The AM Forum

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

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.

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

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.

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
KM1H

I thought I had read that linear efficiency ran around 50% ?

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.

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

 

How is the input VSWR of the amp?

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
WD5JKO

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.

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

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.

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.

I seem to remember early software versions of power SDR had problems with  under modulation on AM.

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

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.

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

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.

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.

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         

6080s would make a nice direct coupled screen modulator too.

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
KM1H

Yeah Carl,

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.   ;D

If considering the 6080, pls. consider the 6336. Over twice the capacity (400V/400mA/30W vs 250V/125mA/13W) and some have graphite anodes.

Because some of the RCA tube manuals have errors in the regulator section regarding type numbers, I have attached a corrected table.

A good candidate for the transistor idea might be the HD1750FX, rated 1700V/24A. But any of these are going to take some base drive, since they have Hfe of about 20 at a low current of 0.5A. Most don't specify the Hfe at those low currents because they are designed for switching. Going to need some protection as well. Maybe a Darlington would help, like a BU941.

Before I did all that fuss I'd try to use a tube. Or a transformer coupling from a good amplifier. The best fidelity sound I ever got was using a 32V/4A:120V power transformer to step up an audio amp. The thing to be watched when transformer-coupling a cathde modulator is to avoid driving the cathode positive to the grid to the extent of saturating the tube's current with audio rather than RF during modulation peaks.

I have a crap load of 6080's, always wanted to do something with them.
I use one to regulate the screen voltage in the 813 rig (adjustable and regulated), but they must make great driver tubes for class B modulators.

I got on for a very short while last nite, started out on the 32V3, the wife said I was taking out the TV, not sure if it was live, the dvr, or vhs tape, but I switched to the flex barefoot and that was ok.
Not sure if it was the 32v3 or just the power difference.

I was just wanting to play with the flex with something more than the 25 watts out, and since the 813 deck is all adjustable, I thought I would try it, but I think things are too critical in adjustment when they are grid driven.

I have used the flex to excite both the 3x4D32 rig on 40 meters, and the 813 rig on 80 meters without problems.

Maybe I should think of building a 4x813 grounded grid amp, maybe 80 meters only, that would be simple.

I dont have a lot of big tubes around, just a bunch of 100TH and 100TL's, a bunch of 4-125's, bunch of 813's and a bunch of 811/812a's.

The power supplies are already built, so that saves a lot of work, just need to put four 813's and an output section together, homebrew filiment choke?

Time to read up on grounded grid amplifiers...

Brett

Brett,

   That combo would sure work well and do an easy 250 watts output on AM, or 1KW + on SSB. Since you are talking a one band rig, consider resonating the filament choke. I recall this was done in an old Bill Orr handbook (1947?) with a 304TL G-G amp (6 db gain).  I bet the Flex would have enough PEP to drive this thing you are considering. Now, do you have a 10V @ 20A filament transformer?

Regards,
Jim
WD5JKO

I looked through the iron collection last nite.
I do not have a 10 volt 20 amp transformer or anything close.

So I am looking for one, a clean nice looking one, I have stuff to trade for one, some utc LS audio iron, cg chokes, etc, or can pay cash.

The tank circuit stuff I think I can do, make a coil out of copper tubing.
I might be able to make it go 80 and 40 meters as I have big switches for the coil.

I have the tubes, not sure about the sockets.

What to do about the filiment choke?
I have a home brew one out of some hamfest special, wound on a ferrite rod, the wire looks big enough.

Anyone know how much power it would take to drive four 813's in grounded grid?

Brett

I don't have a 10v 20a, but I have a pair of Thordarson 21F19 rated at 10v 12a ct.

They are NOS in the original boxes un-touched by a soldering iron.

I would be willing to trade for any kind of UTC LS series line to grid transformer -- low level is fine.

Not sure that would be a good deal for you, though as those LS input transformers are in demand by the Audiophools big time.

Dave

Dave,
I have a few LS mic to line, mic to grid, as well as some plates to grids bigger (20 watt?) ones.
I dont know how I would use 2 seperate filiment transformers into one choke and 4 tubes though....

I found I have 4 813 sockets, the plate choke, need to check for the pie net caps...

Brett


 

I was thinking that since the filament transformers are identical, you could get away with paralleling them for 20+ amps.

What think others on this?

Rather than parallel them, use one transformer for each 813.  Then you could monitor cathode current on each individual tube, if you wish. 

If you plan to use just one filament choke, parallel the transformers and parallel all tube filaments.  The transformers will work just fine, just make sure to get the secondaries phased correctly.

73,  Jack, W9GT

I dont think I would paralell two filiment transformers, that sounds like a bodge to me.
Fair radio sells a Hammond trans 10 vct at 20 amps which is light for 4 813's but would work.
I always like to run things at no more then 75% load....

Brett

Do what you want, but it works fine for me.  Been doing that for years in one rig and never had a problem.  Homebrewing frequently requires making the best out of what you can come up with in the way of parts.  Unfortunately, the perfect solution is not always possible or affordable.

73,  Jack, W9GT

I learned a long time ago when Jack Speaks...this Jack Listens real Close.....

73
Jack.

A lightly loaded filament transformer usually produces excessive voltage, especially at high line voltage.

With around 3 KV a 100W rice box will drive 4 GG 813's to 1500W thru 20M if using a tuned input. About 1100-1200 thru a .01.

Carl
KM1H

They need bias at that voltage?

Brett

With a wide range of choices from well used to NOS and multiple vendors and construction its hard to say. Best bet is to static test for 4 that are closely matched and use bias if necessary which wont be much in any case.

Carl
KM1H

4-125s are a better RF tube than an 813 when you go above 40 meters. They will work well in GG.

That sounds like 813's that are getting soft. Ive had no problem up thru 15M where the power is dropping a bit. Output 20-160M is pretty constant.

The 4-125 is a nice tube. I have a pair running 450W out on 2M as a FM amp for contests. The big amp and the long yagis stay down at the low end of the band.

Carl
KM1H

I have no plans on using it above 40.
I have about ten 813's and four 4-125's and one 4-250 plus a bunch of 100TH's and TL's.

I have four 813 sockets, and four 4-125 sockets, a nice plate choke, chassis, front panel, meters, a 5vct at 30 amp filiment trans.

So I suppose I could do the four 4-125's in grounded grid but think 813's would be better.

Brett

Id start with 813's, you can always change over.

The 813 likes HV better than 1000-1500V, it really wakes up over 2500V and its a good way to get close to expected output from slightly tired ones. I havent pushed them beyond 3KV but others have reported no problem at 3200V on AM. Its always a good idea to high-pot unknowns anyway. Ive run mine that way up to 5200V; didnt have the 13kv unit finished at the time.

Carl
KM1H

The power supply will do 3000 volts at 500 ma ccs.
It should be ok with 700 ma icas.

Four 4-400's would be interesting, but I no longer have any.

