The AM Forum

I have the Clegg Zeus RF deck and modulator pwr sup that I'm having difficulties with. The problem is the modulation envelope as observed on a scope. The envelope pattern is from my perspective really ugly. All caps in the audio chain have been replaced, tubes checked on my Heath tube tester, the 6BX7's swapped around with NOS units since the tubes that were in the unit when I got it were below the min level.

1- In my mod/pwr sup unit that came with the Zeus, R331 a 47k fix resistor on cathode of v306 pin 7 is a 50k pot.

2- with the rf deck front panel audio pot R1 (cal on schematic) set along with the rear panel on the RF deck R2 to give a sinusoidal pattern with a scope on the grids of the 811's I get about 20% mod on the scope envelope pattern. Increasing either of the audio gain pots on the rf deck to get more mod on the envelope pattern yields a really ugly pattern. See attached photos of envelope pattern.

3- the 811's are chinese. I don't have any other 811's to swap into the mod/pwr sup chassis for comparison.

4- with a bird dummy load, and bird 43 I see 100w of rf out.

Any thoughts from the group would be greatly appreciated.

Jim WB8HMD

It looks to be over 100% modulation.

This is a time I’d want to separate the af from the rf. Find some power resistors that add up to the modulation impedance (I’m not familiar with the Zeus ooerating parameters so I can’t offer a number).  Hang those across the mod tx secondary disconnected from the rf section. Pull the final. Scope that secondary, using a divider or hv probe as needed.

Doing this will tell you if the modulator is puking or the rf is an issue.

Ed

Check the idling current for the 811's. I think there is a jack on the modulator chassis to insert a current meter.

Check the PA grid dive is between 5-7 ma.

I have disabled the clipping circuit in my Zeus modulator as its job is to produce clipping/distortion in the audio.

73 Jerry W1ZB

  Hi, Jim. It's good to hear from another Zeus operator out there; it's an awesome rig. I agree with Ed: eliminating the potential for RF on the audio is a good idea, even more so since your meager 100W output leads me to suspect that there might be something amiss on the RF deck—I get a solid 130W from mine, with a fresh tube. Having said that, I don't think RF gets into the Zeus audio too easily, with modulator being a separate unit, but it's still a good idea to eliminate that.
  Jerry's advice to check the idle current is good, also. I'd definitely want to get in there with a scope and try to ascertain at which stage the distortion is happening.
  I have to respectfully disagree with Jerry on the issue of the clipper, though: to me, that little clipper circuit is a superb bit of engineering. Yes, clipping inevitably results in clipping distortion—it's the nature of the thing—but if driven properly the distortion is minimal with Clegg's design. The circuit does a wonderful job of controlling negative peaks while still allowing for healthy positive peaks, and unless your audio chain is capable of keeping negative peaks in check I'd be cautious about eliminating the clipper. When I tried that with mine, I found it difficult to avoid exceeding 100% negative, and I went back to the clipper; still getting very good audio reports. But, YMMV, as they say.
  I don't have my scope handy here in the shack, but here's a picture of my Zeus's output on my REA mod monitor, on a whistle. Disregard the positive mod percent meter; I'm a lousy whistler and the needle was jumping everywhere.

sorry, I dont follow. Thought the Zeus was a six and two meter AM transmitter that used a pair of 6146 tubes in the modulator? Never saw one in person so its academic to me but just what do you do with a high power six and two meter AM transmitter? is there any activity on six doing AM? thought all the activity up there was SSB?

  Six and two meter AM, yes. RF final is a 4X150 (or 4CX250) and the separate power supply/modulator uses a pair of push/pull 811's. The matching receiver is the Interceptor. There's a decent amount of six meter AM activity here in the Northeast—I just signed off our regular weekly six meter AM net; we typically get anywhere from three to eight participants, and there's another local net that runs at the same time a few KC above ours. There's a six meter AM net in Florida on Sunday evenings, and sometimes when the band is open I check into that one.

Typical magazine ad for the Zeus is below.
Matching receiver is the Clegg Interceptor or Clegg Interceptor B.

