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811A transmitter




 
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W4RFM
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« on: November 13, 2018, 08:32:47 PM »

I need the collective input of the group. 
I have an Apache modulation transformer, (11K to 3K) and I have two new 6CA7's, (like the Apache came with) for modulators.  Using separate power supplies, I plan to run the 6CA7's at 750 volts, to develop 80 to 100 watts of audio, couple that to the Apache Mod iron, and modulate an 811A tube in the Class C r f stage, operating at 900 volts @ 250 mA, with a 70% efficiency giving me a 150 watt or so signal. Your thoughts?

Reference 6CA7 / EL34 tube AB1 P/P 725 Vdc @168 mA - 90 watts at 11,000 Zout

Biggest question, will an 811A be happy running at only 900 volts on the plate? Or does it care?
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« Reply #1 on: November 13, 2018, 10:54:18 PM »

The even bigger question is will you be able to modulate to 100%.

The Apache mod xfmr has a 3.66:1 impedance step-down (11k to 3K). That's a 1.91:1 voltage step-down (square root of impedance ratio). With 750 volts on the mod 6CA7 you may get ~1200 volts swing. Stepped down by 1.91:1 that's only 628 volts. You'll need 900 volts to hit 100% modulation.

I learned this here:
http://amfone.net/Amforum/index.php?topic=26630.0

Don
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« Reply #2 on: November 13, 2018, 11:29:04 PM »

The 811A really performs better at ~ 1300 volts in Class C.

The other question is, does the Apache Mod transformer have the insulation capabilities for 1300V with respect to ground? Does the Apache Mod transformer have the insulation capabilities for 900V with respect to ground?

Either way, I think I would insulate the Mod transformer.


With ~ 73% Efficiency, here are two possibilities:


AC0OB

* AC0OB 811A Class C Stages.pdf (144.09 KB - downloaded 19 times.)
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« Reply #3 on: November 14, 2018, 10:30:49 AM »

So if I am going to put the 6CA7's and transformer to use, it looks like I should build an "improved" Apache clone, maybe add a third 6146 (as Wireless Girl suggests), and run everything the way Benton Harbor planned it. 725V on the modulators and 600 on the 6146's. Yes, no?  I just want a little something to check into some nets with.
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« Reply #4 on: November 14, 2018, 11:09:41 AM »

Instead of three 6146 tubes, may I suggest a pair of 4D32 bottles?  They strap with moderate plate voltage!  You might also consider a cathode follower driver (6080 or 6AS7) for the modulators, and run them a bit into AB2.
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« Reply #5 on: November 14, 2018, 01:59:04 PM »

I like the 4D32 idea a lot..  Impressive little bottle.
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BOB / W4RFM
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« Reply #6 on: November 14, 2018, 05:21:20 PM »

With 725 volt on modulator and 600 volts on the RF final you should have an "effective" 1.58:1 voltage ratio. That calculates to about 734 volt swing - good enough to modulate the 600 volts. The impedance ratio of the transformer, 3.66:1, remains unchanged (it's strictly turns ratio squared). Keep this in mind when selecting modulator tubes.

The problem with the 6CA7 (and many other octal "audio" tubes) is the plate is on pin 3 and the filament is right next to it on pin 2. This may cause high voltage arcing if you go too high. Careful wiring and good tube sockets are recommended.

Consider using screen driven TV "sweep" tubes for modulators as was done by WA1QIX here:
http://amfone.net/Amforum/index.php?topic=30337.0

Don
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« Reply #7 on: November 14, 2018, 05:49:08 PM »

No, no, no to the 6CA7s.
They're not going to be happy with 750vdc on the plates and they're not going to swing 100 watts on
a good day going downhill - not for long.
Especially not the Chinese tubes of today.

A pair of 807s run in AB2 will run hot but do 100watts with 750vdc on the plates.
But that's too little, imho.

Better to run a pair of 811s in the modulators.
OR a big pair of sweep tubes that will not punk out when you need current drawn!

Possible a pair of 4D32 as modulators too.

I run a pair of 6DQ5 sweep tubes in my 3 tube 6146 RF section of my Viking Valiant and manage
a 130watt carrier with more than 100% positive peaks.

You probably want to run "Heising" and keep the DC off the mod iron.

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« Reply #8 on: November 15, 2018, 01:12:43 AM »

I like the 4D32 idea a lot..  Impressive little bottle.

Here is a potential Class C stage with the 4D32, a tough tube in my experience.

I have had zero problems modulating with my Svetlana 6CA7's at 700+ volts.


Schematic updated for a VpF of 740V.


