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811A transmitter




 
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Author Topic: 811A transmitter  (Read 3166 times)
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DMOD
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« Reply #25 on: November 17, 2018, 03:32:09 PM »

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


Phil - AC0OB

* 6DQ5 Modulator.pdf (82.14 KB - downloaded 19 times.)
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« Reply #26 on: November 17, 2018, 04:49:42 PM »

I'm a fan of 6DQ5's so what do you think?
By now, anyone that doesn't like sweep tubes as modulators should be tagged as a "Sweep Tube Denier"!

That's STD, for short. Tongue
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« Reply #27 on: November 17, 2018, 05:13:56 PM »

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

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« Reply #28 on: November 19, 2018, 08:34:34 PM »

I'm a fan of 6DQ5's so what do you think?
By now, anyone that doesn't like sweep tubes as modulators should be tagged as a "Sweep Tube Denier"!

That's STD, for short. Tongue

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

Phil
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« Reply #29 on: November 19, 2018, 08:59:54 PM »

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

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.
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« Reply #30 on: November 19, 2018, 10:36:17 PM »

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
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« Reply #31 on: November 20, 2018, 10:01:49 PM »

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.
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« Reply #32 on: November 20, 2018, 10:29:11 PM »

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.
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« Reply #33 on: November 22, 2018, 12:39:08 PM »

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
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« Reply #34 on: November 22, 2018, 05:03:11 PM »

One thing I have noticed , is that most of the suggestions call for running the given tube at it's/their maximum limit.  Why?
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! Grin
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« Reply #35 on: November 23, 2018, 12:14:19 AM »

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
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« Reply #36 on: November 23, 2018, 09:06:39 AM »

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


Phil - AC0OB

Lots of things to like about it!
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« Reply #37 on: November 23, 2018, 09:58:36 AM »

Seems to me that plate impedance plays a role in tube choice...
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. Grin
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« Reply #38 on: November 24, 2018, 09:43:45 PM »

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
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« Reply #39 on: November 24, 2018, 10:24:24 PM »

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

* 6DQ6 in AB1.pdf (72.79 KB - downloaded 22 times.)
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« Reply #40 on: November 25, 2018, 10:35:06 PM »

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).
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« Reply #41 on: November 26, 2018, 06:15:22 PM »

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'!
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« Reply #42 on: November 27, 2018, 06:33:29 PM »

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
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