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screen modulation again...




 
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DMOD
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« Reply #250 on: December 01, 2015, 08:39:51 PM »

Since we have never seen a circuit how can we tell?  Cheesy

Even a small amount of ac on the speech amplifier or the modulator's DC supply will show up as an amplified AC waveform somewhere in the chain.

Make sure each supply for the speech amp and modulator is decoupled by an RC filter.  

In addition, bias supplies should have good regulation and should be "stiff."

Insure you have a solid "star" pattern grounding system as well. I once found that daisy chain grounding can cause problems.

Phil - AC0OB

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« Reply #251 on: December 01, 2015, 09:59:14 PM »

The diagrams for the modulator circuits are in this thread, but with no connection to the modulator, the hum is the same.
Its not IN the modulators.
I think, in plate modulated transmitters the mod iron inductance filters hum out.
My 32V uses the same audio drive system with not a trace of hum.
If I take the 4-400 deck and screen modulate it, it has hum.
If I plate modulate it, it does not.
80 mf in the power supply, choke input.

Everything runs to a single point ground system.
The 4x150 seems to have more hum then the 4-125 or 4-400 rf decks.

I would try and run the filaments off dc but that is a lot of current....
 
 
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DMOD
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« Reply #252 on: December 01, 2015, 11:47:37 PM »

Quote
Quote
If I take the 4-400 deck and screen modulate it, it has hum.

That may be a clue.

Recall that when looking into a screen node you will see a higher average resistance than if you were looking into a plate load.

For a simple example, let's say we're plate modulating a pair of 6146's at 600V@300mA so the modulator sees an average resistive load of 2k.

Now lets screen modulate those same tubes at 100V@7mA. The modulator sees an average resistive load of 15k, or a factor of 7.5 times the average resistance of a plate modulated system.

A rule of thumb is the higher the impedance of a circuit the more susceptible it is to EM interference.

Let's say you are modulating the screens of tube whatever with a cathode follower. The whole circuit from the cathode follower node to the screen needs to be a low resistance circuit.

Try connecting an RG-59 cable from the modulation point to the screen connection while avoiding any high current filament circuits via proper routing. And make sure both ends of the shield grounds are at the same potential (voltage).

Quote
The diagrams for the modulator circuits are in this thread

How are you modulating the 4-400 screens? I know you keep saying that but most of us are graphical image/schematically driven.

Phil - AC0OB



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« Reply #253 on: December 02, 2015, 08:17:20 AM »

The 4-400's are being modulated by the solid state modulator:


http://amfone.net/Amforum/index.php?action=dlattach;topic=35375.0;attach=40659;image

The last RF deck I built (quad of 4-125's) I shielded almost everything and it still has some hum.
Maybe shielded filament wires would help.
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« Reply #254 on: December 02, 2015, 10:27:46 AM »

The last RF deck I built (quad of 4-125's) I shielded almost everything and it still has some hum.
Maybe shielded filament wires would help.


shielding for rf reasons means grounding at both ends ...

shielding for audio reasons usually means grounding at one end only because if you ground at both ends any ac currents flowing thru the chassis are now flowing in the shield and couple to the inner conductor ... in audio preamplifiers where signals are in millivolts, grounding is usually at a single point only for minimum hum

the problem is more prevalent with the screen modulator because of its extended low freq response compared to typical plate modulated rig which does not...

grounding can get tricky......  javascript:void(0);
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« Reply #255 on: December 02, 2015, 10:32:54 AM »

As it is now, I ground things at one end only.
Coax gets grounded at both ends.
Maybe I will try shielded filament wires.
I thought about that but did not see a nice way to do it.
Maybe just use rg8x...
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DMOD
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« Reply #256 on: December 02, 2015, 06:02:41 PM »

I know the original author of the SS modulator wanted the lowest component count to do the job, but if I were using it I would make the modifications shown in the pdf file below and here is why:

1. the transistors used are HV bipolars with relatively high input impedances, making them susceptible to noise and hum,

2. there is no RF filtering at any gain points, so I have placed caps to kill any RF,

3. none of the base pots that set base bias are referenced to ground, making them susceptible to noise and hum, so ground referencing resistors were added,

4. the original audio input capacitor is too large, hence modifying the input network to accept frequencies from 75Hz on up, above the 60Hz hum frequency,

5. source voltage filtering to kill any hum,

The selected transistors used are operating near the ragged edge of their Vceo specs, so this circuit would be better suited for HV fets or vacuum tubes, IMHO.