Phil

* AC0OB 4D32 Class C Stage.pdf (115.29 KB - downloaded 40 times.)
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« Reply #9 on: November 15, 2018, 09:41:09 AM »

---No, no, no to the 6CA7s.
They're not going to be happy with 750vdc on the plates and they're not going to swing 100 watts on
a good day going downhill - not for long.---

I am quite surprised by this comment Bear. The EL34 or 6CA7 is spect. upto 800V at the plate and is one of the most used audio tubes in Europe with a very reliable and good reputation. I am interested where this negative experience comes from. I used them as well and and they never gave me problems. For high quality audio I prefer the KT77 in UL, quite similar but for reasons of linearity.
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« Reply #10 on: November 15, 2018, 10:22:21 AM »

My 2 ₵'s,

The 811a is not a good tube for Class C Plate Modulated Service since the grid has to swing too far positive to get the plate current near saturation. Something like an 812 is a better candidate. Either type would need neutralization.

As to the 6CA7, besides bottle fragility of some of the imported tubes, they have the pin 3 problem. The plate is at pin 3, and the screen is at pin 4. On pin 2 is one side of the filament. Then the path from pin 3 to chassis ground varies with socket type, but is often insufficient. The big ceramic Johnson socket is a problem with those clearances. Some other socket brands have made attempts to beef up the insulation path between pins to ground and adjacent pins.

I once made a 100W modulator using a pair of 8417's. Worked great except the big Johnson Ceramic sockets arc'd from pin 3 to ground until I spaced the socket below chassis level by about 1/8".

I once tried the Teslovak E34L bought through Penta Labs. This was a brute since it had the 6L6GC tri-metal plate. NO HOT SPOTS with moderate overload. Some of other brand EL-34's would go gassy with one iteration of a localized red hot spot plate.

Jim
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« Reply #11 on: November 15, 2018, 11:15:08 AM »

May be it is the brand of the EL34. I use in general Svetlana and never had problems, with 800V at the plate. But a pair of old Philips did perform as well without problems. I tried also Sovtec and didn't find a problem. When repairing guitar amps, I normally put a pair of Sovtec tubes in it and never got one back. We are using VERY many EL34 in guitar amps in Europe, and I know that there are quite a lot of inferior chines tubes around. Perhaps these spoil the name of the famous EL34
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« Reply #12 on: November 15, 2018, 12:23:17 PM »

Have you "hollow state" guys considered a more contemporary solution, namely a class D solid state modulator. There are a plethora of high power class D audio amp chips available.  The TDA8953 is but one of many.  It costs about $8 and can deliver up to 420W of mono audio if connected as Bridge Tied Load (BTL) with a minimal number of additional components.  This could then drive a mod transformer 8 0hms up to whatever ohms to get the required voltage for a series secondary feeding B+ to the Tube final.  There has to be some HiFi tube output transformers around to do the job.  Just a thought is all.

https://www.nxp.com/docs/en/data-sheet/TDA8953.pdf

73s  Nigel
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« Reply #13 on: November 15, 2018, 08:45:35 PM »

I appreciate this info.

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« Reply #14 on: November 15, 2018, 09:07:51 PM »

(1) So I just throw away the mod transformer I have, find another one that will match all these changes and start over.  thanks.

  I would not do that. Since that mod transformer is there, make a design that compliments the transformer capabilities, and turns ratio. Some others have already made that suggestion with a sample circuit.

Jim
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« Reply #15 on: November 15, 2018, 09:59:26 PM »

(1) So I just throw away the mod transformer I have, find another one that will match all these changes and start over.  thanks.



Build it and they will come.  Grin  Hollow-state, thermionic devices are much more interesting and challenging.

I have a couple of 4D32's if you need them, but no sockets unfortunately. I also have some 12BY7A's drivers if you need them.

If you can find some Svetlana or Phillips 6CA7's  they will modulate just fine. Cool


Phil - AC0OB



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« Reply #16 on: November 15, 2018, 10:55:55 PM »

Ahhhh...  the 4D32.  I can't say enough good things about them.  Here's a project that put out an easy 500 watts of carrier, 140%+ modulation, class C using four 4D32s in parallel - a PDM tube rig.  2200 VDC on the rig. Those tubes are like mini 813s.

I highly recommend this tube.

The 6LF6 sweep tube is also a contender.

Thread:
http://amfone.net/Amforum/index.php?topic=35596.0


* 4D32 PDM Rig.jpg (326.24 KB, 1280x960 - viewed 75 times.)

* 4D32 Complete PDM rig.jpg (333.8 KB, 960x1280 - viewed 61 times.)
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« Reply #17 on: November 15, 2018, 11:24:59 PM »

Go with the modulation transformer you have.  Use the 6CA7s you have.  Reduce the plate voltage a smidgen if you experience arcing between plate and filament pins at the socket.  No one will perceive any difference in signal strength if you drop the plate voltage to a safe level.

