new to me 811A modulator project

[kb3ouk]

Just got this off of ebay. Its a little rough. The mod iron is an Inca N-21, modulators are a pair of 811As. So far I can tell it does need a pair of 811s (one had a bad filament, the other is questionable), new filter caps (one is a GE pyranol cap marked 8 muf 1000 volts that looks ok, the other is unmarked and rusty and looks like it seeped out around the insulators at some point in its life), and some wiring fixed. I drew out a schematic, my guess is the thing wasn't built for much power, the HV transformer is an 800 volt center tapped unit, the only marking I can find on it is TX-4247. No clue what the second filter cap is or what the filter chokes are, both are Thordarson chokes I know. Also, I have no idea whats up with the mica cap across the primary of the mod iron, and my guess is the variable resistor must be the bleeder (its a huge wirewound variable one). I'm looking to use it with the 814 RF deck that I'm gathering parts to build, I'm wondering if I could power the 814 off of the same power supply? I'll post pictures later. And if you are wondering what is up with the 5R4's filament transformer circuit, that transformer is actually a 6.3 volt unit and the pot must've been used to drop the primary voltage down so that it only puts out 5 volts.

[N2DTS]

Are you even going to get 700 volts out of that thing?
I doubt it.
811's like 1200+ volts on the plates.
320 ma on peaks with a high vacuum rectifier is going to suck, solid state or MV rectifiers would be better with less voltage drop, I would scrap the entire thing.

For a modulator, you want a stiff 1200 to 1500 volt supply.

[w7fox]

You should get around 150 watts of audio as it stands, don't know what your plans are.  The cap across the mod. transformer is probably intended to cut high audio frequencies.  Good luck.

73,
Chris

[w8khk]

Your schematic is also missing a center tap on the secondary of the driver transformer.  This should be grounded if running the 811As at zero bias, or connect to a negative bias supply if running higher plate voltage.

[kb3ouk]

The biggest thing I wanted out of it was the plate and mod iron. What I might end up doing is solid state the power supply and replace the 811s with 809s, which is what I originally wanted to do anyway was a pair of 809s modulating an 814 until I found this thing. I was only looking to build a 75 to 100 watt transmitter and that's all the more audio power I need to ask from this. And I don't remember seeing a center tap on the driver, which that may not be staying in there anyway.

[kb3ouk]

Here's the pictures.

[kb3ouk]

I'd like to be able to use the existing plate iron and mod iron, but maybe change the tubes out to 809s. I'd also like to use the modulator plate supply for the final too (yes I know, probably not the best solution but it's far better than running the crap I have been for the last several years). I'm looking to get about 1000 volts from it, which means the current filter has to go. I'm thinking about going with a full wave solid state rectifier and maybe a cap input filter, good idea or not? As it is right now that would give me a little over 1100 volts I believe, but the transformer has 3 taps on the primary, 105, 115, and 125 volts, our line voltage here is about 117 to 119, so I think if I put it on the highest tap then that should drop the voltage down some into the 1000s. Also thinking about getting rid of the existing driver transformer and putting a backwards audio amp transformer in there and then driving it with an amp I already have that has mic inputs on it. First thing I want to do is strip that power supply section off of there and see how much room I have to work with. Going to a solid state rectifier means I can also get that mess with the pot and 6.3 volt transformer out of there and that should lighten the thing up some, as it sits it weighs like 77 pounds and is a pain to move around, I'd prefer to have a little less weight since I imagine it will get moved a lot. Also want to get rid of that cap on the mod iron, I looked and its a .002 mfd mica.

[VE3AJM]

Your schematic is also missing a center tap on the secondary of the driver transformer.  This should be grounded if running the 811As at zero bias, or connect to a negative bias supply if running higher plate voltage.

Bias can also be applied if needed through the centre tap of the 811a filament transformer using a zener diode etc. A small voltage above ground there appears as a negative voltage at the grids of the tubes. Try and locate any specs on the plate transformer and the Inca mod iron. Their ratings might limit what you want to do with this.

Looks like the modulator has possibilities. My Clegg Zeus modulator/PS only puts 900v on a pair of 811as. They work fine of course. Good luck with it.

Al VE3AJM

[kb3ouk]

I'm still trying to figure out who made the plate iron, the only markings on it are TX 4247 on the top under the terminals and then the terminals are marked 0, 105, 115, and 125 on the primary side and 800, 0, 800 on the secondary, so as it is right now the thing probably does only make about 700 volts, if not less than that. What I really want to know is what that iron can handle. Should I keep the choke input filter, or could I switch it out to a cap input? Either way I'm going to probably solid state the supply. Also looking for specs on the Inca N-21, I could at least put a voltage across the windings and try to figure out what the turns ratios are, but that doesn't help with current ratings, how much audio it can handle, etc. I might keep the 811s too, if I switched to 809s they would definitely help lessen the load on the iron, but the 811s wouldn't be pushing it as hard either if I was only asking 100 watts or so from them.