Brett

Just leave a little extra room between the tubes so you have options. Quad 813s does sound cool. Blow a little air past them and they will love you back.

I have all the parts to build the 4x813 amp, except the filiment transformer, but dont know if its a good idea, as I read on the web that the 813 and other older tubes are not very clean in output when used in grounded grid, not like tubes designed for grounded grid service.
a pair of 3-500z tubes would likely be much better?

Or just stick up a liquor store and get a real amp?

Brett

I haven't built or used an 813 amp, but I can say that you can sure find a lot of crap on the web.

For  just about any tube or circuit you can find someone who posts how useless it is, and then there will be 10 other guys who agree or post a 1/2 a$$ed analysis why the original poster was right (or wrong), and then you find a post from some old guy with a two letter call who says that in the 1950's he built a rig using that circuit or tube and it worked great and he got great signal reports.

Lots of tubes get used for lots of things that they were never intended for.

I have an old Eimac 6c21 which is a huge old WWII triode similar to a 450tl designed as a radar modulator. The regular datasheets don't even list use as a class C telegraphy or telephony spec -- just a lot of radar pulse specs. But then, I found an Eimac datasheet with a much newer copyright date where they rated the 6c21 as a gg linear amp. It was certainly never designed for that use.

I'm hoping some day to build an amp around that old beast.

I'd go by what the people here on amforum say rather than articles on the internet.

As I said, I'm not an expert, and can't recommend 813s myself, but there are plenty of people here who have (and do) use them successfully

And AMforum members have a lot more credibility (and expertise) than random internet articles.

That is true, but just because someone builds something and it works, does not mean its clean.
I know there is a big difference in intermod products between various tubes.
The problem with 813's is there is nothing published about using them in grounded grid.
From what I read, a tuned input helps clean things up some, but I would not like to build something that generates a lot of trash.

Brett

Info

The 1970 Radio Handbook, ed. W. Orr, page 712 has the following.

"Some of the best g-g performers are the 811A, 813,7094, 4-125A, 4-250, 40400A and 4-1000."

There is a mention of signal to distortion ratio and also the ' x-xxxZ type tubes.

As Napoleon Dynamite would say, 'I like the Radio Handbook '.


klc

Yes, but if you look in the 'care and feeding of power grid tubes' some of the tubes you listed are poor when it comes to imd.
The book I have is old, and does not list the modern types, or the 813, but the 4-xxx series are poor imd performers.

When talking IMD, does that show up as grunge around the signal, or would it be audible?
Say you take a low distortion AM signal and feed it into a poor IMD amp, does it SOUND poor, or just get wide?

Brett

One dunce on QRZ tried to imply that. Same dunce has never worked with said 813.

Another dunce on QRZ tried saying there was not much chance of finding good 813s.

Both of these guys seem to have an answer for EVERYTHING.

That's the biggest problem with that site. People like you (Brett) remember most of what you read and it sticks in the back of your mind. Take information with a grain of salt unless it is peer reviewed or from a trusted source.

There isn't anything wrong with 813s in grounded grid service. They have been used by a lot of us as class B modulators with good results. The grounded grid configuration will be cleaner than simple grid driven modulators without feedback.

I have an old 4 holer 813 here. It had been used previously by another hamateur for condomtesting on ssb where I calculated dissipation of each tube to be over 300 watts!

Brett,
As mentioned in a previous post, I did the 813 linear thing. At the time, I had available to me a spectrum analyzer. The amp was clean or within amateur practice. You should not have any problem operating a 2x813 or 4x813 GG rig. Granted it is not the best way to increase power level but it worked OK Fine. The 813 is a sturdy cheap ass tube.

I suppose I will order a filiment trans from fair radio and give it a try!

Brett

My Three 813's worked fine GG so an I just copied the circuit off of a Dentron circuit i liked an used the same or close as I could find components...but I do have a 3 kw supply I can draw on for alot of things that helps... :D
it'sa kick butt supply..LOL...Pick a voltage you wanna operate at and build to that spec...build heavier the sky's the limit here and the right hands are here to help...so your good to go start building an quit worrying.

Get-R-Done.. :)

73
Jack.

I have some massive components, on a 17X17 chassis, there is not much room left for the tubes the caps are so large....

The power supply is already built, I dont need any sort of TR relay since I use the station one, and plan on driving it with the flex, using a seperate receive antenna port. PTT is already done, so I just have to build the rf deck and hook it up.

It sounds like a tuned input is the way to go, which complicates things somewhat, unless I can get one circuit to cover 80 and 40 meters....or would the built in antenna tuner in the flex work?

I plan on making 2 big plug in tank coils out of copper tubing, one for 40, the other for 80 meters.

If I had more sockets, I could do 6 813's....

Brett

Bill Orr reported the 813 as one of the cleaner tubes for GG with 3rd order in the -33 dB range at 2500V. Thats not too shabby for a 70 year old tube and a lot better than some of the Russian crap some idiots keep praising.

Plus its still GIGO and your average ricebox cant even get out of the high 20's

Carl
KM1H

Here is the layout so far, with the 40 meter coil in...

About the meters, what will be the plate current and grid current for 4 813's in grounded grid?

Brett

Nice looking amplifier, Brett!

With 4kv on it you cud get out an easy 2KW+ pep.  I've always liked 813's in most any config -  linear GG, class C or class B triode-connected modulator service.  Six 813's - why not?  I have the tubes and sockets to do twelve. Almost did it. Quiet amp with little blower noise. But I opted out for Quadzilla instead to get a good glass fix.. ;D


Carl, did you say the IMD on some of the Russian tubes was poor?  I have long wondered how that wud work out now that there are many in the field. The prices have soared compared to a few years ago.

So what are you hearing and seeing for IMD these days with the Russian tubes?

I've heard a few on that were truly bad news, but didn't know if the amp had a problem itself or it was the tube.... :o


BTW, to answer a question from a previous post - If the 3rd order IMD is about -15 to -20db or better, you wud not hear the distorsion on freq, just up the band. IIRC, a -30db 3rd IMD is equal to .1% distorsion. That's literally impossible to hear on frequency, especially on a noisy band.

T

Tom, Ive personally only used the Svetlana 4CX400A and 4CX800, the latter especially needing a load of idle current to get IMD out of the 20's.

Ive seen SA plots from several triodes, from people I know and respect, that are pure crap in the high teens. I dont have the types handy ( I can never get their Ruskie designations memorized) but they are ones commonly used in various conversions and HB, usually at way higher than spec voltages.

Remember, none of those tubes were designed for linear service. They are all for military radar, and Class C comm gear. Try and find curves on any of them.

IMD may not be that important to you basement dwellers but a crappy amp can ruin it for all on 10/15M and up where I tend to spend a lot of time. Up there I even bitch about 4CX250B's ;D  On the busy low bands it all adds up until at some point it is just band noise (white noise) since it keeps mixing and remixing . Its similar to what CATV trunking amps have to deal with and there the IMD has to be in the 40's or better.

