Reply
 
LinkBack Thread Tools Search this Thread Display Modes
  #11   Report Post  
Old November 28th 08, 05:39 PM posted to rec.radio.amateur.homebrew
external usenet poster
 
First recorded activity by RadioBanter: Nov 2008
Posts: 77
Default Passive grid linear

On Nov 27, 1:45*pm, ken scharf wrote:
Anybody here ever build a passive grid linear amp?
That's where you simply terminate the grid of a tetrode with a 50 ohm
resistor and just drive it directly. *Svetlana recommended that
configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should
work well with more commone surplus 4cx250's.

I was thinking of using a 4:1 or 9:1 balum and increasing the grid
resistor value to 200 or 450 ohms to reduce the amount of driving power
required. *With the 4cx250 a peak grid drive of about 50 volts is
required in class AB1. *With a 50 ohm termination the driving power
would be 50 watts, with a 200 ohm *termination 12.5 watts, and with the
450 ohm termination 5.6 watts. *That's actually just the power sucked up
in the resistor, but the tube requires less than 1 watt of drive itself
in class AB1. *Some power might be lost in the balum, so maybe the
actual driving power might increase by a watt or two. * Since I wanted
this to be a final for a QRP rig the larger terminating resistance
looked like a better way to go. *The only issue is how high can you go
with the termination resistance and keep the tube stable without
neutralization being required?


Ive seen one amp that I know worked using a 4cx250B using a 16:1
transformer. He used tha amp on 6M. I think he still added a little
capacitance to tune out some of the inductance in the transformers.
The best I remember he really didnt have to do this. He just did it so
he would see a perfect 1:1 SWR beteween the exciter and amp. I
modified an old 2X 6146 amp with tunned grid by ripping out the tuned
circuit and replacing it with a 200 ohm resistor. 5 watts was more
than enough drive.

Jimmie
  #12   Report Post  
Old November 28th 08, 06:02 PM posted to rec.radio.amateur.homebrew
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 202
Default Passive grid linear

On Fri, 28 Nov 2008 10:06:43 -0500, ken scharf wrote:

raypsi wrote:
On Nov 28, 12:00 am, Tim Wescott wrote:
On Thu, 27 Nov 2008 13:45:33 -0500, ken scharf wrote:
Anybody here ever build a passive grid linear amp? That's where you
simply terminate the grid of a tetrode with a 50 ohm resistor and
just drive it directly. Svetlana recommended that configuration with
their 4cx400, 4xc800 and 4cx1600 tubes, and it should work well with
more commone surplus 4cx250's.
I was thinking of using a 4:1 or 9:1 balum and increasing the grid
resistor value to 200 or 450 ohms to reduce the amount of driving
power required. With the 4cx250 a peak grid drive of about 50 volts
is required in class AB1. With a 50 ohm termination the driving
power would be 50 watts, with a 200 ohm termination 12.5 watts, and
with the 450 ohm termination 5.6 watts. That's actually just the
power sucked up in the resistor, but the tube requires less than 1
watt of drive itself in class AB1. Some power might be lost in the
balum, so maybe the actual driving power might increase by a watt or
two. Since I wanted this to be a final for a QRP rig the larger
terminating resistance looked like a better way to go. The only
issue is how high can you go with the termination resistance and keep
the tube stable without neutralization being required?
This should jump out at you with the proper circuit analysis. Just
analyze the circuit with a parallel tank load on the plate, and look
for negative resistance showing up at the grid. Your grid swamping
resistor would need to take care of the worst-case negative
resistance.

Then spend a month tearing your hair out over parasitics that increase
the plate-grid coupling over and above what the data sheet says they
are.

--
Tim Wescott
Control systems and communications
consultinghttp://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes,http://www.wescottdesign.com/actfes/actfes.html


Hey OM:

I don't see plate grid coupling with a grounded screen and the cathode
at negative screen voltage? eh?

73 OM
n8zu

I think he means that any feedback which would induce instability can be
translated as a negative resistance seen at the grid (all an oscillator
is after all, is a negative resistance generator).


Yes, that's what I meant. And if you're looking to figure out the
largest parallel grid resistance that will still effectively swamp out
the instability, I think that's the best approach to take.

