Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Proper measurement of bias




 
The AM Forum
April 21, 2018, 03:39:07 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 [2]   Go Down
  Print  
Author Topic: Proper measurement of bias  (Read 924 times)
0 Members and 1 Guest are viewing this topic.
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7558


"Let's go sailing, Tommy!" - Yaz


« Reply #25 on: April 11, 2018, 04:06:59 PM »

Regarding the ratio of fixed vs: grid leak bias...   (Fixed bias + grid leak = recommended class C voltage at grid RF choke.)

I've found this ratio is not critical for high fidelity performance as long as the fixed bias "barely" cuts off the tube enough so that when the RF drive is gone, (grid leak voltage = 0) the finals do not go past maximum dissipation. I use a Variac on my fixed supply and simply set it so that the 813s idle at about class B. Pretend the amplifier is a linear amp and set the idle for this plate current with no drive. It is now protected against loss-of-drive meltdowns.

Then adjust the grid leak resistor value so that when the rig is running, the recommended class C voltage appears at the grid RF choke.  (fixed + grid leak combined)

Some hams feel that the most grid leak is best... I say use the minimum fixed as protection and then get the rest from the grid leak, whatever that works out to be.  I know some hams who run 100% grid leak bias and use no fixed - instead they use a failsafe shutdown protection circuit when a certain plate current is exceeded. (No drive condition)

The bottom line is to set the rig up right, you need to run IMD, THD and triangle tone tests while looking at the scope and SDR spec analyzer. By carefully playing with all parameters - the grid voltage ratio, plate loading, plate Q, screen current, screen voltage, grid current, RF drive, HOW FAR into Class C/D you can run it for max efficiency by increasing the grid leak resistor and increasing RF drive, increasing audio negative feedback, etc., will have the biggest effect on fidelity, PA cleanliness and PA efficiency. You will find how high and clean your audio peaks can go by experimenting with all parameters.  It does make a difference.

The grid leak and fixed bias is but a small portion of what needs to be adjusted to fully optimize a rig for class C AM.

T
Logged

“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1122


« Reply #26 on: April 11, 2018, 05:15:37 PM »

The only issue I have is the value of the grid resistor.  Both your circuit and JJs is the series design which is what I use and what I keep suggesting to Clark to use.

Problem is neither you or Tom are taking into account the fixed bias voltage.  I know JJs 2x813 design.  I was just looking at the RCA tube manual.  They have a single 6146 xmtr in the back pages.  Usual grid leak resistor is about 27K for the 6146.  They have only a 12K grid resistor but the xmtr also has a fixed -45V bias supply.


I beg to differ as we ARE accounting for both voltages.



This is the point I keep trying to make.  The fixed bias gets added to the grid leak voltage.  I keep saying that you should reduce the value of the usual grid resistor to a value to only drop the remaining voltage needed to reach the correct neg grid voltage for class C.


OK, what equations or models would help you do that?

And I said earlier and in the circuits I showed the bias supply voltages DO add to the grid leak voltages in a series circuit.

And the facts you are missing are these:

The peak RF drive to the grid is the primary determining factor for the value of the grid voltage in a series circuit.

Too low a value of grid leak resistance means the voltage developed across the grid is too low, may never bias the tube into class C, and requires a large amount of grid drive power because the grid impedance is too low.

Too high a value of grid leak resistance may allow the grid-to filament/cathode potential to go into arc-over territory and exceed the grid input power (heating) values.

One can control the plate current of a Triode Final in two ways: 1) by adjusting the RF drive, 2) by adjusting the control grid bias voltage.

In a Tetrode, the plate current is controlled by: 1) by adjusting the RF drive, 2) by adjusting the control grid bias voltage, and 3) adjusting the screen grid voltage.

In either case, what we want to do is to allow about 140 degrees of positive grid drive  to pulse the final for class C.

BTW, the tube manuals and the tube cookbooks are only starting points for tube stage design.

Just because you see a 7k value mentioned on a schematic doesn't mean it is suitable for yours or anyone else's  design.


Phil

Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
KA2DZT
Member

Offline Offline

Posts: 2105


« Reply #27 on: April 11, 2018, 05:53:45 PM »

OK Phil thanks for the added information.  I did note that you were taking into account for the fixed bias in my follow up comment.  Tom also posted a detailed explanation on the method he uses for adjusting both fixed and grid leak bias.

The number of xmtrs I've built I never copy anything from other xtmr designs.  I do all my own designing.  This way when it doesn't work I can't blame anyone else.

Fred
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1122


« Reply #28 on: April 11, 2018, 05:56:51 PM »


...The grid leak and fixed bias is but a small portion of what needs to be adjusted to fully optimize a rig for class C AM.