You coming up for the canoe events Friday/Sat?

Carl
KM1H

 

A good control grid current limit is the class C oscillator spec if the concern is tube life. 10mA for the control grid and 40mA for the screen. Will you connect the supressor to the filament's RF for a little degenration or to GND? I think you got an experiment to do.

Carl has a good point. can you up the Q on the pi-net to help with the distortion?

BTW - just keep adding 813's and you won't need a plate tuning cap. It is tricky to get a pair to work efficiently on 10-12M but the lower bands should be fine.

In the 1962 handbook, the article on page 194, "One Band Kilowatt Amplifiers" showed a stack of parallel 813 amps with a common power supply. This was called the "ne plus ultra" by the decently educated author.
The 10M one had a tap in the pi coil for the tuning cap, so that it was plate-PI-coil-C-tuning-PI-coil-C-loading. This was said to help with the large plate capacitance of the 813 pair. These were not GG amps but control-grid-driven amps with bias and screen voltage. Maybe this will help with the plate circuit design.

The HT-44 "CB leenyar" of a friend that had two 813's in GG converted from the original unobtanium GG 7094's was never very efficient on 10M. It also tore up TV sets real good so attention to the plate Q is a possible help.

Nope, a "reasonable" Q will have little to no effect on IMD cuz IMD mainly affects the freqs within +- 10kc or so. Q would have to be VERY very high to come in that close in the tank bandwidth curve to have any effect on generated side products.

OK on the Russian tubes. Wouldn't ya know it. Well, I thought there were some decent ones that were designed for GG triode service. How about that GU-34B? or what is it that has the big copper anode good for 2500w that's a very loose version of the 8877? How are they doing for IMD?

Yes, I'll be heading up for the water balloon fights and snowman contest Thurs nite. The Huzman will be here tmw morning and we want to play with some toys during the day. He wants to get on the DX portions and have some fun at night.  I'm looking forward to seeing his impression of the 40M 2x2x2 on the air. He'll be on as HUZ in VA so it oughta be fun.  If he likes being king for a night, ;D I'm hoping to get him to put up a 2el 75M wire Yagi and build up a new linear. There might be some parts at HossTraders, ya never know.

See ya there, OM.

T

 

rotate the plate tuning cap 180 degrees plates up so the connection to the stators is shorter.

I will have to see if the cap can be inverted.
The tubes will mount under the chassis, lowering their height, plus the caps need to be aligned so the shafts are at the same height on the front panel.

Brett

The old timers advice of short and wide plate leads and longer grid leads still works especially with tall triodes. If you can keep them from becoming a TPTG oscillator you have half the battle licked.

Tom, I dont know one Ruskie designation from another. I'll dig some stuff out and we can yak about it while attaching chains on the vehicles.

For all pratical purposes IMD is a function of the tubes internal construction and applied voltages and currents. Dont confuse IMD with operator caused splatter, totally different animals.

Carl
KM1H

All it takes to invert the cap is a couple L brackets held by the screws coming through the end plates. Inverted will also reduce the minimum C required to get on 10M. Also an easy way to adjust the height.

I have a pair of 813's built GG that I saved from certain doom. Pretty nicely built except for the stand alone P/S, which has a pretty anemic looking plate transformer and sloppy construction. I finally got some Millen connectors for it, so I may work on it this weekend. I'll take some pictures of it and post them on another thread.
Phil

There was Pictures of mine on here somewhere But I can't find'em I know Da gone well I put'em on here...maybe i'll take sum more an post'em....

73
Jack.

What is everyone doing for the rf input?
From what I read, a tuned input cleans things up and provides more power output.

I have an MB40L thing, supposed to cover 80 to 10 meters with one knob, has a link input, and single terminal RF output. It was two circuits, one series coil and cap, one paralell coil and cap, hooked up together to cover 80 to 10 with one tuning knob and no switches.

I have also seen a circuit where a cap tunes a coil the filiment power runs through, and also a pie net type circuit.

Or would the antenna tuna in the flex do it all?

Since its supposed to rain a lot this weekend, I may start drilling and blasting....

Brett

I suppose I have to study more about IMD.

I do know the MB-40, if that is like the MB-150?. The HF band is divided in two and folded back across the shaft rotation, with no tuning position harmonically related to the other band. So at any position, it will resonate to two unrelated frequencies. The range on the 150 is 3.45 to 8.5 and 12-30. I'd suggest an escutcheon!

I have an 150, but have never built it in because of the lack of 160M and the gap from 8 to 12 MHz. These things might be fixable with a relay and some caps though the L may be lacking on the low end. I put the mnaual up, I hope it might help. Did you have the manual on the MB-40? A comparison would be very interesting.

One possible way to use it comes to mind: use the low-Z (link) connection as the tuned circuit for the input, and leave the hi-z connections alone. This is what I did to grid-dip my MB-150 to play with it.

Could the Q of the MB-40 be be lowered as people wish with GG amps by loading the high side with a non-reactive load? It (MB-150 anyways) otherwise is rather sharp.

IIRC though it is a hi-Z to lo-Z device intended to go from feedline to plate, or driver to grids. Not really for driving a 50-ohm to a cathode. I don't know if it would do for a GG input coupler.

Here is a schemo on it witn 813's, but not GG.
http://www.wb4gwa.netfirms.com/final.htm

You want a low Q Pi or L network for a tuned input to the cathodes.

That Milton Bradley thing is a nice toy for another project. I have one here and now know what it does. Thanks!

The Flex tuner can probably handle the task at the expense of ever so slightly more (unnoticeable) distortion because the match doesn't exist right inside the amplifier.

I sell a 6 band PC board for GG inputs for $20 shipped in the USA. You provide the toroids, wire, mica compression trimmers, and 12VDC DIP relays.

Carl
KM1H

Brett,
Very nice looking deck but I have my doubts that you'll get sufficient capacity on 160 or maybe even 80 with the plate tune cap you've mounted for 4 each 813's in parallel.  Check with any online Pi net calculator.
Your cap seems to be a couple of 100pf in parallel.  You'll need more than that unless your run 5kv+.  ;D

You could parallel the tune cap with some decent 200pf/10 to 15 kv door-knobs.  Maybe you've already planned to do that on 160 anyway. (the old last band switch position to a loading pad trick, trick #611-405A in a series.)

I'd try to find a good 500 pf, 7.5 to 10kv or better vac. variable. May have to pad 160M  anyway.  4 813's really suck down final load resistance.

Well, I got it done and tested, and at 2000 volts, I get 200 watts carrier out with about 900 watts pep.
That is four 813 tubes at 2000 volts in grounded grid.

I need to rack mount it and hook it up to everything, and make some power supply changes to get up to 2500 volts or more.
A nice boost for the flex from 25 to 200 watts carrier out.

The only thing I had to buy was the filament transformer....

Brett
 

Brett

Got a final pic with the 4 x 813 in its home?

