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G8MNY > TECH 15.07.03 11:05l 101 Lines 5157 Bytes #999 (0) @ WW
BID : 5720_GB7CIP
Read: DB0FHN GUEST OE7FMI
Subj: 4CX250B PA regulated supplies
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OK0PPL<RZ6HXA<SP7MGD<IW8PGT<IK1ZNW<GB7CRV<GB7CIP
Sent: 030714/2220Z @:GB7CIP.#32.GBR.EU #:5720 [Caterham] $:5720_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
In the tetrode valve PAs like the 4CX250Bs a hard (stiff) grid & screen power
rails are needed for optimum power & linearity, eg >400W @ >-20dB intermod
products, or better than a bank of transistors can do.
I found that a simple NPN emitter follower with a diode does a very good job
both for the screen & grid supplies.
GRID SUPPLY
For class AB2 working the negative grid supply needs to be able to handle grid
currents (more -ve voltages generated from rectified RF drive) without any
significant change in the bias voltage. In this design the sudden peak current
from the grid RF drive is safely conducted to ground without upsetting the bias
pot voltage. So the optimum no crossover distortion valve bias for about 100mA
anode cirrent can be maintained.
0v -----------------------------------------
! ! ! !
! 30V--- R !
+ ! 1W \ / 1K !
22uF === ---/ ! ! +
160v --- ZENERS !------ ! === 100uF
! --- / 10K !/c --- 100V
! 40v\ / \!--------! !
! 1W ---/ /BIAS ! !\e___!
70V 1N4003 ! ! \POT ! ! -30 to -70V
AC ---!<!--------R----------- --!<!-------------------Meter---- GRID
-100v 4K7 -70v + 10mA -
The 100V 20mA NPN transistor, current amplifies the bias pot, so that the peaks
of grid current flow to ground through the NPN transistor & the 1K, the 1K
dumps some of the power enabling the use of a small low power transistor.
A diode from the pot to the grid (base-emitter) routes -ve volts through to the
grid when there is no grid current. No sudden AC step change from the
transistor to diode voltage is allowed due to the 100uF on the grid output. The
DC must be fed to the grid via an RF choke (or low value wire wound R) & not
via a high value resistor (other wise you waste all the effort of this design)
if the peak drive grid current is going to drive the valve properly. Grid
current meter of 10mA can be put in series with the grid PS note the polarity.
N.B. on a 4CX250B a mean grid current of 2.5mA (0mA on a 4CX350!) is the
recommended value for minimum 3rd order IP product. In class AB2 there could be
half RF cycle peak current of about 50mA (2.5mA Mean), which is needed to give
a good half cycle RF peak anode current of greater than 1 Amp (350mA mean on
anode meter over the whole cycle).
Also recommended is the use of a RF power dump resistor across the grid tuned
circuit to stop the onset of grid current current upsetting the drive rig SWR
assuming you have more than 5W of drive.
SCREEN SUPPLY
A similar arrangement as the grid supply is needed for the Screen, but this
transistor provides the normal HT so a 500V 100mA NPN transistor (video/line
O/P stage transistor). This device follows the voltage across a 4 neon bulb
stack (or less reliable Zener stack), to give a regulated voltage of approx
+280V. A diode across the base-emitter also provides -ve current dump ability
from the screen into neon stack under certain tuning/load conditions.
1N4007
300V ---!>!---------------------- +420V
AC ! ! ! !
! 1M 56K !
! ! ! +280v !/c !----!>!-!
! ! !--RB-----! !-!<!----! PTT
! ! neon ! !\e__ +280V ! ! Relay
! ! ! ! !----------Meter------@ /---- Valve
! ! neon -!<!---! +25mA- / !
+ ! ---! ! @ !
22u === neon 1u --- ! VDR
500v --- ! 300v--- 1K !
! neon ! ! !
! ! ! ! !
0V --------------------------------------------------------------------
Again no sudden AC change from the transistor to diode step voltage is allowed
due to the 1uF on the screen output. A normal mains 300V VDR is recommended on
the valve base to dump any spikes from the anode 2KV supply that occasionally
appear on the screen grid! (this is why neons are more reliable than Zeners)
As the neons usually provide a -ve impedance characteristic series base
resister RB of a few K ohms can make the O/P zero ohms. A simple DC load test
can fine the best value for RB.
Screen Grid current meter of 25mA can be put in series with this PS, but should
be protected with 2 1N4001 diodes.
The PTT relay should connect the power to the valve to turn the valve on for
TX. It is recommended that this relay operates after the aerial C/O relay, but
releases before the aerial C/O. Suitable diode steered CRs can do this.
Also note that the screen power should not be connected if the valve...
has not warmed up. (takes 1min),
has No Anode supply,
has no bias supply.
Again simple relay inhibit electronics can do all this.
/QSL
73 De John, G8MNY @ GB7CIP
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