The AM Forum

Working towards something higher power, I'm looking to see if I can barter for a more suitable power Pentode for RF final.

An 813 looks like a nice tube but it requires very high voltages.....is there any less intense running Pentode suitable for RF final, which is of similar vintage and easily available for barter?

I might be able to get away with screen or suppressor modulation so for now I'm not going to worry about high power audio tubes (audiophiles have raised the prices anyway).

Can anyone recommend a tube of similar vintage which isn't too hard to get?

Thanks and 73s
geo
VE3GZB

How about the 807?  Classic!

http://home.clara.net/rod.beavon/807.htm (http://home.clara.net/rod.beavon/807.htm)

http://www.mclink.it/com/audiomatica/tubes/807.htm

Are they still easy to get?

http://www.rfparts.com/tubetran.html (http://www.rfparts.com/tubetran.html)

They can be found.

Why is everyone so hung up on pentodes.  I nearly fell into the same trap till I discovered the simplicity and forgiveness of triodes.

Check out my projects at http://schmidling.com/radio.htm

100W from an 811, 200 from two, 250 from an 810 and 400 from two 8000's  The first two tubes readily available brand new from China or swap and trades.  The 8000 is more expensive and harder to find but the 810 works just as well.  I have had them all on 160 to 20 meters.

Think beyond the crowd.  Triodes are beautiful.

js

Triodes are indeed simple....but neutralization is a PITA....Pentodes help reduce the urgent need for neutralization and simplify the circuitry, also they can have much higher gain than Triodes.

But yea, I like Triodes too. But I wouldn't want to use one in my current rig.....too much feedback to the Master Oscillator would result because neutralizaiton remains a PITA.

Also you can screen or suppressor modulate a Pentode, something I'll probably end up doing.

Tubes often forgotten are the 829B, 4-65, 4-125 all of which can be run at 600 volts and won't break your bank. 4D32's are out there but somewhat scarce.

So what power out poot are you looking for? All the above should do 60 watts or better and if you need more power, think parallel.

For now I think 100 watts input would help me get my signal out of the valley. I'm not 100% sure of course.

I'd really like to stay running low power but even with 20 watts input I find it impossible to make any contacts.

Similar vintage?
I don't know how many times "we've" mentioned or recommended a 4D32 at this power and voltage level.
Hey, just a couple of threads ago too.  ;D

Hmmmm, looks like a good one!

I don't have any immediate plans to work 10 meters.....besides my receiver won't hit 10 meters!

40 and 80 are good enough for me now!

What's wrong with 6146's?   A pair will easily deliver over 100W.  Used, good one's are cheap at festers,  or I can put you in touch with Carl, VA3CGM, who has loads of 'em, new and used.

I think if u consider all the alternatives........the 813 is your best bet......BUT 4-65's and 4-125's  are good also.
And 813's are still available.

JMO..........Bill

Just saw this thread...  here's a nice 813 rig to consider, and coincidently a posting I made on your previous thread about more power:

-----------

Geo,

This rig may be too big of a jump right now, but it would be no big deal for you in a year or so.

http://www.amwindow.org/tech/htm/813/813.htm

This plate modulated 813's X triode connected 813's rig has been built by quite a few and will easily do 500 watts carrier out or as little as 100w out, depending on the plate voltage and loading.

I've found in most hobbies it's better to stretch and go for something that's a little out of reach, and then grow into it. This rig is big enuff for even the very experienced AMer. At least start collecting the parts for something like this.

You appear to have the motivation and building skills to pull it off. Be always aware that this level of HV can kill you easily, so be careful with whatever you build.


Tom, K1JJ

PITA is in the eyes of the beholder.  A neut cap is trivial compared to supplying and modulating the screen supply.

>Pentodes help reduce the urgent need for neutralization and simplify the circuitry..

How is providing screen supply a simplification of the circuitry?

> also they can have much higher gain than Triodes.

A possible advantage if drive is limited but a definite disadvantage as far as circuit layout and shielding is concerned.

Separating the grid circuit from the plate circuit requires no more than orienting the tank coils at right angles to each other with a triode.

My 811/810 rig is built entirely on wood chassis and masonite front panels and works well up to 20 meters.  Try doing that with a screen grid tube.

>But yea, I like Triodes too. But I wouldn't want to use one in my current rig.....too much feedback to the Master Oscillator would result because neutralizaiton remains a PITA.

