G8MNY > TECH 08.02.19 11:45l 425 Lines 23547 Bytes #2 (0) @ WW
BID : 58405_GB7CIP
Subj: T500M 12V 500W HF Linear
Sent: 190208/1031Z @:GB7CIP.#32.GBR.EURO #:58405 [Caterham Surrey GBR]
To : TECH@WW
By G8MNY (Updated Sep 18)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
A few years ago I bought an old 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, & cost around ť2000 in 1977)
///////////////////////│ 2-30MHz, 4x 150W push pull amps in parallel.
/////////////////////// │ > 10dB gain, 70W max drive. (typical 15-40W)
/////////////////////// /│ IMD 3rd Order >-32dB @-500W, >-36dB @ 400W.
/////////////////////// / │ PA harmonics to better than -43dB.
/////////////////////// / │ 13.6V @ 75 Amps needed for full 600W output!
/////////////////////// / / 1kW DC input, Infinite SWR rated, <2:1 recom.
││││││││││││││││││││││││ / / 15A charger & car battery will power it (SSB).
┌┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┴┐ / Thermal 70°C heatsink shutdown. CW/SSB use!
│T500M __ ____ │ / Over current 75A trip (high SWR & over drive).
│ o< [__] [____] │/ Manual & Remote operation (On & Band select).
└────────────────────────┘ 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
S C H E M A T I C
Rx & through path
RIG____/ ┌─────┐50ŕ┌────────┐ ┌────────┐50ŕ ┌──────┐ \___ANT
| 70W└─────┘30W└─┬─┬─┬─┬┘ ┌───┐ └┬─┬─┬─┬─┘600W│ | └──────┘ | │ |ptt
| Max Max │ │ │ └──┤PA1├──┘ │ │ │ │ ┌──────┐ │
| │ │ │200ŕ└───┘200ŕ│ │ │ ├─/─┤Filter├─\─┤
| │ │ │ ┌───┐ │ │ │ │ | └──────┘ | │
PTT>───┘ Drive ┌────┐ │ │ └────┤PA2├────┘ │ │ │ ┌──────┐ │
/─┤Bias├─> │ │ 200ŕ└───┘200ŕ │ │ ├─/─┤Filter├─\─┤
│ └────┘ │ │ ┌───┐ │ │ │ | └──────┘ | │
Trip │ │ └──────┤PA3├──────┘ │ │ ┌──────┐ │
┌──┐ DC│ │ 200ŕ└───┘200ŕ │ ├─/─┤Filter├─\─┤
12V___│/_│_/─┴─> │ ┌───┐ │ │ | └──────┘ | │
└──┘ | └────────┤PA4├────────┘ │ ┌──────┐ │
| 200ŕ└───┘200ŕ └─/─┤Filter├─\─┘
Band Switch| | └──────┘ |
or Remote> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
L A Y O U T (Bottom cover off)
│ ── ── ── ── ── ── ── ── ── ── ── ── ── ── ── ── ──│
Ů│~~~~~── Pot PushPull | ┌─────┐ ┌─────┐ ├─┐
Ţ│100 │ Bias =()=┤▒▒▒▒│Output │Relay│ >15MHz Filter│Relay│ ├─┘Rig
Ţ│Amp │Circuit [▒] PA1 Transformers ~~~~~ ~~~~~ │SO239
Ů│Meter│ [▒] =()=┤▒▒▒▒│ | ┌─────┐ ┌─────┐ │
│-----~~5R [▒]Input │Relay│8-15MHz Filter│Relay│ │▄█ 13.6V
│ [▒]Spliter=()=┤▒▒▒▒│ | ~~~~~ ~~~~~ /│+▀ DC 75A
│──────. [▒] [▒] PA2 │┌────┐ ┌─────┐ ┌─────┐Ţ│-▄ Wing