Phil - AC0OB

* SS Screen Modulation Driver.pdf (36.94 KB - downloaded 241 times.)
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« Reply #257 on: December 02, 2015, 09:06:14 PM »

The problem is the hum is NOT in the modulators.
I also use tube modulators and they do the same thing.

Some of it might be the fan in the 4X150 rig, if I tap on the RF deck with no audio it modulates the carrier a LOT.

I just got done redoing the filaments in that deck, ran them in coax cable, no real difference in the hum.

Maybe the fan is inducing hum into the tubes, and/or the filament transformer is.
I have a pot on the fan to adjust the speed, if I turn it up high, 60 Hz comes up on the sdr display.

I suspect it has something to do with the way the rig is run in screen modulated service, loaded heavy and low (static) screen voltage.

The 4X150's need the fan...
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« Reply #258 on: December 02, 2015, 10:49:45 PM »

How about kill the fan on the deck and use a oscillating desk fan,  floor fan,  something to remove the oscillator (fan) from the equation.

Sounds like microphonics.   Maybe a DC fan if this works?

I had good luck with 43 beads at the fan,  multiple wraps,  to keep rf out of the DC fans on my 16 pill mobile legal limit amp.


--Shane
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« Reply #259 on: December 02, 2015, 11:17:27 PM »

As a test, I disconnected the filament transformer on the RF deck and used the one in the control deck, plus I moved the fan from inside the chassis to the outside.
These are forced air tubes and need airflow though them, so a fan/blower has to be someplace although I could route air to the deck through a hose.

No major change in the hum levels which are 120 Hz mostly.

Changing the plate voltage does not change it.
Changing the screen voltage into or out of the modulator does not change it.
Changing the drive level does not change it.
Changing the bias or grid leak does not change it.

Can hum in the exciter pass through the transmitter?


Power supply has over 100 mf and is a choke input full wave center tap design.

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« Reply #260 on: December 10, 2015, 02:42:53 PM »

Phil, 

That Continental schematic has flown around in my head for a couple years.   I've been collecting parts to replicate a miniature version.

Single 4cx5ka.  Instead of tubes series mod on the screen,  pulse width modulated using hv mosfets.

I have tube,  socket,  3A CCS Dahl and the fil xformer and a choke,  for good measure.

Ought to be good for legal limit am.   

--Shane
KD6VXI

If I had that tube I would definitely go for it.

Phil - AC0OB
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« Reply #261 on: December 11, 2015, 09:54:33 PM »

I did some experiments last week, changing power supplies around.
I discovered I had changed the high voltage power supply to cap input (small cap) to get more voltage out of it.
I switched power supplies and the hum was much lower so I moved transformers around to give a higher voltage, and changed the power supply to small cap, choke, big cap, choke, big cap, hum very low.
I can not get 3000 volts under load out with full wave center tapped choke input with the transformers I have.
I am not fond of bridge supplies so I have a cap input setup.

Unlike plate modulation, you can use high voltage as the peak voltage the rf deck see's is the plate voltage.
4-400's work well at 3000 volts, they might work even better at 3500 or even 4000 volts.
They want a choke input power supply to have very low hum levels though.

The 4x150 rig uses all the stuff from the 3x4D32 rf deck and 811a modulator, designed for 1200 volts output, but shoved up to 1800 volts for the screen modulated 4x150's.

If you build something to screen modulate, make the power supply choke input and good for the highest voltage a tube will take.
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« Reply #262 on: December 12, 2015, 06:45:47 AM »

I have been down the hum road when repowering old military sets (and I have built some screen modulators as well) The hum in all likelyhood has absolutely nothing to due with screen modulation. For me, my worst source of hum came when building choke filtered HV supplies with solid state rectifiers. I read an explanation on line by a former designer of why he no longer used chokes in his Hv power supplies for ham equipment. I removed mine. I put a soft start resistor in the circuit at the beginning of the rectifiers. Just a few ohms , hi wattage. I removed the bridge and built a full wave rectifier using multi diodes properly by passed by 01s. I also used multi caps series with proper resistors and I went to a voltage divider supply. Is it perfect? NO but it is so much better I don't get the "hum reports". I used a PS design for my linear from the old SSB ARRL book as I had found it to be a very quiet supply. Using that as an example I rebuilt my mil radio PSU to match best I could and was rewarded with a workable radio. I also discovered I am on a dirty mains supply. Somedays are wonderful. Some aren't. Not all noise is generated in your own home. I have not had any issues with hum in my screen mod rigs and I have 3 of them , Two homebrew. I suspect had I used old rectifier tubes (Like a pair of 80s) and the old diagrams I would have had no or little trouble. PSUs , gotta be the number one source in most cases. It is my belief now that solid state is too abrupt and the design needs to be a bit different for that. Tubes were if you will allow me more graceful