T

It is a small portion of the total design but is also an important part of the design.

Too many times people think tube books and cookbooks are God given and if a 7k works well here, it is going work elsewhere. That is technically not correct.

Many more people are now interested in AM and are interested in homebrewing their own rigs, but are also a bit more technically inclined than the previous cookbook generation and are looking for technical guidance in building each stage.

So I think we owe them models and technical explanations rather just saying, "here, it works for me so just follow my template and don't ask questions."

Questions are good as it means they are attempting to add to their understanding of the subject.


Phil - AC0OB
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1122


« Reply #29 on: April 11, 2018, 06:02:58 PM »


...The number of xmtrs I've built I never copy anything from other xtmr designs.  I do all my own designing.  This way when it doesn't work I can't blame anyone else.

Fred

That's fantastic Fred because you learn so much more than the tube cookbooks can tell you.

BTW, the 0.875 factor comes from empirical (on the bench) data from the design of many transmitters, both plate modulated and Screen Grid modulated.

Phil - AC0OB
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
K1JJ
Contributing
Member
*
Offline Offline

Posts: 7558


"Let's go sailing, Tommy!" - Yaz


« Reply #30 on: April 11, 2018, 06:40:14 PM »

Yep, we gots it now.

Actually a universal design that would work with simple calculations for any class C AM rig:

For a given HV, in the spec sheets:  1) find the recommended grid bias voltage in linear service   2) find the recommended bias for class C service   3) find the recommended grid current for class C.

Take the class C bias voltage then subtract the (fixed) linear bias voltage to arrive at the required grid leak voltage.

Resistor grid leak = E/I   =     Required grid leak voltage (drop across resistor) / class C grid current.

Set the fixed bias to the linear spec voltage (Variac) and install the calculated grid leak resistor. (grid leak adds its voltage in series, of course)   With the right amount of RF drive, you should have the correct grid bias at the tube pin at the correct grid current.

Simple as that.

T
Logged

“Remember, remember, this is now, and now, and now. Live it, feel it, cling to it. I want to become acutely aware of all I’ve taken for granted.”  -  Sylvia Plath

Favorite Song - Trololo thru the years:  
https://www.youtube.com/watch?v=IVqUecYGnoM
WD5JKO
Member

Offline Offline

Posts: 1677


WD5JKO


« Reply #31 on: April 11, 2018, 07:53:23 PM »



Another good discussion on this topic back in 2012. Some former heavyweights weigh in. I like Bill KD0HG's bias circuit using a 6BQ5 shunt regulator.

http://amfone.net/Amforum/index.php?topic=31784.msg247501#msg247501

Jim
Wd5JKO
Logged
KA2DZT
Member

Offline Offline

Posts: 2105


« Reply #32 on: April 11, 2018, 09:08:23 PM »



Another good discussion on this topic back in 2012. Some former heavyweights weigh in. I like Bill KD0HG's bias circuit using a 6BQ5 shunt regulator.

http://amfone.net/Amforum/index.php?topic=31784.msg247501#msg247501

Jim
Wd5JKO

I read the whole thread,  seems to be same debate that we have on this thread.
Logged
N1BCG
Member

Offline Offline

Posts: 559


« Reply #33 on: April 14, 2018, 09:44:48 AM »

Again, a hearty thanks for the suggestions! The re-working of the bias circuit was a success with the RF deck working well, the meter reading grid I accurately, and protection against drive failure.

Stay tuned. I have another circuit redesign challenge...
Logged
w9jsw
Two shots of Whisky
Member

Offline Offline

Posts: 54



« Reply #34 on: April 14, 2018, 12:02:05 PM »

Can you post your final schematic, pls...
Logged
N1BCG
Member

Offline Offline

Posts: 559


« Reply #35 on: April 15, 2018, 09:14:23 AM »

It turned out to be the bias supply, ammeter, grid leak resistor, and RF choke in series with bypass caps to chassis. Basically what Phil posted...

http://amfone.net/Amforum/index.php?action=dlattach;topic=43814.0;attach=58196

In my case, R2 is 7k, C1 is 0.05, C2 is 0.01, C3 is 0.02, and Lfg is 2.5mH because of the original design requirements of the transmitter.
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1122


« Reply #36 on: April 15, 2018, 12:40:37 PM »

Sumptin like this Clark?  Smiley


* Speech Amplifier 6SN7.pdf (62.76 KB - downloaded 8 times.)
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
N1BCG
Member

Offline Offline

Posts: 559


« Reply #37 on: April 15, 2018, 12:51:59 PM »

Ah, good. Now I can toss my hand-drawn version. Testing the 6SN7 circuit now...
Logged
Pages: 1 [2]   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 0.081 seconds with 18 queries.