│75A DC ├──[▒] [▒] =()=┤▒▒▒▒││ DC │ │Relay│ 5-8MHz Filter│Relay│█│▀█ Nuts
Ů│TRIP & ├()shunt▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄Relay│▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄█│
/│ON/OFF │__▄▀▀▀▀ =()=┤▒▒▒▒│└────┘ ┌─────┐ ┌─────┐ │12 Way
▀│======' PushPull[▒] PA3 │ | │Relay│ 3-5MHz Filter│Relay│ │Jones
│Drive│ Driver[▒] =()=┤▒▒▒▒│ [▒▒]Output~~~ ~~~~~ │Socket
│Relay│ Transformers [▒▒]Combiner─┐ ┌─────┐ │
│~~~~~ =()=┤▒▒▒▒│ [▒▒]| │Relay│ 2-3MHz Filter│Relay│ │
│| ThermSw [▒] PA4 │ [▒▒] ~~~~~ _____ │SO239
█│|Band [▒] =()=┤▒▒▒▒│ | │ Ant │ ├─┐Ant
│|Switch P.A. PCB :RF lead: FILTER PCB │Relay│ ├─┘
│ ── ── ── ── ── ── ── ── ── ── ── ── ── ── ── ──~~──│
PA protection is from a 75A fast magnetic trip for bad SWR & over drive, it has
an external calibrated shunt, & a thermal 70°C auto resetting cut off switch.
The 100A ammeter uses 10cm of the thick DC lead as its 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 large very well rated input & output matching transformers (no saturating
IMD products!) consisting of 3 & 5 ferrite rings stacked on each of the 2 brass
tubes for the 1 turn low Z side of the 5:1 turns ratio. The smaller 2x 3 ring
transformers are used for the driver & much larger 2x 5 ring ones for the
outputs. A large amount of RF NFB (for good linearity) is provided by 47ŕ 5W &
u1 C between each collector & base of the 8 transistors.
ONE of 4 PA AMPS Input & Output Transformers use...
NFB┌─47─┐ 1:5 Input 3 rings/side, Output 5 rings/side
200ŕ 5:1 u1=== ├──┬─────┐ ______ Collector──┤▒▒ ▒▒ ▒▒ ▒▒ ▒▒├+PCB
─────┐ ┌──────┴──┤< e │ │|( 200ŕ or Base tube~~~~~~~~~~~~~~tube
5W 5 )|│ ┴ │1n2 _)|( 5 Turn /////______________ \\\\\
Turn )|(__ NFB === │ )|( Output 5 |||||│▒▒ ▒▒ ▒▒ ▒▒ ▒▒│ |||||
Input)|( │ u1┌─47─┐ │ │ │|( >150W turns||||| ├+|||||
)|│ │ === ├──┴───)─┘|( |||||│▒▒ ▒▒ ▒▒ ▒▒ ▒▒│ |||||
)|└──)───┴──┤<PT9847 │ ┴ \\\\\~~~~~~~~~~~~~~~/////
┴ ├─bias │e ├───┬────+12V tube______________tube
.6V @.7A ===2u2 │ u1=== ===1mF @18A Collector──┤▒▒ ▒▒ ▒▒ ▒▒ ▒▒├+PCB
──────────┴────────┴──────┴───┴───── 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
┌──────>PA1>───────────────┐ Splitter & combiner both have out of
(|_100R_ _100R_|) balance dump 100R to soak up any amp
_(| 1W │ 200ŕ │ 5W |)_ differences for best stability &
│ (|_100R_│ AMPS │_100R_|) │ linearity. Ferrite ring & tube
│ (| 1W │ │ 5W |) │ construction like the transformers.
