The AM Forum

I'm planning a homebrew AM TX based on a box of NOS sweeps i've got. (4X EL500)

Ideal would be xtal controlled on 75M, coz I have that crystal handy, and high level modulated. (modulation Iron is no problem, i'm almost a competent transformer winder.) I would go with a VFO, or  VXO, but to start, rockbound will hopefully keep me out of trouble drifting too badly.

my question is threefold:

1. Any AM'rs in the south central KY area?

2. if no, how can i test my transmitter practically, in order to maximize my chances of success?

3. How to best modulate 4X EL500 (24W Pdiss triode-mode) to maximize my carrier power?

No replies? Not even a "go home and read your ARRL handbook!"? I'm starting to wonder if this is a "i've got it, and you're gonna have to learn it" group.

This group is a pretty helpful bunch, but easily distracted by a variety of other topics. Give it a chance. There are lots of very accomplished homebrewers here. I bet you'll be besieged by opinions in a day or so.

If you have reviewed any of the posts in this forum you would see that we are an extremely helpful group.  There are many who might offer opinions that don't check in every day.  I'd suggest that, as a new member, you not begin by throwing stones at the group.

Perhaps the reason you haven't received any answers is that nobody here has done this.  I don't know of anyone who's recently built a sweep tube rig.  Not to say you have a bad plan - I fully support the concept that ya run what ya brung... and if you have some mamb jambo sweepers laying around, why not?

Why not use a pair of them for the modulators and the other pair for the finals?  With 24W plate diss you should be able to score a hundred - hundred and fifty watts out in class C plate modulated.

Good luck and 73 - John

I didn't answer you because I had no clue what kind of tubes you were rapping about and had no useful info to give you.

Best way is with a scope and a dummy load. There are many ways to couple the scope, but the easiest is a couple turns of wire around the coax at the transmitter.
For audio tests just use a receiver with no antenna and listen to urself with headfones.

If you're inclined to use more advanced techniques this article from K1JJ offers good advise:

 Sweeping Your Transmitter  (http://amfone.net/ECSound/K1JJ1.htm)

Perhaps this schematic for an EL-90 transmitter will help.

 EL-90 Transmitter  (http://www.amwindow.org/tech/htm/eltx.htm)

Whatever you decide, there's plenty of help here and on www.AMWindow.org (link at the top of the home page here)

Patience is a virtue. We don't have a radio doctor monitoring the site 24/7 Jacob. But, give it some time and we usually get around toit !!

Buddly

I seem to recall a rig built by a south american (ham?) that used 16  or more sweep tubes in the final modulated by another handful in PP parallel.....anyone remember where/who ???cool stuff

Hello Steve, the rig you spoke about is on LU8EHA home, you search the call sign on google and you will find that you want!

John FG5FC

Most sweep tubes are pretty low impedance tubes (IE lower voltage higher current) This should make for an interesting scenario for the power supply and mod transformers.

A handfull modded by a handfull of sweep tubes will definately pose an interesting challenge winding a mod transformer for it. If you use a factory made one you may need to go to one several times the power rating that you are running in order to get the current handling capacity you will need. Although with the lower operating voltages, insulation breakdown problems will be less of an issue.

this may be a good application for some kind of series PWM modulation.

                                             The Slab Bacon

Hello
Looking at some specs about the EL500's doesn't seem too promising.

          Class Va Vg2 Vg1 Ia Ig2
Line output 75 200 -10.0 440 37.0

This was copied from WWW.TDSL.duncancamps.com tube data.

Homebrewing is a lot fun. It's quite a challenge beyond my capabilities and patience, but there are rewarding results.
It just seems to me that the tubes you are considering aren't going to make legal limit AM. Would you consider other tubes? How big is that junque box of yours?? You have a good basic design according to your first post.
Fred

 el500 DATA  (http://www.nj7p.org/Tube4.php?tube=el500)

Crosses to A 6GB5 horizontal deflection driver. 17W Plate dissipation.

