All the supplies will be solid-state except for the exciter and the speech amp / driver.

[KA2DZT]

Yes,  that will work.  With the 175K, the bleeder current will be about 11ma on each leg.

[KA2DZT]

With 175K you wouldn't even need 100W resistors.  Two 50 watters would still run way under their rated wattage.

[W9ZSL]

Like I said, that feels right!  You can do all the math, but sometimes intuition and a "gut" feeling makes perfect sense.  Will keep you posted.  Thanks all!

[Opcom]

Have you tried Duncanamps.com PSUD? It is a power supply simulator and gets pretty close. It's made for ease of use. Then you can tell what effect various bleeders will have, etc.

[W9ZSL]

I'll keep that in mind Op.  I'll probably bread-board the whole supply before I build it anyway.

[W9ZSL]

Fred, I think my best bet is just go with the KISS principle putting the chokes in series and using the one 100K, 250 watt bleeder.  It will at least give me a start. I have to do this before I become a SK. The good thing about all the iron is it's not used.

[KA2DZT]

Yes, that's what I would do.  When I built my HB 813/811s xmtr I used one HV supply.  It has a Thordarson HV xfmr (P-6411) and two Stancor 500ma chokes in series with 100K 200W bleeder.  I pulled the HV for the 811s off the first choke and the 813 through both chokes.  Works FB.  The power supply is about the easiest part of the project.  Put together correctly, very little can go wrong.  I designed the entire xmtr myself, from the VFO to the antenna.

I spend more time building stuff than I ever spend on the air.  I have thousands of parts here and I'm trying to use them up before I go SK.  I have probably more than 1000 xfmrs and chokes (counting the small ones) here so I have to keep going.

For the 813 final I just used a series screen resistor off the modulated HV B+.  This is the easiest way to modulate the screen.  When you shut off the HV so goes the screen voltage.  You don't have to worry about having screen voltage without plate voltage which will damage the screen in only a few seconds.

Try to keep the project simple.  If you're building an AM rig then just focus on AM.  I saw you mentioned something about also using the 813 final for SSB as a linear.  This will greatly complicate what you're trying to build.  Plus a single 813 as a linear is probably not worth the effort in added output power vs a 100 watt rice box.

Fred