There's also at least one 6 meter AM net in Eastern PA.

I had the pleasure and experience, as a high school kid, of seeing the Clegg Zeus and Clegg Interceptor being manufactured.

  A very significant outlay of cash back in 1962; roughly the equivalent of the top-of-the-line Kenwood TS-990 of our day, and that's not including the price of the Interceptor receiver to compliment it. Not for the faint of wallet, back in those days.

I will add some further info to supplement my initial information. The transition from low level mod envelope nice looking pattern abruptly transitions from maybe 20~30% to the ugly mod envelope at the point where the audio signal goes from a nice sine wave to clipping or plat topping. There isn't a smooth transition of deeper depth in the mod envelope, it just goes to the ugly mode pretty quick. I never see the nice progression of deeper and deeper trough, it's rather instantaneous once the mod audio starts to clip. 

HMD

  Okay, my apologies for not being more attentive when I first read your post—I missed what was probably the most important point: R331 has been replaced by a 50K pot! Do you know what value that pot is set to right now? That resistor, originally a 47K, is the cathode resistor in the cathode follower circuit of V306; a crucial part of the clipper circuit. It looks like someone has monkeyed around with the clipper circuit in an attempt to "improve" it. That circuit works very well as designed—Clegg's engineers knew what they were doing—and I'd sooner disable it all together, as Jerry suggests, than alter its function.
  The actual clipping element in the circuit is the back-to-back diode pair, CR302/CR303, in series with the incoming low-level audio. Increasing the bias to this diode pair allows more audio voltage to reach the phase-inverter, V301; decreasing the bias restricts the audio voltage. Lowering the value of V306's cathode resistor with that 50K pot will decrease the static bias on the diode pair, in effect lowering the threshold at which clipping begins to occur.
  Besides returning R331 to a 47K fixed resistor, I would suggest going through that entire clipping circuit and restoring everything back to the schematic values: R303, which feeds the bias to the diode pair; R330, the cathode resistor of the peak-detector V305; resistors R319 through R322; and capacitor C310, which sets the very important time-constant of the clipper.
  I think this will solve your problem. Once the clipper is functioning properly, I suggest giving it a chance; if you don't like the way it sounds, then by all means take Jerry's advice and disable it—there's much to be said for both sides in the clipper vs non-clipper debate.
  One additional point: if your testing involves injecting a sine wave into the audio circuit to test the modulation, do it with caution—it can punish a modulation transformer severely, and Clegg isn't selling replacements anymore.

I used my DVM at the cathode pin of V306, and it's set to 50k. Looking in the area of the clipper circuit and the 2 diodes, It doesn't appear that there has been any other mods, all the solder joints look like original and not reheated, or components changed with other parts of values.

My thinking at this point is the chinese 811's, and I don't have any other 811's to try, or the mod xformer is already toast.

If I speak into a mic connected to the front of the transmitter I see similar patterns on the scope as compared to a sine wave input. As the mic gain is increased, and monitoring the mod envelope while listening to myself on the interceptor and headphones is very week at lower mic gain settings as I would expect as evidenced by the mod envelope, and as I increase the mic gain to the point of clipping it sounds distorted, not clean as I would expect, the mod envelope on the scope looks distorted like it does with the 1kc tone.

The transition from low mod to distortion is very abrupt going from low mod to distorted mod envelope very quickly.

HMD

  How is this pot wired—is the wiper connected to one of the pot's other terminals, so that it's essentially a variable resistor to ground, or are the two fixed terminals tied to ground and the tube's cathode, with the slider connected to the clipping diodes?
  An idea: use both channels of your scope—one looking at the RF envelope and the other at the low-level audio signal just after the diode pair (the junction of CR303's cathode and R304 would be convenient), and check to see if clipping of the low-level audio and the distortion of the RF envelope begin at the same time.
  Yes, it certainly could be those Chinese 811's (did you check the idle current as Jerry suggested?) but seeing that someone has obviously been hacking around in the clipper circuit, and that the distortion begins abruptly upon reaching a certain level of audio, I would be suspicious of the clipper circuit before anything else at this point.