Maybe a good choice for the 50C5 rig on the AM Window.

 50C5 PW Rig  (http://www.amwindow.org/tech/htm/50c5/50c5.htm)

Then drive an amplifier with it !!!

I remember a fellow by the name of Mike, N3HJQ form the Phila. area, would be on a HB sweep tube rig on 40 meters with about 45 watts output. He later upgraded to a pair of 6146's. I don't know what your resources are but I think you would be better off with a single or a pair of 6146's modulated by sweep tubes.

Sorry for sounding brusque, I did not intend to be throwing stones. As for patience, you'll find few who have more than I do, I'm just used to (rec.audio.tubes) style replies, in where everyone has something to say, but nothing useful. This, coupled with prior experience with some very unhelpful AMRAD operators, got me more than a little worried that I was yet again in a dead-end forum.

Obviously this is not the case here! Yes, the EL500 crosses to a 6GB5. Yes, it does show a 17W Plate dissipation, but if that's a 17W plate then the 807 is a 10 watter...I've personally run them in class AB1 push-pull audio amplifiers obtaining just over 35W output, at 250V and 100mA bias for 25W dissipation. Time will tell just how well they hold up.

As to other tubes in the box, I've got a few 6BG6, one 6BQ6GTB, one 6DQ5. Power isn't as important to me as getting on the air with good signal quality, I can always upgrade to a bigger rig later, after I've got my feet wet.

By the way, this will be the first "real" transmitter I've built, but not the first, ya know? (6V6 power oscillator plate modulated by another 6V6 with a monstrous 1:1 interstage I wound a while back was the first, but trioded 6V6's don't make much power, and signal quality was astonishingly horrid. I've chosen AM because it's easy to receive, and I like the idea of AM and tubes...which i have plenty of parts for!

Ok
I understand a little better what you are trying to put together. A low power AM TX 15-50 watts might be good at peak sunspot cycle on 40M, but not now, and definitely not in the 75M zoo nighttime. You need RF power (100W min to Legal limit MAX) to overcome band noise and crowded conditions. The antenna is the heart of the station and has to be as high as you can get it. There are times during the early eves that you can get by on 75M with a low power station, but when the highway gets congested then it is a real struggle to hear the Rangers and other 25 watt stations.
Fred

Noted Jacob and thanks.

Do any of the ideas mentioned help ? I think those tubes wud work pretty well in the 50C5 K1JJ transmitter. Whatyathink?

This is a cute little circuit that could be adapted to your tubes: http://www.amwindow.org/tech/htm/wb9eckseriesmod.htm

Again, this would do best driving a linear amplifier. This design is from Bob WB9ECK

73's Mike WU2D

hello again
Low power will be very frustrating for you and the others trying to hear your signal. Try low power during the early eves before conditions crumble.
Fred

Early evening is exactly when I was planning to work! Maybe I've got a chance at this after all. I figured that if I can get the TX off the ground and transmitting with one output tube, then try to-rework it for PP parallel.

If it's truly godawful, then I'll start over with a 6AG7 master osc driving a pair of 6BG6/807, plate modulated by some 6V6 or 6DX8 :)

I'm only a general, so I'm restricted on the 75M band, I think. is AM legal on 40M, specifically 7200kc? (I already have that crystal, and a VXO can tune around a little so I have some limited flexibility...)

Picked up some hefty iron tonight at Walt Disney World, when I get back home to Kentucky tomorrow i'll test it out and see what it'll do. Might be perfect for this...

Have you thought about 160?  There is a group of AM'ers that meet regularly in the evening on about 1985.  Several of them use homebrew  rigs.  A couple of the stations are Doug, WB4TFQ in Centertown and Bill N4VMY in Leitchfield.

Yes Jacob, AM is legal on all Amateur phone frequencies. Just keep within the general class allocations.

