Modulator Schematic

[W7SOE]

I have drawn up a schematic of the modulator section of the "new" homebrew transmitter.  I would appreciate any insight into its operation from the experts here.  It is two 811A's that modulate a pair of 813's.

It has an input for 'bias" that is labeled "9V".  This goes to the Stancor A-4765 transformer. 

I am still working on a schematic for the RF deck.......

Thanks

Rich

[WBear2GCR]

A few questions.

Why is there a pot across the filament line with a meter connected to the wiper going to ground?
What is the value of the pot?
What is the B+ voltage on the 811As?
How much bias does the "book" call for to bias 811As properly given ur B+ voltage?
You show drive as "500 ohm audio" - what is your proposed source for "500 ohm audio"?
How much voltage swing and current are needed at the "500 ohm audio" input point?
How much or what are the drive requirements for an 811A in modulator service?
What class are you running the 811As in - B1, B2, AB1, AB2?
How much swing do you have to generate at the secondary of the mod iron to achieve 100% modulation given the design parameters of your proposed final? And, will this modulator design give you that?

Also it is generally useful to use a standard transformer symbol in the schematic, and a side frame showing the pinouts if needed.

 

             _-_-bear

PS. there are many schematics/articles in print for 811A modulators, have you referred to them? Definitely some in old handbooks... etc.

[The Slab Bacon]

And......er....A... furthermore............. a pair of 811s would be kinda marginal at best to modulate a pair of 813s at full strap!!

[W7SOE]

Thank you Bear!  All good questions, I look forward to learning all about this.

Why is there a pot across the filament line with a meter connected to the wiper going to ground?
That is an adjustable wire-wound resistor, I'll have to check on the value.  So it is basically fixed.  Purpose?


What is the B+ voltage on the 811As?
That comes from the modulator PS, adjustable from 1KV to 1.6 KV (unloaded).  Not sure what it will be loaded.....

How much bias does the "book" call for to bias 811As properly given ur B+ voltage?
I will have to investigate, thanks for the direction...

You show drive as "500 ohm audio" - what is your proposed source for "500 ohm audio"?
Plan du jour is to use my Viking II as both the RF exciter and audio source.  The JVII manual describes how to extract modulated audio at 500 ohms.

How much voltage swing and current are needed at the "500 ohm audio" input point?
No idea...

How much or what are the drive requirements for an 811A in modulator service?
No idea.  Where do I look for this information?

What class are you running the 811As in - B1, B2, AB1, AB2?
I am not sure.  I understand what these are but how do you know by looking at the hardware?

How much swing do you have to generate at the secondary of the mod iron to achieve 100% modulation given the design parameters of your proposed final? And, will this modulator design give you that?
Perhaps that will become more apparent after I finish the RF deck schematic.  The B+ supply for the finals is also adjustable to ~1.6KV unloaded.  Perhaps I can reduce the final B+ if I am not getting 100% modulation..

Thanks so much for the help.

Rich

[W7SOE]

Oh yeah, I would have put in a proper diagrams for the mod and interstage transformers but I don't have that information.....

Rich

[The Slab Bacon]

that is pretty much a classic "class B modulator" schematic from one of the older ARRL handbooks. It looks like the primary winding on the mod tranny is in series with the secondary in parallel. This is common for 2 of something modded by 2 of something similar.

The driver transformer is one of those nice stancor multi tap /variable ratio driver transformers. I have several of them. The 2 wires marked bias in the drawing should be twisted together as they form the secondary center tap, and the biass applied between them and ground.

0-9v is pretty common for 811As depending on what the plate voltage is that they are seeing. They are high Mu triodes.

A JVII will produce WAY more audio than you need to drive those 811s. Keep in mind that they are high mu tubes and will only require a few watts of audio to swing the monky.

And the variable resistor between the filament leads to ground is for balance and hum reduction, kind of an artificial center tap for the filament transformer.

                                                     The Slab Bacon

[Steve - WB3HUZ]

Thank you Bear!  All good questions, I look forward to learning all about this.

Why is there a pot across the filament line with a meter connected to the wiper going to ground?

This goes through a meter (measures the 811 plate current). My guess is the pot is set for equal voltage either side of center, since the filament transformer is not center-tapped.

