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The RCA "807 in Special Triode Connection" gets a new life. With Sweep Tubes!




 
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Author Topic: The RCA "807 in Special Triode Connection" gets a new life. With Sweep Tubes!  (Read 1421 times)
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KK4YY
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« Reply #25 on: November 25, 2018, 03:14:09 PM »

How about a separate filament transformer for the 6BL7?

Sure, if the swing excedes the heater/cathode voltage rating, a floating or biased filament transformer may be needed. Fortunately, if the 6BL7 doesn't cut it, a 6080 can be swapped in without rewiring the socket, as you've written, so it's easy enough to try them both.

It's my understanding that crazy drive requires less voltage swing than screen drive alone. So, maybe I get away with a 6BL7 as a CF and save 6W of filament power.

I plan to use MOSFET source followers for my initial trails to set a baseline of performance in my DX-100 test bed rig. For a future, new build (where there's more room) I'd be looking at the hollow-state CF possibility.
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KK4YY
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« Reply #26 on: November 25, 2018, 03:24:53 PM »

Unless you have really big sweep tubes, then you might consider a pair of 304-TLs as drivers for each phase.

I'm a little confused. I think you mean 304TL output tubes. They'd work, if I could ever find them, but being triodes the aren't compatible with crazy drive which requires a sweep tube pentode. Push-pull-parallel sweep tubes can be used for greater power.
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« Reply #27 on: November 25, 2018, 03:25:13 PM »

Opcom, I like your CQ article with the 6AS7 drivers.  Interesting use of the split secondary driver transformer, very similar to the way many broadcast transmitters interface from the 600 ohm line to the push-pull voltage gain stage.  The RCA BTA-1M series of transmitters do this, followed by an 807 for a cathode follower driving the 833s.

I got a basket-case Valiant (supposed to be complete/working) at the Huntsville hamfest.  It was so bad, with bent chassis, that I gutted the audio section and re-designed it to use a UTC S-21 modulation transformer, and the original modulation transformer (all windings in series) as a mod reactor.  Instead of the pair or 6146s, I put in four 1625s in push-pull parallel, with a 6SN7 cathode follower for a driver.  Another 6SN7 for a gain stage, preceded by a 12AU7 phase splitter and UTC A-10 for 600 ohm line input.  That worked out really well, great response, low distortion, but I wish I knew about crazy drive then.  This thread got me thinking about how I might resurrect my other BTA-1MX.  Maybe eliminate all the 807s and audio transformer, and redesign the audio from the ground up.
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« Reply #28 on: November 25, 2018, 03:28:27 PM »

Unless you have really big sweep tubes, then you might consider a pair of 304-TLs as drivers for each phase.

I'm a little confused. I think you mean 304TL output tubes. They'd work, if I could ever find them, but being triodes the aren't compatible with crazy drive which requires a sweep tube pentode. Push-pull-parallel sweep tubes can be used for greater power.
Sorry, I did not mean to confuse you.  Most of my post was straightforward.  But the comment about a pair of 304TLs as a driver for each phase was just my failed attempt at humor, as if a 6336 was insufficient.  I guess I better not give up my day job, comedy is obviously not my forte!

In any case, very interesting thread.  That article from CQ hit the nail on the head as far as cathode follower drivers are concerned!
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« Reply #29 on: November 25, 2018, 03:46:32 PM »

Fellow member BEEFUS taught me that a low Mu tube might not be the best as a cathode follower. Here is why:

The output impedance can be approximated as:

Zo = 1/gm

The gain can be approximated as:

A = mu / (mu + 1)

Reference:  http://www.valvewizard.co.uk/accf.html

This means that a 6080/6AS7 in a cathode follower will gave a gain of about 0.66 whereas a higher Mu tube might get up to 0.90. This might not seem like a big difference, but getting a bigger swing of audio to overcome a 1/3 drop can be a lot.


Now if there was a tube with high Gm and High Mu that also had some peak current cathode emission capability.

Look at the 6DJ8....or maybe the Russian types D3A (triode strapped), 6n6p, or the 6C45P. Other more common tubes like the 6BQ5, and 6W6 (triode strapped) might have merit as well. The 6W6 is a sleeper here..itself a real candidate for Crazy drive.

Jim
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* 6c45pi-sovtek.pdf (225.29 KB - downloaded 10 times.)
* 6W6GT.pdf (585.32 KB - downloaded 11 times.)
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« Reply #30 on: November 25, 2018, 04:47:26 PM »

I much prefer a cathode follower to drive the grids positive instead of an inter stage or driver transformer.  I never looked at the 6BL7 before.  Nice tube.  Reasonable filament current at 1.5 amps, and nice voltage capabilities too. 