Hi Jacob,
I looked up your call on QRZ.
160M is usually a nice place for good ole buzzard QSO's but you will definitely need near legal limit and a lot more antenna. I hope you do not have space limitations for growing aerials. It's ashame that 40M is so iffy at this time in the sunspot cycle. But 75m early eves and 40M, when it's not long, is great for low power. The meeting freq for 40M is 7290 and 1885 for 160M. 75M is 3880 or 85...can't remember...I stay away from the crowded condx. The lower (3705 or 10) is great.....for now.
Fred

Ok, the 'Disney Iron' is a power transformer putting out 450-0-450 at 250mA, complete with 100V bias tap, 12.6V, 6.3V, 5Vct, and weighs in at 40lbs. Considering that I have a pair of 807's, I think that it would probably end up better to use those instead. What kind of power can I expect to run on a pair of 807s? For plate modulation, I'd want to have just over 1/2 my max carrier power available from the modulator, so could I get by modulating with a pair of 6V6?

Anyone have an older copy of the ARRL handbook with a transmitter something like this? I seem to recall one, buti can't find it in the 1941 version that I have.

J,

Here are a few sites that may be of use to you

 a online  tube look up
http://www.nj7p.org/Tube.php
 
and a source of a few handbooks, ets

http://www.pmillett.com/tecnical_books_online.htm

The Millett site has Orr's handbook, along with 2? ARRrrrL handbooks.

For modulators, a pair of 807's in a triode conection can give you 100+ watts of audio.  The RCA transmitting tube manual has this ckt, along with a 'whole bunch' of others....

http://www.amwindow.org/tech/htm/813/813.htm        Here is an interesting set up.  It has some design concepts that may be in line with what you are trying to accomplish.

klc

Thatz a prety nice transformer. with solid state rectifiers and a full wave center tap rectification it should make around 500v for the B+ supply. A pair of 807s should easily make 50-60 watts of carrier output at that voltage without much problems.

As far as modding them, a pair if 6V6s wouldnt cut it at that power level. a pair of 6L6s or another pair of 807s would be a much better choice. Keep in mind that by textbook definition you should have 1/2 of the DC input to the final available in audio power to modulate 100%.
In real world experiences, You should have about an equal amount of audio power to the amount of carrier power output to have some headroom and not be working things too hard.

A pair of 6V6s might modulate a single 807 if you didnt load it too heavily, and you beat the hell out of the 6V6s.  An 807 is really nothing more that a 6L6 with a 5 pin base and a plate cap. The inner gutz are the same. 807s were designed as a gussied up 6L6 for RF service with shorter pin leads and a plate cap to take higher voltages and again, keep the leads shorter. the plate curves are identical.

                                                                         The Slab Bacon

                       

For a homebrew, plate modulated transmitter, of modest power (less than 100 watts of input at carrier), you are going to be far better off if you design the modulator with tubes that are able to deliver significantly more (e.g., 50-100% more) audio power than the minimum requirement of 50% of the carrier-level rf input power ... even though you won't actually use that much audio power. Likewise, if you can, use a modulation transformer that is capable of delivering significantly more modulated audio power (without excessive saturation at low frequencies) than you would use if you were going to mass produce the transmitter for the type of highly-price-sensitive market that Johnson and Heath were serving in the late 50's and early 60's.

The cost of the components will be marginally more, but you will be able to get much better modulation fidelity.

For example, you can run the modulator tubes in AB1 (no grid current, except on occasional, high audio peaks)... which makes them easier to drive, with low distortion, with a capacitively-coupled, low-impedance, transformerless driver.