That is an adjustable wire-wound resistor, I'll have to check on the value.  So it is basically fixed.  Purpose?


What is the B+ voltage on the 811As?
That comes from the modulator PS, adjustable from 1KV to 1.6 KV (unloaded).  Not sure what it will be loaded.....

How much bias does the "book" call for to bias 811As properly given ur B+ voltage?
I will have to investigate, thanks for the direction...

811A's can be run zero bias up to 1250 volts. I think -4.5v is specified at 1500 volts.

You show drive as "500 ohm audio" - what is your proposed source for "500 ohm audio"?
Plan du jour is to use my Viking II as both the RF exciter and audio source.  The JVII manual describes how to extract modulated audio at 500 ohms.

How much voltage swing and current are needed at the "500 ohm audio" input point?
No idea...

How much or what are the drive requirements for an 811A in modulator service?
No idea.  Where do I look for this information?

What class are you running the 811As in - B1, B2, AB1, AB2?
I am not sure.  I understand what these are but how do you know by looking at the hardware?

How much swing do you have to generate at the secondary of the mod iron to achieve 100% modulation given the design parameters of your proposed final? And, will this modulator design give you that?
Perhaps that will become more apparent after I finish the RF deck schematic.  The B+ supply for the finals is also adjustable to ~1.6KV unloaded.  Perhaps I can reduce the final B+ if I am not getting 100% modulation..

Thanks so much for the help.

Rich

[W7SOE]

Hmmm, looking at the 811a spec, how would I know that it requires zero bias up to 1250V?

If I am going to use it with zero bias does the bias connection go to ground as it is the center tap of the driver transformer (thanks slab)?  What would be a good method if I decide to add the 9V bias for higher voltages?  For example what kind current would I have to supply?

Thanks for the patience guys.  Learning.....

Rich

[W7SOE]

Ooops, there it is.  under "single-tone" modulation.

rich

[Steve - WB3HUZ]

Yup, just ground the center-tap of the driver tranny.

[WBear2GCR]

The folks on here are both knowledgable and forthcoming...

Fwiw, I was hoping that Rich would go forth and read the handbook(s) and other sources to find the answers!

But...

Rich, the answer to the last question(s) is to be found in/on the spec sheet for the 811A, which can be had in a transmitting tube handbook, or these days online. Download it and the one for the 813.

At least with the spec sheet you can set up the tube to run within standard factory specifications, which no matter what is safe wrt blowing up toobes and long life.

You did not say if you had looked at handbooks and articles on 811 modulators or not?? I'd say that is a very good idea.

Also, consider putting a stage in front of the 811s to permit line level to drive the rig?
The traditional set up is/was 2A3s or 6A3s, but nowadays you can use things like 6V6s or 6L6s or similar tubes. It is fine to strap them for triode to avoid the extra screen supply.

Again, look online and in the old handbooks for many examples of this same basic modulator - much is explained in those old articles and books.

It probably goes without saying but the VOLTAGES in the rig ur thinking about building are DEADLY x___x , so be sure you understand proper precautions when working with a real rig with these sorts of voltages (and current of course) present.

                   _-_-bear

[W7SOE]

Thank yo Bear.  Yes, I will RTFM and not bother y'all any more until then.  I just got the RCA TT5 tube manual and an ARRL handbook from 1962.
Thanks again.

Rich

[WBear2GCR]

No no... don't get the wrong idea here!

It is just best to have some references to read, that's all.
Ask away!

          _-_-bear

[k4kyv]

As modulators, high mu triodes like the 811-A run in class-B at or near zero-bias.

The tapped resistor in the filament line should be low resistance, something in the order of 10-20 ohms.  It doesn't have to be variable, a pair of resistors of equal value wired in series would work.  Apparently, the resistor is used because there is no midtap on the 6.3 volt filament transformer, which is not uncommon.  The resistor should be low enough in value to have negligible drop at the cathode current, but high enough not to substantially add to the load on the filament transformer.  20 ohms, for example, would pull only about 300 mills of additional current from the transformer, but @ 250 m.a. of cathode current, the drop would be 1.25 volts.  Effectively, the two sides of the tapped resistor are in parallel as cathode current dropping resistors, but in series across the filament winding.

