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G8MNY > TECH 01.01.17 14:51l 412 Lines 22632 Bytes #999 (0) @ WW
BID : 29906_GB7CIP
Read: GUEST
Subj: T500M 12V 500W HF Linear
Path: IW8PGT<CX2SA<GB7CIP
Sent: 170101/1123Z @:GB7CIP.#32.GBR.EURO #:29906 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Jul 16)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
A few years ago I bought an old (1977) large commercial Trans World Electronics
Inc, 12V HF Amp for "MEDIUM POWER Air/Ship/Army" use. at a local junk sale.
(actually used in BBC Engineering Vans)
///////////////////////³ 2-30MHz, 4x 150W push pull amps in parallel.
/////////////////////// ³ > 10dB gain, 70W max drive. (typical 15-40W)
/////////////////////// /³ IMD 3rd Order -32dB @-500W, -36dB @ 400W.
/////////////////////// / ³ PA harmonics to better than -43dB.
/////////////////////// / ³ 13.6V @ 75 Amps needed for full 600W output!
/////////////////////// / / 1kW DC input, Infinite SWR rated, <2:1 recom.
³³³³³³³³³³³³³³³³³³³³³³³³ / / 15A charger & car battery will power it (SSB).
ÚÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ¿ / Thermal 70øC heatsink shutdown.
³T500M __ ____ ³ / Over current 75A trip (high SWR & over drive).
³ o<ð [__] [____] ³/ Manual & Remote operation (On & Band select).
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ Weight 8kg.
It was cheap as it had a fault, it came with the handbook, so I expected a
problem or two. On examination it basically worked OK "no blown amps", but it
had a faulty band switch. That was just a "light contact" on the single wafer
switch, causing non operation, or no "band filter relays selected" (no RF
output path!) & easily fixed by bending contact once the switch was stripped
down.
S C H E M A T I C
Rx & low power through path
DriveÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿Ant
RIG____/ ÚÄÄÄÄÄ¿50êÚÄÄÄÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÄÄ¿50ê ÚÄÄÄÄÄÄ¿ \___ANT
|ÀÄÄÄ´AttenÃÄÄÄ´Splitter³ ³CombinerÃÄÄÄÄÂÄ/Ä´FilterÃÄ\ÄÂÙ|
| 70WÀÄÄÄÄÄÙ30WÀÄÂÄÂÄÂÄÂÙ ÚÄÄÄ¿ ÀÂÄÂÄÂÄÂÄÙ600W³ | ÀÄÄÄÄÄÄÙ | ³ |ptt
| Max Max ³ ³ ³ ÀÄÄ´PA1ÃÄÄÙ ³ ³ ³ ³ ÚÄÄÄÄÄÄ¿ ³
| ³ ³ ³200êÀÄÄÄÙ200ê³ ³ ³ ÃÄ/Ä´FilterÃÄ\Ä´
| ³ ³ ³ ÚÄÄÄ¿ ³ ³ ³ ³ | ÀÄÄÄÄÄÄÙ | ³
PTT>ÄÄÄÙ Drive ÚÄÄÄÄ¿ ³ ³ ÀÄÄÄÄ´PA2ÃÄÄÄÄÙ ³ ³ ³ ÚÄÄÄÄÄÄ¿ ³
/Ä´BiasÃÄ> ³ ³ 200êÀÄÄÄÙ200ê ³ ³ ÃÄ/Ä´FilterÃÄ\Ä´
³ ÀÄÄÄÄÙ ³ ³ ÚÄÄÄ¿ ³ ³ ³ | ÀÄÄÄÄÄÄÙ | ³
Trip ³ ³ ÀÄÄÄÄÄÄ´PA3ÃÄÄÄÄÄÄÙ ³ ³ ÚÄÄÄÄÄÄ¿ ³
ÚÄÄ¿ DC³ ³ 200êÀÄÄÄÙ200ê ³ ÃÄ/Ä´FilterÃÄ\Ä´
12V___³/_³_/ÄÁÄ> ³ ÚÄÄÄ¿ ³ ³ | ÀÄÄÄÄÄÄÙ | ³