I (as well as many others) have done this with my EFJ Ranger. I have substituted 6550's for the 6L6's that came with it. I have removed the interstage audio coupling transformer, and replaced it with an AC-coupled electronic phase splitter. In my Ranger, the remaining performance bottleneck is the modulation transformer... which limits the low frequency response (due to its too-low a value of magnetizing inductance), and which also produces substantial amounts of saturation-related distortion if I apply a low frequency audio sine wave. My Ranger works very well now, even with the remaining limitations of the modulation transformer (I don't have huge amounts of low frequency content in my voice)... but the fidelity is even better when I use an external modulator, which has an 11-pound modulation transformer. The modulator (6L6's) and the modulation transformer of the original Ranger are good enough for "communications quality" audio, but not good enough for high fidelity audio.


Stu

Looks like I'll be using the cheaper cousin of the 807, the 6BG6GA. I have a pair of them, so I'll need 60-70W of AF to fully modulate them, with overhead. Ouch, that's by far the beefiest AF amp I'll have done, the most powerful so far has been 25W.

Probably gonna use 6BG6's driving 6BG6's, with a 6AG7 40M crystal osc/VXO, with loop feedback from the RF stage to the AF stage: attenuate and sample the output, demodulate the AM, apply AF as negative feedback.

I fully support this idea!  Use up some of them thar "junk" tubes and put 'em to good use.  There's little demand for 6BG6s anymore.

This may be put off for a while, as I have a car to rebuild, and a N2 laser that has bit me again (I homebrew lots of stuff, especially lasers and electronics.) The car is a '72 Datsun 240Z, my newest toy :)

After spending some time in the Tube Shack (my junk room) I've found a pair of NOS 6DQ5. Now, I seem to recall that a single 6DQ5 can make about 75W class C, and plate modulated that'd mean I would need about 40W of AF drive. I've got a 25W class A audio amp based on 6BQ6GTB's that if converted to class AB1, could probably make around 40W. I think that this transmitter appeared in the 1941-1945 editions of the Handbook, (I think I've figured out that ARRL is a bad word around here...) If anyone cares to look at it.

That's the same tube line up used by the Gonset G-76 and it worked pretty well. I think the 6DQ6's were triode connected in the G-76.

There's really no such thing as a "bad word" around here insofar as organizations are concerned.  Everyone has their opinions, of course, and that's what makes for lively and interesting discourse here.   But nobody's gonna sock ya in the jaw for using one one of the best electronics bibles around  ;D

I love the little G-76s they're really kool rigs! Your close, Steve. The G-76 uses a 6DQ5 modded by a pair of 6DQ6s in a rather odd class B configuration. The control grids are tied directly to ground, and the audio drive is fed to the screen grids with no biass. It makes plenty of audio, but is kinda wierd lookin on paper.

                                                                          The Slab Bacon

Yes, as I said, "I think the 6DQ6's were triode connected in the G-76."

Of course, I can't read since, Jacob was considering 6BQ5's, a completely different animal, but very nice tubes. Personally, I'd go with the 6DQ6s, since they would produce more audio.

Just happened to run across this info in the February 1980 edition of QST.

I would guess that for plate modulated service you could run about 2/3 to 3/4 the power/plate current.

Ah, thanks Steve! 6JE6 is simply a duodecar version of 6DQ5, IIRC, and NJ7P seems to support that. Now I have a set of usable operating parameters, and albeit for a single tube, I bet that if I parallel two 6DQ5 I can get about 15/8ths the output current of a single tube, and probably about double the power. makes for a heavy mod transformer, but I've got some chunk iron here to rewind, so it's not a *huge* deal.

On the first attempt, I'm shooting for 70W carrier, then as I get some experience, I'll add another final and adjust the modulator accordingly, and go for 140-150W carrier. This being my first real transmitter, I'm worried about tuning up and finding no one around on 7.200Mhz. (my only crystal that falls in the 40m band...I've got a 3.686Mhz that I could double to get 7.372, but that puts me out of band, something I shouldn't do :) )

I guess I should go ahead and rebuild this 40M receiver into a proper cabinet first, though. If ya can't hear 'em, how ya gonna work 'em? I think the old saying goes...