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Half Gallon linear




 
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Author Topic: Half Gallon linear  (Read 844 times)
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wa4nlw
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« on: November 14, 2019, 10:48:00 AM »

Good day all,
I came across what seems to be a project that i might be able to accomplish. I the March 1958 CQ mag I found an article on the "Half Gallon linear" I am hoping that this will be able to work with my "Heil Pine Board Transmitter", and that with my very limited experience I am able to build it. it says that 500w output is normal with 45 to 50w of input, and is a grounded grid setup. The "PBP Transmitter" is only putting out about 9w but hopefully there is a way to boost that for the amp. I welcome all constructive comments and alternatives. This would be my first "REAL" building project, and i expect the learning curve to be quite steep. Any advice would be appreciated and if someone would care to assume an Elmer role to assist me that would be great. I have included scans of the article below.

73 and thank you very much
Matt WA4NLW

* hgamp1.pdf (1095.45 KB - downloaded 166 times.)
* hgamp2.pdf (1093.97 KB - downloaded 102 times.)
* hgamp3.pdf (1049.92 KB - downloaded 101 times.)
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kb3ouk
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« Reply #1 on: November 14, 2019, 05:57:58 PM »

The old rules gave the power limit by input, not output, so that is a 500 watt input amp, should be good for about 300 watts PEP output, assuming 60% efficiency, which is probably close to what you'll get from it. AM pep is 4x the carrier power, so if you're getting 9 watts carrier from the pineboard rig, that's about 36 watts peak which is close to what it sounds like that amp wants for drive, and you should get somewhere around 75 watts carrier out of the amp, a good rule of thumb is don't run any more carrier than half the total plate dissipation of the tubes, an 811A is 65 watts dissipation per tube.
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K1JJ
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« Reply #2 on: November 14, 2019, 07:15:56 PM »

Hi Matt,

Based on practical on-air experience:  I once had a Collins 30L-1 linear that uses four 811As in parallel.  On AM, I ran it at about 100 watts carrier out with X6 audio peaks. (600 W pep out)  This was the maximum before flat-topping and overheating the finals.  This is about 600 watts pep output for four tubes.

If we reduce that by 1/2 for two tubes as you plan, this is about 50 watts carrier and  300 watts pep out unless they are pushed hard.

We take a big beating in heat and poor efficiency when running AM linear.

I would recommend running a pair of 813 tubes in the same grounded grid configuration. An 813 is rated at 125 watts dissipation without air and is about 200 watts diss with a lantern chimney blowing a good breeze thru. A pair of 813s in parallel will about double or triple your output power.  An air-blown pair of 813s on AM can easily deliver 100-150 watts of carrier on AM that really puts you into a strong class of signals. Run the HV as high as you can, even up to 3 KV for best positive peaks and cleanliness.

BTW, I would add a loading capacitor to the output of your schematic to make it a pi-network.  A 4-section ganged 365 pF receiving capacitor will do fine there.  A tuned input will also help IMD and lower drive requirements 1 DB or so.

Tom, K1JJ


* Collins 30L-1.jpg (179.63 KB, 1280x720 - viewed 101 times.)

* 813 Linear Amplifier.jpg (79.51 KB, 525x528 - viewed 150 times.)
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WBear2GCR
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« Reply #3 on: November 16, 2019, 09:33:35 PM »

Pair of 813s in Linear service?

If you search here, there's a mod to a Heathkit SB-230(?) the one that used the conduction
cooled tube. Don KK4YY put a pair of 813s into that linear, replacing the ceramic conduction
cooled tube.

An approach like that might be a good "get ur feet wet" way to go with Linears?

You get to start with a complete linear, power supply, output tuned circuit & chassis with
switches and metering. These particular Heathkits do not command a high price, since
nobody wants the not-so-great and hyper expensive conduction cooled tube. (btw, others
have switched it out for a huskier Ruskie conduction cooled tube - but that's another
thing entirely)

                 _-_-bear

PS. At first glance this seems, appears, to be a very simple amp. But not shown
are the power supply and some means to key the linear. Also, there is no
tuned input, which may be advisable from a variety of reasons. So, keep these
things in mind. Also this old design does not show a shielded, enclosed RF & HV
chassis. That's something that is pretty much mandatory.
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_-_- bear WB2GCR                   http://www.bearlabs.com
Steve - K4HX
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« Reply #4 on: November 16, 2019, 10:53:17 PM »

Run the 813s as triode and you can run that zero bias up to 2kV. Grab the schematic of the B&W 1000 linear. It should give you all the details.
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WD5JKO
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WD5JKO


« Reply #5 on: November 17, 2019, 10:53:32 AM »

I like the simplicity of the original link. Looks like a bare bones project to be built at low cost.

The filament choke gives way to modern replacements that are readily available.