ÀÄÄÙ | ÀÄÄÄÄÄÄÄÄ´PA4ÃÄÄÄÄÄÄÄÄÙ ³ ÚÄÄÄÄÄÄ¿ ³
| 200êÀÄÄÄÙ200ê ÀÄ/Ä´FilterÃÄ\ÄÙ
Band Switch| | ÀÄÄÄÄÄÄÙ |
or Remote> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
L A Y O U T (Bottom cover off)
ÚÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ Äij
Þ³~~~~~ÄÄ Pot PushPull | ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ÃÄ¿
ݳ100 ³ Bias =()=´±±±±³Output ³Relay³ >15MHz Filter³Relay³ ÃÄÙRig
ݳAmp ³Circuit [±] PA1 Transformers ~~~~~ ~~~~~ ³SO239
Þ³Meter³ [±] =()=´±±±±³ | ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ³
³-----~~5R [±]Input ³Relay³8-15MHz Filter³Relay³ ³ÜÛ 13.6V
³ [±]Spliter=()=´±±±±³ | ~~~~~ ~~~~~ /³+ß DC 75A
³ÄÄÄÄÄÄ. [±] [±] PA2 ³ÚÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ÚÄÄÄÄĿݳ-Ü Wing
³75A DC ÃÄÄ[±] [±] =()=´±±±±³³ DC ³ ³Relay³ 5-8MHz Filter³Relay³Û³ßÛ Nuts
Þ³TRIP & Ã()shuntÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜRelay³ÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÜÛ³
/³ON/OFF ³__Üßßßß =()=´±±±±³ÀÄÄÄÄÙ ÚÄÄÄÄÄ¿ ÚÄÄÄÄÄ¿ ³12 Way
ß³======' PushPull[±] PA3 ³ | ³Relay³ 3-5MHz Filter³Relay³ ³Jones
³Drive³ Driver[±] =()=´±±±±³ [±±]Output~~~ ~~~~~ ³Socket
³Relay³ Transformers [±±]CombinerÄ¿ ÚÄÄÄÄÄ¿ ³
³~~~~~ =()=´±±±±³ [±±]| ³Relay³ 2-3MHz Filter³Relay³ ³
³| ThermSw [±] PA4 ³ [±±] ~~~~~ _____ ³SO239
Û³|Band [±] =()=´±±±±³ | ³ Ant ³ ÃÄ¿Ant
³|Switch P.A. PCB :RF lead: FILTER PCB ³Relay³ ÃÄÙ
³ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ ÄÄ~~Äij
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÙ
PA protection is from a 75A fast magnetic trip for bad SWR & over drive, it has
an external calibrated shunt, & a thermal 70øC auto resetting cut off switch.
The 100A ammeter uses 10cm of the thick DC lead as it's calibrated shunt.
The bias supply is a simple 2 transistor thermally tracked circuit provides up
to 2.2A of current @ 0.69V for the 4 class AB push pull amps. Excluding other
currents, PA quiescent current should be 1.6-2A, (which gives the best two tone
linearity results at around that level. See 5/)
P A R A L L E L A M P S
Four identical push pull Amps (>150W/Amp) use 2x PT9847 100W HF transistors
with input & output matching transformers consisting of 5 or 3 ferrite rings
stacked on 2x 1/2 turn brass tubes for the low Z side of the 5:1 turns ratio,
smaller ones are used for the driver & much larger ones for the outputs. A
large amount of RF NFB is (for good linearity) provided by 47ê 5W & u1
collector to base on each transistor.
Input & Output Transformers use...
