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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 1422 times)
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KK4YY
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« on: November 20, 2018, 06:49:42 PM »


   Whilst perusing an online audio forum (diyaudio.com) I came across an interesting circuit arrangement. Similar to the old RCA "Type 807 in Special Triode Connection" circuit, audio drive is applied to the screen and control grids of a pentode but with a difference.

   Similar to the RCA method, drive is directly to the screen, and through a resistor to the control grid. But this new method adds a second resistor from the control grid to ground. Unlike the RCA method, tubes like a 807 are not good candidates. However, TV "sweep" tubes show excellent results!

   Most sweep tubes have a relatively low screen voltage limit and a "Triode Amplification Factor" (as stated on the datasheets) of between 3 and 4 or so. Those characteristics are what make this drive method work well.

   How does it work? I can't explain it better that the author "smoking-amp" who has performed many hours of evaluation and refers to the method as "Crazy Drive".

   He posts many plate curves that he ran on various tubes. Unfortunately, these cannot be viewed by non-members of that forum. But I can tell you that he gets some fantastic linearity when using Crazy Drive!

   I plan to try this in the near future. Anyone else? Comments?

Here's a link to his description:
https://www.diyaudio.com/forums/tubes-valves/307517-ruminations-screen-drive-crazy-drive-21.html#post5531436
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« Reply #1 on: November 20, 2018, 09:52:50 PM »

That is very interesting and will work great with most sweep tubes.
Depending on the tube his voltage divider ratio may need adjustment. Some sweep tubes don't even need grid drive if driven by the screens only and a little distortion is not a big deal.

He's going for linearity in the stage -those audio folks don't like much NFB, but for modulators NFB is fine.

I'd be concerned in the circuit below linked, with control grid dissipation - there is shown a divider having 1.78K between G1 and the driven G2, and I would not want to be that control grid with a loudmouth at the mike.
https://www.diyaudio.com/forums/tubes-valves/307517-ruminations-screen-drive-crazy-drive-post5513530.html


The special class B arrangement used 20K in series with the control grids to limit the control grid current.

Anyway, modulation-minded folks could join there and read that topic, there are lots of things to consider.
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« Reply #2 on: November 21, 2018, 08:21:20 AM »

I saw that thread... didn't see the schematic though, or the specifics of the way to set it up.
Somewhat confused as to how the grid gets negative bias with a resistor to the cathode?
It is to the cathode, isn't it?
I could see it to ground, IF there is cathode bias... maybe.
Anyhow, unclear. Not sure where the drawing is...

The 6HS6(?) curves were scary linear.

And, I guess that this sort of linearity isn't really essential in a modulator, but it would not hurt.

I'm not a fan of feedback around transformers.
It causes problems in the way transients are handled, aka ringing and overshoot.
Again, probably not a real issue in modulators, but I don't like it anyhow.

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« Reply #3 on: November 21, 2018, 04:41:57 PM »



Is there a specific link to the schematic being discussed?


Phil
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KK4YY
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« Reply #4 on: November 21, 2018, 05:34:21 PM »

Somewhat confused as to how the grid gets negative bias with a resistor to the cathode?
In "normal" screen driven circuits that I've seen, the control grid is simply grounded. Crazy Drive utilities that grid to linearize the overall gm. The resistors set up a voltage divider to provide a proportionate drive to the control grid. I believe that getting the ratio and total resistance of the divider correct will determine the amount of drive compensation and limit the grid current respectively.

And, I guess that this sort of linearity isn't really essential in a modulator, but it would not hurt.
Yes, just going to screen drive is the big step. Seems to make the kinks go away. Another advantage of crazy drive, as the author mentions elsewhere, is the it is easier to drive than screen drive alone. I'm sure a low impedance driver is still required (preferably a mosfet) but apparently less voltage swing is needed.

To me, if I were going to go with screen drive anyway, it only takes 4 resistors to make it crazy drive. The open question is the resistor values. Some work needs to be done to find the proper values. If that were known, it would be a simple matter to convert any screen driven sweep tube modulator (of which there are many) to crazy drive. Maybe not a great benefit, but certainly an inexpensive one.
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« Reply #5 on: November 21, 2018, 05:43:59 PM »

Is there a specific link to the schematic being discussed?
Schematics and photos can be found in the other forum by its members. I don't want to post them here directly - they're not mine. But the circuit is an easy one to describe.

