Ideas for Simple Medium Power AM Rig

[VE3LYX]

I built a rig last spring and got it working reasonable just today. It is a VFO , a hartley in one half of a 12sl7. That is fed to the grid of a 50l6. The other side of the tube is the modulator fed by a electro mechanical carbon mic circuit using a # 3 hand mic. A parallel tuned circuit is in the output. A pot allows one to find the best balance between screen voltage and modulation input. As far as working it does but power output is very low. Stil the signal is very readable. SO I built a 6l6 linear. It would drive it but still power was less then a watt. It wasnt getting enuf drive. SO today I made a driver (6v6) or basically another stage to drive the linear.  It works. But power is not huge. Maybe 5 watts out on a good day. Partly because you have to run the tubes in class A mode. Partly because a VFO doesnt have the suds a Crystal controled Power osc does. Partly because of the simple screen modulation circuit. It was an interesting experiment and it works however it is not really practical (although I will be using it on 7115 to 7120 kcs mornings before 10 and afternoons before 4. ) This set is a transciever having the same two tubes in the other half as a regen and audio amp and PA. It is an AC DC set.
I have another 50l6 TX I use on Cw . It is a crystal controlled rig and makes pretty good power for a one tube deal. I think it could be plate modulated without too much trouble  however first I want to try cathode modlation as it is cathode keyed so that would be a no brainer. Moderate power homebrew AM fascinates me. I use a regen also with the 50l6 crystal controlled set. The 813 rig idea is intriquing . Maybe  a 6146 crystal osc with plate modulation would be a fun build.
Don
I have

[W3RSW]

Crystals used with RCA's rig back then were physically larger than the FT 243's most of you are thinking of, let alone HC 6 u's.  An 813 even in an electron coupled (the gentlest to crystals) circuit would crack one of your good buddy rocks instantly unless you were a very good tune-up expert. 

I would seriously use a 6CL6 or 6AG7 as an EC Osc. Into the 813 for a one or two band rig, fundamental or first harmonic.

[KC2ZFA]

Here's the MOPA King:

http://n4trb.com/AmateurRadio/GE_HamNews/issues/GE%20Ham%20News%20Vol%2003%20No%205.pdf

[VE3LYX]

Today I did cleanup of the whole circuit (The Pa not the main rig) to move it from a experiment to permanent.  I added a 100ma bulb in the parallel 6l6s plate supply just before the RF choke . Probably should have bought a 150ma as it gets a bit bright but lasted a 1/2hr with no trouble. Dims when you tune to resonance. I also added a key jack for the B- circuit so I can drop the amp out when not speaking. I moved the tubes to even spacing (I use a modular breadboad design for tubes and terminal strips then prewire as much as possible the screw the assembly in place. )When I was done cleaning up all the jury-rigged connections and had everything tucked way power output came up considerable. Showing on my mickey mouse (RS ) meter between 7 and 10 watts. Plates are 310 volts under reasonant  load and appears judging from the 100 ma bulb about 60 to 70 ma. (Almost full bright but not quite. ) I called CQ for a bit around 7115 but no takers today. Anyway, It is done and working. I am thrilled. There were moments I wondered. Tis a real adventure when the only schmatic is the one you draw!
So if you hear a PW in and around 7115 to 7140 could be me. I use a WWII carbon mic so you will be able to tell.
Don Ve3LYX
Here is the top view of the amp. The rear black 6l6 is the driver and the two "tall boy" EH 6l6s are the power tubes. They are supposedly good even to 600 volts. I have a HB step adjustable supply for that. I will check it out tomorrow to see if I can get even 575V with enough current for a test. Let me see, if I double the volts................ mmmmmmmmmmmmm.

[Steve - K4HX]

USA cannot work fone below 7125 (7130 for AM).

[KM1H]

Im surprised nobody mentioned the 4D22/4D32 in the 7 years of this threads life.

Two in the final with a 5763 or 2E26 driver and a pair in the modulator in AB2 with a 12BH7 driver.

