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Author Topic: ARC-5 works great on AM...  (Read 93410 times)
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WB3JOK
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« Reply #50 on: February 06, 2015, 07:07:49 PM »

Yes, I also have made a couple of 250-mile QSO's with my 3-6 Mc. transmitter barefoot, so I agree it's possible. If there's propagation, if the alligators in 5-land and the foreign broadcasters aren't wiping out the band, and perhaps most importantly, if someone's listening  Grin

I've been trying without success to find out what Bristol spline sizes are needed for the various setscrews in the receiver and transmitter. Any ideas? The wrenches are rather expensive (at least compared to Allen hex wrenches) and I don't know if the "SS-508" set sold on ebay has the right sizes.
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WB3JOK
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« Reply #51 on: February 06, 2015, 10:58:41 PM »

Here's some pics of the transmitter output with a sine wave applied to the mic jack.
Audio: 1 KHz

1st pic: approx 50% mod.
2nd: 100% mod. There's a tiny "kink" that appears as the envelope approaches zero, otherwise looks good.
3rd: 100% mod, undercoupled final tank (loading too low), other settings the same.


* P02-06-15_21.44.jpg (140.55 KB, 1280x960 - viewed 808 times.)

* P02-06-15_21.44[1].jpg (151.4 KB, 1280x960 - viewed 827 times.)

* P02-06-15_21.45.jpg (143.77 KB, 1280x960 - viewed 833 times.)
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WB3JOK
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« Reply #52 on: February 06, 2015, 11:05:12 PM »

Now with a sine wave input of 250 Hz.

1st pic: approx. 50% mod. Looks reasonably clean.
2nd pic: distortion visible at 100% mod

And at 100 Hz:

3rd pic: looks crappy at 100% mod  Tongue

I couldn't capture a good trace while speaking into the mic but it looks much better than this.

If I can make a QSO, it'd be interesting to hear reports of what the low-freq distortion actually sounds like... above 400 Hz where most of the speech energy and intelligibility is, it's clean.


* P02-06-15_21.46.jpg (146.97 KB, 1280x960 - viewed 848 times.)

* P02-06-15_21.46[1].jpg (151.18 KB, 1280x960 - viewed 820 times.)

* P02-06-15_21.47[2].jpg (146.2 KB, 1280x960 - viewed 826 times.)
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WD5JKO
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« Reply #53 on: February 07, 2015, 10:46:30 AM »



   That low frequency distortion will likely go away at a lower level, such as at 20% modulation. The idea is to keep the lows, but at a level such that they are heard, but the level is low enough to not distort. Many folks EQ this way anyway while accentuating to 1-2 khz region. I was playing around with this idea with the Retro-75 transmitter since its overloaded modulation transformer could not pass much below 300 hz unless the level was cut way back.

   You might consider making both RF and AF taps such as was done at this thread:

http://amfone.net/Amforum/index.php?topic=37653.0

    This way you can easily observe a trapezoid OR modulator envelope pattern on the scope. The trapezoid is key to measuring RF linearity independent from modulator distortion. You can have a perfect trapezoid when the modulator is heavily overloaded. Then once you get a decent trapezoid, then look at the modulator. All too often we skip the trap and try to clean up the modulator when the main culprit is poor RF linearity of the final modulated stage.

   Most modulated class C amplifiers can get improvement to the RF linearity by juggling the drive level, screen modulation level, and phase of the screen modulation. For example, with the 6146 plate modulated, the amount of screen modulation is often excessive, and there can be appreciable phase shift at the higher audio frequencies above 1 Khz. This phase shift is from the screen dropping resistor and the screen bypass capacitor. Some or all of this might apply to the ARC-5.

Jim
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N2DTS
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« Reply #54 on: February 07, 2015, 11:17:47 AM »

I bet most of that is in the transformer you are using to modulate the screens with.
Without a transformer, there is nothing to distort the signal except the power supply and the way the screen modulates the carrier (tube type).
Not that an arc 5 has to sound like a broadcast radio, but it CAN.
 