A pentode is screen driven (such as a 6DQ5). A resistor is placed between the screen grid and the control grid. A second resistor is placed between the control grid and ground. That simple.
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« Reply #6 on: November 21, 2018, 05:54:44 PM »

Anyway, modulation-minded folks could join there and read that topic, there are lots of things to consider.
You bet. There are some very, very sharp engineers on the board. I think there's a wide area of cross-over between AMfone and diyAudio. I know we each tend to snub our noses at each other sometimes, that's human nature. Tribalism they call it. But that's for a psychology forum! Roll Eyes
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« Reply #7 on: November 22, 2018, 10:59:10 AM »

Is there a specific link to the schematic being discussed?
Phil

Phill,

   This is one link:
https://www.diyaudio.com/forums/tubes-valves/307517-ruminations-screen-drive-crazy-drive-2.html

I downloaded the LT Spice file, and converted it to a .JPG file. I attach it here. My library is missing the pots used, but it is clear enough to figure that out.

The Fet source follower driving the output tubes as been something that big George came up with at TubeLab many years ago. One of his famous typologies is a "weak" type 45 amplifier with a solid state constant current load, and that driving the FET source follower. The Power amp (single ended) can be then operated class A2 instead of A1. This dramatically increases the power while the low source impedance drive keeps distortion low. Then without NFB, the distortion from the 45 amplifier will cancel out much of the distortion from the Power output tube (perhaps it is a big triode like a 300b or 845).

Jim
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* 6DQ5_Crazy_Drive.jpg (251 KB, 1916x938 - viewed 86 times.)
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KK4YY
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« Reply #8 on: November 22, 2018, 11:44:44 AM »

Referring to that schematic:

Notice the idle current is set to only about 38% of the Pd. I believe that crazy drive (if set up optimally) may lower crossover distortion enough to allow an even lower idle current than that to be used. More experimentation needs to be done.

-Low idle current
-Better linearity
-Easier to drive
-Inexpensive to implement

What's not to like?
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« Reply #9 on: November 22, 2018, 04:34:44 PM »

Is there a specific link to the schematic being discussed?
Phil

Phill,

 
...I downloaded the LT Spice file, and converted it to a .JPG file. I attach it here. My library is missing the pots used, but it is clear enough to figure that out.

The Fet source follower driving the output tubes as been something that big George came up with at TubeLab many years ago. One of his famous typologies is a "weak" type 45 amplifier with a solid state constant current load, and that driving the FET source follower. The Power amp (single ended) can be then operated class A2 instead of A1. This dramatically increases the power while the low source impedance drive keeps distortion low. Then without NFB, the distortion from the 45 amplifier will cancel out much of the distortion from the Power output tube (perhaps it is a big triode like a 300b or 845).

Jim
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Thanks Jim,

I had heard of screen drive for 6DQ5's but had never seen an actual circuit implementation.


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KK4YY
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« Reply #10 on: November 22, 2018, 05:31:56 PM »

I had heard of screen drive for 6DQ5's but had never seen an actual circuit implementation.
Here's a classic 6DQ5 screen driven modulator from WA1QIX
http://amfone.net/Amforum/index.php?topic=30337.0
Methinks it could easily be modified for crazy drive. In fact, Steve even mentions driving both grids in the notes of his schematic!
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« Reply #11 on: November 23, 2018, 10:36:16 AM »

Note that the schematic posted is DC coupled to the screen... so it is in class A2 or AB2.

Also, there is a cathode resistor - presumably that provides the (some?) requisite negative bias, which
IF it's in mostly or all Class A is not much... but maybe it's not really doing that much.

Trying to run that in more like AB2 might be impossible, as it likely biases where it biases,
plus or minus some cathode bias...

Solo screen drive that I have seen (no grid) isn't all that linear. Also, it seems that not all tubes
produce the same wonderful linearity that the "magic" circuit is tried with.

But it does produce a fairly wonderful set of curves for the 6HS6 tube.

             
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KK4YY
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« Reply #12 on: November 23, 2018, 11:33:14 AM »

Also, there is a cathode resistor - presumably that provides the (some?) requisite negative bias, which IF it's in mostly or all Class A is not much... but maybe it's not really doing that much.
The cathode resistors are 1 ohm. I think that's only used to monitor cathode current when setting the bias for the correct idle current. Note the +/- rails for biasing of the screen. I'm thinking of it as a triode with the screen being the active, driven and biased grid and ignoring the control grid essentially. The control grid is used only for its ability to help linearize the gm. The schematic states 20mA(9.3W) at idle per tube with 42W output so I suspect its class AB(?).