Al

I will post some pix tonight.
I have lots of work to do, pulling both power supply decks for the 2x813 AM rig and modulator, and swap transformers, the one in the RF deck will give 3000 volts, the one in the mod deck does 2200 volts under load, so I need to swap them out, I will be using the mod deck power supply to power the amp, and besides changing the iron, need to check the voltage rating of the oil filled caps on the deck.

I also need to install some feed through insulators between the two cabinets, and make up an amp key cable between the flex and the cabinet/ amp.

I also need to slow the amp fan down, its too noisy.
I plan on making a better 80 meter coil, I am using one for the 3x4D32 rig at the moment.

At 200 watts out, the tubes show no color, and the signal sounded very good in the mod monitor.
Four 813's put out a nice amount of heat, so the amp will make a nice shack heater for the winter.


I was a bit surprised at how many good QSO's I had with just 25 watt out.

Brett

pix...

pix

Hi Brett,

The amp looks FB. Nice attention to details and you used quality hardware, OM.

That amp shud serve you for a long time.

One good point is how quiet it will run, since 813's require such minimal air cooling.


BTW, I'm in the process of converting Quadzilla (4X1 linear) over to a class C plate modulated rig using a 4X1 modulated by a pair. Shud have it on in a week or so.  Like you, I'm in the midst of the building fun right now and always good to see others' work for inspiration.

Later -

T

Quadzilla Squared, cum awn.  8)

Very nice, it gets my blessing ;D

Carl

A 4-1000 modulated by a pair?
Sure you will have enough audio?
You might have to turn it down to 300 watts carrier.... :P

I am starting to scrape the bottom of the parts barrel.
You need to start getting creative unless you want to spend big bux on parts.
I have some stuff set aside for a homebrew CW rig for 80 and 40 meters, a trans receiver of sorts, but dont know if I will ever get going on it, maybe I will when it snows, sure would like to get back into CW.


I was thinking about getting a 3cx1200 type amp and running 220 volts into the shack, but the 4x813 amp should do.

I put a 400 ohm resistor in the fan power and got it so it moves air but is almost silent.
I really like rigs with no fans, the push pull 812a rig was best, no variacs (buzz), no fans, a silent radio.
The 3x4D32 rig has no fans, so its quiet, the 2x813 with 4cx250b modulator has blowers and fans but on variacs so I can slow them down to a low noise level.

The amp is in the rack right next to the operating position, so it needs to be quiet.
I took out the antenna tuner, the 32V3 and homebrew receiver number 1 and put the amp in there, no place to put that old stuff...  4 racks (and no room for more) and a desk full of stuff...

Brett

Brett,
Just a safety suggestion; The B+ connection on the outside of the chassis, the HV wire you connect to it, you may want to put some sort of shield or boot on it so you or anyone else doesn't accidentally  come in contact with the exposed metal.

Nice job! And clean too!

I was thinking the same thing Bob. How about a right angle spark plug boot.

All my equipment is like that, but its all in rack cabinets with doors that lock.

I have some amp questions, never having had any, or anything grounded grid....

I get 200 watts carrier out and about 900 to 1000 watts pep at about 2000 volts.
I can load it heavier and drive it less, or load it lighter and drive it more, and seem to get the same results withing a wide range,  what is the best way to run it?

I dont seem to get much of a dip in plate current, but get a nice peak in power output. That seems odd.


I only seem to get about 1100 watts output on ssb, is that low?

I can only seem to get about 1100 watts pep out on AM or ssb, even if I turn it up to 2500 volts.
I suspect I could turn it up to 3000 volts on ssb and get more since the duty cycle is low, but thought I could get more out at 2500 volts....

Brett


 

    For AM, I would log the variables and compute the plate efficiency while looking at linearity with a scope. Tune for best linearity at acceptable efficiency. With the Flex 5000 driver I suppose doing Trapezoid pattern would be a challenge? if not, using the Trapezoid is very handy for tuning any form of AM linear or efficiency modulated amplifier. If the exciter is very clean, and a Flex 5000 should be, then maybe modulate with a triangle wave, and use the scope as normal envelope pattern. This might be easier than doing the Trapezoid pattern when adjusting the big AMP.


 Maybe. I wonder how long the coax is between the amp and the Flex 5000 ATU? Without a tuned input to the 813 cathodes, you are likely loading down the drive signal such that every other half cycle is compressed. A tuned input in the AMP, or a very short cable should help. Some of the big AMP builders say that using RG-8 over RG-58 on the input makes a difference when a tuned input is omitted in the amplifier.

That AMP sure looks great Brett,

Jim
WD5JKO

I dont think I have anything that generates a triangle wave, I will have to look.
The flex has some built in tests, 2 tone, freq sweep, but I dont think it does a triangle.
Maybe the 2 tone and the spectrum analyzer will show something....

I ran it up to 3000 volts last nite and got about 1600 watts pep out on sideband.

The tuner in the flex gets a good match to the amp, and the coax is rg8x and about 5 feet long, I will try some rg214 between the radio and the cabinet.
I dont think I want to add a tuned input to the amp, as that would make another thing to tune, and I dont think I have space to add a circuit and have it look right, add a switch, etc.
I dont understand why more drive does not do the same thing, more voltage, right?
Drive requirements are quite low, about 10 watts on AM, 30 watts pep on ssb....


The plate current meter seems worthless for tuning up, it basicly does nothing, the power meter is what I have been using, it shows changes in loading and tuning, the meter on the power supply matches the meter in the amp, but they just read about 400 ma at 200 watts carrier, and while you can load it heavier, and play with the plate tuning, it does not change much, other than go up a little bit when you load it heavier....

I wonder if I seperated the various grid returns off ground would improve things, but the design I found on the web did not do that...

I got it all intigrated into the station last nite, and changed the power supply trans, and one cap in the power supply from 3000 volts to 4000 volts so all the caps are good for 4000 volts.

I also did some tests last nite and the xyl did not start screaming about me getting into the TV, I wonder if that will hold up. If it does, I might be able to get on a bit at nite....

Brett

In leanyour mode the yamplifier should be loaded heavily enough that you shouldnt see a real large dip at resonance, but you should see something. Try loading it lightly and check your dip. then bring up the loading, and touch up the tuning again after you load it up. Dont forget in leanyour mode, even though you are only running a couple hundred watts of carrier, you have to have it loaded heavily enough to make the audio peaks.. (henceforth the small dip at reduced drive)

Here is another thought I was pondering............... A quad of 813s is still capable of LESS dissipation then a quad of 572Bs (500w vs 640) why not use a quad of 572Bs instead. They are very happy in GG service and will make the same or more power at a lower plate voltage??  (1800-2000V) The 813s are prolly gonna want somewhere around 3000v to get all warm and fuzzy.

Keep in mind that a pair of 813s will easily make well in excess of 1000w PEP in plate modulated service. (IE the K1JJ "Tesla 360")

Just a thought to ponder.......................

                                                      The Slab Bacon

                                                      

Well, I have a bunch of 813's, but only two 572b's.