NFBÚÄ47Ä¿ 1:5 Input 3 rings/side, Output 5 rings/side
200ê 5:1 u1=== ÃÄÄÂÄÄÄÄÄ¿ ______ CollectorÄÄ´±± ±± ±± ±± ±±Ã+PCB
ÄÄÄÄÄ¿ ÚÄÄÄÄÄÄÁÄÄ´< e ³ ³|( 200ê or Base tube~~~~~~~~~~~~~~tube
5W 5 )|³ Á ³1n2 _)|( 5 Turn /////______________ \\\\\
Turn )|(__ NFB === ³ )|( Output 5 |||||³±± ±± ±± ±± ±±³ |||||
Input)|( ³ u1ÚÄ47Ä¿ ³ ³ ³|( >150W turns||||| Ã+|||||
)|³ ³ === ÃÄÄÁÄÄÄ)ÄÙ|( |||||³±± ±± ±± ±± ±±³ |||||
)|ÀÄÄ)ÄÄÄÁÄÄ´<PT9847 ³ Á \\\\\~~~~~~~~~~~~~~~/////
Á ÃÄbias ³e ÃÄÄÄÂÄÄÄÄ+12V tube______________tube
.6V @.7A ===2u2 ³ u1=== ===1mF @20A CollectorÄÄ´±± ±± ±± ±± ±±Ã+PCB
ÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÁÄÄÄÁÄÄÄÄÄ or Base Ferrite ring stack
The inputs & outputs are wired up from the drive splitter & output combiner
with staggered wire lead lengths, so all the RF signals ends up exactly in
phase.
ÚÄÄÄÄÄÄ>PA1>ÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿ Splitter & combiner both have out of
(|_100R_ _100R_|) balance dump 100R to soak up any amp
_(| 1W ³ 200ê ³ 5W |)_ differences for best stability &
³ (|_100R_³ AMPS ³_100R_|) ³ linearaity. Ferrite ring & tube
³ (| 1W ³ ³ 5W |) ³ construction like transformers.
50ê ³ ÀÄÄÄÄÄÄÄ)Ä>PA2>ÄÄÄÄ)ÄÄÄÄÄÄÄÙ ³ ÚÄÄÄÄÄÄÄÄÄÂÄÄÄÄÂÄÄÄÄÄÄÄÄÄ¿
Drive>´SPLITTER ³ ³ COMBINERÃ>50ê 100ê 100ê 100ê 100ê
25W ³ ÚÄÄÄÄÄÄÄ)ÄÄÄÄ>PA3>Ä)ÄÄÄÄÄÄÄ¿ ³600W ³ ___ ___ ³ ³ ___ ___ ³
³ (|_100R_³ ³_100R_|) ³ _³|___X___|³_ _³|___X___|³_
³_(| 1W ³ 200ê ³ 5W |)_³ ±³|³± ±³|³± ±³|³± ±³|³±
(|_100R_³ AMPS ³_100R_|) ±³|³± ±³|³± ±³|³± ±³|³±
(| 1W 5W |) ~~|~~~~~~~|~~~~~~|~~~~~~~|~~\Drive
ÀÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ>PA4>ÄÄÄÄÄÄÙ PA1 PA2 PA3 PA4 Output
F I L T E R S
There are 10 relays that in pairs switch in 1 of the 5 QRO 2 section PI low
pass band filters, & reduce the quite high PA harmonics to > -43dB.
N.B. there is no PA RF output path without a pair of band relays operated!
Filter In Filter Out
______Relay Relay______ Band C1/C3 C2
From /ÄÄÂÄÄ())))ÄÄÂÄÄ())))ÄÄÂÄÄ\ To 2-3MHz 390+430p 750+680+270p
Combiner³ ³ L ³ L ³ ³ Aerial 3-5MHz 390+120p 750+270p
³ === === === ³ Relay 5-8MHz 270+47p 430+200p
³ ³C1 ³C2 C3³ ³ 8-15MHz 82+82p 200+150p
ÄÄÄÄÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÁÄÄÁÄÄÄÄ 15-30MHz 82p 56+120p
The Cs are all 2-3kV RF types. The Ls are wound 2cm ferrite rings, or air for
the highest range. Using several Cs in parrellel not only gets the odd filter
value, but also gives greater current handeling & reduced lead inductance.