Solo screen drive that I have seen (no grid) isn't all that linear. Also, it seems that not all tubes produce the same wonderful linearity that the "magic" circuit is tried with.
Again, the key to crazy drive is using tubes with characteristics that are common to sweep tubes - low triode mode amplification factor (3 to 4 or so). For reasons beyond my limited understanding(!), that's when the proportional driving of the two grids can effectively linearize the overall transconductance of the tube.
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« Reply #13 on: November 23, 2018, 12:04:51 PM »

I suspect the crazy drive circuit has the ability to linearize a beam power tube better than the dubiously named ultra-linear circuit. Sweep tubes are a poor choice for UL mode anyway as the plate voltage must be kept very low (due to low screen voltage limits) sacrificing output power and basically wasting a good tube.

The outstanding characteristic of sweep tubes is their high peak cathode current rating (this at the sacrifice of using more filament power) and relatively low average plate dissipation. So, in a service where there's a high peak to average requirement, such as a modulator for voice, they're a good choice. Combine that with a way to make them very linear, their small physical size, the many different ones available, relatively low cost, and they become very very hard to beat.

More experimentation needs to be done with the crazy drive method. I plan to do some, and I'm hoping others will take up the task as well. Hey, if I'm lucky, one of you will figure it all out and save me the trouble! Grin Roll Eyes
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« Reply #14 on: November 23, 2018, 01:58:04 PM »

I recall Pete W1VZR used 6DQ5's in a Valiant with Screen drive. Back in the 1980's, he had one of the best sounding AM rigs I had ever heard. Is Pete still around?

The Gonset transmitters sometimes use screen drive to the modulators. I believe a Harvey Wells transmitter also does so. I attach some ideas put into my G-76 to enhance the modulator. It would be fun to try adding some in phase audio to the control grids.

The mods I did allow for 130% positive peaks with a sine wave, and with voice peaks, I can go to 140% or more. I use a soft clip on the negative peaks that gradually kicks in at around 70% modulation, and gets progressively more intense at 90% modulation. I attached few documents about that. I have more, scope traces, 1 tone, and 2 tone tests as seen on a pan adapter. Seems very clean until the Negative Peak Attenuator goes into full swing.
Jim
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* G-76_Modulator_Modifications1.jpg (95.16 KB, 1024x768 - viewed 48 times.)
* G-76_Speech_Amp.rtf (7.26 KB - downloaded 11 times.)

* 2_Diode_Ckt_Schematic2.jpg (142.45 KB, 800x600 - viewed 43 times.)
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« Reply #15 on: November 23, 2018, 02:00:41 PM »

 A few plots attached of the G76 at 50% modulation. The plots at 75% are a little worse, but not bad. The last scope image is with sine wave modulation adjusted up to 130% positive peak. See the attenuated, but not clipped negative peak that does not exceed 100%. Lower the audio level down to around 75%, and the modulation negative peak attenuator does nothing.


* G76_1_Tone_Harmonic_50_Percent_Mod (2).PNG (28.77 KB, 711x357 - viewed 19 times.)

* G76_2_Tone_IMD_50_Percent_Mod (2).PNG (36.55 KB, 713x381 - viewed 20 times.)

* mod_130.jpg.JPG (45.56 KB, 400x300 - viewed 25 times.)
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« Reply #16 on: November 23, 2018, 08:10:17 PM »

It would be fun to try adding some in phase audio to the control grids.
Yes, the G-76 may be a good candidate for crazy drive. I don't know if transformer coupling will be stiff enough to drive it but it would be easy enough to try (only 4 resistors). The control grid is grounded on TX but it's hard to see what they're doing with it on RX. You'd need to figure that out. Obviously it wouldn't be grounded after modification. It would be at the tap of a resistive voltage divider from the screen to ground. It would be interesting to wire it up with a switch or relay so you could A/B it, if you're so inclined.
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« Reply #17 on: November 24, 2018, 02:27:05 PM »



Here is another dual grid drive circuit from an old Handbook. See attachment. For a mobile application, it had several advantages,: no bias supply, no screen supply, low modulator idle current, and high efficiency. This is similar to the zero bias 6N7 based 10 watt modulators.


* 807 Modulator.jpg (47.77 KB, 576x284 - viewed 38 times.)
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« Reply #18 on: November 24, 2018, 02:50:13 PM »

Yes, that uses the same connection (20k - 22k resistor between grids) to the 807 as the circuit that I refer to in the Title. I've attached an image of it from the RCA Transmitting Tube Manual TT-5.

