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2x 813 amplifier?




 
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N2DTS
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« on: September 30, 2009, 09:26:18 AM »

I am thinking about trying to run my homebrew 813 pair as an amplifier for AM, instead of plate modulating it, and wonder what power level it would be good for.
Its grid driven, the handbooks say in AB2 its good for 650 watts out at 2500 volts.
That would be about 162 watts carrier?
If I ran it at 150 watts carrier output, it would be ok?

If I set everything for AB2 service, what is the best way to tune it up?

I might add a 3rd 813, that would give me about 243 watts carrier output...
I wonder if 4 would fit....

Brett
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KE6DF
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« Reply #1 on: September 30, 2009, 10:21:33 AM »

I think the general rule of thumb is that in a linear running AM the tubes can produce output of about 1/2 of their plate dissipation rating.

So for 813s the PD rating is 125W if I remember right so a pair could produce 125W of AM.

You need a hefty power supply.

And the 125w PD number for 813s is an ICAS rating. The CCS rating is 100w. But 813s get abused a lot and seem to take it.
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KD6VXI
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« Reply #2 on: September 30, 2009, 11:07:34 AM »

I am thinking about trying to run my homebrew 813 pair as an amplifier for AM, instead of plate modulating it, and wonder what power level it would be good for.
Its grid driven, the handbooks say in AB2 its good for 650 watts out at 2500 volts.
That would be about 162 watts carrier?
If I ran it at 150 watts carrier output, it would be ok?

If I set everything for AB2 service, what is the best way to tune it up?

I might add a 3rd 813, that would give me about 243 watts carrier output...
I wonder if 4 would fit....

Brett


Orr's 23rd edition of the Radio Handbook has a 2X813 linear in it.

2500 volts on the anodes, 2kw PEP input.  160/80 meters.

PEP in = 70 watts.

For both tubes, 400 mils on the plate, 110 mils on the grids.

Both tubes should glow a light cherry, according to the article.


--Shane
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ka3zlr
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« Reply #3 on: September 30, 2009, 11:09:27 AM »

Mine has three in it it's on the forum here I think in the picture section it's copy of a dentron but uses 813's there was more effort on a heavy power supply for head room an I've had it up 300 CCS when it was new has a little age on it now with the right supply and good heavy components 200 watts shud be no problem your not gona hurt them 813's build it heavy that thing will loaf at a 150...

73
Jack.
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KM1H
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« Reply #4 on: September 30, 2009, 11:10:47 AM »

Calculate the DC input and toss away 2/3 of that as heat and the remaining 1/3 as output. At 1000W input you will have 666W of Pd. Not good for 813's.
You have a small leeway with voice but not if running processing. A small fan moving some air will keep the tubes contented once you settle on the input.

My own tests indicate that carbon plate 813's have a more realistic Pd of 200-225W with just a small air flow that should be left on for a minute after a buzzard transmission before powering down. Metal plate versions get rather orange but cool down faster.

A pair of 4-400's will survive at 1200W input but need more air.

Carl
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KC4VWU
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« Reply #5 on: September 30, 2009, 12:35:58 PM »

I thought I had read that linear efficiency ran around 50% ?
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K5UJ
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« Reply #6 on: September 30, 2009, 01:27:24 PM »

Carrier is 33% if you are lucky, may be less.  Depends on how well you dip and load.  But because it's AB and not C may be harder to find as much dip.   My general amp rule for AM is run carrier at 25% of tubes' spec'd plate dissipation.   There are lots of variables though such as whether you modulate 100% positive or more, how long you transmit and how much cooling you have on the power supply and RF deck.    Check the plate h.v. choke is it wound on ceramic or plastic?  Ceramic better.  I think Ameritron had prob. with plastic choke coil form melting.   what is CCS current spec. for plate transformer?  that give U idea of how many ma you can devote to carrier & how much modulation power current draw you can do for a while.  say ur B+ trans. rated at 1/2 amp CCS.  you can probably run carrier draw of 300  ma and do some modulation for several minutes (5 to 10) provided you have lots of air moving through power supply and tubes.  Am I conservative? maybe.   Now, true I don't know much at all about 813 tubes therefore YMMV.
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N2DTS
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« Reply #7 on: September 30, 2009, 02:15:31 PM »

I tried it.
Something is not right though, using grid input and the flex set to 1 watt output, it overdrives the 813's.
I put an attenuator in line and could tune it up but had odd results, maybe a flex problem.
While I could get 100 or 200 watts of carrier out of it, the pep was only twice the carrier.

Drive levels may be critical.

The power supply is good for 3000 volts at 500 ma CCS, thats real CCS, nothing gets warm.
The 813's are forced air cooled (filtered air) with a variable speed blower, so I could likely run them at 150 watts carrier out without problems.

