4-65A Modulator

[W9ZSL]

Yeah, I know.  I'm kind of a pest but I've spent years collecting parts to build a "dream rig" and now it gets down to the most efficient combinations.  I will use a single 813.  I planned on modulating with a pair of 811A's...but wait!

Just for sheets and giggles, I have a couple 4-65A bottles.  When used in a modulator they can have some serious output and will work with a wide range of plate voltages. It takes very little to drive them.  Of course, you have to consider screen voltage as opposed to 811A's.  On the plus side a single 6V6 into a UTC S-8 driver I originally planned on building would be simpler and make a lot more sense.  This just keeps evolving.  Coming up with the screen voltage is no problem.  S-22 mod iron with the 813 PA.  

The P-P 6V6 driver described in my other thread is far more complex than this beast and it would work with both modulators.  You already commented on the driver and I changed the diagram to reflect your input. This kills two birds with one stone.  Don't use 811A's in the modulator, period.  Sometimes the obvious isn't.  

Comments?

[N2DTS]

Build both.
An 813 likes 2000 volts at 200 ma, so its handy to make a modulator that runs 2000 volts on the plates.
You also want a modulator that makes as much audio power as carrier output power of the RF deck.

A good AM rig would be a pair of 813's as they can run forever at 400 watts carrier output.

Modulators that work at 2000 volts:
813
4-125, 250,400,
4x150a/4cx250b (in AB1)
100th

4-65 likes 1800 volts in AB2 per the tube data and only does 270 watts.
A single 813 at 1800 volts will do about 275 watts and the 4-65's will do for that.

[W9ZSL]

One 813 at 1800 volts and a pair of 4-65A's at either 1250 or 1500 volts is the limit.  I could use a pair of 811A's for the modulator at the same voltages but driving them would require more power.

[KA2DZT]

Run the modulator on the same supply as the 813 if you can.  Your supply would need to supply about 300ma.  What plate xfmr are you using??  Use the 4-65As as they can run at 1800 volts.  811As are limited to around 1500 volts.

Running the 813 at 1800 volts and the modulator at 1250 volts you'll never make 100% modulation.

Fred

[N2DTS]

I think you could modulate an 813 at 2000 volts with an 811a modulator running  1200 to 1500 volts with the right turns ratio in the mod iron.

I do not think its the best setup, but it would work.
I like having about the same voltage on both sides of the mod transformer.

These days, the mod transformer seems to be the big limitation, a really good high power mod transformer is hard to come by and you do not want to push them too much or screw up and blow one.

I have a stanley mod transformer I got years ago, from fair radio I think, 8000 to 2800 ohms at 300 ma and 1750 working volts, marked 200 to 15,000 Hz and tried it with 811's and the 2x4x150 rf deck a few days ago.
It worked but the low frequency stuff sounded very nasty.
So its useless really.
The old Thordarson 11m77's were quite good I think, the cvm 5 was very poor on the low end response, and after trying various setups, I kind of like screen modulation.
No phase shift, and no response changes, the screen modulated rigs seem to need very little eq of the audio to sound very good.

[w8khk]

If you are considering running a pair of 4-65A tubes as modulators, or even a quad in push-pull parallel, if you run them class AB1, no driving power is required.  This mode would allow you to eliminate the driver transformer.  This would reduce overall phase shift, and improve frequency response, while allowing more stability if you decide to use negative feedback in your design.

If you wish to pursue this approach, you might want to take a look at an article entitled "500 watts of audio from AB1" by Mandoli, ex W6HAM and Atkins, W6VX.  The article was printed on page 13 of QST for March 1948.

While the implementation used a pair of 4-250 tubes as modulators, it could easily be scaled to a pair or quad of 4-65A tubes.  It only used four receiving tubes from microphone input to the modulator grids, and no transformers.   Tube lineup was a 6J7, 6J5, followed by a pair of 6SJ7s in a phase inverter similar to the design you posted.  Hope this gives you some good ideas.  GL with your projects.

Note.... I altered the thread subject to 4-65A, not 4-64A....

[Opcom]

A TX here uses two 4-65's in AB1 as modulators at 1500VDC with a 14K p-p load.
The modulation transformer is rated 90W.
The TX is rated 90W carrier, 100% modulation.

[N2DTS]

AB1 was how all the old tube hifi amps used to be set up.
You need to regulate the screen voltage, not hard with VR tubes.
My 4x150 modulator deck runs AB1 and sounds cleaner then the class B 811's to me.
4-65's would look great glowing behind some glass, pulsating with audio.

[fg5fc]

Hello
Why not a pair of 572b as modulators?