H E A T S I N K
At 25øC ambient in free air, the very large heatsink does not need a fan on 30%
duty SSB Tx cycle, despite only the front part getting quite hot. But carrier
modes are to be avoided (input attenuator overheats on lower bands!) or the
temperature might rise above the thermal 70øC auto resetting cut off switch.
M O D I F I C A T I O N S
1/ LED INDICATORS, REDUCED STANDBY CURRENT & GIVE RELAY SEQUENCING.
Current was quite high in Rx mode, I found all the relays would operate OK
down to 7V. So I added series Rs to reduce the currents by 30% for the slow to
operate ones, & I used the R's added voltage drop to light 2 status LEDs too.
ÚAmmeter¿ ON Drive
+12V>ÄTRIPÁÄShuntÄÁÂÄÄÄÄÄ\ÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄÂÄÄÄÄÄÄÄÂÄÄÄÄÄÄÂÄÄÄ\ÄÄÄ>Bias
75A ÚÄÄÄÄÁÄÄÄÄ¿ ÚÄÄÁÄÄÄ¿ === PA ÚÄÄÁÄÄ¿ÚÄÄÁÄÄ¿ Regulator
³ DC ON ³ ³Filter³ _³_3000uF ³Drive³³ Ant ³
³Contactor³ ³Relays³ /// ³Relay³³Relay³
ÀÄÄÄÄÂÄÄÄÄÙ ÀÄÂÂÂÂÂÙ ÀÄÄÂÄÄÙÀÄÄÂÄÄÙ
ÚÄ100Ä´ ³³³³³ Red ÚÄ100Ä´ ³
Green _³_ 1W oooooo Tx _³_ ³ ³
ON <=\_/ 75R /³\ LED<=\_/ 33R ³
LED ÀÄÄÄÂÂÅ¿ ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÙ ³
oooooo e\³ ³
/³\ PNP ÃÄððÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄ<PTT
³ 2N2905/³ Fbead 80mA
_³_ _³_
/// /// Relay back EMF diodes & RF caps not shown
Added components mounted on or near the band switch.
Input & output filter relays (not in Rx path) are NOW only operated when the
PTT is active, from a PNP emitter follower. The drive relay that also puts on
the PA Bias, is last to operate wih a series LED too is also buffered. But the
Ant relay must be faster, so it is left directly on the PTT line! These
modification save about 300mA on standby & helps keeps the filter relay
contacts clean! It also reduces the PTT current to 80mA (limited PTT current
on my exciter's reed relay). And the slight voltage differences is all that is
needed to ensure the relays all operate in the right sequence order, so no
QRO RF contact splats.
2/ RIPPLE SMOOTHIMG & RF on DC LEADS
Only 3x 1000uF was fitted on my PA's +12V rail, the diagram showed 3x 2200uF,
& having a large bag of similar 1000uF caps, I added 7 more symmetrically
stacked up around the 4 amplifiers to give 10,000uF in all, much more than
that might weld up the DC contactor! Each of these Caps can give a few amps at
audio, reducing some of the battery lead AF ripple current.
To stop RF on the DC leads (don't want any in the shack), I added a 1uF non
eletrolytic internally across the DC terminals & another 0.5uF from +12V to
the nearby "RF In" ground.
3/ DC LOSSES
This QRO amplifier has very high currents, a drop of 1V = 100W less peak RF
power! DC lead losses & the use of unsoldered crimp connectors all adds up.
