The AM Forum

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?

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 (http://amfone.net/Amforum/index.php?topic=26630.0)

Don

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

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.

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.

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

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 (http://amfone.net/Amforum/index.php?topic=30337.0)

Don

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.

                                 _-_-bear

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

---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.

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
Wd5JKO

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

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

I appreciate this info.

  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
Wd5JKO

Build it and they will come.  ;D  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. 8)


Phil - AC0OB

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

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.

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. :D

                                      _-_-

...or keep the 6CA7's and the apache mod xfmr, and throw away the 811A. ;) 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).

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.

Why not give some consideration to the sweep tubes??
Substantially more monkey swing in a similar size bottle... really.

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. :-[

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 (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.

  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

KB1VWC

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.

I'm a fan of 6DQ5's so what do you think?


Phil - AC0OB

By now, anyone that doesn't like sweep tubes as modulators should be tagged as a "Sweep Tube Denier"!

That's STD, for short. :P

6DQ5's, 6CD6's or 6CB5's are pretty much interchangeable in the "ground the control grid feed the screen" type of modulator. There are some slight pin-out differences, but wired correctly, you can interchange the three at will.  It's whatever is cheapest or what you have on hand. The 6CB5 has the highest plate dissipation. I used them just because I had a large number of them. I did melt a few until correct bias and proper grounding techniques took over. If you could find them, the 8236 graphite plated 6DQ5 would be nice to use, but they seem to go for stupid money. Or if your not trying to stuff them into an Apache, go for the big boy 6LW6's. All are octal socket beasts  that make copious amount's of audio.


  Steve

KB1VWC

I like that and may have to use it the next time I come across an Anti-Sweep Tube User, or "ASTU."  ;D

Phil

I use Svetlana EL34s as well. They are fine but I think no longer made. I suggest ceramic sockets above 650V based on experience.

There are cheaper tubes like the 6CD6/6BG6 plate cap included, that will take 800V all day and need only 150V max on G2 to run oodles of plate current. You might want 4 in pp-par for 100W+ from them, but they are much more durable than 807s for audio, and have larger cathodes, and work great. With 11K as the load, high voltage tubes are a must so sweep tubes should be considered. No need to use the special class B arrangement on the 6CD6, AB1 driving the control grids will run as much plate current as you want, see the curves. They are also not designed for high G2 voltages. Will they take it? no idea!

There is the 120W "special class B arrangement" for 807 and similar types where the screens are treated as the driven grids, with the control grids lightly driven via 20K resistors, but you need high voltage drive and there is grid current, so it complicates things. And it is pushing the tubes.

I'd vote for non-zero-bias tubes for the class C side like three 6146s or one or two 4D32s.

I like the 4D32 from the specs, seem very nice rugged tubes. But hardly available in Europe. When I had a few, for sure I should use them. Like to make a simple 2 stage AM transmitter with cathode modulation in the final (so no big modulators, just a small FET). A 813 may make 100 Watts carrier, but I can't get HV transformers here in Costa Rica
I investigated a little about the EL34, and it seems that the 6CA7 was introduced in the USA by Svetlana as a copy of the EL34. Now there are many copies even constructed as beam tetrode with that name. So I lost track but it coincides with the experience that the older tubes are more reliable. Problematic 6CA7 tubes may be bad or strange copies and no real EL34

What are the popular/common power vacuum tubes in Europe and in Costa Rica? I have no idea what can be bought in Costa Rica.

Hope customs/crooks don't mess you up again.

In Europe the most popular AF power tubes are EL34, KT88, KT77, 6L6 and less the 6V6  The most popular smaller tube is the EL84. Vintage tubes are mostly English, German , Dutch tubes and USA tubes
For RF the 6146 and the 807 are very popular. But nowadays the Russian tubes are used a lot for high power applications. For ham stuff I used the 4-400 (2 in parallel, grounded cathode), the 4CX250, the 4X150 and the TB2.5/300 (two in parallel)
Here in Costa Rica you simply can't buy electronics at all. You have to drive 5 hours to San Jose to find a resistor (if you are lucky). All stuff has to be bought from Ebay, Aliexpress etc. and that doubles the price, (tax and transport). Boatanchors are impossible due to transport cost. And new stuff can be stolen by the customs in Puerto Limon. There are possibilities to buy ham transmitters, but all imported from the USA, so you have to add approx 50% to the standard price. Even coax is not available, only for TV applications. I needed Sprinkler wire to connect my antenna rotor, not available in Costa Rica. So I bought it at Amazon and had it sent to my neighbor that lives 3 months in Costa Rica and 3 months in Arizona. When he returns, he takes it in his luggage.
So for non-Chinese materials you need someone that brings it when visiting Costa Rica or sending it.
For hams, a little difficult if you don't bring all your materials including your junkbox with you.

One thing I have noticed , is that most of the suggestions call for running the given tube at it's/their maximum limit.  Why? Originally I would have been happy with a 100-110 watt carrier with modulation. Just something that I built, that I can check into nets with.

Now I have decided to use the 811A all right, but as is more traditional - as modulators,  and a tried and true 4-125A TX design from the west coast Radio Handbook volume 14 (1956) that brings me back to where I was 4 years ago, before I bought the Gates BC-500K. (I still dont have 240 vac hooked up downstairs yet).