don
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« Reply #263 on: December 12, 2015, 08:24:30 AM »

this continues to be an interesting thread ....

you might evaluate for your capacitor input supplies using a series resistor, say 50 Ohms, and a second capacitor to form a pi filter .... this really reduces the power supply ripple .... in my case from 20Vpp to .5Vpp in one application...

your o-scope will let you know .... its a one-eyed snitch
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« Reply #264 on: December 12, 2015, 09:09:22 AM »


   It's worth mentioning that a 3KV supply if looked upon with a scope could cause carnage to the scope, probe, and OM unless the probe is a 1000X type. Of the 1000X types, there are the ones for a DVM that are just for DC, and are not AC compensated. These will have a serious rolloff at 1 Khz. Then there is the more expensive type that are AC compensated, and good to perhaps 50 Mhz.  There is also a line of 100X scope probes that are much less expensive. These are often good to 1.5 to 2 KV DC, and have a derating curve for AC as the frequency goes up.

Jim
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« Reply #265 on: December 12, 2015, 06:57:15 PM »

I tried this with the Cheyenne, I tried a 10 ohm resistor, then a 100 ohm resistor with a cap before and after it, that is, out of drake supply, into resistor, out of resistor with a 24 uf cap, then to the radio.
I needed to drop some voltage as well,  the 100 ohm got me down to 700 volts.
No real improvement with the hum.



this continues to be an interesting thread ....

you might evaluate for your capacitor input supplies using a series resistor, say 50 Ohms, and a second capacitor to form a pi filter .... this really reduces the power supply ripple .... in my case from 20Vpp to .5Vpp in one application...

your o-scope will let you know .... its a one-eyed snitch
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« Reply #266 on: December 12, 2015, 07:08:14 PM »

Very interesting Don.
I will have to experiment with some of these ideas.

On my current setup with the 4-400's the hum is very low, but it took a cap/choke, cap, choke big cap setup.


I have been down the hum road when repowering old military sets (and I have built some screen modulators as well) The hum in all likelyhood has absolutely nothing to due with screen modulation. For me, my worst source of hum came when building choke filtered HV supplies with solid state rectifiers. I read an explanation on line by a former designer of why he no longer used chokes in his Hv power supplies for ham equipment. I removed mine. I put a soft start resistor in the circuit at the beginning of the rectifiers. Just a few ohms , hi wattage. I removed the bridge and built a full wave rectifier using multi diodes properly by passed by 01s. I also used multi caps series with proper resistors and I went to a voltage divider supply. Is it perfect? NO but it is so much better I don't get the "hum reports". I used a PS design for my linear from the old SSB ARRL book as I had found it to be a very quiet supply. Using that as an example I rebuilt my mil radio PSU to match best I could and was rewarded with a workable radio. I also discovered I am on a dirty mains supply. Somedays are wonderful. Some aren't. Not all noise is generated in your own home. I have not had any issues with hum in my screen mod rigs and I have 3 of them , Two homebrew. I suspect had I used old rectifier tubes (Like a pair of 80s) and the old diagrams I would have had no or little trouble. PSUs , gotta be the number one source in most cases. It is my belief now that solid state is too abrupt and the design needs to be a bit different for that. Tubes were if you will allow me more graceful

don
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« Reply #267 on: December 17, 2015, 08:48:33 PM »

Started gathering parts for another RF control deck.
I have too many RF decks (5) and modulators (7) and some power supplies (3) but only 2 control decks.
The control decks do bias (fixed variac and adjustable grid leak), screen voltage (variac 0-1000 volts plus a dropping resistor (pot)), screen overload protection, keying and metering.