I failed to mention in my last update about idling current for the 811's that should be 50~70ma as I recall. I am not able to hit the min value it's more like 35 to 40ma.

HMD

  I seem to recall that, way back when I first got my Zeus, I wasn't able to quite get the idle current into the middle of that spec, either. But, at least to my humble knowledge (maybe someone with more experience playing with 811 modulators will chime in), somewhere around 40ma should keep them happily in class B. It's close enough, anyway, that it's most certainly not the cause of your problem.

I'm new to the game here but on another forum we were discussing using the 4CX250B as a final in AM service. I understand they can be finicky because they require in phase modulation of the screen. Someone suggested the Clegg Zeus. Maybe sniffing around that clamper tube (6BX5?) may be causing issues if it is not in the modulator alone.

I've been thinking about the clamper tube as well, and have given some thought to just pulling it to see what happens to the mod. As I understand the clamper tubes function, if either the mod exceeds some set point value, or current set point limit, the clamper tube starts to roll back the screen grid voltage. Or am I thinking of something else ?

HMD

  Sort of, but not exactly. Most class C transmitters use a grid-leak resistor to generate the grid bias for the final amplifier tube, sometimes called self-bias; the grid current flowing through that resistor causes a voltage drop across it, which is the bias voltage. The problem is that this grid current is only present when the tube is being driven with RF—if you lose drive, there's no longer any grid current flowing, and thus no grid bias. With no grid bias, the plate current soars and the tube destroys itself.
  The clamp tube is connected so that the grid bias voltage also appears on the clamp tube's grid, and holds it in non-conduction, but the loss of bias voltage on the final tube's grid will allow it to go into conduction, at which point it pulls the screen voltage down to the point where the tube can't conduct enough plate current to injure itself. There are variations along that theme, but that's the general idea.
  Under normal conditions a clamp tube shouldn't respond at all to modulation or screen voltage—it only acts when the final tube's grid loses RF excitation. But, in your Zeus, things are obviously not normal. So, maybe...

I've been doing some poking around the modulator deck on my zeus since my last post. I did the resistance and voltage checks per the docs in the info I have. The resistance values I measure are very close to the ones in the docs, as are the voltage measurements except for V313 the Clamp Tube, a 6BX7. Pins 1 and 4 which are the grids are -65vdc on my unit while the info in the Zeus document specifices -140vdc. My question is, where does that -140v bias voltage come form ? I don't see a path from the bias supply to V313's grids ? Can someone enlighten me as to how the -140vdc is derived and from where ?

HMD

  That bias voltage would come from the voltage divider consisting of R23, R25, and R26 on the RF deck schematic. Those resistors, essentially, comprise the grid-leak path for the final amplifier tube.
  Did you disconnect the RF deck from the modulator power supply before taking those readings?
  Normally, the voltage drop through that grid-leak path would be sufficient to maintain V313 in non-conduction, but once disconnected from that, those grids will be floating and the tube probably in some degree of conduction, so whatever voltage you're measuring on the grids would be dependent on the internal resistance of your meter, through which grid current will be trying to flow.
  In the manual, the use of a VTVM is specified for the RF deck voltage measurements, and I assume that was the intent for the modulator power supply as well, so unless you're using a VTVM, the voltage you measure on those floating grids might be just about anything.

For resistance measurements, the RF and Pwr/Mod deck were disconnected as it says in the docs. The voltage measurements were taken with a Fluke which is high impedance, but maybe not 10/11 meg ohm like a vtvm. Voltage measurements were made with the trans tuned up as described in the docs, and into a 50ohm load (Bird Termaline) that measures 51.6 ohms. I have not looked at it with a network analyzer but accept it as being flat, and little to no reactance at 2m.