50ŕ │ └───────)─>PA2>────)───────┘ │ ┌─────────┬────┬─────────┐
Drive>┤SPLITTER │ │ COMBINER├>50ŕ 100ŕ 100ŕ 100ŕ 100ŕ 1 or
25W │ ┌───────)────>PA3>─)───────┐ │600W │ ___ ___ │ │ ___ ___ │ 5W
│ (|_100R_│ │_100R_|) │ _│|___X___|│_ _│|___X___|│_
│_(| 1W │ 200ŕ │ 5W |)_│ ▒│|│▒ ▒│|│▒ ▒│|│▒ ▒│|│▒
(|_100R_│ AMPS │_100R_|) ▒│|│▒ ▒│|│▒ ▒│|│▒ ▒│|│▒
(| 1W 5W |) ~~|~~~~~~~|~~~~~~|~~~~~~~|~~\Drive/
└───────────────>PA4>──────┘ PA1 PA2 PA3 PA4 Output
F I L T E R S
There are 10 relays that in pairs, switch in 1 of 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 /──┬──())))──┬──())))──┬──\ To 2-3MHz 390+430p 750+680+270p
Combiner│ │ L │ L │ │ Aerial 3-5MHz 390+120p 750+270p
│ === === === │ Relay 5-8MHz 270+47p 430+200p
│ │C1 │C2 C3│ │ 8-15MHz 82+82p 200+150p
──────┴──┴─────────┴─────────┴──┴──── 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 parallel not only gets the odd filter
values, but also gives greater current handling, reduced lead inductance &
swamps any self C resonances.
H E A T S I N K
At 25°C 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 70°C 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.
Rx mode current was quite high, 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.
SWITCH┌Ammeter┐ ON Drive
75A ┌────┴────┐ ┌──┴───┐ === PAs ┌──┴──┐┌──┴──┐ Regulator
│ DC ON │ │Filter│ _│_3mF │Drive││ Ant │
│Contactor│ │Relays│ /// │Relay││Relay│
└────┬────┘ └─┬┬┬┬┬┘ └──┬──┘└──┬──┘
┌─100─┤ │││││ Red ┌─100─┤ │
Green _│_ 1W oooooo Tx _│_ │ │
ON <=\_/ 75R /│\ LED<=\_/ 33R │
LED └───┬┬┼┬┐ ├──────────────┴─────┘ │
oooooo e\│ │
OFF/│\ PNP ├───────────────────────┴──────<PTT
& BAND│ 2N2905/│ Fbead 80mA
/// /// 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 the added PNP emitter follower. The drive relay also puts
on the PA Bias, is last to operate with 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 on identical
relays is all that is needed to ensure the relays all operate in the right
sequence order, so no QRO RF contact splats.
2/ RIPPLE SMOOTHING & 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 gives a few amps at
audio, reducing some of the battery lead AF ripple current.
PA 12V Bus now has 10x 1mF Contatctor Trip
kit 3A +│ +│ +│ +│ +│ +│ +│ +│ +│ +│ │ │
see/4 === === === === === === === === === === .5u=== ===1u
┴ ┴ ┴ ┴ ┴ ┴ ┴ ┴ ┴ ┴ ┴ ┴
To stop RF on the DC leads (don't want any in the shack), I also added a 1uF
non electrolytic 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 the painted
panel. 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 heavy leads to a 75A SMPSU, or "starting grade
cables" to a large battery, or 30A leads to 2 small 24AH batteries & 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!
R4 turns on Q2, when Q2 emitter > 0.6V, Q1 turns on reducing Q2 base drive.
Value of R3 is used to set the exact bias voltage, R4 & supply voltage also
affect the bias 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 (Drive relay)
│ R5 This circuit was slightly
R4 Added 5R unstable when scoping
270 47n 10W Scope for R5 (5R), so I added a small
│ ┌┤├---│ <--1MHz oscillation! capacitor base to collector
│ | │/c on the large NPN to stop it.
│ C6 === Q2 │\e NPN +690mV @ 2.2A
│ 2u2_│_ ├────┬───┬─> via RFC to
│ /// │ │ │ Amp input
NPN c\│ │ │ │ transformers
TIP29 ├───────────┤ │ │
e/│ Q1 _│_ +│ 2R2
PA BIAS │ \_/ === │ Components were not the
Quescent└>5R D2 │ C7│ │ same as original diagram!