One big isolation transformer with voltage doubler and quadrupler for all the B+
or right off the line for the brave

Carl

[VE3LYX]

True, Ok, I will use my 7140 marker and work there as well. I am going to try late tonite just to see. Probably the band will be full of broadcasters but I want to see.
Don
BTW Carl, this transceiver is right of the line while the linear runs on a regular PS.

[Opcom]

Yep, no reason we can't use class C1 instead of class C2.  We can get more plate current from a given tube in class C2 - but if we have enough plate current, we have enough plate current, and sweep tubes with high screen voltage give a lot of plate current at zero grid volts.  Generally in class C you need peaks about four times the DC plate current.

C1 will work, used it for gating pulses. No idea how it stacks up for RF but with enough bias to make for a narrow pulse and enough HV and screen volts, it'll do a good job!

[Steve - K4HX]

N2DTS built a rig with two 4D32s in the final. Can't remember the modulators.

Im surprised nobody mentioned the 4D22/4D32 in the 7 years of this threads life.

Two in the final with a 5763 or 2E26 driver and a pair in the modulator in AB2 with a 12BH7 driver.

One big isolation transformer with voltage doubler and quadrupler for all the B+
or right off the line for the brave

Carl

[Steve - K4HX]

It's a crap shoot. You may have less QRM below 7125 but also less people to return your CQ. Very few BC stations (some nights none) in the 7125-7200 range these days.

True, Ok, I will use my 7140 marker and work there as well. I am going to try late tonite just to see. Probably the band will be full of broadcasters but I want to see.
Don
BTW Carl, this transceiver is right of the line while the linear runs on a regular PS.

[VE3LYX]

It was pretty noisy but I gave it a shot anyway for 15 minutes off and on. I am just a hair under 7140. Will try in the morning around 9 am. Can't monitor now because I have enuf power to drive my rxs into overdrive. I need a crystal set of 7140. :>)
Don

[W3RSW]

Steve,
 Yes, N2DTS worked me with a pair of 4d32's mod. by pair of 811A's., Dec. '06.
Then I logged him with three 4D32's mod. by I believe the same 811's in Jan. '07 and I think discussion mentioned wanting to use a pair of 4D32's as modulators. Maybe he eventually did this.

His reasoning is a relatively lot of power using relatively low plate voltages.  Have to agree and am also surprised more don't use them, particulary in the big dump of them on the market not too long ago as mentioned.

your friend over the mountains,
-Radial Rick.

[Steve - K4HX]

Yep. And not much drive is need with the 4D32s too.

Hope to hear you on the air soon Radial Rick. 

[WU2D]

A 6AG7 oscillator operating straight through will drive an 813 and would be much gentler on the crystal.

Darrell, WA5VGO

6L6's are notorious and even a 6V6 will crack a crystal if you are not careful. These monster beam power pentode oscillators are novel but really are for BIG blanks like those WW2 BC-610 jobs only.

Mike WU2D

[KM1H]

Any tube can work with a FT-243 (Not one of those Carling specials with a HC6 mounted inside) as long as the current is kept low. Thats what the pilot lamp in series with the crystal was used for.....a visual indicator and a fuse for dummies

QST ran a test on octal tube oscillators pre WW2 and concluded the 6AG7 was the best but I dont remember all the criteria involved. The 9 pin version would be the 6CL6.

Carl

[VE3LYX]

Was working on another one today. 6sa7 osc with modulation and 2 6sl7 tubes. One a speech amp supplying modulation to the 6sa7 and one a two stage rf PA boosting the output to a useable level.  6sa7 seems to have laid down on me. Was oscillating but would die out . Now nothing. I dont have another so I will have to look around for a spare to try. I have checked the circuit several times and found a few mistakes but I may have overlooked one even yet  except it was oscillating yesterday.  Anyway other than find a 1/4 jack for the mic input and fixing the osc or swapping the tube it is done. It should have enuf tickle to work with my triple 6l6 PA. I am hopeful. Here it is. I started this a year or so ago. I also built, today, an HF crystal set for in shack monitoring. I bought nothing. Everything was in my junk box. This low power basic style stuff really has my interest. The comments on the 4 D 32 tubes I found quite interesting as well. 100 watts at 600v. Decent!
Don Ve3LYX

[Carl WA1KPD]

Was working on another one today. 6sa7 osc with modulation and 2 6sl7 tubes. One a speech amp supplying modulation to the 6sa7 and one a two stage rf PA boosting the output to a useable level.

