Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Transformerless screen coupling from PP plates?




 
The AM Forum
January 23, 2021, 06:02:28 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Transformerless screen coupling from PP plates?  (Read 627 times)
0 Members and 1 Guest are viewing this topic.
SA2CLC
Member

Offline Offline

Posts: 49


« on: January 12, 2021, 02:16:23 PM »

So, as usual, there is not enough transformers in the junk box.
Wanted to test a very simple dsb circuit using 6l6's as a balanced modulator, with G1 in PP, plates in parallell, and modulation applied to G2 in PP.
All schematics call for an interstage transformer, but I have non. Hifi stuff seem to have directy coupled control grids, with resistors forming a center tap. What if I do something like that, but drive the screens directly from the plates of a 6sn7?
Anyone got experience in that?
Please have a look at the schematic I cobbled together.
//Tommy


* 20210112_201433.jpg (2758.56 KB, 3456x3456 - viewed 119 times.)
Logged
MikeKE0ZUinkcmo
Member

Offline Offline

Posts: 399



« Reply #1 on: January 12, 2021, 10:10:14 PM »

Interesting, and preliminary test results are?
Logged

Mike KE0ZU

Bold Text and PICS are usually links

https://mikeharrison.smugmug.com/
WBear2GCR
Member

Offline Offline

Posts: 4002


Brrrr- it's cold in the shack! Fire up the BIG RIG


WWW
« Reply #2 on: January 13, 2021, 03:11:28 PM »

It appears to be capacitively coupled, not directly coupled?

To my eyes, the screens on the first tube look rather well coupled to RF
ground via those 0.04ufd caps?  The two 100kohm resistors need to have
a positive voltage on the screens??

Balance may or may not prove to be an issue?

Not sure what class you want to run the tubes in, but I see no provision for
any bias control or setting?

                       _-_-bear
Logged

_-_- bear WB2GCR                   http://www.bearlabs.com
SA2CLC
Member

Offline Offline

Posts: 49


« Reply #3 on: January 14, 2021, 03:14:18 PM »

Ok, to confirm, it does work. Added cathode bias to the 6sn7 push-pull drivers. Going to clean up the wiring a bit before I test it some more.


* 16106552216347035875531806578417.jpg (3080.12 KB, 3456x3456 - viewed 56 times.)
Logged
SA2CLC
Member

Offline Offline

Posts: 49


« Reply #4 on: January 18, 2021, 02:35:20 AM »

Ok, It does work, but there is nowhere near the voltage swing required to drive the screens enough to get any useful power output.
By applying 275v to one of the screens, I get a steady 8w output. Measured peak-peak voltage at the screens driven by 6sn7 and a  1khz tone is around 54volts, so still a long way to go.
Tested a backwards 70v line transformer, managed to get around 90v pk-pk, driving it from a signal generator. Power out was about half a watt.

Might play with the anode resistors at the 6SN7s but there will not be enough swing whatever.

Transformer is the way to go Smiley
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7717



WWW
« Reply #5 on: January 18, 2021, 10:03:12 AM »

The screens of push pull 6L6s can take 15-20mA 'dc' so the peaks will be higher. The plate current of the 6SN7 is much lower, so there isn't enough oomph there to drive the screens well. Maybe using a cathode follower after the 6SN7 voltage amp would help.
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
SA2CLC
Member

Offline Offline

Posts: 49


« Reply #6 on: January 19, 2021, 05:28:56 AM »

I have a bunch of 6V6s that might do the trick, although I feel I know too little to implement it.
How do I calculate the needed circuitry?

Should I consider the voltage drop across the cathode resistor as the key value?

If, for example, I drive a triode to draw 20ma, I should see 300v between cathode and a 15k resistor to ground?
Biased to draw 10ma during zero signal condition should yield around 150v, and put the modulator halves in balance, cancelling any rf output.
A more negative voltage on the grid would reduce the voltage between the cathode and resistor even more, giving me the possibility to get a 300v PK-PK signal at the output?