I have the 2x 4x150 rf deck hooked up with my 811 modulator with the only good mod transformer I have, running it all at 1500 volts (350 watts carrier).
I would like to swap out the 4x150 deck with the 4D32 deck and screen modulate the 4x150 deck at 1800 to 2000 volts with the QIX screen modulator (200 to 250 watts out).
I like the way that setup works.
 
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« Reply #268 on: December 21, 2015, 07:50:03 PM »

Hooked up the 4x 4-125 rf deck to the solid state screen modulator and gave it a workout today.
3000 volts, about 150ma I think, 200 watts out, 1000 watts pep,slight dull red color showing on the plates.

I also changed the power supply in the pair of 4cx150a transmitter to bridge choke input setup and tested the rig at 1500 volts. 811a modulators with some bias, 300 ma plate current, 350 watts carrier and 1500 watts pep with the variac at 68% of maximum.
450 watts in on the RF deck (roughly) , 350 watts out, 50 watts dissipation per 4cx150 out of 250, or 100 watts out of 500 allowed for the pair.
I am tempted to try the screen modulators on the pair at 2000 volts on the plates...  
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« Reply #269 on: December 30, 2015, 12:55:54 PM »

I did try the DX60/qix modulator and the solid state modulator on the 4x150 pair with the new power supply setup at 2000 volts and get VERY good results, sounds very good to me and runs 200 watts carrier and up to about 1000 watts pep.

The solid state (W2IMX) modulator also works fine, but I find it interesting to run a 200 watt carrier modulated by two miniature 9 pin tubes.

I wonder if the dx60 modulator could drive three 4x150's at 300 watts but don't plan to build a three tube deck to test it...

So, don't have a mod transformer but want good sound?
Build the dx60/QIX modulator and modulate a 4x150/4cx250 for 100 watts out, or two tubes for 200 watts out, very low drive, line level audio or d104 in.
 
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« Reply #270 on: January 01, 2016, 12:52:03 AM »


I wonder if the dx60 modulator could drive three 4x150's at 300 watts but don't plan to build a three tube deck to test it...

So, don't have a mod transformer but want good sound?
Build the dx60/QIX modulator and modulate a 4x150/4cx250 for 100 watts out, or two tubes for 200 watts out, very low drive, line level audio or d104 in.
 

What was your quiescent (resting) DC voltage and P-P audio voltage to the screens of the 4X150's?
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« Reply #271 on: January 01, 2016, 01:07:10 PM »

180 volts is the resting output voltage of the modulator at maximum power output.
That is 200 watts carrier and about 1000 watts pep, 2000 plate volts and 300 ma on the 4x150's.

Not sure what the voltage swing is at 100% modulation.
The modulator has no problem doing 150% positive and has a negative limiter built in.

Average current is light, the modulator I have in line now does not have a current meter in it but from memory the other modulator was doing something like 5 or 10 ma.
About 750 volts into the modulator.

The 4x150's are working hard at roughly 600 watts input (2000 volts, 300ma plate current) and 200 watts output.
I think the system sounds quite clean and good though.

The 6EW7 modulator tube seems quite happy.

The modulator:

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« Reply #272 on: January 01, 2016, 01:08:35 PM »

The transmitter:
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« Reply #273 on: January 01, 2016, 02:15:22 PM »

How do you know that your transmitter is humming?
If you listen with your receiver you are probably receiving a signal that is not coming from the antenna.
I had the same problem and the humming was due to the fact that I was monitoring a signal  radiated by the transmitter power supply cable.
RF inside the transmitter cabinet coupled to the HV power supply transformer, was modulated by the diodes of the supply circuit and radiated by the mains cable into the room..... I filtered the mains circuit and solved the problem.
So ask to a nearby ham what is really radiated by the antenna.
Obviously this is only a possibility, the one that happened to me!!!

Happy new year
Giorgio
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« Reply #274 on: January 01, 2016, 03:30:27 PM »

I got reports from people while in qso with them.
Screen mod rigs seem to need very good filtering as there is no mod iron inductance to filter the B+ maybe, but I got the hum down to very low levels by really filtering the power supplies.
I did a full wave bridge with choke, cap, choke, cap, big cap (50UF) on one supply, and did a full wave center tap, cap, choke, cap, choke, cap, big cap (40 UF) on the other.
I did cap input on that supply because the chokes would buzz loudly (sound waves) without the input cap.

Maybe the solid state brick diodes are harsh like Don says.
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