Again, dont get me wrong I appreciate all the comments and suggestions.  MANY heads are much better than my one.  Happy Holidays.
Bob W4RFM

Efficiency.

From the tube data sheet, the 4-125A  will provide 300W output for 380W input. That's 78.9% efficient. At 300W input, you'll have 225W output. for an efficiency of 75%. That doesn't sound like much until you look at the plate dissipation. 75W less output but only 5W less plate dissipation (80W vs 75W Pd) so the tube runs practically as hot either way.

Again from its tube data sheet, a 4-65A, as an example, will provide 225W output for 270W input. An efficiency of 83.3%. So, only 45W Pd vs 75W Pd for the 4-125A at the same (225W) output level. But even the 4-65A isn't magic. At 70W input it provides 50W output for an efficiency of only 71.4%

Thus, at an output of 225W the 4-65A would be a better choice than a 4-125A as it wastes 30W less power on its plate and 11.5W less filament power. That's 41.5W less wasted electricity for a transmitter with 225W output.

If that's not convincing enough, running a 4-125A at 100-110W output and a plate voltage limited by what 811A's can handle (1500V) so the apache mod iron can still be used, you're probably going to see much worse efficiency. The 4-125A data sheet doesn't even go there.

The good news is: the 4-65A data sheet does go there. At a plate voltage of 1500V the 4-65A will give 145W output at an efficiency of 80.5% Stellar efficiency. Still better, 4-65a's can be had for small money.

As far as tried and true designs, nothing comes close to an 813 modulated by 811A's. But not with an apache mod xfmr and not at 100W output.

In the end the choice is yours. Transmitters are fun to build. Transmitters that work well on the air are a compliment to their builders. Not a man among us doesn't want you to see you succeed in this endeavor and give you great on-air reports.

All the best,
Don

P.S. If I had a Gates BC-500K sitting here I'd have the shack wired for 240 volts before you'd finished reading this post! ;D

Nice info Don!

Seems to me that plate impedance plays a role in tube choice IF you have a fixed mod
iron to use. The sweep tubes will look like a much lower Z than something like the 4-65
or 4-125, also the sweep tubes will run on lower plate voltage... I think the 811 will be
pushed rather hard above ~1200vdc B+... once I start looking at the 811 my eye wanders
to the 805/838 or an 813 strapped for triode (or not)...

The ability to modulate over 100% positive (using a 3diode limiter, or other negative
peak limiter) is a big plus. FYI and imho.

                   _-_-bear

Lots of things to like about it!

Agreed. Impedance matching will maximize the transfer of power. But this is of little consequence if there isn't enough voltage swing to achieve 100% (or more) modulation. So, the turns ratio of the mod iron (in conjunction with the voltages applied to modulator and RF amp) needs to be within a workable range. To me, that's the first step. There are many others.

We can certainly go far afield with this discussion, but I'm trying to limit my comments to the OPs original and subsequent questions. This isn't always easy to do as we each have our own predilections as to how something would/should/could be done. To me, it's wonderful that everyone can create something different - something unique. I'm trying help the OP facilitate that - to provide a framework where workable solutions can be found.

Pushing the margins of a thread is bound to occur and isn't necessarily a bad thing. Personally, I'm trying to contain my enthusiasm. ;D

Hey Don!
Yeah, but... by the same token you could have ample voltage swing, but if you can't transfer the
watts, it won't matter either! In the end you need enough of both, or extra of one or the other
but not less than needed. That's why the sweepie toobies are inviting, they'll run on a wide
range of B+ voltages and they'll swing stupid amounts of peak current.

Sure wish the Chinese or Ruskies would put one of the big ones back into production!!


                 _-_-bear

Bear,

I totally agree.

And, just today, I found a pdf file (attached) showing test results of the little 6DQ6A. At 560V a pair has 86W output in AB1. Then wouldn't a quad produce 172W? So who needs big ones?

Don

I forgot to add that I recently Ebayed a mod transformer that has a 15K primary @ 250 ma, with multiple secondaries from 18K down to 8K, @ 150ma, states 126 watts continuous, and a peak working voltage of 2850, sooo.....
that plays into the 4-125 / 811A scenario. (And others of course).

Yes, I see that transformer in the sold listings on ebay. Looks like a good unit for what you're doing. And you got it at a very good price too! That will give you much more versatility than the apache unit. Now you're cookin'!

So, are you going to split the supplies? I took a look at the 4-125 circuit. Clean design. Needs a 350v bias in addition to the HV. My exciter needs the same voltage. 

I really like this thread. Been looking for a single tube RF deck to match with the 811/Art-13 mod transformer parts I have. Your modulator is similar.

John

I just took some time to re - read all the comments on this former plan of mine.  I appreciate all the input. I feel funny about asking for help, because as a 40 year broadcast engineer of AM's & FM's I "should" know all this, but in my defense, the boxes I worked on did not have to be re designed or re built, they just ran...I knew enough theory to figure out what was wrong, but that was/is my limit.
Thanks again to everyone, there is a world of knowledge available on this forum.

Sweep Tubes; Attention Don, Phil, and Steve, I am no longer in denial, I have been raiding Ebay for sweep tubes, and I now have a collection of NOS 8x 6CD6, 2x6DQ5, 2x6CB6, and for good measure a pair of new 6883's.  Now I am ready to build a modulator! :D