Clegg Zeus modulator

[wb8hmd]

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

[n8fvj]

It looks to be over 100% modulation.

[K8DI]

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

[w1zb]

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

[KD1SH]

  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.

[KA3EKH]

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?

[KD1SH]

  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.

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?

[Pete, WA2CWA]

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.

[KD1SH]

  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.

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.

[wb8hmd]

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

[KD1SH]

  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.

[wb8hmd]

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

[KD1SH]

  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 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

[wb8hmd]

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

[KD1SH]

  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 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

[W3SLK]

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.

[wb8hmd]

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

[KD1SH]

  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 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

[wb8hmd]

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

[KD1SH]

  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.

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

[wb8hmd]

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

[KD1SH]

   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.

[W3SLK]

KD1SH said:

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.

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

[wb8hmd]

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.

[KD1SH]

   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.

KD1SH said:

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.

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