Set 1-2A R3 │ 2u2│ │
There is no ALC system on this AMP, & I am used to old Valve amp with a power
front panel ALC control. With PA ALC, the driver power is automatically set to
the wanted level, & with the PA turned off you are back to full bare foot
power. So I designed this ALC circuit for this PA...
R4 5R │
270 PA 10W │
Circuit │ 2n2 │
├──┤├┐ │ Front panel
PNP e\│ │ 10K 50W-600W
2N3703 ├──┴─>POWER ALC Control
on POT /│ POT
4K7 |│| ///
│ │Fbead Mounted on 12 way JONES plug
Input -ve│ │ Fbeed
RF on >─1K──┤<├─┤<├─┤ └─────────────────│<├─┬────┬───>ALC to rig
Drive 1N4148 │-20V long wire 1N4148│ 100 0V to -10V
Atten === with DC leads │ │ -
2n2│ 4K7 === 4u7
_│_ _│_ _│_+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 4K7, 4u7 & 100R give a sensible ALC time constant action.
MY CLEVER ALC DESIGN!
My ALC works very well compared 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 commercial PA, increases the PA's drive power, as the amp gain falls off
at full power. This sudden increase in bias current, occurs just before the
PA actually clips. So a useful & accurate maximum drive threshold point that
caters for any SWR, supply voltage, or Rig power setting.
7/ INPUT SWR
The input frequency compensating attenuator circuit was not as the diagram &
the SWR was not all that good, despite all components testing out OK.
SWR Original Input Match SWR Improved Input Match @ 50W
1.7┤ .˙'˙. 1.7┤
1.5┤''''''''''' '˙.. 1.5┤
1.3┤ ''''˙˙˙˙˙ 1.3┤ ..˙''˙. ..˙'
1.1┤ 1.1┤'''''''''' ''''''''
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
40W L2( === ├─220─┤ Splitter 40W │ L2( === ├─200─┤ Splitter
( │56p ├─220─┤ === ( │56p ├─200─┤
┌───┬┴──┬┴──┐ └─220─┤ 100p│ ┌──┴┬─┴─┐ ├─200─┤
200 200 200 200 220 │ 220 220 220 └─200─┘
_│_ _│_ _│_ _│_ _│_ _│_ _│_ _│_ _│_
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 up to the sudden (like AF Amps) 600W clipping level. This due to
the effective NFB keeping good linearity until it fails. But even brief full
carrier testing on lower bands did provide smoke from the underrated input
The 2 tone test showed very good linearity to 400W PEP, so I think the quoted
IMD figures look right. On air tests with SDR displays show the amp is very
clean & no other sidebands/spatter could be detected @ S9+30dB etc.
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
dB Topbands with dB Topband with
0_│ f1 15-30MHz Filter 0_│ f1 2-3MHz Filter
-10_│ │ -10_│ │
-20_│ │ f3 -20_│ │
-30_│ │ f2 │ -30_│ │
-40_│ │ │ │ f4 f5 -40_│ │ f2
-50_│ │ │ │ │ │ f6 -50_│ │ │
-60_│ │ │ │ │ │ │ f7 -60_│ │ │ f3
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
On actual testing into aerial via a high Q QRO ATU, I found it was possible 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 might be
indicating a Tx PA "parasitic" or aerial "arcing"!
H A Z A R D S
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!
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 flatten one end, solder to cable (on cooker).
__________ heat shrink
Drilled ___ _____________
Hole ____/ │~STARTER CABLE
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!
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 & plenty 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 sense 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!)
In my shack I now use a DEL 13.8V 75A SMPSU with thick short leads for this PA.
Reports were all pritty fantastic, good clear comms quality AF from the old
IC735 with its hard AF clipper mic processor, 2.4kHz SSB filter & 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", "DEL A870P7 SMPSU 12V 70A",
"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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