So with the amplifier into a dummy load, I use a DVM on 2V range from battery
-ve & +ve to highlight where the voltage was being lost... drops on the leads,
contactor, & tags. (If RF gets up your meter use 1k R in series as RF stopper
at probe end)
Metal case connection of the -ve terminal had not been used, it could reduce
the internal earth wire loss to near zero. It was just bolted on painted
panels. So I ground off the paint around the earth post, greased the bare
aluminium to keep the air away, & bolted it up tightly. I did the same to back
panel to heatsink screws with lock washers etc.
External DC cables, I use short "starting grade cables" to a large battery, or
30A leads to 2 small 24AH batteries & a 30A PSU, see "battery leads" below,
4/ DC FUSE
There was no low current fuse, so I soldered in a 3A one in the small wiring
feed (to the band switch) to reduce the risk of an internal fire!
5/ BIAS
R4 turns on Q2, when Q2 emmiter > 0.6V, Q1 turns on reducing Q2 base drive.
Value of R3 is used to see the exact bias voltage, R4 & supply voltage also
affect the bias to slightly. R5 limits the max current, D2 is a safety
feature. Q1 & D2 are thermally connected to the PA on the same heatsink.
PTT Switched
+12V ÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ R5 This circuit was slightly
R4 Added 5R unstable when scoping
270 47n 10W Scope for R5 (5R) so I added a small
³ Ú´Ã---³ <--1MHz oscillation! capacitor base to collector
³ | ³/c on the large MPN to stop it.
ÃÄÄÄÄÄÂÄÄÄÄÄ´ TIP33A
³ C6 === Q2 ³\e NPN +690mV @ 2.2A
³ 2u2_³_ ÃÄÄÄÄÂÄÄÄÂÄ> via RFC to
³ /// ³ ³ ³ Amp input
NPN c\³ ³ ³ ³ transformers
TIP29 ÃÄÄÄÄÄÄÄÄÄÄÄ´ ³ ³
e/³ Q1 _³_ +³ 2R2
PA BIAS ³ \_/ === ³ Components were not the
QuescentÀ>5R D2 ³ C7³ ³ same as original diagram!
Set 1-2A R3 ³ 2u2³ ³
ÄÄÄÄÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÁÄÄ
6/ ALC
There is no ALC system on this AMP, & I am used to old Valve amp with a power
ALC control. With PA ALC, the driver power is automatically set to the wanted
level, & with the PA turned off your back to full bare foot power. So I
designed this ALC circuit for this PA...
+12V Tx>ÄÂÄÄÄÄÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄÄ¿
R4 5R ³
270 PA 10W ³
ÀÄ>Bias<ÄÁÄÄ270ÄÄ¿ 15K
Circuit ³ 2n2 ³
ÃÄĴÿ ³ Front panel
PNP e\³ ³ 10K 50W-600W
2N3703 ÃÄÄÁÄ>POWER ALC Control
on POT /³ POT
ÚÄÄÄÄÄ´ _³_
4K7 |³| ///
³ ³Fbead Mounted on 12 way JONES plug
Input -ve³ ³ Fbeed
RF on >Ä1kÄÄ´<ÃÄ´<ÃÄ´ ÀÄÄÄÄÄÄÄÄÄÄÄÄððij<ÃÄÂÄÄÄÄÂÄÄÄ>ALC to rig
Drive 1N4148 ³ long wire 1N4148³ 100 0V to -10V
Atten === ³ ³ -
2n2³ 4K7 === 4u7
_³_ _³_ _³_+20V
/// /// ///
The -ve supply for the ALC is derived from the RF on the attenuator after the
DRIVE relay. The -ve after the 4K7 it is normally clamped to +ve by the PNP.
But when the PA bias current (limited to 2.2A by 10W 5R) reaching the 8 PA
bases, gives a voltage lower than that set on POWER POT & the clamping stops,
letting the -ve through. The series diode & 4K7 load mounted on the rear Jones
socket ensures only -ve voltages are given to the exciter to reduce power
drive. The 4u7 & 100R give a sensiable ALC time constant action.