This is my rig at 30 Hz with screen modulation:


* P1220196.JPG (3728.08 KB, 4288x3216 - viewed 870 times.)
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WB3JOK
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« Reply #55 on: February 07, 2015, 12:27:44 PM »

Yeah, I can never stop with good enough... I think I'm going to eliminate the transformer.
The neat thing is that I can make the change in situ, using many of the same components.
This is what I'm proposing to do (see attachment) - anyone see obvious "gotchas"?

After looking at the GE data sheet, the existing 6AQ5 driver can be triode-connected instead of changing to a 6DE7 like the DX-60 circuit. The OB2 will now be used to provide a regulated -108V, and I can either use the existing 115v isolation transformer with a bridge/cap to feed it, or just rip it out entirely and tap a half-wave rectifier off one leg of the 480CT HV trans. It only needs to supply keep-alive current (5 ma).

It looks like 200V p-p will be more than enough screen voltage to get 100% mod at 560V on the plates. So the plate of the 6AQ5 will rest at a nominal 100V, which is about half RF power output, and can swing to ground since its cathode will be tied to the regulated -108. There will be 5 ma quiescent plate current, dissipating 1 watt in the tube. If the PA screens ever show negative current, this circuit can absorb it.

The 6AQ5 grid bias, which according to the triode curves needs to be about -40V to conduct 5 ma, can be provided by a pot from the -108 since there is no grid current.

It looks like I may actually have too much voltage gain in the speech amp... the 6AQ5 should only need a few volts of grid swing to provide full swing to the PA screens. Any thoughts before I start messing with it?  Smiley

Edit: Jim, the low-freq distortion does indeed go away at lower modulation levels, as shown in the pics. I don't see a problem with the communications mic which tends to emphasize the midrange anyway.

* revised ARC-5 screen modulator.pdf (19.27 KB - downloaded 385 times.)
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N2DTS
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« Reply #56 on: February 07, 2015, 09:07:35 PM »

I suspect that might work well.
Two things you need for good operation, enough voltage swing and the ability to set the resting carrier level.
Then it all comes down to how the tube works with screen modulation, some work much better then others.

I do wonder what the ideal transformer would look like.
The right impedances, the ability to handle DC on the screen side, maybe push pull on the audio side, push pull off the cathodes? and HI FI.

Single ended would have dc on both sides...

The nice thing about no transformer is no phase shift so you can use wide band feedback to clean things up a lot.
You should add it to that circuit, it helps a LOT.

Sure is fun to play with after years of plate modulation...
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WB3JOK
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« Reply #57 on: February 07, 2015, 11:32:50 PM »

I spent the evening rewiring the modulator for transformerless operation, and making stupid mistakes. It's been a long time since I designed any tube circuits  Roll Eyes

First (after removing the transformer) I discovered that I can't use rectifiers to create negative bias in the economy PS circuit since the CT is not ground! Dumb mistake, I should have known better. So I put the transformer back in, and wired up a bridge rectifier and capacitor for the OB2 supply.

Smoke test good - but no output. I had made an elementary mistake - since the cathode of the 6AQ5 is at -105 volts, setting the grid at -68 volts is in the wrong direction (grid positive w.r.t. cathode!)... should have been -145. -:-X For a temporary fix, I wired the grid pot to the unregulated supply feeding the OB2 (about -145 volts). Now it works and the frequency-related distortion is completely gone, as expected.

I can't get the peak screen voltage to go over about 160 volts. I can push it to 200 volts but the waveform is beginning to flatten). I think the peak current required is in excess of what the plate resistor can flow, but making it smaller will increase the dissipation in the driver. Compromises in a simple design, as usual... anyhow with the loading controls optimized for my 50 ohm dummy load,  there is just 7 watts carrier and 16W peak (75v p-p into 50 ohms). The 1625's are certainly capable of more carrier and PEP without overheating, but the screens need more drive! I was getting that with the transformer but as we have seen, although the 6AQ5 has plenty of drive power available, the transformer causes distortion at low frequencies.