My main rig for the last 25 years has been a pair of 813's which will do 700 watts carrier and off the 2kw scale on the power meter.

I am jumping on the flex bandwagon though, and need some power out of it.
Its just so interesting to use, and it has lots of stuff to play with, you can set the TX bandwidth to whatever you want, 0Hz to 20KHz, or anything inbetween, it has little or no distortion, and can be flat in response and has audio processing built in, with more stuff coming.
I do use it as an exciter for the 813 rig (rf) and use its receiver, which is the best receiver I ever had by far, and for AM, is likely the best you can buy.

Actually, I think they have some sort of subliminal program running that brainwashes you.

I always took a lot of pride in operating an all homebrew station, receivers, transmitters, antenna's, but I got to say, if I had to get rid of all that and just use a flex 5000 and an amp, I would be happy....

Which got me thinking the other day (makes my head hurt) I am 51 years old now, and if I had to move, or wanted to move, what a mess I would have with a basement full of rack cabinets filled with all that iron and worthless homebrew stuff.
No one would want it, even for parts say 20 years from now, and I would have to pay someone to lug it out of the basement and put it in the dumpster.
In 20 years, there will likely be very little interest in tube radio, and there will be a glut of restored vintage old AM gear that no one wants on the market as old farts retire, crap out, etc and have to get rid of the basements full of gear....
Its not like there are loads of teenagers lusting after Johnson equipment....Vintage tube stuff is a dead end...

Brett

 

I would think you have a low tank Q if the dip isn't deep. Try shorting a turn of the tank coil and see if it improves. Another way to check is measuer the values of the plate and load caps at full power and look up what they should be at the operating Z. (power it down to measure them ::))
The power going up when you increased the voltage because it could run at a higher Z

Yes, I should experiment with the coil.
As it is, I think the plate tuning cap is 3/4 closed.

But, I get a great peak in power output as I tune the plate...it seems quite sharp....

Brett

Don't count us all on that hand.  KE7TRP, myself, a pair of friends in SoCal licensed in the last 6 months...  We ALL love homebrewing and restoring. 

There is also a BUNCH of 11 meter operators that have radios that are museum pieces...  And they are ALWAYS looking for more radios.  I know of someone in the great Northwest that has an entire Johnson AND Collins collection, and they all work.  He chooses to run his Flex, but every radio he has, works, and goes inline at times.

Some of it is attitudinal.  Youngsters get crap from "old timers" because we didn't have to learn CW, the tests are easier, etc.  I have a friend who is now an active amateur that isn't doing ANYTHING different than he did on 11 meters, except using more bands now.  And the big turnoff to him 5 years ago?  The attitudes he encountered tuning as an SWL.  I'm not bashing EVERYONE, but it IS a problem, and one that gladly is slowly dying off (bad choice of words).

Yeah, in 20 years there won't be the market their is for them now, but 20 years ago, we where throwing them in the trash because solid state was all the rage.  People WILL get tired of silicon again, and you STILL have EMP to worry about. 

Remember, Tubes have guidelines, transistors have ratings!  Those of us in the know won't be switching ANY time soon....  Even as it gets harder and harder to find parts (it was MUCH cheaper to build a kilowatt in the 80s, even CHEAPER to build a 10 kilowatt (8 3-500Zs was < 300 dollars in 81), people will still be running these things.

I remember 15 years ago, when the internet was just starting to be useful to the general public (and even then, not many had immediate access to it), that was going to be the end of the mail system, the end of the telephones, etc.  Cell's where a dying breed.

Don't count us 30something whippersnappers out of it, I'll be running iron for YEARS to come.


--Shane

Those of in the know, know how to design using solid state components that can take a beating.....but tubes are still cool

Heck, when I was 20 or 30 years younger I dreamed of having nothing but solid state stuff. Not sure what change my attitude, maybe it was working with tube audio gear, but that change it.  Then hearing AM'ers talk AM, tubes and iron and homebrewing. Then there was something about seeing a device glow (a good glow not a bad glow).  Tubes being hot are cool!   

I did some experiments with the tank coil, no change in the lack of any dip.
I get a nice sharp peak in power output, but loaded light or heavy, no dip at all.

I must be missing something...

The grids are to ground, the current meter is in the filiment return, which should show grid current, the power supply meter should read correctly though.

I will change the meter in the amp to the power supply return terminal and see what I get.

Brett

Either you left out a couple words or the amp could be taking off

Brett,
        If you get to the point where you can't stand all that old gear collecting dust in the basement anymore, just give me a call. I've got a truck!

Phil

The amp seems clean, and no rf in, no rf out, and it sounds good in the mod monitor.
I did not think grounded grid amps tended to have problems with parasitics and so on....
It also works ok on ssb, with no odd stuff going on.

Brett


 

Maybe your metering is not configured properly and you see grid current plus cathode current.
Sounds like you can crank it higher since you pass the XYL radiated susceptibility test. 

That is what I thought, and changed the meter shunt to the power supply return and got a grid current reading, which seemed to not change with tuning.

No rf in, very little plate current. Bias is set at 7.5 volts using diodes in the fil center tap return, a 50 k resistor in standby.
2000 volts on the plates.
Put a watt in, get some current, some power out, no dip at all, but a sharp peak in power out.
Put a little more power in, get more power out, same peak in power out, lack of dip.

I can adjust it so I get 150 watts carrier and 800 watts pep, over a range of loading and power input settings, but at the extremes, peak power falls off, like it should.

I have all the supressor grid directly to ground, the other two grids go to ground through 10 ohm resistors.
I thought it would be best to go to ground, with short paths, rather than cap the grids to ground for rf and put the dc to the fil center tap, which would take the grids current out of the plate current picture.

I checked it on the spec an last nite and it looked ok, seems to work ok, why no dip?
No dip at all, the plate current goes up slightly when the plate tuning cap is moved in one direction, it seems to do nothing in the other direction, output power decreases evenly on both sides of the power peak....
Loading works normal, more loading, more power out, more current.



Brett

What is the value of your plate tuning cap. I suspect it is too small and the circuit is acting like an L network or a low Q pi network. I think the plate tune should be at least 300 pf but have not run the numbers.

I figure at 2500V anf 2400 watts in for 1500 out
Plate 280
load 1281
L= 8uh

I bet that inductor is much bigger than 7.8

I think the plate cap is 300 pf.
2000 volts in, 300 ma or less, with 60 ma grid current, so its more like 220 ma in, but say 600 watts in, 150 out, 450 watts plate dis for 4 tubes with 125 watt dis.
The plate tuning cap is 7/8 closed.

I shorted a turn and could not get it to resonance...but that might not have been done right.

Brett
 

that plate tuning cap is the good old national 100 DA... 100pf per section, I have 2 of them here.0

I counted abt 17 turns on that coil.

Try putting 500 pf across the loading cap and see what happens. If the power drops down but dip is deeper try shorting the coil or just spread the turns.