MY CLEVER ALC DESIGN!
My ALC works very well compaired with manually keeping the drive power always
low enough at all times so the PA never clips. By using bias current demand,
it is quite effective at keeping the PA operating in it's linear region by
reducing the driver power. This is because the large amount of NFB used in
this commecial PA, increases the PA's drive power, as the amp gain falls off
at full power. The sudden increase in bias current occures before the PA
actually clips. So a usefull & accurate maximum drive threshold point that
caters for any SWR, supply voltage, or Rig power setting etc!
7/ INPUT SWR
The input frequency compensating attenuator circuit was not as the diagram &
the SWR was not all that good, dispite all components testing out OK.
SWR Original Input Match SWR Improved Input Match @ 50W
1.7´ .ú'ú. 1.7´
1.5´''''''''''' 'ú.. 1.5´
1.3´ ''''úúúúú 1.3´ ..ú''ú. ..ú'
1.1´ 1.1´'''''''''' ''''''''
ÀÂÄÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄ ÀÂÄÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄ
1.8 3.5 5 7 10 14 18 21 24 28MHz 1.8 3.5 5 7 10 14 18 21 24 28MHz
L1 39p 25W L1 39p 25W
>ÄÄ())ÄÄÄÂÄÄÄÂÄÄÄÄÂÄ´ÃÄÄÂÄ>Drive >Ä())ÄÂÄÄÄÄÄÄÂÄÄÂÄÄÄÄÂÄ´ÃÄÄÂÄ>Drive
40W L2( === ÃÄ220Ä´ Splitter 40W ³ L2( === ÃÄ200Ä´ Splitter
( ³56p ÃÄ220Ä´ === ( ³56p ÃÄ200Ä´
ÚÄÄÄÂÁÄÄÂÁÄÄ¿ ÀÄ220Ä´ 100p³ ÚÄÄÁÂÄÁÄ¿ ÃÄ200Ä´
200 200 200 200 220 ³ 220 220 220 ÀÄ200ÄÙ
_³_ _³_ _³_ _³_ _³_ _³_ _³_ _³_ _³_
Actual Circuit New Circuit
There was a bump @ 10MHz & that is from the drive splitter load. L2 & 56pF
disconnects the added load, as the 39pF bypasses the series attenuator Rs on
the higher frequencies, to flatten the amp gain. The original diagram did not
have 220R to ground, but had 20pF to ground @ the L1/2 junction. I found
making this a 100pF (Tx grade) was better on 10m band & changing the load Rs
around gave a better lower band input match.
Flat gain is less important than driver rig linearity, due to poor load. The
input SWR will change with drive level (higher Z at more power), as the RF NFB
level reduces, correcting each amplifier gain, as each amplifier works harder.
T E S T I N G
At a club meeting, 2 of these amplifiers (modified & unmodified) were tested
with 2 tone linearity test & with a spectrum analyser for harmonics. Both amps
performed well to the sudden (like AF Amps) 600W clipping level. This due to
the effective NFB keeping good lineararity until it fails. But even brief full
carrier testing on lower bands did provide smoke from the underrated input
attenuator!
The 2 tone test showed very good linearity to 400W PEP, so I think the quoted
IMD looks right. And on air tests with SDR displays show the amp is very clean.
The harmonics tests on a spectrum analyser showed the need to have the "right"
low pass filter selected, as these un-tuned broadband amps are quite harmonic
rich otherwise!
dB Topbands with dB Topband with
0_³ f1 15-30MHz Filter 0_³ f1 2-3MHz Filter
-10_³ ³ -10_³ ³
-20_³ ³ f3 -20_³ ³
-30_³ ³ f2 ³ -30_³ ³
-40_³ ³ ³ ³ f4 f5 -40_³ ³ f2
-50_³ ³ ³ ³ ³ ³ f6 -50_³ ³ ³
-60_³ ³ ³ ³ ³ ³ ³ f7 -60_³ ³ ³ f3
ÀÄÄÁÄÄÁÄÄÁÄÄÁÄÄÁÄÄÁÄÄÁÄ ÀÄÄÁÄÄÁÄÄÁÄÄÄÄÄÄÄÄÄÄÄ
On Topband the 2-3MHz LPF is not really that good for the 2nd harmonic! Higher
bands fared better with filter performance. Of course no problem at all after a
good ATU.