Many pre-WW2 broadcast receivers used an identical scheme for the directly heated filaments on the audio power output stage, using tubes like the 45, 47, 71-A, etc.

Check the tube charts.  811-A's can run at zero bias up to about 1000 volts, but @ 1250 or higher, they require a few volts bias to bring the static plate current down within ratings.  I do the same thing with the 805 modulators in one of my homebrew transmitters.  I use a solid state bridge  rectifier across a small filament transformer, and use a low-voltage electrolytic filter cap, and a small wirewound pot (50-100 ohms? - forgot) to pull off the bias voltage.  The internal resistance of the supply is low enough not to have to worry about regulation.

The modulator should be OK for a pair of 813's if you don't load them down enough to exceed what the 811-A's are capable of modulating.  They should do OK up to about 500 watts DC input.  Running the 813's  conservatively will prolong tube life.

The 811A's should be driven by an audio source with low internal resistance.  A 10-15 watt solid state amplifier should be OK.  If you use a tube amp, use a pair of 6B4G's or 2A3's, or a pair of 6L6's with negative feedback.  Without the feedback, beam power tubes make very poor class-B drivers.  A pair of 6L6's can be triode connected by tying the screen to the plate, but the internal plate resistance is about double that of 6B4's or 2A3's. If you plan to use a tube type driver,  better results would be obtained with a push-pull plates to push-pull grids class-B driver transformer.  The driver xfmr is the weak link in the chain of most tube type class-B modulators, since it must maintain the low internal resistance of the driver stage.  A 500-ohm output transformer feeding a second 500-ohm input transformer doubles the leakage reactance and internal resistance and other losses in the transformer.  This effectively is like adding resistors in series with the grids of the tubes, and the varying modulator grid current over the audio cycle distorts the audio waveform at the modulator grids, and results in increased audio distortion at the modulator output.

There are a number of solid state circuits available for driving the class-B grids directly without a transformer.  Also, there is the Gates transformerless direct-coupled cathode follower circuit used in their post-1960 line of 250-1000 watt broadcast transmitters.

[The Slab Bacon]

Bear, If he is using a JVII and that strapping driver transformer, he doesnt need any more amplification before hitting the grids of the 811s, he's allready got way more drive power than he needs. It may be a 500 ohm input, but it is allready way way more than line level.

Rich, an easy way to tell if you need more or less biass is to see what the resting plate current is for the modulators. If it exceeds the rated zero signal class B plate current rating, then you need more biass. that simple!! for Zero biass operation just ground the center tap of the driver secondary. As far as the biass supply goes it has to be capable of the required bias voltage at whatever the maximum signal grid current rating of the tube is.

                                                      The Slab Bacon

[N3DRB The Derb]

and the current draw is supa low. I used battery bias on my 805's in my HB rig; a cat o 9 tails ever ready.

I'm worried he will blow the grids of teh 811's out of place. 

[N3DRB The Derb]

BTW, have the audiophools latched onto the 805 yet? You could still get them cheap when I built my rig. All of them wanted 810's at the time, to have more 'tessitura' I guess.

I love teh 805. Bitchin modulator tube. Monkys start to swing when you got a pair of them.

The plate voltage on the 813's should be lowered. The 811's should be able to take 1750 with a goodly amount o bias. exceeds the ratings but they will take it at 50% duty cycle. Must swing munky to strap.

[k4kyv]

YEP, 805s are sky high now, the market is so good that the Chinese make them now.

The 838 is identical to the 805, except that the plate lead comes out of the base instead of a separate plate cap.  I have some of both, and the plate structures appear identical.

The plate cap allows the voltage rating to be higher than that of an 838.

[WBear2GCR]

Don,

Very similar, but a slight diff when it comes time to drive them. Iirc, the 805 drives easier.

The one's on my shelf are waiting for a good application!

              _-_-bear

[Steve - WB3HUZ]

Ask away. But truth be told, you'll learn more reading books than asking us know it alls!! ;-)

No no... don't get the wrong idea here!

It is just best to have some references to read, that's all.
Ask away!

          _-_-bear

[The Slab Bacon]

Not to mention the fact that most on here are old enough to fall into the category of 'forgot it all'.

Iz it the memory that goes first or the hair?? I forgot