Interesting that it uses Grid Leak Bias, I did not catch that point. I will investigate the divider chain and the resistance values now -vs- what's on the schematic.  What I find curious is that the -65v is about what is in the tube data sheets for a 4cx250, and the -140 is for a 4x150 type tube. I have not physically looked at the tube in my Zeus. Because it uses grid leak bias, is it possible my Zeus has a 250 in it and not a 150 based on my measurements ?

HMD

   Any chance you might have been looking at specs for different classes of operation for the 4X150 and 4CX250? For plate modulated class C operation, both tubes should have very nearly the same grid bias; both right around -100v. A grid voltage of -65v on either tube would put them into the class AB1 area. A grid voltage of -140v would probably work, but would put the tube further into class C than you'd really need to be, and require more drive to get over the fence.

KD1SH said:I agree! The 4CX250 has always been a 'drop-in' replacement for the 4X150. No reason the bias voltages should have to change!

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.

   I actually replaced the 4X150 in my Zeus with a 4CX250, when the original started to get tired. Drop-in replacement; no changes needed. There are variants—letter suffixes—in the family, that mostly entail heater voltages and physical ruggedness, but other than that most are drop-ins. That all changes with the 4CX300 and 4CX350. Their grids, both control and screen as I recall, are more delicate, and the 4CX300 even has a different base altogether.

  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.

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

  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.

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
WB8HMD

  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.

  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,

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

   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.

  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 and others who contributed their thoughts and ideas.

This past sat I went up to k9keu's qth who also has a Zeus. I took my zeus, 811's, my power umbilical cable, the spare 6bx7 tubes that I have, my bird watmetter and 1/4kw slugs to cover both 6m an 2m.

What John and I found by A to B supstituion between or zeus rf decks and his power supply boiled down to bad 811's that were in my pwr/mod deck when I got it.

On his TV7/u tube tester, all of his 811's tested with a gm of 40 or 41. The chart in the TV7/u says min of 36. The 1st of my 811's had a gm of 32, while the other one had a gm of 3 or 4, barely off the pin of the meter.

John was gratious enought to send me home with an extra set of 811's he has, and I put them into my pwr/mod deck for testing.

Bingo, bad chinese 811's was the root cause.

Photos attached off my scope showing the mod envelope of bad 811's -vs- good 811's.

I will get a set of new (is that possible) of 811's, and return his back to him.

  Glad to hear that! You can still get new-old-stock 811's through various outlets on the web. Enjoy your Zeus—it's an awesome rig; arguably the best commercially made six-meter AM rig ever. The matching Interceptor receiver is impressive, too; mine consistently outperforms my modern Yaesu and Icom rigs on AM. And there's nothing like those 811's filaments glowing away under my bench on a cold winter evening!

There were two matching receivers for the Clegg Zeus. One was the Clegg Interceptor and the other was the Clegg Interceptor B.

  I've got one functioning Zeus/Interceptor pair here in the shack, and several more down in the workshop awaiting their turn on the bench; at least one of the Interceptors is an older non-B model. Very similar, but as I remember, the original had a Q-multiplier and audio filter, which the "B" replaced with a selectable wide/narrow filter. Both have the superb Eddystone flywheel dial as on the Zeus.
  The only complaint I have about the Interceptor is that the AGC isn't all that effective, often leading to a frantic dash for the volume control when a strong station joins the net. I've gone through the alignment procedure and checked the AGC circuit, but all seems to work as designed.

A class AB final with modulation applied to it will make the carrier rise and fall at an audio rate.  IE, you'll get the swing.

But no mixing takes place, sans any IMD.  So, no modulation is superimposed on the carrier.  You are just making the B+ rise and fall at an audio rate.


I proved this a decade ago with a friend.  He swore that a linear amp with a modulator would make a fine transmitter.  It didn't.

He grounded the base of the driver and finals (it was a 1 x 4 cb amp) and voila!  Modulation!

And solid state bipolar transistors.  For best linearity, you need to modulate TWO stages.   The drive and final amplifier!   This is t necessary with MOSFET devices.

I figured this was commonly knjown, but maybe not.

Glad you found the problem.

--Shane
WP2ASS / ex KD6VXI