6L6 AM rig

[VE3LYX]

 I will be testing a 6l6 Am transmitter over the next week on 40M in the '290 area. It is a real peanut whistle so if you hear it please let me know. A sig report is all I need . I will not keep you.
don

[WU2D]

Don, I did up a 6AG7 and 837 transmitters with suppressor grid modulation off a carbonium. It worked pretty slick but not much output.

[VE3LYX]

I like that! Do you still have it? I get a lot of enjoyment out of building and trying things. Obviously you do as well. I see an ARC5 was sacrificed some where in the process.
don
My 6l6 is the aluminum one at the top with the funny fluted knob on the plate tank variable cap. It also uses a carbon mic. It is a Hartley variation. Seems to work well but also lacks power for daily use however I still haven't lost hope. I may install a key jack  so I can use it on CW and then switch once contact is established. don

[WU2D]

Many an ARC-5 has given up their parts for ham projects over the years. Sometimes I find a pile of ARC-5 parts in a box at hamfests.

I converted it from 6AG7 to to an 837 rig and it does 5 Watts out now. Basically you set the suppressor voltage which is negative bias to the point where the output power goes to 50% of full blast out. So if you have a 600V supply, you basically need around -150V or so and a way to adjust it. That Bell transformer hooked up backwards takes the 12VAC and steps it up where it is rectified into the negative bias. The carbon mic needs bias as well.

A big boy version of this would be a 6AG7 crystal oscillator driving an 814 and suppressor grid modulating it for about 30 Watts out.

Mike

[WU2D]

The toggle switch in the upper left not shown in the schematic is a phase reversal sw which makes a big difference as always, but especially with a little rig. I never got the negative cycle loading to work quite right with this rig. That is the circuitry to the left with the extra pot.

[w1vtp]

Nice design concept, Mike.  I like it

[WA1LGQ]

Mike do you have good positive modulation peaks with that setup? My GRC-9 is suppressor modulated and the pos peaks out about 50%.
Larry

[WU2D]

Well they will not any more positive than the peak power set level at full output with the Supp Grid bias around zero. My guess is that in the GRC-9, the bias is not set critically, but is at some conservative bias value which guarantees at least 50% modulation for all of the conditions. Yours might be set up a bit tight and you may be out of phase. Take a peek at the manual and see if you can measure the bias setting in the final. Can you flat top it? Do you have enough gain?

As implied, beyond the 6AG7 and 837 and 814; the 2E22 is another good candidate tube for suppressor grid modulation since it was designed specifically for that purpose.

[VE3LYX]

I have a PT15 tubed transmitter I have been working on. The PT15 was also specificallly designed for supressor grid modulation and used in the T1154 Lanc Bomber radio. I found a training schematic which has a single tube and decided to build it see. The British Schematics are confusing (unnecessarily complicated) and so I take long breaks of a month or so although The osc is running the final supplied.
Anyway back to the 6l6. I decided to add CW and installed a Key jack in the Ground circuit. Didn't expect any trouble but it doesn't like the key cord. It was heard though in NC first try on CW.You know if I build a one or two tuber and can talk that far away I am more then content. If I need more I can mate it t my twin 811A Amp. As I age I have grown oh so weary of bigger better wider louder fancier and am enthralled with "the way it was."
don
for those into the PTO thread. I dug out that PTO tuned rig late last night and listened on 80M for about an hour. Still like it. On 14 inches of wire I could hear lots and easily separate all stations with its slug tuning. It also uses an absorption loop to control regeneration which is why or how I understood the Tx loop modulation method as they both work the same way.

[VE3LYX]

Don, I did up a 6AG7 and 837 transmitters with suppressor grid modulation off a carbonium. It worked pretty slick but not much output.

I am glad you showed us that. It caused me to dig out a transceiver I built a few years ago that was screen modulated and worked but also lacked output. I did work VE6PG twice but he has exceptional skills in Rx antenna technology. Oherwise as you said with yours it worked pretty slick but lacked output. However last week I got a heard you email from NC with my single 6l6 rig on Cw. I thought if I can be heard there with the Cw rig since the transceivers Tx is also a 6l6 final on a similar supply it should at least make it half that on AM. And so I dug back in after a few years of letting it sit. One picks up a few things freehand homebrewing and  I made some corrections. Seems to be coming along good. I now have a solid 7.5 watts out which would tickle my linear if need be. At least enough to give it some legs. Your project re-inspired me. Today I am working at producing more power in the modulator circuit.
thanks again,
Update. Today was a total success. Power is up and modulator is working much better. conditions on 40m and 80m today were crap so I will wait for a good day to try it out.  
don