[W9ZSL]

I actually looked at an old AM Forum thread that discusses 4-65A tubes as modulators.  I think I'll stick with the single 813 / P-P 811A combination using the P-P 6V6 driver which puts out 12 watts.  That amp was discussed on another thread recently.  I double-checked my plate transformers and can easily do 2000 volts for the 813 with 1350 for the 811A's.  If the 2 KV was rated at more than  300 MA, I could use one supply for both since the 4-65As wouldn't give me enough output at the lower voltage.

[N2DTS]

If you have 300ma to play with, the 813 at 200 ma and a modulator of 4-65's would be ok because its icas and the average current of the modulator is low.

[KA2DZT]

I actually looked at an old AM Forum thread that discusses 4-65A tubes as modulators.  I think I'll stick with the single 813 / P-P 811A combination using the P-P 6V6 driver which puts out 12 watts.  That amp was discussed on another thread recently.  I double-checked my plate transformers and can easily do 2000 volts for the 813 with 1350 for the 811A's.  If the 2 KV was rated at more than  300 MA, I could use one supply for both since the 4-65As wouldn't give me enough output at the lower voltage.

Use the 2KV xfmr for the PA and the modulators with the 4-65s.  The modulators on average don't put that much load on the plate xfmr.  What plate xfmr do you have??  Some are much better than others.

The 811As at 1350 volts you will have a much harder time trying to make 100% modulation.

Fred

[W9ZSL]

It's a Thordarson T-16P03.  It has a dual secondary.  With 120 on the primary, I've tested it with rectifiers (solid state), 12Hy choke input filter, 12MFD, and a 100 K bleeder. I get 2250 VDC on one secondary and 1840 on the other, no load.  It's 300 MA. It's 4600 and 3750 VCT.  I have both swinging and smoothing chokes plus plenty of HV caps so I could do a two-stage filter to separate the RF and Mod if necessary.  Sure would make life simpler.  I'm not sure if both the 4-65A's are good.  I know one is NOS.

Of course, I'd have to come up with a driver and wouldn't need driver iron.  I've seen a circuit for a real simple inverter used with a pair of 813's.  I don't want to use 813's in a modulator though.  I'd have to buy a pair and a new filament transformer and can't afford that right now.  I wouldn't use tubes in the screen supply.

[w4bfs]

if you decide to use the 4-65 be sure to test the filaments for normal V & I (6.3V and 4A) ... most I have tested have lost some vacuum and did not light up properly

[W9ZSL]

I've heard they can get gassy.  Pun intended. 

[KA2DZT]

The 16 series Thordarsons are a little heavier duty than the later 19 series.  I have some 19 series xfmrs and I'm not too impress with them, not much iron in the core.  In my HB 813, 811As xmtr I use an earlier Thordarson P-6411, it's rated at 280ma, but has much more iron in the core.  The P-6411 has a tapped primary.  It's rated at 1000 DCV and 1250 DCV depending on which primary terminals used and after a two stage filter (LCLC).  Primary is 115vac, with 120 volts it makes about 1150 DCV with SS rectifiers, using the lower voltage primary terminals.  I increase the DC voltage by adding another 350VDC (from another xfmr) to the CT of the P-6411.  This brings up the plate voltage to about 1500 volts.  I use it for both the PA and the 811As.  The 811As are powered off the first choke and the 813 through the second choke of the LCLC filter.  Even though the P-6411 is only rated at 280ma, the xfmr doesn't even work up a sweat running the xmtr for hours.

You should be able to power both the 813 and the 4-65s from the 16P03.

Fred

[W9ZSL]

Fred, would you recommend using a two-stage filter with the swinger followed by the smoothing?

[KA2DZT]

Fred, would you recommend using a two-stage filter with the swinger followed by the smoothing?

Sure, if you have them.  You don't need a lot of filtering for the push-pull modulators, so feed that off the swinging choke and the PA off the smoothing choke.  About 15mfd on the swinger and more on the smoothing.  If you plan to key on and off the HV supply with xmit and receive the chokes help to reduce the inrush current on the supply.

Taking the mod current off the first choke helps improve voltage regulation for the PA stage.  You need chokes with a high enough voltage rating.  Also use chokes that do not have a high DC resistance, under 100 ohms for each choke would be OK.  Most xmitting chokes should be under 100 ohms.

You didn't mention what chokes you have, some are much better that others.

Fred

[N2DTS]

Also, keep in mind you can come off the rectifiers and go to each choke, then to a cap, and have one choke do the modulator and one do the rf deck, all the current does not have to flow though one or both chokes.

The DC high voltage does not need much filtering, but should be stiff and have as much energy storage as you can do (for positive peaks).
Step start is a must for energy storage, even if you leave the supply on and key the cathodes.
4uf per deck and you likely do not need step start.

[W9ZSL]

Yeah.  Use the swinger on the mod and smoothing on the RF. I have a LOT of caps!  What about the bleeder/s?  