On actual testing into aerial via a high Q QRO ATU, I found it was possable to
get slight PA parasitic oscillation (of the RF envelope) at very high power. It
never did this into my dummy load or an actual aerial on Spectrum Analyser! But
with the final tweak to the input attenuator, it tested OK across all bands,
with ATU tuning over a range of SWRs. So sudden high SWR in a tuner may
indicate a Tx "parasitic" as well as aerial "arcing" failure!
H A Z A R D S
Current Loops:
Although 12V is fairly safe (compared to 230V or 3kV), with high currents
anything metal is a hazard! This includes the PL259 plug & mains earth wiring!
I put heat shrink sleeving on 259 plug rig lead near the +12V terminal. Care
must be taken to ensure the "75A" does not flow around unsuitable leads in
parallel e.g. Mains PSU earths!
Battery leads:
With single battery, use short "starter gauge" cables, with soldered on copper
tabs/lugs, made from thin Copper sheet 0.5mm, wound on a 8mm drill 1.5 turns.
Then flattern one end, solder to cable (on cooker).
__________ heat shrink
Drilled ___ _____________
Hole ____/ ³~STARTER CABLE
~~~~ÄÄÄÄÙ~ÄÄÄÄÄÄÄÄÄÄÄÄÄ
~~~~~~~~~~~~
Clean up & apply heat shrink sleeve or tape. Drill hole for PA & battery
connections. Mark up + & - with coloured tape. Apply water repellent grease to
tabs, bolt tread, washers etc.
Lead Acid Batteries:
Other than high current & fire hazard of melted leads, batteries have Sulphuric
Acid that always seems to get out & damage cloths etc, you can replace clothes,
but eyes are something else! Take care!
H2 Anti Explosion Tip:
Always "blow" at the battery, before making/unmaking connections, this simple
action reduces the chance of hydrogen being around for sparks to ignite!
High Power RF:
At these powers RF leakage from loose PL259, high Filter & Aerial voltages are
dangerous! Double checking connectors & everything is SAFE before keying up, &
testing is essential. Otherwise you will soon learn about deep RF burns & gain
"Respect for the RF" the hard way!
RF Chokes:
Wind coax or balanced aerial leads to make "RF chokes" near shack end, this
helps keep shack RF fields & RF lead currents down!
I N U S E
Running it /P for 8 days at a summer camp on HF with autocaller & pleanty of
pile ups, I did find a fan system was useful, to cool the front part of the PA
(used a small Germanium transistor to sence temp & a high gain Tip Silicon to
operate fans in series.)
A 25A linear PSU floating batteries worked well. The rig was floated on another
battery & PSU. This did allow a much smaller petrol generator (650W 2 stoke)
to be used rather than a 2.3kW 4 stroke & QRO Valve Amp. (That was used for
cold night to keep the operating tent warm!)
Reports were all pritty fantastic, good clear comms quality AF from the old
IC735 with its hard AF clipper mic processor, & a strong signal. A local looked
at the remote Hack Green SDR website radio, only to comment "my /P station much
was stronger than his & narrower!"
See my Tech buls on "AF 2 Tone Test Osc Design", "Transistor PA Biasing", "Lead
Acid Batteries", "Variable Speed Thermal Fan", "12V 75A Del SMPSU Mods", "2nd
Car Battery for /M & /P", "Rig DC Power & RF Hazards", "Using 2 HF PAs" &
"NORTHERN 650W 2 Stroke Genny".
Why Don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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