I need to play with bias voltages and the plate resistor to get more peak screen voltage (and current) while minimizing the wasted heat in the 6AQ5. In triode configuration I probably don't need 100 volts across it to be able to pull the screens to ground. I may have to resort to a zener diode or two, although I was hoping to do it with hollow-state devices other than the rectifiers Wink an NE-2 is the right voltage but won't handle the required current (10 ma or so).

Also, there is still a little "kink" in the bottom of the envelope as it approaches 100% modulation. Some investigation with my scope (one channel on the screen voltage, the other on the RF output) and I discovered there is still output from the 1625's even at a screen voltage of zero!  Shocked I didn't think this is even possible.

In order to achieve full 100% negative mod, the screen voltage actually has to go negative about -10 volts. I'm only running about 560 volts on the plates. Can someone tell me what is going on? Screen emission perhaps?  Huh

Edit: another thought... the ARC-5 has a 15k grid-leak shunted by a .05 uf can capacitor. Those caps are known to become leaky after only 70+ years, so maybe the 1625's are underbiased. Still surprised to see output with zero screen voltage though.

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N2DTS
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« Reply #58 on: February 08, 2015, 12:41:32 AM »

On most tubes, you have to modulate the screen negative, not just down to zero.
Also adjust the grid drive and bias.

I built the dx60/qix modulator and it did not work great unless I lowered the grid drive a lot.
I then added the feedback and then the grid drive did not matter much at all!

Look at the KS3K dx60 mods in the transmitter section and add that feedback from the screen to an early stage, amazing things may happen.

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N2DTS
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« Reply #59 on: February 08, 2015, 12:49:44 AM »

Oh, I just noticed you are taking the screen modulation off the plate side of the tube.
Most screen modulators are cathode followers to get the low impedance drive to the screens, they are a variable load, just like class B grids, no load at all then the screen draws current and there IS a load.
Another thing I found that helps a lot is a load on the modulator, 100K resistor from the modulator output to ground.
 
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WB3JOK
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« Reply #60 on: February 08, 2015, 12:54:04 AM »

Great minds think alike.  Grin I just figured that out independently a few minutes ago, recalling that screens take more current the more positive they get, and reexamined the DX-60 circuit... sure enough, it's a cathode follower!

Time for version 3.0 after I get back from a business trip, on Wed. evening. Watch this space for updates Smiley

Mods, should this thread be in the Technical section? Feel free to move it Wink
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VE3LYX
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« Reply #61 on: February 08, 2015, 07:54:29 AM »

What are the wave form like at 90%?
I ask because the Dx60 circuit has been mentioned several times. I have one and seriously doubt it could do 100%.
Anything above 70% should sound decent.
As for someone needs to be listening. = Email me when you want to try. My call letters plus @outlook.com and I will be there if at all possible
don
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« Reply #62 on: February 08, 2015, 11:03:50 AM »

This is the dx60 circuit with some simple mods:
http://n2dts.smugmug.com/Ham-radio/i-XjX39zC/A

100% is easy with screen modulation, simple circuits can give well over 100% in both directions.
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WB3JOK
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« Reply #63 on: February 08, 2015, 01:34:22 PM »

I reconfigured the circuit as a cathode follower and made a few other minor changes (attached).
It looks very clean now, still requiring about -15V on the screens (at 560-580V plate) to cut off completely.
Looks just as good at 50 Hz as at 2 KHz Smiley I think that is "good enough for now".

I can get 56V pp carrier and 128V pp fully modulated. According to my trusty calculator into 50 ohms that is 8 watts carrier and 40 watts PEP. It's a bit cleaner on the scope when driven less vigorously, say 6 watts carrier and 30W PEP.

There is some AM activity going on now around 7290 and I tried to join... someone said "I heard a little carrier out there but couldn't make out any audio"  Sad Not a great propagation day from central MO it seems.

Time to head for work until Wed. evening anyway... when I get back I may hook it up to the 4x4-125A amp and try that on for size!