200 pf is not enough unless the loading is pretty light. The inductor looks kind of large. You will make some real power once you get the tank circuit right.
I have been burned by running the Q down and a number of others have been there also.
Find yourself a 300 pf or dual 150 pf 7000 volt Johnson and mount it stator up. 
Then start lopping off turns and watch the power go up. You may need more loading if it is close to fully meshed now.

Brett,

I have a big ass Johnson here thats at least 200 pf pr section, and at least 5kv. I'll go take a picture of it and post it, it's yours free if you want it - just paypal me some shipping money - I'm so broke at the moment I cant afford to send it out unless I get it first.  ::)

lemme get this picture.....standby.... BTW thats a 8 inch ruler for scale...this mofo is large

Brett,

Yes, as Frank mentioned, we can't stress enuff the importance of using a lot of C in the tank for both C1 and C2 for a linear amp.

For years I used breadslicers and compensated by using overly large inductors. When I finally got some vacuum var caps, my amps came to life.

In fact only recently I thought my GG triode ssb amp was running well. Turned out I needed to add another 500pf to C2 and the efficency (and power out)  came up dramatically. I ran out of C even in a vac cap. Many of the C2's out there are 1500pf. Sometimes on 75M you will need more with a big amp. The 813's are of higher impedance, thus less C is required, but I've found even with that class of amp I've always underestimated the tank C values.

Frank's suggestions will get you seeing better efficiency. Keep adding C and playing with the inductance until the power peaks and then drops off and you know you are close.

I find the power peak when tuning resonance is sharp when the Q is right, but it can be made sharper so that the Q is too high. For a linear, the plate current meter doesn't tell you dick when dipping - it's the power meter that tells the real story.

T

813s are higher impedance but when you have 4 in parallel you go below 2 Kohms in QRO land of class AB. Your dip should be close to max power out usually at a tad more C than dip. When you operate at higher Q the inductor circulating current goes up so it needs to be low resistance.

Derb,
I will pay more than shipping.
Let me know where to pay pal the money to.

It might not fit, but I can try it.

I think the results I am getting are normal as far as input power and output power go.
Say you had a single 3-500z type tube, that is good for about 150 watts out, no?

I stretched the coil and added 40 pf to the plate tuning cap, no change in the lack of plate current dip.
I DO get a nice power peak.

Brett

Forget about carrier power when tuning the amp.

The best way is to use a pulse with a very narrow duty cycle to see the peak power without stressing the amp.. Even a 60wpm CW string of dits is a start. Look at a peak reading power meter and make your adjustments. That's where the amp gets its highest efficiency and the tank circuit should be a good match. There will be a sweet spot in the L/C ratio and tank adjustments where you will see, say 2KW out with a certain drive. Adding more drive just causes saturation... and other L/C combos produce less than 2KW out.  That's how to tune and find the best L/C in the real world.

Then when you set you AM carrier you are assured of having the maximum headroom available for a maximum carrier setting.

Later you can add a little more C2 to load it more heavily for better linearity.

T

I measured the values:
plate tuning cap, 200 pf at max.
Coil, .006 mh,
loading cap, over 2000 pf.

Normal settings seem to be around 150pf and 1100 pf.

The zl1axb design I used uses a 100 pf plate tuning cap and 1100 pf loading cap.
They dont say about the coil...

Brett

More tests....
I put a 40 meter coil in, and get the exact same results, 150 watts out, 800 or 900 watts pep, no dip in the plate current at all. I have the flex set to 70% carrier which explains the high pep.

I dont think anything is wrong with the pie net part of the amp.
Into the dummy load, the plate tuning is set to 60%, the loading is at 72%, output power peak is sharp.

Brett

Brett, on a tetrode amp you wont get much plate current dip when the amp is loaded. it does not act like a triode where no matter how hard its loaded you can still get a good dip. Tetrodes and Pentodes simply dont do that. You get a good dip when the amp is unloaded, and it progressively gets smaller the more you load it.

Heinz & Kaufman recommends that you tune the RF stage of their tantalum plate tubes by the color of the plate, not by using a plate mil meter, for that very reason. you cant do that wih 813's, but basically fool around with the output tank until you get the max blowsmoke you can get out of it, balls to the wall. use a output meter to tune it. H& K recommended a rf ammeter, but we got Bird wattmeters these days. It would be a great idea to send cw dits at 60 wpm instead of making a cb keydown, come on.  ;D

Think power transfer. You want the most blowsmoke out for the least inpoot in.  ;D Lookit the little snippett. You chasing a ghost.

let me know if you still want the cap.

Derb,
The 813s are set up as triodes.
Brett you have to decide on the power you can or want to run. 4 813s should do 1500 watts pep if your power supply is up to it.
Take your plate voltage and divide it by the peak plate current. Then multiply that number by 1.6 for class AB. This is your plate load Z. Assume your exciter will do 100 watts. Then pump 100 watts into the linear and tune it up. 4-813s should handle that drive and also handle making output power. Just watch your input power, output power and the color of the tube plates. This is where you set it up for PEP.
Once you know the peak input power and plate load Z then you have to decide on the plate Q.  12 is a good number. Then it is a simple matter to get component values from a table in most handbooks and a zillion places on line.
Say the table is set up for a Q of 10 and you want 12. just multiply cap values by 1.2 and divide inductor values by 1.2.
At full bore and 2500 volts your plate Z is going to be around 1300 to 1400 ohms. This is pretty low so another thing to look at is the plate coupling cap. I would think 2000 to 4000pf. If your inductor is really 6 uh I bet you are tuning up on 40 meters. It really wants to be around 8 uh.
If you have a signal generator pull the plate caps off and pump a signal into the loading cap with a resistor across the
 plate cap equal to the load Z. Monitor the resistor with a scope.

ack. what I get for not reading the whole post.  :-X

And I should finish my coffee before I post. Divide plate voltage by plate current. take that value and divide it by 1.6 to get load Z at class AB.
Brett, a pi network will work over a wide range but as the Z goes down the values get more critical. I'm not sure of the gain of an 813 in GG but 4 of them should make some real power. i would like to know the gain of an 813 in GG if anyone knows but suspect it should be at least 10 dB.

Well, first off, I had a nice long qso with Eric and Nick today, with plenty of old buzzard transmissions and everything held together.

I also dont have any problem getting lots of PEP out of the amp, at 2500 volts it will do 1100 watts out, at 3000 volts it will do about 1600 or more.

I run it at 2000 volts and about 250 to 300ma on AM. That gives about 125 to 150 watts carrier out.

That gives a plate load of 4166 ohms.
My Bill Orr book says, for 4000 ohms, a plate tuning cap value of 135pf, an inductance of 15.5uh and a load capacitance of 1100pf.
Those cap values are VERY close to what I have dialed in, the coil measures lower, but I dont think the meter I have really measures micro henrys.

It is correct that i cant get much more than 900 watts pep out of it at 2000 volts, but it does not have a tuned input, so output will be lower than it could be...

