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Passive grid linear



 
 
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  #1  
Old November 27th 08, 07:45 PM posted to rec.radio.amateur.homebrew
ken scharf
external usenet poster
 
Posts: 182
Default Passive grid linear

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?
  #2  
Old November 27th 08, 08:28 PM posted to rec.radio.amateur.homebrew
Grumpy The Mule
external usenet poster
 
Posts: 87
Default Passive grid linear

Howdy,


I've only read about them. G2DAF wrote extensively
on pasive grid linears.

I think the ultimate value of grid resistor will
depend on the grid capacitance and layout parasitics.
The articles I've read use 300 Ohms as the grid swamping
resistor but with a 4CX250 450 Ohms might be possible.

Here is an article you may find useful.
http://www.radioamator.ro/articole/files/291_1.pdf

I have a bucket full of 4CX250B's and would like
to try passive grid someday. Please Let us know
how yours shapes up if you decide to build one.


73,
Grumpy


ken scharf wrote in
news
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?


  #3  
Old November 27th 08, 10:13 PM posted to rec.radio.amateur.homebrew
ken scharf
external usenet poster
 
Posts: 182
Default Passive grid linear

Grumpy The Mule wrote:
Howdy,


I've only read about them. G2DAF wrote extensively
on pasive grid linears.

I think the ultimate value of grid resistor will
depend on the grid capacitance and layout parasitics.
The articles I've read use 300 Ohms as the grid swamping
resistor but with a 4CX250 450 Ohms might be possible.

Here is an article you may find useful.
http://www.radioamator.ro/articole/files/291_1.pdf

I have a bucket full of 4CX250B's and would like
to try passive grid someday. Please Let us know
how yours shapes up if you decide to build one.


73,
Grumpy


ken scharf wrote in
news
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?


That's an interesting article. He shows no matching network between his
exciter and the linear so there would be a 6:1 SWR with the 300 ohm
termination, though a tube type transmitter with a Pi network might
still be able to load into it. Deriving the screen voltage from the rf
drive seems a hybrid method of doing grounded grid. I was thinking of
an amplifier with normal bias and screen supplies which would reduce the
drive required.

I have a pair of used 7034's (4cx250's with glass seals) but I have no
idea how much life (if any) are left in them. My idea was to build an
amplifier chassis that could be used to experiment with several tube
configurations. I've found a source for a 400VA toroidal power
transformer with two 550v and two 6.3v (5A) secondaries and a 120/240 v
primary for about $65 postpaid. In a bridge rectifier configuration
with a capacitor input filter using large capacitors it should yield
about 1400 volts under load. This would be a suitable plate voltage for
a single 811A/572B or a 4CX250A. With the bridge rectifier
configuration the center tap of the HV secondary will provide 700 volts
that can be regulated down to the 300-350 volts required for the screen
of a 4CX250. One of the 6v filament windings would power the tube
(unless I find some 4CX250F/G's which would require an extra 25v
transformer) and the other winding could be used with a voltage
multiplier for the bias supply (or drive another filament transformer
bassackwards for that purpose).

So I could build up a power supply and tank circuit that could serve
either configuration, and swap out one of two chassis; one for a single
811A/572B and one for a 4CX250.350/400A. The target amp would be
between 150 to 250 watts output (a nice afterburner for a 10-20 watt QRP
rig I'm thinking about).
  #4  
Old November 28th 08, 12:18 AM posted to rec.radio.amateur.homebrew
Ian White GM3SEK
external usenet poster
 
Posts: 233
Default Passive grid linear

Grumpy The Mule wrote:

I've only read about them. G2DAF wrote extensively on pasive grid
linears.

I think the ultimate value of grid resistor will depend on the grid
capacitance and layout parasitics. The articles I've read use 300 Ohms
as the grid swamping resistor but with a 4CX250 450 Ohms might be
possible.

Here is an article you may find useful.
http://www.radioamator.ro/articole/files/291_1.pdf

I have a bucket full of 4CX250B's and would like to try passive grid
someday. Please Let us know how yours shapes up if you decide to build
one.


The defining feature of a G2DAF amplifier is not the untuned passive
grid input, but the method of grid biasing. It has no control grid bias
and the screen voltage is developed by rectifying the RF drive.

