Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Negative Cycle Loading on Suppressor Grid Modulator




 
The AM Forum
April 12, 2021, 12:42:59 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: Negative Cycle Loading on Suppressor Grid Modulator  (Read 4042 times)
0 Members and 1 Guest are viewing this topic.
WU2D
Contributing
Member
*
Offline Offline

Posts: 1767


CW is just a narrower version of AM


« on: March 05, 2011, 08:44:38 AM »

This is hurting my head. I want to put conventional negative cycle loading via a diode and clamp voltage on a suppressor grid modulated 837 tube.

The resting G3 bias is -35 VDC and the modulating voltage applied though a small transformer swings and instantaneously changes the bias to effect the efficiency.

Conditions: Plate voltage is 350 and G2 is at 200V. G1 is grid leak biased.

I have at my disposal -150V, + 10V and + 350V.

Mike WU2D
Logged

These are the good old days of AM
WD5JKO
Member

Offline Offline

Posts: 1896


WD5JKO


« Reply #1 on: March 05, 2011, 06:40:32 PM »



Mike,

*Disclaimer: just waking up here, so what I am going to type might not be fully reasoned out*

   Since you have a -150v supply, rig up a pot from that to vary the G3 bias, and see what it takes to cut off the tube (no RF output). Use a pot that will pass a milliamps or so as a bleeder. Bypass the wiper with a capacitor (a few mfd), and then connect a diode (anode to the pot wiper). Then when the modulation minimum goes MORE negative than the voltage at the pot wiper, the diode will conduct and clamp the voltage. Adjust the pot to set the "keep alive" value to ~ - 95% modulation or so.

Here the source impedance of the AC G3 voltage comes into play. If it is stiff (low-Z) the method I outlined will not work well. I prefer to have a soft clipper that kicks in earlier, maybe 85% negative modulation such that we preserve the waveform somewhat (no hard clip) while limiting a loud audio passage to less than 100% negative modulation. To do this we add a series resistance with the diode, and it works as a volatge divider with the AC waveform source impedance.

I did something similar to a 6146 rig I have where I used a diode limiter to clamp G2 voltage to swing no less than ~ +20v. Worked like a hose, keeping the carrier from chopping up with big audio peaks in the wrong direction.

Jim
WD5JKO

   
Logged
WU2D
Contributing
Member
*
Offline Offline

Posts: 1767


CW is just a narrower version of AM


« Reply #2 on: March 05, 2011, 08:55:31 PM »

OK Jim I see what you are doing. I drew it up and have attached it. You must have the main pot to set the bias to the point of about half power out as a starting point. The suppressor is not like the screen or plate and I doubt it draws much current or has a significant impedance. I may need to add a resistor to ground from the grid.

I will give it a try tomorrow. Thanks.

Mike WU2D


* 6SK7_AM.jpg (172.51 KB, 2460x1525 - viewed 496 times.)
Logged

These are the good old days of AM
Mike K8WEU
Guest
« Reply #3 on: March 06, 2011, 01:36:07 AM »

Hey Mike... following your progress on the supressor grid modulation, 1 tube transmitter.  Thought this could be a part of what used to be called a mini rushbox -- started out with a dual triode concept -- one triode oscillator cw transmitter and one triode regenerative receiver.  Maybe a 2 tube rig for 75 meters could be effective using a supressor modulated transmitter and a regen receiver (of course made to look 1930s). Always find your concepts interesting.  Talked with you a couple of years ago at AWA in Rochester. Let us know of your progress. 73 Mike, K8WEU. BTW just built a Retro 75 --lots of fun.
Logged
WU2D
Contributing
Member
*
Offline Offline

Posts: 1767


CW is just a narrower version of AM


« Reply #4 on: March 06, 2011, 07:33:42 AM »

Mike - 30's?

I am thinking about 10M AM - must be the sunspot.

Certainly a 1930's dual triode tube could be put into service as an old fashioned XCVR with the one section as the transmitter and the other triode as the speech amp Hiesing modulator off a carbon mic and then through clever switching, one triode converts to a super-regen with the triode as the mini-speaker amp or vs-versa. But the triode oscillator would be hard to modulate without FMing. You really need a pentode, especially for 10M.

Yes you could use a 6BA8 or even a 6U8 but I think a 4 dollar compactron TV tube like a 6LU8 would give you some juice because it is a 12Watt diss beam power tube and a high mu triode in one envelope with both handing higher voltage.

You won't be QSO'ing Germany with that too soon but that could be the basis for a dandy 10M, 5 Watt job.

Mike WU2D


* 6LU8.JPG (56.6 KB, 312x580 - viewed 348 times.)
Logged

These are the good old days of AM
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 0.05 seconds with 18 queries.