While crazy drive does not appear to help much with the 807, it seems to work very well with sweep tubes.

Don


* 807_special_triode.png (760.13 KB, 587x441 - viewed 48 times.)
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« Reply #19 on: November 24, 2018, 09:04:06 PM »

The worth of sweep tubes is in those that require only a low G2 voltage for maximum plate current.

The driving voltage to the screen (and grid combo) has to be quite high for the 807, and much lower for the sweep tubes.

The tube manuals show which tubes of various kinds use a low screen voltage.

With a small OPT run backwards 700-800V peak grid to grid voltage drive for 807s shown in the handbook, and other screen-grid tubes like them, is not too hard. There are some class B driver transformers that might work but those seem a bit rare to get.

For small stuff and when it's wanted cathode followers to eliminate the need for a driver transformer, the peak screen to screen voltage of 300-400V is easier to get.

Also consider the headroom wanted from the driver as 'good practice' and sweep tubes are even more attractive. It may not be unreasonable to use a 6080 for those who prefer tubes to transistors.
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« Reply #20 on: November 24, 2018, 09:41:16 PM »

It may not be unreasonable to use a 6080 for those who prefer tubes to transistors.
Hard to beat a mosfet source follower, and I may use them, although the "purist" in me would like to use a cathode follower. Yes, the 6080 comes to mind (I have a bunch of them) but I'm looking at the 6BL7GT for a future project. It has a 500V plate voltage limit, lower filament current, slightly higher gm, and similar Pd vs the 6080/6AS7.

I have one NOS 6BL7GT in my "inventory" which I inherited from my father that he had for his TV repair shop back in the 1950's. Always a favorite of mine to put some of his old stuff on the air so there may be a cognitive bias in my choice. Wink
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« Reply #21 on: November 25, 2018, 10:08:08 AM »


6080 seems like WAY too much tube for a driver... way way way.

How many watts do you think is needed?
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« Reply #22 on: November 25, 2018, 11:24:26 AM »


6080 seems like WAY too much tube for a driver... way way way.

How many watts do you think is needed?
I think the object is to have a cathode follower with a low impedance output as a driver. Both the 6080 and the 6BL7GT would have an output impedance of about 140 ohms. If you know of any tubes that can better that, I'd like to what they are.
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« Reply #23 on: November 25, 2018, 02:13:04 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. 
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« Reply #24 on: November 25, 2018, 02:20:37 PM »


6080 seems like WAY too much tube for a driver... way way way.

How many watts do you think is needed?

I don't understand an objection to the 6080 for power reasons, OK on objection for voltage, it's officially not rated so much. The 6080 has been used at > 400 supply volts in ham stuff but that's off label. It also has a +/-300V heater to cathode voltage rating.

Aside from the heater power, the anode drop for a given current (30V@100mA) is low compared to the 6BL7 (200V@100mA) and it may compensate the power budget a little there.

Either way the probable 6-10 Watts drawn by the modulator screens is comparable with the heater wattage of either tube (10 or 16W), and either cathode follower tube has to run class A1 or AB1, in order to get away from cutoff/crossover distortion and make it easy to bias the modulator accurately so there is some idling plate current there, and that's a few more watts. For either tube, probably some negative supply through a cathode resistor to get it right (attachment).

For power all I can say is 2-3x whatever is required as "good practice" when driving class B-like grids with variable impedance. The voltage swing for the G2's of the modulator is more or less fixed, it only remains to supply the peak current without distortion. Power supply is cheap and many tubes are cheap and junkboxes are filled with both. Don't skimp on what you build because you might have to do it over later.

I have no accurate idea what the peak screen grid current would be but it will change upward with drive as the instant plate voltage lowers and the upswinging of this effect gets sharper and sharper as the Modulator is driven harder or the load impedance is lowered. Too many variables. Whom is building it will have to decide what to do.

But that's just my opinion and my way of doing. Most audio or RF circuits I build would not be cost-competitive for sale. I agree that there are many cost-down to be made, but the worst thing that will happen because someone used a larger tube is that it will last longer.

I think the object is to have a cathode follower with a low impedance output as a driver. Both the 6080 and the 6BL7GT would have an output impedance of about 140 ohms. If you know of any tubes that can better that, I'd like to what they are.

How about a separate filament transformer for the 6BL7?

* 6080 driver.pdf (1519.74 KB - downloaded 15 times.)
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