Could you please post or send me a schematic for that circuit?
Thanks
Carl
/KPD

[W3RSW]

QST ran a test on octal tube oscillators pre WW2 and concluded the 6AG7 was the best but I dont remember all the criteria involved. The 9 pin version would be the 6CL6.

Here's a site with completer rundown on the 6CL6 which is a 9 pin minature copy of the 6AG7, also why an ECO is a great oscillator and all kinds of neat retro stuff.  ...resurrected and modernized an old 813 transmitter and everything !

-Includes references to QST's tests too.
I think the whole site was referred to in an earlier series of postings on AMfone.
I've kept the link for some years and have wanted to build up something similar starting with the 6CL6.

http://faculty.frostburg.edu/phys/latta/ee/6cl6xmtr/6cl6why6cl6.html

[VE3LYX]

Could you please post or send me a schematic for that circuit?
I can send you a link for the basic circuit. From the 6sa7 forward (6sl7) is my own and is yet to be drawn. Bsically first triode section biased properly with an Rf choke in the plate lead from B + and coupled to the next stage (of the same tube) by  150 pf cap where the process is repeated which then feeds a parallel tank circuit which coules the output to the antenna (or big amp) with a 5 turn link wound overtop and centred. I have removed the PA tube when trying to get the vfo back going. I will send the link right here in a minute. The idea started life as a Am broadcaster for home use.

Don Ve3LYX
I can't remember where I got the circuit. It says on it AM Transmitter 2. Because the basic cicuit isnt mine I dont think I could post it here as an attachment. I can however send it to you privately.

[VE3LYX]

I have been beating on a hb transciever for almost a year and only had one QSo with it. I built a new linear but never had success with it. it seemed good and would po the watt meter. (Mickey mouse plastic Rs thing) but i suspected that was in shack radiation rather then real antnena output. SO i got my old standby, never lies watt meter. AKA light bulb with 50 hm coax and hooked it up. Not even a twinkle. i went through the Pa and the one thing I found was the final plate tune wasnt very definate. Seemed like it wasnt on frequency. Not resonant. I looked over the circuit and test, bypassed and fooled around t no avail. I finally looked at the RFc  in the plae circuit. it was a homemade one. I often make them for regens . I wondered if it was shy a few turns. (or perhaps a few hundred turns). i dud though my junk box and found one from a 1923 radio I had stripped. I swapped it for my homebrew. Instant success.  Plate circuit tunes sharply dipping my indiactor ncely on resonance. (a 100 ma bulb) So now the acid test. I hooked up the bulb dummy and turned it on. Success. Bulb lights to about half brilliance on resonance. Since it is link coupled it has to be Rf and not stray stuff either. I ran the power supply clip up to full voltage and reset it all. Draws about 100 ma  (plate current indicator lamp fully bright on resonance. ( too bright either side of it. I will be working to need a 150 Ma indiactr today so if any are around I will be testing on 7120 and 7140 throughout the morning. till about2 pm EST. if your not good on 7120 a sig report here would be welcome. I was going to give up on it but couldnt quite do it. ! too soon old too late Schmart
Don VE3LYX

[VE3LYX]

I now have power up to a pratical level. Today I had a guest operator stop by and had them watch everything while I bricked it and went for a quick drive wih a portable recvr.  About 4 1/2 minutes. About a 1/2 mile away the rx was stil a bit overloaded but at about 2 miles away the carrier could still be heard nicley. Hurried back before the smoke rolled.  It has been my experience that anything I can hear at that spot will work 400 to 600 miles on a good night with a sked. So now that I know it is putting out a decent signal it is just a matter of time.
Don

[VE3LYX]