The output tank with a tapped coil will work fine so long as the tap point is adjustable. The Central Electronics 20A does the same thing with a fixed tap. Going to a Pi-Net would work except those variable caps these days new are getting pricey. As stated earlier by another, a 3 gang Broadcast Receiver variable (0-1000pf total) would do fine. If cost and room are constraints, the fixed tap works.

As to gain, an amplifier like this with an untuned input will have around 10db gain. A few db more if the input is tuned. So 9W in gives about 90w out. As stated earlier, this is a little hot for two 811's running AM linear.

If it were my project, I'd tune the input to increase the gain a little, and more importantly improve the linearity. Then I'd move the bias upward close to cutoff. At 1500v B+, that would be around -12v. This will reduce the gain, and increase the resting carrier efficiency. Some call this Class BC AM Linear Amplification. I do this here with a Gonset GSB-201 using a quad of 811A's built by the Ruskies. I can run 200W out AM with headroom for 100% modulation, or about 150w out for AM with enhanced positive peak headroom. The 811 plates show some color, but not full red. If I go back to -4.5v bias, and reduce the drive for the same RF carrier output, then the 811's are full red, and positive peak ability is diminished.

So IMHO, class BC AM Linear, an 811A is good for at least 40 watts RF output per tube. I can get up to 50 watts per tube here with my 20+ year old Russian 811A's.

Should be said though, not all 811A's out there are created equal. Some of the Chicon tubes I've tried worked great at first, and then had a steady decline.

I added two attachments describing Class BC.

Jim
Wd5JKO


* Class BC Explained1.jpg (81.47 KB, 530x641 - viewed 79 times.)

* Class_BC_Explained2.jpg (61.8 KB, 497x654 - viewed 90 times.)
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KK4YY
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« Reply #6 on: November 17, 2019, 05:43:16 PM »

Pair of 813s in Linear service?

If you search here, there's a mod to a Heathkit SB-230(?) the one that used the conduction cooled tube. Don KK4YY put a pair of 813s into that linear, replacing the ceramic conduction cooled tube.

An approach like that might be a good "get ur feet wet" way to go with Linears?

You get to start with a complete linear, power supply, output tuned circuit & chassis with switches and metering. These particular Heathkits do not command a high price, since nobody wants the not-so-great and hyper expensive conduction cooled tube. (btw, others have switched it out for a huskier Ruskie conduction cooled tube - but that's another thing entirely)

                 _-_-bear
Matt,

I run that pair of 813's at just above 2kV and they idle nicely at zero bias. I keep the HV on and bias the tubes off with about -50VDC (iirc) during standby.  Keeping the HV on eliminates the surges and clunks of switching it. There are other keying methods but I had a bias supply winding on the power transformer (which I originally planned to also use for operating bias).

For many builders, the biggest challenge is the metal work. As tempting as it may be, this is not a pine board project not that I haven't seen it done, but it was scary. So scary in fact, that I made a metal enclosure for a friend, who built one that way, and gave it to him. A total metal enclosure is a must-have from a safety standpoint. You can get creative here... a discarded microwave oven for an enclosure... an aluminum cake-pan for a chassis.  It's not hard to spend half your amp budget on a pre-fabricated enclosure. There's no shame in adapting something else for the purpose.

The CQ article you reference is a bit deceiving. It omits many things that would be needed. Take a look at some commercial amp designs and get a feel for what a complete amp looks like. There are some nice homemade amp designs to learn from as well.

Everyone who has done this has been there. Every amp "expert" started with no experience. Don't let that stop you. There's nothing like the feeling of seeing your wattmeter go BAP! up against the pin with an amp you built yourself. And it's equally gratifying to switch that meter up to the high power range. Smiley

Good luck!


Don
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N4DJ
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« Reply #7 on: December 09, 2019, 08:35:27 AM »

Back in the late 60's early 70's I built a linear amplifier with a pair of 813s in grounded grid. All the grids were simply grounded at the tube socket except the control grid was run through a meter. There was a 73 magazine article that gave me the idea. I think the title of the article was called "The Big Bang". Somewhere I have the magazine. I drove the amp with a Viking ranger II. When I asked the guys on 20 meters (14,205) how it sounded they said it sounded just like a Big Ranger! As best as I can remember I had a 300 to 400 watt unmodulated carrier. That's probably input power with no modulation.  I wound my own filament choke and 20 meter tank coil with copper tubing using my mothers rolling pin.
That amp used probably the minimum number of parts possible. Several .01 bypass capacitors, filament choke, RF plate choke with a couple 500 pf Doorknob capacitors, 2.5 mh rfc at the output, two 813s with sockets, two variable capacitors and tank coils (tubing for 20 meters and B&W coil for 160-40), and a filament transformer. I had an external 2000 volt supply using about 14 diodes in a voltage doubler with 6 or 8 450 volt electrolytics for a filter. It worked great on CW, SSB and AM. The rig ran an even 1000 watts DC input on CW with a good power supply. Max power for those days!
Don
N4DJ
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