Referring to that modulator using 813's, would there be much difference in the driver if I went with 4-65A's?  Since the voltage to the plates of both types of tubes would be the same, it seems to me that the only difference would be making sure the 4-65A's had the correct bias and screen volts. The 813 diagram shows -68 volts through a 220K resistor to the 813's.  Bias is no problem since I have plenty of parts to build a separate supply for the modulator. The other consideration would be the feedback. Again, it's covered parts-wise.

So the question is, what changes would I have to make with the dual 6SJ7 driver if I substitute the 4-65A's for the 813's with 2 KV on the plates?  No problem with the screen supply either. Got that covered also...separate supply.

[KA2DZT]

Also, keep in mind you can come off the rectifiers and go to each choke, then to a cap, and have one choke do the modulator and one do the rf deck, all the current does not have to flow though one or both chokes.

The DC high voltage does not need much filtering, but should be stiff and have as much energy storage as you can do (for positive peaks).
Step start is a must for energy storage, even if you leave the supply on and key the cathodes.
4uf per deck and you likely do not need step start.

Yes, that is another way it can be done.  My 6146/6550s is set up that way.  Two chokes for the 6146 and a separate choke for the 6550s.  My 813/811s is also set up to run that way when I use 1800 volts for the 813 and only 1500 volts for the 811s, otherwise when it runs on only 1500 volts for both it runs with only two chokes.  The 811s run off the first choke, the 813 through both chokes.

Fred

[KA2DZT]

Yeah.  Use the swinger on the mod and smoothing on the RF. I have a LOT of caps!  What about the bleeder/s?  

Referring to that modulator using 813's, would there be much difference in the driver if I went with 4-65A's?  Since the voltage to the plates of both types of tubes would be the same, it seems to me that the only difference would be making sure the 4-65A's had the correct bias and screen volts. The 813 diagram shows -68 volts through a 220K resistor to the 813's.  Bias is no problem since I have plenty of parts to build a separate supply for the modulator. The other consideration would be the feedback. Again, it's covered parts-wise.

So the question is, what changes would I have to make with the dual 6SJ7 driver if I substitute the 4-65A's for the 813's with 2 KV on the plates?  No problem with the screen supply either. Got that covered also...separate supply.

If both the 813s and the 4-65s are running AB1 you may not need any changes to the 6SJ7 circuit, just look at the difference in drive voltage to the grids of both tubes.  If the voltages are close, make no changes.  You will need to check the differences in bias and screen voltage.  You may want to make a separate adjustable bias supply and a separate screen supply regulated.  Not too crazy about the PS shown in the schematic.

The two 220K resistors are just the grid resistors and they should fine for the 4-65s.  The other two 220K resistors are the plate load resistors for the 6SJ7s.

Fred

[W9ZSL]

Fred,

Surprise, surprise!  I forgot that I have a completely-matched set of Thordarson iron. EVERY PIECE IS NOS.  I have the T-16P03, T-19C36 5-20 Hy swinging at 300 MA, 80 Ohms and the 19C43 smoothing at 12 Hy, 300 MA also 80 ohms. Like I said from the beginning, I've been collecting parts for over 10 years. All this iron is MINT.  No problem with power supplies for bias and screen.  If the chokes are in series that's one thing since I can tap between them and use just one bleeder.  Seems like a logical way to go.

MK

[N2DTS]

Then build the supplies.
Make it so you can adjust the output voltage using relays on primary taps, a variac, and or dual voltage outputs.
Then build an RF control deck, bias and screen supplies, metering, adjustable grid leak and fixed bias on the front panel, variac screen supply, big wire wound screen dropping resistor, adjustable high current overload for the screen.

Then you just need a simple RF deck and a modulator.
And you can build all sorts of RF decks to try, and modulators, and even screen modulation.

[KA2DZT]

Fred,

Surprise, surprise!  I forgot that I have a completely-matched set of Thordarson iron. EVERY PIECE IS NOS.  I have the T-16P03, T-19C36 5-20 Hy swinging at 300 MA, 80 Ohms and the 19C43 smoothing at 12 Hy, 300 MA also 80 ohms. Like I said from the beginning, I've been collecting parts for over 10 years. All this iron is MINT.  No problem with power supplies for bias and screen.  If the chokes are in series that's one thing since I can tap between them and use just one bleeder.  Seems like a logical way to go.

MK

OK FB, those chokes should be fine, (I have a number of them also) they are rated for 2KV DC working.  The RMS TEST voltage is 5KV but that computes down to the 2KV DC.

Both chokes in series, use the one bleeder off the smoothing choke.  The 100K is FB, same amount I use (two 50K 100 watters in series).  Just remember even with bleeders on the caps, NEVER work on the HV supply without shorting all the caps to ground.

Fred