//Tommy the tractor technichian


Logged
SA2CLC
Member

Offline Offline

Posts: 49


« Reply #7 on: January 19, 2021, 03:33:58 PM »

Ok, tried 6v6's configured as in the Altec Lansing 1570B.
Seems to work ok, with the cathodes connected to a centertapped choke, providing around 195v pk-pk to the screens. Had it been driving control grids it would probably be enough. Power output from the 6L6's was about 3w.Had an unusable transformer, with too low impedance primary, turned out that the 6v6's cathodes would drive that pretty good instead of the center tapped choke. Connecting the secondary of that transformer (abt 450v pk-pk) to the screens gave a good 17w out from the 6L6's with a tweak of the drive power.  Nearing my target of 20w output.
Oh the fun with gator clips, test leads with 300v on them, and a total mess on the bench Smiley
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7717



WWW
« Reply #8 on: January 19, 2021, 04:24:26 PM »

 Smiley

Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 7717



WWW
« Reply #9 on: January 20, 2021, 01:20:27 AM »

Don Stoner wrote a sideband book which includes double sideband generation and has a few balanced modulator circuits of the type discussed here, with tubes from the small sizes to Octal sweep tubes to the 6146.
I recommend it, good reading. The ARRL seems to have disregarded the high level DSB in their handbooks.

A carrier in every garage and two sidebands in every pot!

"New Sideband Handbook" by Don Stoner, CQ Technical Series No. 103, Cowan Publishing 1958.

I had looked into transformerless circuits. Glad you found a suitable transformer!

Moot point now but what I had in mind was an adaptation of a July 1955 CQ article on direct coupled drive of 810 grids. It's pertinent because a large voltage swing is needed for the 810 grid. -the author claims 185V peak and 0.25A peak, though I think he may have been overdriving.

The 6L6 likes high screen voltages, as opposed to the sweep tubes and the 6146. For this reason the adaptation I did may not be ideal for your circuit, as running from the 300V, the positive cathode follower swing will be limited. If I'm right, a screen-grid cathode follower's output is limited when the cathode tries to rise above its screen voltage. Looks like no problem when stepping up with a transformer,

A medium current triode might be a better for direct coupled cathode follower perhaps, and I should not have been suggesting the 6V6 for that, however it worked out OK with the transformer.

The direct coupled cathode follower circuit also requires bias adjustments and reasonably regulated bias supply, so the trade-off for avoiding the transformer is more complexity and an additional adjustment.

Maybe a few more peak volts on the 6L6 screens will get the 20W.


* cathode follower.png (193.92 KB, 1604x800 - viewed 23 times.)
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
SA2CLC
Member

Offline Offline

Posts: 49


« Reply #10 on: January 21, 2021, 09:50:09 PM »

Thanks!
I appreciate the effort with nicely drawn schematic. Like the bias arrangement.
Yeah, the tube choices, perhaps, was not the wisest, happened to be what I had enough pairs of in octal form. I do have a bunch of PL36/25E5 sweep tubes but their 25v heater is kind of tricky.
Anyway, I'm cleaning up the wiring, tuning some resistor values, then on for some on-air tests and see if the carrier is suppressed enough.
Logged
W3SLK
Member

Offline Offline

Posts: 2435

Just another member member.


« Reply #11 on: January 21, 2021, 10:52:54 PM »

SA2CLC said:
Quote
Thanks!
I appreciate the effort with nicely drawn schematic. Like the bias arrangement.
Yeah, the tube choices, perhaps, was not the wisest, happened to be what I had enough pairs of in octal form. I do have a bunch of PL36/25E5 sweep tubes but their 25v heater is kind of tricky.
Anyway, I'm cleaning up the wiring, tuning some resistor values, then on for some on-air tests and see if the carrier is suppressed enough.
A 24vac control transformer may fit the bill. I don't know how high your supply line runs. If it is high then no problem at all.
Logged

Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.079 seconds with 19 queries.