Re The 6sa7 Am transmitter circuit. I got my new tube yesteray and got the osc and RF amp running well BUT it would not modulate. I spent late yesterday and some of this morning working on the circuit. I am colour blind so I went through and measured a lot of the resistors I had used because this is my usual fault. I found one too! I thought it was brown, but it was green. (Not good when that is the first colour band!) Anyway it would not modulate. I traced the circuits I had and found nothing. I replaced some .01 caps I had had failure in (bad batch of surplus rejects)
Nothing. I went through the circuits as it was posted on the web. First stage of the speech amp had a 15K resistor in the cathode leg. That seems odd I thought. Second leg was a 4.7K bypassed. I checked a few other similar circuits to see if I was nuts . All less then 5 K . I surmised he meant to enter 1.5K  so I tried it. Instant success.   Probably 2.2K would be typical. So I now have a 40M low power AM rig. I added the second 6sl7 as a driver  Pa and used a parrallel tank in the plate of the PA. I linked it out with a 4 turn link,soldering a 12v 220ma to the link wires for a dummy load. Actually lights the buld fairly brightly at resonance . Now I have to tame it a bit . Shield the VFO from me and add a vernier drive perhaps. Then I will see if it will drive my tripe 6L6 linear.

[W7TFO]

You are doing a gutsy, hands on job there, Don.

PW or QRO, the job is the same. 

Keep up the good work!

73DG

[VE3LYX]

Thanks for the encouragement.
This afternoon I tried cleaning it up. I removed 2/3 of the movable plates from the VFO variable cap. (approx 365pf) and replaced the missing capacity with a fixed 151pf "punkinseed". This brought my tuning in band totally and easier to control. I left the PA tank as is. It seemed to like that change. I then took all the mikes I had and tried them. It only likes the old magnetic stuff similar to an earphone. I had a 1950s telephone mic from an old dial phone. It seems to be something of that nature although it may be intended to use as an earphone. What ever. I took an old AC /DC   set audio transformer. like the ones always mounted on the speaker and connected the high end to the set with a 1/4 plug and the low end to this old telephone piece. Pretty good Audio! I am surprised.  So I am whipping up a homebrew push to talk mic for the rig with the audio transformer in the base. I must have had some RF in the audio before because hooking this in settled the whole rig down on both RF and Audio side. It now tunes smoother, less bothered by my hand. It gets right at it, loading nicely and is much more stable. Even moves the needle on my in shack FS meter! Sounded so good on the monitor Rx I clipped a banana plug to the side of the dummy load , shoved the banana plug in the centre hole of my coax switch, turned it to that position and called CQ on 7145 kc (+ or -) With only a PW I didnt expect an answer but it works good enuf I wouldnt have been totally surpirsed. I am liking it more all the time. Tonite while the XYL is out  may try a 6bl7 in place of the 6sl7 on the RF side. (might be  the wrong number but you know. That old TV sweep dual triode from the 1950s.  same pinout I believe.) I think the 1956 hand book has a rig made with it. Anyway a fun project for sure, this mixer tube AM transmitter.
Don VE3LYX

[VE3LYX]

So I tried the 6bl7. Wow! I lost the dummy load at the instant I hit the key. Wired in a bigger one. Strong for a PW. After about 2 minutes the one side of the 6bl7 , the final side began to develop an orange glow in its plate. I took a picture in the dark . Not knowing what to do, I surmised that since both sections of the tube have the same gird bias, and plate voltage, that it was being driven too hard by the preceeding stage.(of the same tube. ) So I popped a 2k 10 watt resistor in the plate supply for the driver stage  before the RFC. All is good now. Might have been a bit much but I had it in stock. Key down for extended periods with no glow and power is still good.
This rig modulates well. Both my other Am rigs are light on modulation. My DX 60 almost has enuf but not quite.  My homebrew has enuf to be readable but no more. This rig I can set it anywhere I want. From very light to too much. Once I am sure all is well I will stick it on the parallel 6l6EHs linear and see what happens.
Don Ve3LYX
Fun build for me anyway. 
See it is really true! Real radios glow in the dark.