[N2DTS]

I would stay choke input, solid state the supply, and change the mod tubes.
With zero bias triodes, I think they do not work as well at low voltages because you would need to bias them ON more to get it clean.
At 700 volts, you will have almost no resting current.

Fine iron for a 100 or 150 watt rig running both off the supply.
Does the choke have any rating?

A 4D32 would be a good tube to modulate, or a pair.
 

[kb3ouk]

Go with the 809s for the tubes? Not sure what the ratings are for the chokes, I do have part numbers though. Both are Thordarsons, T13C30 and T44918. Maybe someone has a catalog out there that lists those. Also want to change the caps, one shows signs of a slight leak around the insulators, not sure what value it is. The other is what I said earlier, a GE pyranol cap marked at 8 muf 1000 volts. It's not leaking so might be good for now. The final I was looking to use was an 814, I might build a seperate 1000 volt plate supply for it and run it at about 90 watts. 809s are supposed to do 120 watts with zero bias at 700 volts, so I might be able to push the 814 to 1200 volts and get about 130 watts out of it. That mod iron should be able to handle the plate current of the 814, I would probably run it around 140 ma.

[KA2DZT]

That Inca xfmr is probably 75-80 yrs old. hope it's still good.  For a modulator you can use a single choke supply, no need for two chokes.  The Thordarson 13C30 is a standard catalog model but the other one with the five digit part number is a special, no way to tell what its ratings are.  Same is true for the plate iron TX-4 digits, special.

I'll try to look up the ratings for the 13C30 later today.

Fred

[kb3ouk]

Yea I am going to check the mod iron out later. Evidently I'm slightly dyslexic, that plate transformer says XT 4247, not TX. Then what I might do is just pull the leaky cap and the choke before it and just run the first half of the filter

[kb3ouk]

Stripped everything off of the chassis this afternoon. The mod iron appears to be OK. Ended up with a large pile of parts that aren't going back on, like the rectifier socket and filament transformer, driver transformer, the pot used to control the filament voltage, and lots of wire. Once I get the chassis and front panel fixed up I'm going to remount the plate and mod iron, filter choke, modulator filament transformer, and start rewiring everything. Now the only problem is I don't have the RF deck even started yet so I'll have the modulator done but no way to test it.

[VE3AJM]

Looking at your schematic, the fact that only a single 5R4 was used in the original set up HV supply, might indicate that the plate transformer is not rated for that much secondary current, and then again not. Same with the mod transformer. Not rated for much voltage and current. That was a strange/marginal arrangement. You could go cap input or choke input since the supply would only be used for the modulator. FW, FWB etc to get various voltages after solidstating the supply and adding some surge protection. And the primary has taps on it as well. In any case the VA rating of the transformer remains the same whatever it is.

The Inca modulation transformer data might be unobtainable as to voltage and the max dc current through the secondary winding. Theres almost nothing worthwhile online with any details about any Inca transformers. As you say, the impedance transformation can be derived on your own easily. If you're certain that the modulation transformer is good, I would protect it by insulating it above chassis ground, and/or you could use a modified heising arrangement where there would be no DC on the secondary winding, to be on the safe side.

Al VE3AJM

[K1JJ]

The mod iron appears to be OK.

Now the only problem is I don't have the RF deck even started yet so I'll have the modulator done but no way to test it.

No problem.  It's easy to test the modulation deck by substituting the RF deck with a 5K - 7K power resistor. (or calculate whatever value  the RF deck impedance would be under full operation)    You can sample some audio off the power resistor(s)  for the scope with a HV divider or HV probe and run some tones through to see how it looks.


T
  

[kb3ouk]

I was thinking of going with modified heising, finding a choke is the hard part. I want to run the final at 1100 to 1200 volts at 140 ma, so my calculations say I'll need at least a 35 henry choke.

[w1vtp]

Now that you have the iron stripped off, how about better quality photos of the connections on the modulation transformer?  How much does the modulation transformer weigh?   If the transformer is not cross-laminated why bother looking for a modulation reactor?  Use it as is.

So, 1) Better quality photos of the connection (use a better light source or turn on the flash so that Luma 635 has a better chance), 

2) How much does it weigh?

As far as the schematic is concerned, just start over with a proven design out of a handbook.

Al

[w1vtp]

I did a rotation of what I think is the power transformer.  At least I think, in spite of the blurry pic, I see a connection for 115 volts.  I tweaked the rotation and lighting which begs the question what about the other transformer?

Back to this transformer: Be aware that the center tap on this transformer is meant to be at or close to ground potential.  The only real choice for this transformer is center tapped to ground full wave circuit.

[steve_qix]

Depending on your goals, the _ABSOLUTE_ number 1 thing you can do get close to, if not achieve a really low distortion system is to eliminate the driver transformer.