You lost me on that one.

>Also you can screen or suppressor modulate a Pentode, something I'll probably end up doing.

You can cathode modulate a triode with about the same if not better results.

js

If you want to stay under 1000V the 4D32 is still the best bet either singly or a pair. It is a lot more rugged than a pair of 6146's, especially the screen.

Since the USAF dumped thousands they are no longer pricey.

You can also get 100W AM out of a 814/VT-154, another cheap tube.

Carl
KM1H

I just tested the first power supply this evening (see attached). It's still in shape to be powered up (gently of course so to let the cap dielectric "reform" if required).

It actually requires TWO 5R4 rectifiers, one in each leg of the HV secondary, plates are in parallel for any given individual rectifier. Luckily I had a second one kicking around.

No-load voltage is around 800VDC (I'm using the old Avometer to measure it because it's the only thing I have around that will stand this kind of juice.....I don't trust digital meters to high voltage since my trusty industrial BK Precision DMM, rated to 1500VDC, it fried itself measuring about 750VDC).

So while I'm still testing power supplies, I have to keep in mind that I may or may not go over 1000 volts, so I should select my tubes accordingly to keep some safe overhead in mind.

I contacted a friend of mine and we may arrange some kind of parts swap if he can dig out a pair of same power tubes (preferably Pentodes but if stuck I'll resort to Triodes).

I bought two at Dayton in the early/mid 90's for $20.

I saw them numerous times afterward for a similar price. A ton of them came on the surplus market around then and they were plentiful and cheap.

4D32 -- 20 bucks a pop everywhere nowadays... rf parts, nebraska surpluss... great news for my Johnson Viking I (if i don't tear into it for parts before I become inspired to repair)

I said earlier in the thread that (un)Fair Radio was selling them for ~$20!

If you're worried about the HV requirements of an 813, I'd vote for the 4D32.  Nebraska surplus still sell 'em for $18 ea if you buy 3.  I did and they look pretty good.  Brett, N2DTS has a beautiful legal limit AM transmitter that uses 3 of these workhorses in parallel running about 1200 volts on the plates.  In an email to me he said,  "At 1200 volts, I get 300 watts out, 1200 watts pep, and nothing gets hot."  Timmy HLR likes 1000 volts on the plate.  They seem to like the higher voltage to get best efficiency and have good mod head room.  Not for the faint at heart but an intriguing idea.

Getting back to the 813, my first 500 watt 2 X 813 amplifier ran at 1500 volts on the plate and was a cool (what is considering today) legal limit AM transmitter.  It did double service as a RTTY transmitter and could transmit RTTY graphics that could last as long as 30 minutes with no complaints whatever.  I think the 813 is the easiest and most documented RF amplifier ever.

There is nothing wrong with running triodes either.  Neutralization doesn't have to be a PITA.  There is plenty of info in the older handbooks on the subject. It just takes some care in setting up the neut.. at the beginning.   I'm running an EFJ Courier that uses a pair of Russian 811A's in linear service and so requires neutralization -- I have had the same setting of the neut.. cap for decades and it is as stable as a rock.  I'd consider running that in plate mod service but the RF components just won't hack it.  There are some good construction ideas out there for 811A / 812A's in the 1940's handbooks.  I might suggest avoiding push pull in favor of parallel config so you wouldn't have to have two balanced tank circuits.  Just a balanced input circuit for the connection to the neut... cap going to the plate.  Then you could use a PI net output circuit.

• 813's = good documentation along with great construction ideas.  It'll take a wide range of plate voltages
• 4D32s (3 of em) will work at lower plate voltage but require some care so that you don't blow them up.  They are a rugged tube, sort've like a mini 813
• 2 X 811A - 812A will give "legal limit," are very rugged and available today as new tubes but requires neutralization, but stay that way for years
• Finally, let's not forget the venerable 833 (or the Russian GU48).  Unfortunately, it doesn't fit your lower plate voltage parameter.

My $0.02 -- Al VTP

George,
I built the K1JJ 813 rig he mentioned in the previous post by Tom.  I chose his design for several reasons.  I spent a lot of time trying to decide what tubes to run. Then researched various circuits based on parts availability and construction issues.  Essentially, I was looking for something that would work with minimal work. Granted it has multiple power supplies but construction wise, K1JJ's setup seemed to fit the bill so that is what I ended up with. The 813's were plentiful and seemed flexible for various situations.  I had a few quirks with the rig to deal with but that was because of a  couple of departures from the original design.  Tom and a host of others here provided the support and got it off the ground. As a neophyte big rig homebrewer (my first homebrew and homebrew tube rig), this is a good rig to build.

