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Author Topic: 2x 813 amplifier?  (Read 129043 times)
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Patrick J. / KD5OEI
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« Reply #25 on: October 04, 2009, 06:29:27 PM »

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        

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.


* RCA VREG TUBES.gif (74.37 KB, 1018x542 - viewed 1187 times.)
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« Reply #26 on: October 04, 2009, 10:06:23 PM »

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



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WD5JKO
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« Reply #27 on: October 04, 2009, 11:24:09 PM »

Maybe I should think of building a 4x813 grounded grid amp, maybe 80 meters only, that would be simple.
snip
just need to put four 813's and an output section together, homebrew filiment choke?
snip
Time to read up on grounded grid amplifiers...

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
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« Reply #28 on: October 06, 2009, 09:09:03 AM »

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
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« Reply #29 on: October 06, 2009, 11:23:13 AM »

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
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« Reply #30 on: October 06, 2009, 01:11:58 PM »

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


 
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« Reply #31 on: October 06, 2009, 01:43:51 PM »

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?
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« Reply #32 on: October 06, 2009, 02:03:59 PM »

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. 
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« Reply #33 on: October 06, 2009, 02:24:43 PM »

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
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« Reply #34 on: October 06, 2009, 03:07:28 PM »

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

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« Reply #35 on: October 06, 2009, 04:13:23 PM »

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
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« Reply #36 on: October 06, 2009, 04:18:09 PM »

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

73
Jack.

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« Reply #37 on: October 06, 2009, 09:34:13 PM »

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
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« Reply #38 on: October 06, 2009, 10:00:15 PM »

They need bias at that voltage?

Brett
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« Reply #39 on: October 08, 2009, 10:05:24 AM »

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
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« Reply #40 on: October 08, 2009, 04:22:57 PM »

4-125s are a better RF tube than an 813 when you go above 40 meters. They will work well in GG.
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« Reply #41 on: October 08, 2009, 05:27:58 PM »

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
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« Reply #42 on: October 08, 2009, 06:28:27 PM »

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


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« Reply #43 on: October 08, 2009, 07:03:58 PM »

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
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« Reply #44 on: October 08, 2009, 07:47:50 PM »

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
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« Reply #45 on: October 08, 2009, 07:57:37 PM »

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.
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« Reply #46 on: October 14, 2009, 08:40:13 AM »

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
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« Reply #47 on: October 14, 2009, 09:29:59 AM »

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.



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.
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« Reply #48 on: October 14, 2009, 09:44:53 AM »

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
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« Reply #49 on: October 14, 2009, 10:12:06 AM »

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

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