And ANY tube
exhibits some degree of plate to grid coupling in the form of stray
capacitance between the elements. Even Pentodes with two grounded (for
rf) elements between the plate and grid show SOME coupling (though in
the case of screen grid tubes the value is reduced to values of a few pf
or less). The grounded grid circuit reduces the feedback even more by
introducing a phase shift as well, but some grounded grid amps still
need some neutralization (especially if you mess things up by putting 4
bottles in parallel.)


In theory, any tube is going to have _some_ coupling straight through the
grids. In practice its probably vanishingly small.

But for many tubes you'll have to spend a long time optimizing the
physical layout of the circuit before you have more coupling through the
tube than you have around it.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
  #13   Report Post  
Old November 28th 08, 06:49 PM posted to rec.radio.amateur.homebrew
external usenet poster
 
First recorded activity by RadioBanter: Nov 2008
Posts: 1
Default Passive grid linear

wrote:
On 28 Nov,
ken scharf wrote:

200W from a PAIR of 813's? You must be running them with less than
1.5KV on the plates. That's the problem with 813's, they don't live up
to their potential (pardon the pun) unless you run them with lots of HV.
The point though is that in conventional grounded grid (all three
'grids' grounded) you still don't need a SG supply and the drive power
should be about the same.


I could get well over 400w (probably nearer 800w) out of mine, with 2,5-3KV
on the anodes.

With G2DAF mode I found the screen grid supply (from the RF drive) produced
lots more TVI than when conventionally used in class AB2. That's why I
changed. I used separate screen and bias supplies, but made sure the anode
supply was present before the screen was energised.

That's with 10W of drive (From a 10W radio) and 2KV on the anode,
maximum output is around 600W with about 40W of drive. You can wet away
with over 3KV on the anode if you are careful.
If you introduce a 4:1 transformer on the input the drive required is
far lower.
I have a KW500 with a single 813 in grounded grid, it produces 40W with
10W in and is a bad load for a transistor TX (No tuned input as it was
designed to work with a valve TX). With passive grid you don't need a
separate tuned input for each band so it keeps it simple.

Steve H


  #14   Report Post  
Old December 8th 08, 07:19 PM posted to rec.radio.amateur.homebrew
external usenet poster
 
First recorded activity by RadioBanter: Nov 2008
Posts: 77
Default Passive grid linear

On Nov 27, 1:45*pm, ken scharf wrote:
Anybody here ever build a passive grid linear amp?
That's where you simply terminate the grid of a tetrode with a 50 ohm
resistor and just drive it directly. *Svetlana recommended that
configuration with their 4cx400, 4xc800 and 4cx1600 tubes, and it should
work well with more commone surplus 4cx250's.

I was thinking of using a 4:1 or 9:1 balum and increasing the grid
resistor value to 200 or 450 ohms to reduce the amount of driving power
required. *With the 4cx250 a peak grid drive of about 50 volts is
required in class AB1. *With a 50 ohm termination the driving power
would be 50 watts, with a 200 ohm *termination 12.5 watts, and with the
450 ohm termination 5.6 watts. *That's actually just the power sucked up
in the resistor, but the tube requires less than 1 watt of drive itself
in class AB1. *Some power might be lost in the balum, so maybe the
actual driving power might increase by a watt or two. * Since I wanted
this to be a final for a QRP rig the larger terminating resistance
looked like a better way to go. *The only issue is how high can you go
with the termination resistance and keep the tube stable without
neutralization being required?


I was recently playing around with a 6DQ6 PA and it did not require
netralization with 2K ohms on the gridI didnt try anything higher than
that. Using broadband transformers will introduce some reactance that
may have to be tuned out. I suspect this may not become a problem
until you get up to 10M or so.

Jimmie
Reply
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules

Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Passive Repeater Bryan Martin Antenna 13 February 10th 06 03:03 PM
Quad 837 Grounded Grid linear amplifier [email protected] Homebrew 3 September 20th 05 10:59 PM
Quad 837 Grounded Grid linear amplifier [email protected] Homebrew 0 September 19th 05 05:45 PM


All times are GMT +1. The time now is 12:12 AM.

Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 RadioBanter.
The comments are property of their posters.
 

About Us

"It's about Radio"

 

Copyright © 2017