The 4D32 was used as a Korean War radar pulse modulator that was scrapped in the 60's. The USAF kept ordering new tubes into the 80's ;D And finally dumped around the mid to late 90's.

The $20 these days is the highest Ive seen them.

Use a pair in the RF modulated by 811A's to keep the triode types happy ::)

I use whatever works for a project and isnt audiohole priced so that eliminates most triodes.

Carl
KM1H

Well, for now I'll just see what I can get that fits my budget (with two teenagers in the house, me the only fulltime breadwinner, mortgage, bills, you can guess what my budget is).

I'll go see my friend on the weekend - he said he might have some tubes he can give me - and I'll take it from there. If I don't have enough voltage to run any given tube to it's fullest power, that's ok....I can still build it and work from there. I'd rather have more tube than less tube, so I still have "headroom" to grow as I am able to afford.

73s
geo
VE3GZB

Geo

Well, this has been a good thread, I think.  I'd suggest two possible approaches.  First, investing a little more time in the scrounge mode and see what you can  come up with by way of a higher voltage PS.  I don't know if you frequent ham fleafests much but it would be fun to seek out something that would give you about 1200 - 1500 VDC @ 350 ma.  This would bring you into the ballpark of the higher power options.  '5HRO has made a good point about your current PS capabilities -- I encourage you to take some time looking for the components for something beefier.  Thing is that an "S" unit is ~6 dB  10 - 12 dB would be an excellent increase in power that would be quite noticeable.

You mentioned previously that you might be considering  other than plate mod.  That brings to mind my second option, I seem to recall my good friend Bob W1SET (recently SK) designed and built a 4D32 screen grid modulated AM rig that ran about 75 watts input.  That might be a good target first step for you to start with.  Just a thought.  I've attached some photos of Bob's xmtr - one I hope to restore to operation on 75

Good luck and good "hunting"  Al VTP

Very VERY nice indeed!! I can see that the PA doesn't require a lot of watts to drive it....all of the other tubes are considerably smaller. Screen or Suppressor modulation is appealing for the reason that the modulation requirements are humbler....granted it's a bigger strain on the PA, but that's why I'd pick one tube to be strong to withstand it.

Well, I checked over the second power supply this evening, had to do some repairs before I powered it up. (See attached photo)

It uses only one rectifier - a type 80! Can you imagine! Such a massive power supply using such a vintage rectifier!

It's a choke input filter, output caps (the two grey rectangular boxes in the middle) are each 8uf 600WVDC.

Output voltage is 450VDC. AC RMS into the 80 plate to plate under idiling load (50K bleeder) runs 750VAC. Without the rectifier the plate to plate AC runs just a bit over 1200V (it just about pegs my Avometer, thank goodness I still have it, what a durabeast!).

The Xformer with the price tag (tape) on it is the HV Xformer. The one in the middle is only filament power. The black shiny box at the far end is the choke (it's value exceeds that of my LC meter).

This might be a good modulation power supply then, good for the 1625 to run in push-pull. The smaller power supply I tested yesterday, it delivers around 800 VDC no load, that might be good for the RF final! The Xformer on that one is a beast, it's actually deformed part of the chassis.

I can't afford to spend on big toys, I have to rely on what I can find via lowest possible cost.

There is still one power supply left...it's very VERY heavy and is in very rough shape.....most of the wiring is shot and I'd hate to rewire it just to find it's got some fatal flaw. I'll take it carefully on that one and see what it can do. One of the Xformers is on the verge of falling off the unit too, what a mess!

Oh well, that's life - if it was in good shape, it would be out of my budget.

Final power supply, I've looked it over this evening. It's not as bas as I first thought, just some of the wiring was ratty. Caps Ohmed out ok and didn't draw strange currents when I fed them 50VDC. So I powered it up with a 100W bulb in series - always put a lamp in series when unsure - and with a pair of '80 rectifiers (yup, imagine!), 550VDC is delivered on the output.

After a bit of time, when it looks like it might be ready, I shunt the 100W series-lamp. Output jumps up to 950VDC and remains there, nice and steady!

WOW!