I generally use a 6SN7, and I have used the 6AS7/6080 as well.  As we get bigger, we need substantially more filament current.  Another step up would be the 6336, but that is probably way overkill for driving sweep tubes.  Nice they all use the same pin configuration.

Unless you have really big sweep tubes, then you might consider a pair of 304-TLs as drivers for each phase. 

ask and you shall receive.

* pass regulator triodes.pdf (46.84 KB - downloaded 23 times.)
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« Reply #31 on: November 25, 2018, 05:05:01 PM »

How about a separate filament transformer for the 6BL7?

Sure, if the swing excedes the heater/cathode voltage rating, a floating or biased filament transformer may be needed. Fortunately, if the 6BL7 doesn't cut it, a 6080 can be swapped in without rewiring the socket, as you've written, so it's easy enough to try them both.

It's my understanding that crazy drive requires less voltage swing than screen drive alone. So, maybe I get away with a 6BL7 as a CF and save 6W of filament power.

I plan to use MOSFET source followers for my initial trails to set a baseline of performance in my DX-100 test bed rig. For a future, new build (where there's more room) I'd be looking at the hollow-state CF possibility.

I had not looked at the socket pinouts but that's interesting! When you do the experiment, can you please look at the modulator grid current in the crazy drive hookup? One circuit had just over 1K Ohm from the screen drive voltage drive to the control grid, then about 10K from the control grid to the cathode. It concerned me about the grid current being too high there.
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KK4YY
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« Reply #32 on: November 25, 2018, 09:54:41 PM »

...the comment about a pair of 304TLs as a driver for each phase was just my failed attempt at humor...
I did, at first, laugh. Then, I wondered if it was a typo. I need to learn not to take life so seriously.

When you do the experiment, can you please look at the modulator grid current in the crazy drive hookup? One circuit had just over 1K Ohm from the screen drive voltage drive to the control grid, then about 10K from the control grid to the cathode. It concerned me about the grid current being too high there.
I'm not at all sure that the resistor values on that schematic are optimum. There probably needs to be some jockeying around of the ratio between the two and the total R of the two. Obviously, one needs to make sure all tube limits are adhered to when doing this. Hopefully this "tuning" process can be refined down to plug-and-play resistor values for any given tube type. It'll take some time.

The gain can be approximated as:

A = mu / (mu + 1)

6BL7 mu=15

A=15 / (15 + 1)=0.9375
Not too bad.

By the way, Patrick, your pass regulator chart made me totally loose it! 3CX3000F1 in deed. Shocked
Take a lesson, Rick. When exaggerating go way, waaaaay, over the top! Grin

Don
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« Reply #33 on: November 25, 2018, 10:24:43 PM »

...the comment about a pair of 304TLs as a driver for each phase was just my failed attempt at humor...
I did, at first, laugh. Then, I wondered if it was a typo. I need to learn not to take life so seriously.

When you do the experiment, can you please look at the modulator grid current in the crazy drive hookup? One circuit had just over 1K Ohm from the screen drive voltage drive to the control grid, then about 10K from the control grid to the cathode. It concerned me about the grid current being too high there.
I'm not at all sure that the resistor values on that schematic are optimum. There probably needs to be some jockeying around of the ratio between the two and the total R of the two. Obviously, one needs to make sure all tube limits are adhered to when doing this. Hopefully this "tuning" process can be refined down to plug-and-play resistor values for any given tube type. It'll take some time.

The gain can be approximated as:

A = mu / (mu + 1)

6BL7 mu=15

A=15 / (15 + 1)=0.9375
Not too bad.

By the way, Patrick, your pass regulator chart made me totally loose it! 3CX3000F1 in deed. Shocked
Take a lesson, Rick. When exaggerating go way, waaaaay, over the top! Grin

Don

Don, I did not see that chart until after my post.  Then I got to looking at the tube numbers.  Something looked WRONG.  Opcom, 6080 - 6SA7 - maybe a pentagrid converter instead of a dual triode 6AS7.  I recall a professor long ago who would leave out some letters when writing on the board, then go back and fill them in - and he would say "Don't mind me, I'm Lisdexic".  Patrick, it is ok if you switch AS and SA, we know what you mean.....