2000 volts, 300 ma, 600 watts input, 150 watts output, 450 watts plate dissipation, four 813's have 500 watts of plate dissipation.
From what I hear, most guys running amps get 300 watts out of a pair of 3-500z type tubes, so i am in the ballpark.

I have not experimented much with running it at 2500 volts or higher on AM.

Brett
 

OK Brett I see. You are setting your load Z based on the impedance at carrier.
When you modulate the plate current will rise but the plate voltage remains the same because there is no modulator. When you modulate the plate Z dives on positive peaks. This tells me you are losing efficiency on positive peaks due to low Q. At a higher Q you may not notice anything different at carrier but I bet the positive peaks improve as long as you have the drive and power supply.
2 3-500Z have 150 watts of emission.  4 - 813s have 200 watts of emission
Blow a little air past them and watch what they will do.

Frank,
When I had the 813 GG amp (1 pair) if I remember correctly I had a gain of around 8 or 9.

Bob,
I wonder if that was tuned or untuned input?

I know a guy who had four 813's in GG with air and 4KV on them. He could easily get out 2KW pep.  He ran it like that for years without problems.

As long as you run some air past them, they will do that for you. The voltage will not bother them since they will handle 3KV in plate modulated service = 6KV + at 100% mod.


BTW, I've found most power tetrodes, when in grounded grid with tuned input are good for about 10db gain or so.  My 4X1 Quadzilla rig was no different.  We need specialized triodes like the 8877, 3cx-1200, 3-500Z, etc., to get into the 13db and higher range. But the GG tetrode does OK with IMD due to the cathode negative feedback. I ran some tests recently, especially on Quadzilla (4X1's in GG, now deceased) that showed the IMD stayed above -30 3rd order, which is good. I found very little difference between the FT-1000D and Quadzilla when the amp was driven reasonably. When hit hard close to saturation, then the garbage begins. The 813's should work out the same. The proof is how clean they run in triode connected modulator service which is basically the same config, minus the cathode negative feedback. (But I always run external NFB on modulators tubes anyway.)

T

Frank, I think it was an untuned input but now that I think about it, 90w in yielded about 600w (carrier only) or so out so I think the gain was lower than 8 or 9.  That was 10 or more years ago. Memory is foggy.

I will have to try some experiments with PEAK power and different coil inductances, although I dont think the amp lacks, pep wise.
And the way I run it, 150 watts carrier, with 800 watts or more pep seems fine.
On ssb it does at least 1600 watts pep out.
But tests are fun, so I will make some.
I also need to make a better bigger coil out of tubing, purely for looks...

Brett

Brett, if you can get out 1600w pep on ssb, then you can do it on AM too.  Find a way to add more cooling, and try the class C diode idea I mentioned to increase dead carrier efficiency. Maybe you can get 300W carrier and 1600w pep on AM. That wud be a nice improvement.  I added Dietz lantern chimneys to mine and it certainly helped bring up the plate dis..

I dont have a lot of air on the tubes, just a muffin fan blowing up at the tube sockets, and the holes around the tubes are bigger than the tubes to allow the air to flow out.
The fan is not running at full speed, as it makes too much noise.

I think I will try the bias increase though.
I could js in about 20 more diodes for testing.

Brett

Ok, I added as a test, 18 more diodes, another 12 volts of bias, and did some tests...
Results were mixed, while I could do 200 watts carrier out, 500 watts in, 300 watts dissipation in the tubes, peak power fell off quite a bit.

I tried every combination of input drive power and loading with no improvement.


Brett

Sounds like the dead carrier power is 40% efficient - that's pretty good. What is it when you run it class B?

Do you have enuff drive peak power to hit the required amplifier peaks?  Just cuz the amp is in class C shouldn't reduce its ability to make peak power. Maybe your exciter is running out of headroom. It will take more drive, as I mentioned.

If you have another linear, you could try it as a test  IPA - but be very careful.

Also look at the input waveform on a scope. Is the input power still rising while the output power saturates?  Use a 1KC audio tone and look at the sine wave carefully for signs of saturation on both the input and output. That shud give you a good clue..

BTW, fired up the 4X1 X 4X1's tonight into the dummy load.  The 75M tank coil was too small - recalculated and gonna replace. Something in the screen circuit shorted to ground. Must be a cap - will check tmw.  The audio didn't work. I found I had wired the SS amp directly to the 4X1 grids...  :o  Should go thru the driver xfmr first.  Other than that and a few minor details like neutralization, it's coming along FB so far.

T

My old 4-1000A linear had a string of zeners to bias it well into class c. This way I could get the best efficiency on CW. Every volt of bias shifts the drive signal down a volt so you need more drive in class C to hit your peak drive power, but your efficiency will go up and in class c and your tank will want more L. Tank Z for class c is Plate Volts/plate current / 1.2.  (1.6 for class AB)

I've been reading this tread with some interest, as I have always have had trouble getting
good efficiency out of my homebrew amps.
You guys stated earlier the importance of using more C less L to get efficiency up.
I suppose that makes it a higher q circuit. I've always designed my HB amps using the ARRL
table for pi-nets, they use a Q of 12. Since my amps run a plate impedance between
3000-5000 ohms, the handbook  suggested L is 12.45-18.56 uh on 80m quite a far distance from the 7uh
Tom mentioned.  I'll be playing around with
this some, in the future...  Thanks.... and GL

Hi Chuck!

Long time, OM.

I try to use a Q of 12 on all my amps too.   In fact I just went thru it last night for my new 4X1 class C amp and found I need about 18uh or so for 75M. (I had about 8 uh in there and found it worked FB on 40M... ;)  I use the following coil calculator and pi-network calc on the web.

http://www.qsl.net/wa2whv/radiocalcs.shtml

http://www.captain.at/electronics/coils/


By going between the two, you can find just what you need to tap off using the particular coil you have on hand or need to wind.

I did my 4X1 coil by seat of the pants and found it was way too small. I was used to the big linears with low plate impedance and forgot I was dealing with a high impedance again... ;D

My comments about using a lot of C meant that I had always underestimated how much C was required to give a Q of 12. In the past I always used too much L cuz I was using breadslicers and never seemed to have enuff C to do the job.  Stick with these calulators above and the tank will work out FB. (Or your ARRL charts)

T

HMMM.. maybe that's the problem Brett is having trying to get PEAK power outa of the newly biased class C amp on AM. IE, the tank impedance needs more L for the peak power impedance match.

BTW, Frank, Brett is experimenting with putting his linear biased into class C for AM use. Read back on my posts here about using the carrier as the "linear bias." It worked for me in the past to achieve a higher dead carrier efficiency (like 40% carrier, 65% peaks)  - and we're hoping he can verify these findings too.

T

I was talking with Stu today and he said I have no dip because the Q is too low, so I did some experiments.
I increased the inductance till the caps (tune and load) were almost nothing, I got a dip in plate current, but noplace near maximum power output.
Otherwise, no change in efficiency, peak power, etc.