* revised ARC-5 screen modulator.pdf (19.37 KB - downloaded 386 times.)
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WB3JOK
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« Reply #64 on: February 08, 2015, 05:11:31 PM »

Another problem just occurred to me while driving to my hotel... now I'm exceeding the heater-cathode rating of the 6AQ5 by quite a bit, since the cathode is swinging several hundred volts above ground  Embarrassed I haven't designed with tubes in so many years I've forgotten to check stuff like this.

There is absolutely no room in that compact chassis (selected because it's just about the footprint of the transmitter) for even a small filament transformer. The easiest solution is to run the 6AQ5 off the 6.3 volt filament trans already tucked underneath (with cathode tied to filament), and power the 12AX7 with its filaments in series from the +28 DC supply via a dropping resistor. Only requires 0.15 amp so the resistor is dissipating 2.5 watts.
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N2DTS
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« Reply #65 on: February 08, 2015, 06:00:16 PM »

What is the filament to cathode rating?
The tube used in the dx60 is ok, I have been running it quite hard without problems.
Was on today with it, 650 volts on the plate, 200 volts resting carrier voltage plus the swing which must be quite large...
6AQ5 is rated at 200 volts.
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WB3JOK
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« Reply #66 on: February 08, 2015, 06:20:03 PM »

The 6DE7 and the 6AQ5 both have 100 volts DC, 200 volts DC+peak ratings. In this design the PA screen resting voltage is currently about 125V and the total swing currently goes from -15V to 260V, which is in excess of the ratings with the heater grounded. I don't have a complete DX-60 schematic so I can't tell if they used a separate, elevated filament winding for the mod tube in the Heathkit. It may be ok even in the long term but I don't like exceeding manufacturer's breakdown ratings, when a short will destroy the tube...

Are two 1625's at 560V capable of significantly more than 40W PEP? Something's running out of breath when I try to push them harder (envelope starts to show a soft flat-topping above 125V p-p at the 50 ohm load). The PS is not sagging. Do I need even more screen swing? Perhaps the 1900 ohm plate resistance of the 6AQ5 in triode connection is getting in the way (the 6DE7 is 925 ohms and only driving one 6146).
Or maybe not enough drive from the 1626 running at 280V? I'm not looking for legal-limit from an ARC-5  Cheesy but would like a little more output without frying the tubes if it's possible.
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N2DTS
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« Reply #67 on: February 08, 2015, 07:41:03 PM »

It varies from tube type to tube type, but it will never be higher then half the power the tubes would run at in plate modulated service.
More voltage on the plates often helps.

You have about 50 watts of plate dissipation to play with, about twice the power output of a DX60 which is about 12 watts, so 24 watts carrier would be expected, about 100 watts pep.
In order to get to 100 watts pep, you would need to be able to get there in say CW service, and may need 750 volts on the plates.
If you want say 50 watts of carrier, you would need to plate modulate it if the parts can handle the peak voltages.
At under 600 volts on the plates power output in any mode is going to be much lower then it could be.


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WB3JOK
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« Reply #68 on: February 08, 2015, 09:18:16 PM »

Thanks. Will see what I have for transformers that'll give me higher plate voltage. (Have you seen the price of new Hammond plate transformers??)  Shocked

This project originally started as a junkbox special and I had available a large (250VA) control transformer with a 480V primary, 120V secondary, which I run "backwards" in the economy PS circuit. That gives about 580V on the plates and half that for the rest of the tube circuitry.

I think the transmitter originally did have a 600V dynamotor providing plate voltage, another reason I built it that way. From the ARC-5 manual, it was rated for 40W CW output and 15W "voice carrier" (but that was with plate+screen modulation from the MD-7 modulator unit). So I'm probably getting about all I can expect with screen modulation at 580V.

