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Author Topic: Clegg Zeus modulator  (Read 2076 times)
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KD1SH
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« Reply #25 on: February 10, 2024, 01:29:50 PM »

  Interesting. For class C plate mod, -150v sounds a bit excessive, but not completely out of the ballpark. That's deeper into class C than I think most would want to go, and your driver would need to swing the monkey that much higher to get him over the wall. But that -60v number sounds suspicious; that's getting well into the linear range—class Ab territory—and don't know that I'd try that with plate modulation.
   Maybe someone who's experimented more with that tube will chime in.

I should have saved the link to the spec sheets I was looking at, but I didn't. For Class C Plate modulated typical operation, the grid on the 150 was given as -150 with a plate of abt 1500 as I recall, while the 250 showed a grid of -60 also as Class C Plate Modulated. I will look again on the web and see if I can find them again. They were for 4x150/4x250 respectively, glass types, not the ceramic types.
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wb8hmd
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« Reply #26 on: February 11, 2024, 12:42:18 PM »

Ok, I dug up the spec sheets (attached) that I was referring to for the 4X150, and the 4CX250. For typical operation of the tubes, the 150 shows for plate modulated class C as typical grid of -150 with a plate of 1kv which is similar to the Zeus. The 250 shows -90 for bias also with 1kv on the plate. and both screens at 250v. They both show these ratings up thru 150MHz.

The 150 is a jpg, while the 250 is a pdf. Hope they come thru

I have not check my zeus for what final is in it, I think they shipped when new with the 150.

I checked the values of the resistors in the divider chain that form the grid leak chain, and the 15ohm 5% R26 is 21.5OHms (in circuit) the others seem to be reasonably within the 10% values given. With this higher R value for R26, my guess is that when setting the drive level to 6 to 7.5ma on the meter, the actual value of grid current is going to be lower than that by a fair amount ?

HMD


* 4X150 data.PNG (1182.95 KB, 854x573 - viewed 22 times.)
* 4CX250 Pg3.pdf (435.71 KB - downloaded 15 times.)
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KD1SH
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« Reply #27 on: February 11, 2024, 03:23:22 PM »

  As Mr. Spock would say—with raised eyebrow—"fascinating." I'll admit that some things are confusing me, here. Not your info, but rather some of the numbers that I come up with. Clegg says to set the grid current, in the tune-up procedure, for 6ma to 7.5ma on the panel meter. The relevant resistors in the grid return path, R23 and R25, (R44 and R26 are low enough to be ignored for grid-voltage purposes; they just extend the range of the meter) add up to 9K. Assuming we set the grid current to 7ma, that would result in a theoretical grid voltage of -63v, which seems odd, to me, for class C operation.
   This doesn't take into account whatever DC resistance is represented by L10, the RF choke, but it would need to be around 5K to get us up to -100v at 7ma, which seems unlikely.
   Resistors R44 and R26, by the way, are only there to turn the 3ma full-scale meter into a 10ma full-scale meter, and my numbers work out fine for that. R26, also, is correctly shown as 15 ohms on the schematic, but as 150 ohms in the parts list.
   To confuse things further, in the voltage measurements for the RF deck, Clegg indicates a desired -100v on the grid of the 4X150, which would seem appropriate, yet they say to set the drive control to minimum when checking these voltages. Since the negative voltage on the grid is created, in effect, by the grid current which flows in response to grid excitation, that grid voltage must naturally increase when the grid drive increased, yet my calculations result in a grid voltage of -63v at the recommended 7ma grid current.
   Either I'm really missing something—admittedly quite likely—or Clegg's documentation is misleading.

Ok, I dug up the spec sheets (attached) that I was referring to for the 4X150, and the 4CX250. For typical operation of the tubes, the 150 shows for plate modulated class C as typical grid of -150 with a plate of 1kv which is similar to the Zeus. The 250 shows -90 for bias also with 1kv on the plate. and both screens at 250v. They both show these ratings up thru 150MHz.

The 150 is a jpg, while the 250 is a pdf. Hope they come thru

I have not check my zeus for what final is in it, I think they shipped when new with the 150.

I checked the values of the resistors in the divider chain that form the grid leak chain, and the 15ohm 5% R26 is 21.5OHms (in circuit) the others seem to be reasonably within the 10% values given. With this higher R value for R26, my guess is that when setting the drive level to 6 to 7.5ma on the meter, the actual value of grid current is going to be lower than that by a fair amount ?

HMD
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wb8hmd
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« Reply #28 on: February 11, 2024, 04:00:16 PM »

Bill,

Interesting indeed. My skizzo and parts list jive for the R26 15 ohms in both locations. The docs I have look like mimeograph paper to me. In the circuit instructions on operation, in the lower right is (Rev 5-23-1), but in the parts list also mimeograph appearing in the upper right it has (EPL-331-2) and lower right (Rev 5-23-1).

In looking at the voltage and resistance measurement points, I went thru and measured mine for both R and V on pwr/mod deck, but only the R values on the RF deck.

My docs for the R and V of the RF deck say that the grid of V6 which I believe is the 150/250, they say Base Index Plug (grid ?) is -100v. For V313 which is the Clamp Tube, the grid is specified at -140v. Where did the extra -40 volts come from, or go to ? I will assume the loss is in the Resistive divider network of R23, R24 on the RF deck, and subsequently would be present at the grid of V313 6BX7 clamp tube since very little current is flowing in that path ?