I think my peak power is down somewhat for 2 reasons, 2000 volts on the plates, and a lack of tuned input.

In the experiments i did with the bias increase, I tried increasing the plate voltage, various settings of drive and loading, without getting the peaks back.
Normal operation gives 150 watts carrier and 800 watts pep.

Brett

More amplifier questions...
Never having had one before, how do they usualy handle the grids?
I have each grid, screen, supressor grid going to ground at the tube socket, the grid and screen through a 10 ohm resistor and a .01 cap (in paralell) to ground.

Would it be better to connect all the grids of each tube directly together and then to ground, all the tubes to a single point ground, or does it not matter?

In qso with Stu and Dave today, dave thought it was a neutralization problem, which I never heard of in grounded grid amplifier, so I am thinking maybe its how I have the grids grounded...

Brett

 

Brett,

On all my linear amplifiers, including two commercial 3-500Z amps, I always tie the grids together with wide copper strap and go directly to a single point on the chassis.  None of my GG amps have needed neutralization  and were always stable. Bias is accomplished through the fil CT, of course.

I never liked those funky inductors or caps in series with the grids for supposed neutralization. Seems like a good place for instabilities.  

As I mentioned before, I usually don't pay attention to the plate current meter in a linear except for idling current. For tuning, I just keep an eye on the power output and watch for a sharp tuning peak when the Q is designed correctly.

T

Thank you!  I do the same, and any amplifier darn near that crosses my bench, I throw the BS away.

Yes, this causes MANY an argument on amplifier reflectors, but I can say this:  I've NEVER had a parasitic in any of my amps with the grids directly grounded.  Not to say that it can't or won't happen, but (knock on wood) nothing thus far.  The YC165 is great like that, no suppressor needed, bolt the grid to the chassis.

People that put the grid "LC" circuits don't take this into account...  You now have an additional c. 26 ohms of resistance (DC) in line with the actual reactance.  Since this WILL effect the operating parameters of the tube, WHY WOULD YOU PUT A 26 OHM RESISTOR IN LINE WITH YOUR BIAS SUPPLY?  The grid is the return for the CT bias in the 3-500 style amplifiers.  Yeah, I like having a variable bias supply on my amplifiers.

NO, ground the grids and be done with it.

--Shane

Brett,

As Tom says, do a DIRECT bond to the chassis.  I do this on all my grounded grid amplifiers, as well as many tetrodes (operated at DC ground), and don't have a lick of problems.  From 4CX250s to the 4x15, I've not had a parasitic go off in one of my amps.  I'm not a magician, nor do I know for a FACT why, but they are all stable.

Having that 10 ohm resistor in series with the grid just meant that you introduced 10 ohms of DC resistance in line with the bias of the tube.  Makes for nice linearity, having the bias jump around a few volts in both directions, ya know?


--Shane

FWIW, I received a gates 2X 4-400 FM rf deck from 7FTO and I thought the way they wired the sockets was pretty nice. 1/2" wide silver plated copper strap for every connection tied together with 2/56 nuts and bolts ( I presume copper) and then soldered. even if the solder would melt,
you'd still have a decent mechanical connection holding it together.

I'll take a pic of the sockets in a bit. I say ground the grids with strap to the chassis. No funny stuff, just ground em.

remember,soldering to a cad steel chassis is only as good as your plating is thick, couple of mills.  For a really good ground connection, drill a small hole in the chassis and put a press to fit wire through the hole and lay both ends of the wire flat against the chassis. treat it like you would treat a plated through hole on a pc board. solder down both ends on both sides after trimming. Now you have a ground to the steel underneath the plating. You could use small nutz and bolts as well. the important thing is bonding to the steel and not just the plating lying on top.

Thanks guys.
I fixed it, but might redo the grids direct to ground with strapping.

What I did was add another plate choke and cap under the chassis, as that is shown on many grounded grid amps, and I also added some spring clips to ground the metal 813 shells.

Now I DO get a dip at peak power output, otherwise, the amp runs the same, same power out, same current and efficiency (or very close).

So one or both of the above eliminated some feedback between the plate and grid circuits.

Brett

 

Aaaah.....  you just triggered a memory here -

I once built a linear that had VERY lazy tuning like that on 160M. It worked "OK" but did not have a plate dip at all and the power out was so-so.  I simply added a few more inches of plate choke windings and the amp came to life on 160.

Evidently in my case,  the inductive reactance was too low for 160M and some power was coupling into and the tank was being influenced by the power supply path to ground, killing the tank Q. 

With your four 813's in parallel, it would seem the requirement for a large value choke would be reduced, but you can't argue with success.

Now add the copper grid straps to groumd, "paint it blue, and ship it"... ;D   (an old VZR saying)

T

Brett, I did not think is was a neutralization problem. All I said was to look at the dip verses max power out to get a feel for how well neutralized the thing is.

Lose the 10 ohm resistors and bond all grids together to ground as mentioned.

Did the existing plate choke show any signs of distress?

Here's how I did my 813 grids.

Very nicely done on the grids!

No distress on the plate choke, its one of the old B+W ceramic jobs, but many amps I look at use 2 chokes because they say its hard to uncouple the low impiedance plate from the power supply.
I don't think the B+W choke was designed for such low impiedances.

I will try the grid change next.

Brett
 

B&W 800 is only 90 uh so would be a bit low for 160. 2 in series should be fine. B&W made an 801 that was 200 uh and the same size. I used one in a 4-1000A rig to convert it to 160M. I bet the rig would have worked at full power running a low plate Z. You are lucky it didn't fry. They are easy to rewind though.  I still think your tank Q is a bit low.

...and I too.

If he hasn't already, Frank usually mentions  turning the plate tune cap. over for a shorter connecting strap.   8)

I did not try turning the cap over, I may later.
The Q should be ok, I used more inductance and have about 75 pf used in the tuning cap.

I bonded the grids all together and then to ground, no change in operation of the amp, which seems to be working fine.
Not an AM power house with 500 watts of plate dissipation, but it gets me from 22 watts carrier and 100 watts pep to 175 watts carrier and 1000 watts pep at well below the maximum tube ratings.

That seems to make quite a difference, but it was sort of amazing how well I was heard at 25 watts.

Brett

Adding inductance will reduce the Q.
Flipping the cap over will help with stability and ability to resonate on the higher bands.
I've seen some nice amps that I know won't work on 10.

I dont plan on using the amp above 20 meters, and don't think I would get on 20, but I might want to check into the flex net up there on ssb...

I have tried lots more and lots less inductance in the pie net, does not seem to make much difference in power or operation.

I have to get some more thick wire, for 40 meters I use copper tubing, but on 80 meters, I cant seem to get enough inductance in the space I have using tubing, without making it very large around.

I did make one tubing coil as long as I could, it might work, but at a high Q, I forget how it tested.

In no case do the coils seem to get warm...

Brett