I gave up on the idea though, as things are touchy and with plate modulation I get 600 watts carrier out easy, so it seems stupid to use it as an amp....

Brett

 
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WA1GFZ
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« Reply #8 on: September 30, 2009, 03:06:03 PM »

How is the input VSWR of the amp?
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WD5JKO
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« Reply #9 on: September 30, 2009, 03:08:07 PM »

For what it is worth, my Central Electronics 600L Linear with a single grid driven 813 will output 70 watts all day, and 100 watts when blushing. The thing will modulate cleanly upward to 100% too. It just takes a watt or two of carrier to drive it, and that is why the 600L is a perfect match to the CE 20A, which can only do 2-3 watts AM cleanly (stock), and maybe 12-15 watts PEP on SSB. The 600L used a 812 to electronically regulate the 813 screen voltage to somewhere around +750V.

73,
Jim
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K3ZS
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« Reply #10 on: September 30, 2009, 03:50:38 PM »

I tried it.
Something is not right though, using grid input and the flex set to 1 watt output, it overdrives the 813's.
I put an attenuator in line and could tune it up but had odd results, maybe a flex problem.
While I could get 100 or 200 watts of carrier out of it, the pep was only twice the carrier.

Drive levels may be critical.

The power supply is good for 3000 volts at 500 ma CCS, thats real CCS, nothing gets warm.
The 813's are forced air cooled (filtered air) with a variable speed blower, so I could likely run them at 150 watts carrier out without problems.

I gave up on the idea though, as things are touchy and with plate modulation I get 600 watts carrier out easy, so it seems stupid to use it as an amp....

Brett

 

Sounds like the amp is not loaded heavy enough.  Increase loading past the point of max power output, you can then increase drive power to get the 200W carrier output.    Your PEP ought to be 4 times your carrier power.
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KD6VXI
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« Reply #11 on: September 30, 2009, 05:02:39 PM »

but had odd results, maybe a flex problem.
While I could get 100 or 200 watts of carrier out of it, the pep was only twice the carrier.

Drive levels may be critical.

Have you checked the AM operation of the FLEX itself, to ensure it's giving you 4 times?

And also, make sure your tuned input is working correctly...  You might be folding back under power.... How does the amp behave when you use it on SSB?  Try tuning it for max suds on SSB, then switch to AM, adjust the carrier for a fifth what the PEP SSB value is, and go from there.

My Kenwood TS440 stock would fold back going into the SB220 under modulation on AM.  It has since been fixed (the tuned input AND the 440).  Caused distorted audio and all sorts of other problems.

--Shane
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« Reply #12 on: September 30, 2009, 05:20:31 PM »

how are u measuring pep?  Not all wattmeters, in fact probably most made for hams that say they measure pep only work right on ssb and cw.   if you inject audio sine wave into rig and see clean sinusoidal envelope on output on scope with carrier just pinching off and nice rounded tops then ur at 100%.  If you see same thing on output of amp then you're running 4 times whatever ur carrier measures on the wattmeter no matter what the watt meter says.
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« Reply #13 on: September 30, 2009, 08:27:58 PM »

Good point Rob on the watt meters.

I use and trust my Tektronix scopes far more than any watt meter.  You can clearly see the true peaks along with all of the other information a good scope provides.

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Rodger WQ9E
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« Reply #14 on: September 30, 2009, 09:05:18 PM »

I seem to remember early software versions of power SDR had problems with  under modulation on AM.
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N2DTS
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« Reply #15 on: September 30, 2009, 10:17:21 PM »

The flex does 100% positive on its own, plus you can set it up to do more, like 150% positive.
But I may get rid of the flex, its not really what I want and not very good in some respects.

Brett


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« Reply #16 on: September 30, 2009, 11:23:57 PM »

I have seen this first-hand, two 813's grounded grid in a modified HT-41 at 2500V.  The 7094's were toast so the new 813 sockets were recessed and the fil xfmr replaced and a B&W fil choke added by a previous owner. This belonged to a CBer. They would run good and hot orange at 100W output carrier, really hot. Also, the HT-41's power supply was not really up to this, regulation wise and PEP was only about 300. I was consulted about this. The power was cranked back to 70W carrier and things were much better. I also convincved him to add a TVI low pass filter. To my knowledge, the 813's never needed to be replaced. They were the graphite anode ones.
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« Reply #17 on: October 01, 2009, 03:02:52 AM »

I've got two linear amps here, both running 2 x 813's, which could be up and running with minimal work. And then, I reasoned with my self and said "why?". The only reason would be to run it with the FT-101, but I figured I'd get more service out of the tubes running them as Class C - P.A. in a transmitter. The only drawback is the lack of mod iron here.
Phil
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« Reply #18 on: October 01, 2009, 04:32:18 PM »