Most passive grid tetrode amplifiers use conventional fixed bias
voltages on g1 and g2. Commercial examples include the Alpha 91b/99 and
the Australian Emtron range.


G2DAF's published articles on amplifiers are on my web site, and the
RSGB historical collection still has his original receiver, transmitter
and amplifier, along with many hand-written notes. The 2 x 4-125A
amplifier that featured in his published articles still exists, but at
some later time he had removed the screen rectifier tubes and converted
the amplifier into a very basic grounded-grid configuration.



--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
  #5  
Old November 28th 08, 01:42 AM posted to rec.radio.amateur.homebrew
ken scharf
external usenet poster
 
Posts: 182
Default Passive grid linear

Ian White GM3SEK wrote:
Grumpy The Mule wrote:

I've only read about them. G2DAF wrote extensively on pasive grid
linears.

I think the ultimate value of grid resistor will depend on the grid
capacitance and layout parasitics. The articles I've read use 300 Ohms
as the grid swamping resistor but with a 4CX250 450 Ohms might be
possible.

Here is an article you may find useful.
http://www.radioamator.ro/articole/files/291_1.pdf

I have a bucket full of 4CX250B's and would like to try passive grid
someday. Please Let us know how yours shapes up if you decide to
build one.


The defining feature of a G2DAF amplifier is not the untuned passive
grid input, but the method of grid biasing. It has no control grid bias
and the screen voltage is developed by rectifying the RF drive.

Most passive grid tetrode amplifiers use conventional fixed bias
voltages on g1 and g2. Commercial examples include the Alpha 91b/99 and
the Australian Emtron range.


G2DAF's published articles on amplifiers are on my web site, and the
RSGB historical collection still has his original receiver, transmitter
and amplifier, along with many hand-written notes. The 2 x 4-125A
amplifier that featured in his published articles still exists, but at
some later time he had removed the screen rectifier tubes and converted
the amplifier into a very basic grounded-grid configuration.



I really don't see any advantage in his circuit over a conventional zero
bias grounded grid amp, at least not with the 4-xxx series of tubes.
With the 4CX250 series it makes some sense since these tubes CAN'T be
run in zero bias grounded grid due to the fragile grid structure in
these tubes.
  #6  
Old November 28th 08, 06:00 AM posted to rec.radio.amateur.homebrew
Tim Wescott
external usenet poster
 
Posts: 202
Default Passive grid linear

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 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
  #7  
Old November 28th 08, 10:45 AM posted to rec.radio.amateur.homebrew
raypsi
external usenet poster
 
Posts: 242
Default Passive grid linear

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
  #8  
Old November 28th 08, 02:27 PM posted to rec.radio.amateur.homebrew
Stev eH[_10_]
external usenet poster
 
Posts: 1
Default Passive grid linear

ken scharf wrote:


I really don't see any advantage in his circuit over a conventional zero
bias grounded grid amp, at least not with the 4-xxx series of tubes.
With the 4CX250 series it makes some sense since these tubes CAN'T be
run in zero bias grounded grid due to the fragile grid structure in
these tubes.

You only needed 20W of drive power and a single HT supply. I have one
using a pair of 813s, 10W from my TS-120 drives it to about 200W output.

It looks to be exactly the same layout as the one on Ian's site but has
had the valve rectifiers replaced with solid state at some time.

Steve H
  #9  
Old November 28th 08, 03:58 PM posted to rec.radio.amateur.homebrew
ken scharf
external usenet poster
 
Posts: 182
Default Passive grid linear

Stev eH wrote:
ken scharf wrote:


I really don't see any advantage in his circuit over a conventional
zero bias grounded grid amp, at least not with the 4-xxx series of
tubes. With the 4CX250 series it makes some sense since these tubes
CAN'T be run in zero bias grounded grid due to the fragile grid
structure in these tubes.

You only needed 20W of drive power and a single HT supply. I have one
using a pair of 813s, 10W from my TS-120 drives it to about 200W output.

It looks to be exactly the same layout as the one on Ian's site but has
had the valve rectifiers replaced with solid state at some time.

Steve H

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.
  #10  
Old November 28th 08, 04:06 PM posted to rec.radio.amateur.homebrew
ken scharf
external usenet poster
 
Posts: 182
Default Passive grid linear

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). 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.)
 




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