So, I do have some 304-TLs.   They have enormous amounts of emission.  But yes, I do have a couple 3CX3000 and one glass 3X3000, going into my next AM transmitter.  The power supply is all finished, 5000 volts at 1 amp CCS, from a Collins FM broadcast rig.  Unfortunately, if I used the 3CX3000s as drivers, I would have nothing to drive, so I showed a bit of moderation in my post.  Not like the new transmitter under construction....  I can't hear you, will you please "turn up the wick?"

On a serious note, I think more should be done and documented on the crazy drive, as well as screen drive.  This may open up many options for clean modulation with simpler driver circuits, and it also just makes it fun and interesting.  Looking forward to seeing your documentation when you perform your testing.  Kudos!
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« Reply #34 on: November 26, 2018, 12:50:13 AM »

It's an unusual grid that will draw 250 ma! (the article calls for 45 watts!)
I guess in that case a large tube will be needed, but that was for an 810 in PP for a kW!!
I seem to recall the 805/838 only needs less than 10 watts, for about 300 watts PP.

The project being considered is somewhat lower in power, so I'm back to smaller
tubes for the driver... Cheesy

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KK4YY
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« Reply #35 on: November 26, 2018, 06:05:47 PM »

On a serious note, I think more should be done and documented on the crazy drive, as well as screen drive.  This may open up many options for clean modulation with simpler driver circuits, and it also just makes it fun and interesting.  Looking forward to seeing your documentation when you perform your testing.  Kudos!
There's whole lot of information on crazy drive in the DIYaudio.com thread, which I referred to in my original post. I spent a great deal of time reading that forum to try and get a handle on all of this. I learned a lot from it but honestly, there's still a whole lot of it that way over my head. There are people in that forum, and this one as well, that are true experts and I'll never be in their league. Embarrassed

That said, I'll do my best with trying to make it work myself and contribute what I find. The responses that have been posted here are very encouraging and I'm glad it's generating interest.  I had hoped that better minds (than mine) would get involved, and this has been the case. I'm pleased (and relieved) that there are others moving forward on this.

I wish I could dedicate more time and money to this, but both are in short supply here lately. So I'm plodding along slowly, but my enthusiasm is unabated!

Don
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KK4YY
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« Reply #36 on: November 26, 2018, 07:14:49 PM »

The project being considered is somewhat lower in power, so I'm back to smaller tubes for the driver...
Bear,
I hope you're right! Smaller is better.

I'm not sure how much voltage or current will be needed for the crazy drive method we're discussing here. It will be similar to screen drive, but the driver needs to work into the resistor network that constitutes the crazy drive circuit, as well as any drive needed for the tubes themselves. The resistor network will certainly draw current, but the tubes might only require voltage. I don't know. Until examples are built, tested, and evaluated, we won't know to a certainty how much drive is required.

My first attempt to build one will be using MOSFET source followers. With this, I should be able to make measurements that will answer the question of how much drive is required.

I hope that others here are working in parallel and can provide some insight after having run some tests.

Don
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« Reply #37 on: November 26, 2018, 09:30:41 PM »


Don, we're into "class 2"  here, the screen and grid are going to draw current.
So, that makes it simple.

Look at the value of the resistor to ground... I didn't take note, that will be the
limiting current draw from the resistors. Everything else will be the tube. We've
got a good ballpark on what the tube's electrodes are going to require from the
tube manual and the results in the thread... the tube is still limited by the max
screen current no matter what. Or else it will look like a roman candle inside that
bottle! Cheesy

But the series resistor is already limiting or more properly scaling the control grid
current, that leaves the screen's current draw to be considered in addition... now that
I think about it the fact that there is a resistor to ground from the control grid scales
the voltage, whereas a series resistor alone would limit the current... if I'm seeing
this properly. (ha ha)
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KK4YY
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« Reply #38 on: November 27, 2018, 05:45:15 PM »

Bear,
Yes, I see it as a voltage divider. The control grid getting a scaled-down voltage. I "think" that's the dominant factor. Hence, the ratio of the resistances is a factor in tuning for minimum distortion. If so, connecting the wiper of a potentiometer to G1 would make it easy if the overall resistance value can be approximated.

Certainly bias set point will matter too. Reading the work posted on DIYaudio, it is suggested that the idle current is relatively low using crazy drive. That's one of the benefits.

I've got a pair of 12GC6 tubes being shipped here that I want to try in the DX-100. I think they will be big enough to modulate it without massive overkill. The DX-100 will be my first crazy drive victim.
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