Grab a couple 4D32s. They will probably be easier to tame than a trio of 6146s.  Plan for good separation and possibly shielding between grid and plate circuits.  You might even get away without having to neutralize that screen grid tube.  You do not need 811s or 1000 plate volts...  You will be heard with this rig.

I suggest building modular, separate chassis for RF, Modulator, and Power Supplies.  Makes it easier to work with, modify, and extend over time.
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« Reply #18 on: November 16, 2018, 12:44:13 AM »

---No, no, no to the 6CA7s.
They're not going to be happy with 750vdc on the plates and they're not going to swing 100 watts on
a good day going downhill - not for long.---

I am quite surprised by this comment Bear. The EL34 or 6CA7 is spect. upto 800V at the plate and is one of the most used audio tubes in Europe with a very reliable and good reputation. I am interested where this negative experience comes from. I used them as well and and they never gave me problems. For high quality audio I prefer the KT77 in UL, quite similar but for reasons of linearity.

Older USA or Mullard tubes do better than the newer ones from different brands... the screens are the big problem, over current.
You need the max screen voltage and current to make max power... they melt down.

The post suggested 100 watts out of a pair. seems like more than they want to do.

My suggestion was maybe some sweep tubes since they can handle huge peak currents and smile. Cheesy

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« Reply #19 on: November 16, 2018, 08:39:38 AM »

So I just throw away the mod transformer I have, find another one that will match all these changes and start over.
...or keep the 6CA7's and the apache mod xfmr, and throw away the 811A. Wink A plate modulated high mu triode like the 811A wouldn't be my first choice.

A single 4D32 in the final would be hard to beat and would be a good match for that mod combo. A single power supply of 600-700 volts and it will work fine. Somewhat better would be separate supplies of ~700V on the 6CA7's and ~600V on the 4D32 (to improve the apache mod xfmr turns ratio for more headroom).
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« Reply #20 on: November 16, 2018, 10:28:27 AM »

Thank you everyone.  I do appreciate the input. I will do the 2x 4D32 thing with the 6CA7's.  Now to gather some pieces-parts.  Happy Thanksgiving everyone.
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« Reply #21 on: November 16, 2018, 05:20:05 PM »

Why not give some consideration to the sweep tubes??
Substantially more monkey swing in a similar size bottle... really.
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« Reply #22 on: November 16, 2018, 08:35:45 PM »

I maintain that a pair 6CA7's and apache mod iron will nicely modulate a single 4D32. You don't need two 4D32's.

The apache mod xfmr is rated at 250mA1 DC on the secondary. Draw much more than that and you'll turn it into doorstop that smells like the La Brea tar pits2. The 4D32 is rated for 600V@240mA=144W input, 100W output3. Two 4D32's would draw 480mA for 288W input, 200W output. You'd need at least 144W of audio to modulate them. A pair of 6CA7's are rated at 100W output maximum3

Result: With two 4D32's you won't be able to modulate to 100% and the 480mA DC will fry the mod xfmr. Embarrassed

Footnotes:
1. Harbach Electronics/Peter W. Dahl Amateur Transformer Database
2. http://articles.latimes.com/2013/oct/28/science/la-sci-la-brea-tar-pits-100-years-20131028
3. These numbers are from the respective datasheets.
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« Reply #23 on: November 16, 2018, 08:46:59 PM »


  I have to agree with Bear...My modified Apache uses triode connected 6CB5's ( I had quite a few) with 1200V on the plates to overcome the lousy step down ratio of the Apache's mod iron. The 6146's run around 800V with the stock Power supply solid stated. The 1200V comes from the same power supply with a steering diode to a cap input filter rather than the choke input that runs to the 6146's. I get 120-125% positive peaks, where a stock Apache has a tough time getting 100% positive. So far, no issues with the voltage breakdown of the mod iron. The 6CA7's are really being pushed at 750V with tube rectifiers. Plate caps out the top of the tubes, eliminate the socket flash over issues. Not hitting 100% negative also obviously helps. There are better mod iron ratio's out there. But if it is what you have, then there are a lot of creative ways to "make the best" out of that transformer.

  Steve

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« Reply #24 on: November 17, 2018, 12:47:43 AM »

...with 1200V on the plates to overcome the lousy step down ratio of the Apache's mod iron. The 6146's run around 800V with the stock Power supply solid stated. The 1200V comes from the same power supply with a steering diode to a cap input filter... 

With this evidence presented that the apache mod iron will handle 1200V, I would happily jump on the sweep tube bandwagon. Particularly when screen driven, they can make great modders and there are many to choose from. The 1200V to 800V ratio gives a very good effective voltage ratio of 1.27:1 with the apache iron. Safer yet, for the xfmr, would be 900V on sweep tubes and 600V on the 4D32. This could be done with the same single supply/steering diode trick.

Many a transmitter has been built around the mod xfmr that one has on-hand.
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