Another ham near me also has a Zeus and we've been comparing notes for V and R values. On his mod deck he gets around -90v,   as I recall on V313 grid, while I get -63v.

In my thinking, with grid voltage that low, it's probably not in class C, but probably in AB maybe ?

In my initial post on this subject I showed what my mod envelope looks like, and I wonder if the crappy envelope is due to the grid being to low and not above -100v or higher ?

Jim
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KD1SH
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« Reply #29 on: February 11, 2024, 05:18:53 PM »



  Unfortunately you can't compare the V313 grid measurement to the V6 grid measurement, since the V313 reading is taken with the RF deck and modulator/power supply not connected to each other, while the V6 grid reading is taken with them connected.

Bill,

My docs for the R and V of the RF deck say that the grid of V6 which I believe is the 150/250, they say Base Index Plug (grid ?) is -100v. For V313 which is the Clamp Tube, the grid is specified at -140v. Where did the extra -40 volts come from, or go to ? I will assume the loss is in the Resistive divider network of R23, R24 on the RF deck, and subsequently would be present at the grid of V313 6BX7 clamp tube since very little current is flowing in that path ?

Jim
WB8HMD
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KD1SH
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« Reply #30 on: February 11, 2024, 05:43:09 PM »

  Yes, I would think that with a grid voltage of around -63v, that tube would be in Ab1, or at least somewhere thereabouts, and operating in a mostly linear fashion.
  I've never tried it so I don't know what the resultant modulated envelope would look like, but it probably wouldn't be pretty.
  When a tube is operated outside of class C, in the linear range, plate voltage vs. RF output will not follow a square-law relationship, where doubling the plate voltage (100% modulation) results in a quadrupling of RF output.
  Effectively, an RF power amplifier, when plate modulated while operating in class C, is a mixer—it mixes the RF signal with the audio signal imposed on the plate voltage, and what comes out of the coax is the original RF signal plus the sum of the two and the difference between the two: carrier and one lower and one upper sideband. Linear stages don't make proper mixers.
  The weird part of all this is that, according to my calculations, if you drive that tube's grid such that the specified 7ma flows, and those resistors in the grid-leak path are correct, as yours are, then -63v is what you'll see on that grid. I'm missing something here, and it's probably right in front of my face, but I'm not seeing it.

Bill,

In my thinking, with grid voltage that low, it's probably not in class C, but probably in AB maybe ?

In my initial post on this subject I showed what my mod envelope looks like, and I wonder if the crappy envelope is due to the grid being to low and not above -100v or higher ?

Jim
WB8HMD
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wb8hmd
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« Reply #31 on: February 12, 2024, 09:27:22 PM »

Bill,

I posed a question to an acquaintance of mine that does AM broadcast consulting for WGN, CBC, and a host of others. I asked what would happen to the envelope if the final were not in class C, but maybe in AB, and he said nothing, but the efficiency would suffer. So it seems I'm back to square 1.

In discussions tonight with another Zeus user, we were comparing notes for the RF and mod/pwr sup decks. In the voltage and resistance charts, it says V313 grid calls for -150v, while on V6 grid it calls out -100v.

I'm wondering if those 2 values are flipped ?

I've also attached what my mod envelope looks like. This is with a 1KHz tone for a very short period of time.

Jim
HMD


* RF Envelope mic at 12.JPG (2345.55 KB, 4032x3024 - viewed 22 times.)
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KD1SH
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« Reply #32 on: February 13, 2024, 09:49:10 AM »

   Well, I'm hoping that someone else with more knowledge than my humble self will join the discussion, but my understanding has always been that tubes operating in the linear region can not be plate modulated with any success.
   But, that being said, it's important to remember that what really matters isn't the actual value of grid bias, but rather the operating angle that results; the bias is simply the means to accomplish that. And further, these "classes" are not, in most cases, rigid concepts. Class B is the only one, really, in which only one value of bias and drive will result in the desired operating angle, which is 180 degrees, no more or no less. The "AB" classes are gray areas—in between zones—and you can be more or less into them. Likewise for class C: anything less than 180 degrees of conduction can be technically considered class C, which gives us a vast playground within which we can play.
   And it's not only the bias that determines our operating angle; the amount of excitation—the grid drive that we apply to the tube—goes hand-in-hand with the bias in determining the operating angle. You can easily drive a tube into or out of an operating class altogether with sufficient drive.

Bill,

I posed a question to an acquaintance of mine that does AM broadcast consulting for WGN, CBC, and a host of others. I asked what would happen to the envelope if the final were not in class C, but maybe in AB, and he said nothing, but the efficiency would suffer. So it seems I'm back to square 1.


Jim
HMD
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KD1SH
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« Reply #33 on: February 13, 2024, 10:07:42 AM »

  Looking at that modulated envelope, I can't help but wonder if you're barking up the wrong haystack, here. That sure looks to me like something that's happening upstream, in the modulator. Have you checked the actual modulated audio signal riding on the B+?  Use extreme caution, obviously, since it's 980 volts as I recall. You'll need a X100 scope probe as well—they're not expensive, and they're a good investment.
   Really, I wouldn't be surprised if you see that same distortion in your modulated B+.

Bill,

I've also attached what my mod envelope looks like. This is with a 1KHz tone for a very short period of time.

Jim
HMD
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