Some are good - and the audio may be "acceptable".  I've personally have never owned one that is even remotely close to good.

The test?  Put a 400hZ triangle wave, modulated 100% and look to see how good it is when demodulated with a local, good quality detector.

I have achieved the good triangle wave "test" with tube transmitters with modulation transformers.  The driver was a direct coupled driver.  That is critical.  Also, this enables you to put negative feedback around the system (including the modulation transformer) and boy does THAT help a less than perfect mod transformer.

But it all depends on your goals for the project.  Anyway, it sounds like a fun and challenging en-devour !

Regards,  Steve

[kb3ouk]

I did a rotation of what I think is the power transformer.  At least I think, in spite of the blurry pic, I see a connection for 115 volts.  I tweaked the rotation and lighting which begs the question what about the other transformer?

Back to this transformer: Be aware that the center tap on this transformer is meant to be at or close to ground potential.  The only real choice for this transformer is center tapped to ground full wave circuit.

Yes that is the power transformer, there are 4 connections on the primary side, marked 0, 105, 115, and 125. the secondary is 800, 0, 800. I'm going to stick with the full wave choke input circuit, just solid stated. The pictures are poor because they were taken on a cell phone with no flash (best camera I have unfortunately).

Now that you have the iron stripped off, how about better quality photos of the connections on the modulation transformer?  How much does the modulation transformer weigh?   If the transformer is not cross-laminated why bother looking for a modulation reactor?  Use it as is.

So, 1) Better quality photos of the connection (use a better light source or turn on the flash so that Luma 635 has a better chance), 

2) How much does it weigh?

As far as the schematic is concerned, just start over with a proven design out of a handbook.

Al

The mod iron weighs around 20 pounds. As far as I can tell, the primary is a single winding with four taps. The secondary appears to be two seperate windings, each winding has 3 taps. Taps 1, 4, and 5 are one winding, and 2, 3, and 6 are the other. The transformer is potted, I had the top cover off this afternoon.

I've already done a good bit of cleaning up, the only things I am keeping are the mod iron and plate iron, filament iron for the modulators, one of the filter chokes (probably the Thordarson 13C30, if I can find the specs on it), and the bleeder resistor. The mica cap (it was a .002 mfd one) is gone, the 5R4 is gone, along with its filament supply, the second choke and cap are going away as well, along with the driver transformer. That driver is a mystery too, it is also an Inca piece, but the secondary side of it has 4 connections, two of them were tied together and grounded, I guess that's the center tap. I missed that when I drew the schematic out originally but noticed it yesterday when I tore everything down yesterday.

Depending on your goals, the _ABSOLUTE_ number 1 thing you can do get close to, if not achieve a really low distortion system is to eliminate the driver transformer.

Some are good - and the audio may be "acceptable".  I've personally have never owned one that is even remotely close to good.

The test?  Put a 400hZ triangle wave, modulated 100% and look to see how good it is when demodulated with a local, good quality detector.

I have achieved the good triangle wave "test" with tube transmitters with modulation transformers.  The driver was a direct coupled driver.  That is critical.  Also, this enables you to put negative feedback around the system (including the modulation transformer) and boy does THAT help a less than perfect mod transformer.

But it all depends on your goals for the project.  Anyway, it sounds like a fun and challenging en-devour !

Regards,  Steve

At the moment it turning out to be more of a learning experience at building something (or in this case, completely reengineering) so I am trying to keep it as simple as possible. I already have a decent amp that I can use as a driver, so I'm just going to get a small push pull output transformer and use that backwards connected to drive the grids of the 809s.

[N2DTS]

You do not need to sound like a broadcast station to have fun.
I thought the rule was 10 watts per pound for mod iron.
20 pounds would be 200 watts?

Depends on how its made though, the cvm-5 had cast iron end bells....

 

[kb3ouk]

You do not need to sound like a broadcast station to have fun.
I thought the rule was 10 watts per pound for mod iron.
20 pounds would be 200 watts?

Depends on how its made though, the cvm-5 had cast iron end bells....

 

That's about what i figured, and i'm only asking about 100 from it. The case is just thin sheet metal with a removable top cover, so the weigh is mostly transformer plus the potting material. I don't really intend on sounding like a broadcast station, as long as its clear and intelligible sounding and can make 100% modulation, thats fine with me. I do intend on using a negative peak limiter, but more as a safety measure for the mod iron.

[steve_qix]

A good idea on the neg peak limiter.  Use the 3-diode neg peak limiter.  That is the best one, and guarantees a load on the mod xfmr at all times (well, unless the final fails, but that's another story!).

[KA2DZT]

The ratings for the Thordarson T-13C30 choke;  swinging choke, 8Hy - 25Hy, 150ma - 0ma, 200ohms, 1600 RMS TEST Voltage.  Choke is no good for HV, it's a choke for voltages under 500VDC.  I would use a different choke.

Fred