I would bet that in the space taken up by that big control transformer, there is room for two smaller ones. A similar 50VA unit, which I have, will handle the small load from the speech amp, VR tubes and oscillator B+. Then all I need is something like 600V secondary at 100VA or less, to supply the 75 watts of 750V for the plates and screens. While of course staying under 900V key-up, because that's what two 450V electrolytics in series can stand off Wink

This is turning into the restoration of my grandfather's axe... I installed a new head and a new handle, but it's still his axe  Grin

It's time to leave well enough alone, and get back to work on the Class AB1 linear, which I designed to take the PW output from this transmitter and bump it up to 200+ watts of carrier!

This beast consists of four 4-125A's in parallel with 3.5 KV on the plates from a beefy 900VA (CCS) Thordarson plate transformer, a spare 4-65A to shunt-regulate the screens, vacuum variables for the pi-net, and grid-driven (50 ohm non-inductive swamping resistor followed by a step-up unun, (don't remember if it's 4:1 or 9:1) so it should be fine with just a few watts of drive and be easy to neutralize despite the high power gain.

Last time I messed with it (a few years ago), it seemed to be amplifying OK, but the modulation from the ARC-5 looked so bad I gave up on the project for a while. Now that I know more about screen modulation and tube PA loading than I did, and have a decent looking "exciter", it's time to have another try at it  Cool

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N2DTS
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« Reply #69 on: February 08, 2015, 10:21:13 PM »

Well, the rule of thumb (if the power supply is good for it) is an amplifier will do about one third carrier output of the plate dissipation of the tubes in the amp.
A pair of 3-500z's is good for about 300 watts carrier.
A single one will be good for about 150 watts, same as four 4-125's.

I think 4-125's screen modulate very well, you could run the same 150 watts out with direct screen modulation.
That has the advantages of needing no screen voltage regulation/protection, very hi fi, and a very small modulator.

15 or 20 watts out is enough to make contacts on 40, I have made contacts on my ten tec at 5 watts carrier, and 100 watts is a nice power level.
I just removed my 811 modulator and put the DX60/QIX modulator in its place.
100 watts out instead of 250, but it sounds so much better, and modulates so much better.
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WB3JOK
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« Reply #70 on: February 08, 2015, 10:47:56 PM »

Only 1/3? I thought that the overall efficiency of an AM linear was 1/3 (at least according to the Handbooks), so the carrier output should be 1/2 the plate dissipation. 750W DC input, 500W plates, 250W carrier. What did I miss?
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« Reply #71 on: February 08, 2015, 11:31:57 PM »

I used to have a 4x813 grounded grid amp I ran with a flex 5000.
Same rating per tube I think, 125 watts, at 150 watts carrier out it was putting out a LOT of heat.
200 watts carrier was really pushing it (it seemed).

In amplifier service you will likely want to run the 4-125's at 2500 volts or more.
For ssb service, I could put 3kv on the 813 plates and get over 2kw pep out.

Grounded grid or grid driven?
Not sure you will have enough power in grounded grid?
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« Reply #72 on: February 09, 2015, 12:24:18 PM »

I went whole hog. A pair of 811A GG is what I use if I need to "Reach" with my ARC5. 12v heater supply fed down the middle. My own concoction.
don
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« Reply #73 on: February 09, 2015, 05:56:30 PM »

Quote
I used to have a 4x813 grounded grid amp I ran with a flex 5000.
Same rating per tube I think, 125 watts, at 150 watts carrier out it was putting out a LOT of heat.
200 watts carrier was really pushing it (it seemed).

4-125A's should be OK up to orange plates as long as they get plenty of air... will see how much power that actually is.


Quote
In amplifier service you will likely want to run the 4-125's at 2500 volts or more.

Already running them at 3500 volts, but I have the ability to put a Variac on the plate transformer and reduce it if needed.

Quote
Grounded grid or grid driven?
Not sure you will have enough power in grounded grid?

Grid driven, for exactly that reason. In AB1 effectively no drive power is required except that for the swamping resistor. Even 6 watts of carrier should be able to drive this amp to full output. I'm not sure my PW'er would have enough "scrote" to drive a big GG amp.
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« Reply #74 on: February 09, 2015, 05:58:21 PM »

3500 volts will make those tubes sing!
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