Brett,
I did the 2x813 linear thing before building the 813 plate modulated rig.  That was about 10 years ago. If I remember correctly, the amp was able to produce a fair amount of power. I don't remember the number. It was a grounded grid configuration and I drove it with a Johnson V2. The down side is having to drive the V2 to full power to get that the output that it produced. It wasn't the best way.  I beat the crap out of it and eventually wasted the power supply transformer. Also, I drove it without an input tuning circuit so the V2 was running at near full power.  Rather than repair it, it morphed into the 813 plate modulated rig I have today.  I always wanted a plate modulated rig anyway so it worked out good in the end.
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« Reply #19 on: October 01, 2009, 08:43:02 PM »

For people with a junk box of big tubes, but no mod iron, a good thing to consider would be cathode modulation.

There are circuits in old handbooks to modulate a pair of 810's, for example, with four parallel 6l6's in the cathode circuit.  No transformer at all.

I wonder if it would work with 813's? Might want to triode-connect them.

You should get more power out than you would using the same tubes for a linear.
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« Reply #20 on: October 02, 2009, 03:21:53 AM »

The article I have for a cathode modulator reads 6Y6's; more gain than the 6L6. For every 200mA of plate current drawn, add a tube in parallel. It uses a cascaded 6SL7 for the mic amp. I've built one with a single tube to modulate an ARC-5 transmitter and it works, but I haven't really been able to play with it yet. Maybe I'll hook it all back up and experiment with it this weekend; the command set is on 40M.   
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The Slab Bacon
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« Reply #21 on: October 02, 2009, 08:01:42 AM »

The article I have for a cathode modulator reads 6Y6's; more gain than the 6L6. For every 200mA of plate current drawn, add a tube in parallel. It uses a cascaded 6SL7 for the mic amp. I've built one with a single tube to modulate an ARC-5 transmitter and it works, but I haven't really been able to play with it yet. Maybe I'll hook it all back up and experiment with it this weekend; the command set is on 40M.   

The reason 6Y6s were popular for cathode mod circuits was their low plate resistance, and still having the gain of a tetrode.

I have always wanted to build a cathode modder using 6080s (or 6AS7Gs) instead. They are very low Mu, but also have the lowest plate resistance of any commomy available tube out there. They also have a very high current handling capacity. 2 of them with everything in parallel could prolly do a 4x1. You would only need a few watts of single ended audio to drive the grids of the 6080s.

Another thought for cathode modulation would be to use one of the newer high voltage NPN power transistors like used in later model solid state TVs for the horiznotal outpoot. High current, high voltage, low impedance............whatz not to like. Build a small solid state speech amp and the whole modulator circuit would fit in an area less than half the size of a pack of cigarettes and mod a 4x1!!

                                                                   The Slab Bacon         
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« Reply #22 on: October 02, 2009, 09:35:17 AM »

6080s would make a nice direct coupled screen modulator too.
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KM1H
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« Reply #23 on: October 02, 2009, 10:42:39 AM »

With any "economy" modulator you have to take into consideration the plate efficiency plus the lack of 100% or higher peak modulation which limits the effective signal at the other guys receiver. IOW, a 100W carrier modulated at 100% has the same talk power as a 400W carrier modulated 50%. Substitute your own power and modulation percentage ratios.

A linear amplifier reproduces what it receives so a high level 100% + modulated input works best.

Ive built and/or used most economy methods. They are fine for converting a CW only rig on the cheap but show up the shortcomings when signals are weak or slop bucket QRMed. I use a modified ( with the most audio I can get without excessive distortion) Knight T-150A at the cottage and have had many tell me I dont have the punch as I do at home with a plate modulated rig of similar power. However it is still better than many of the undermodulated signals often heard.

With one or two 813's and flea power drive its best to get out the calculator, do a lot of reading that explains the difference between theory and attainable modulation percentages, and plate efficiency. Then pick what works best. With other tubes such as 4-400's the choice might well be different.

The economy methods would be control grid, cathode ala Jones, screen, and controlled carrier screen; leaving out a few exotic methods that have surfaced and died quickly.

One other thing, there certainly is no lack of iron for audio up to 120W or so and some industrious searching will find the next step up in the 250-300W range often at very low cost.

Carl
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« Reply #24 on: October 02, 2009, 11:44:16 AM »

Yeah Carl,
Quote
One other thing, there certainly is no lack of iron for audio up to 120W or so and some industrious searching will find the next step up in the 250-300W range often at very low cost.


I guess you could take a couple of 125 watt Triad M12AL's and parallel them too.  (plate to plate, ground to ground, etc.  Don't try to series the windings, may be quite sparky.)

Might be fun to play with the impedance taps.  All resistances cut in half for a rig that'll draw twice the current.   Grin
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RICK  *W3RSW*
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