My last post on this subject explained how to tune up for AM when using an
SSB radio in the AM mode.
** Users should first read the procedure in the manual and there are several
explanations on AM oriented forums as to how to modify that for the drive
power needed. For instance my TS-950SD is detailed for 50W carrier when I
only need 20-25W.
See below for my take on the subject of getting on AM and sounding great.
If you are using an AM transmitter like a Johnson Ranger or other such
vintage rigs as your exciter with your linear amp you have to use a little
different approach to tune the amp properly.
** Im glad you agree.
The linear amplifier still needs to be tuned for the maximum PEP.
The only way that I know how to do this by applying full modulation to the
ranger or other such transmitter while tuning the amplifier.
** Thats the last thing Id do as a starting point. Even a Ranger is too much
drive for much besides an Alpha 77SX with the external transformer, Henry
5K, etc.
You could do that by modulating the transmitter to 100% with a single
tone.
That will give maximum PEP. .
** You could also try following the manual, tune up on CW for a fully loaded
output without exceeding grid and plate currents; that is your maximum
permisable PEP. Switch to AM and reduce the power a bit by increasing
loading to 130ma Ip in a Ranger, but give you a decent amount of overhead (I
use that word a lot)and a good choke input filter followed by a good output
cap will have very little voltage sag. I usually remove the ancient 6-10uF
oil cap or electrolytic and replace with 33-50uF from a series pair or three
electrolytics with sufficient extra voltage headroom; equalizing resistors
are required. After overhauling scores of vintage rigs Ive noticed most had
excessive AC ripple and audible hum on the signal as built. Replacing 866A's
with 3B28's or SS diodes is fine as the voltage drop difference is only a
few volts but since 3B28's have a very long life at such a low voltage why
bother?
** Replacing 5R4's with SS is not a good idea as the extra voltage will also
raise the screen voltage in many designs as well as stress already marginal
modulation transformers. Collins to WRL are all affected and those that dont
want to listen to long established suggestions are soon begging for iron or
sending the burn outs to a rewinder. Modern AC line voltages are already
5-8V above when these rigs were in production.
Then tune the amp for maximum output and you
should be good. Trying to tune the amp with just the carrier is not going
to
get you properly tuned. As discussed in the last post the amp needs to be
tuned at the PEP level not at the carrier level.
Instead of a single tone you could do a long ahhh into the mike and tune
on
that. A little more difficult than the tone though.
** I dont know of any vintage rig that can properly drive most amps with the
power available (some low power mobile and controlled carrier rigs are
exceptions; I have several to chose from as well as many 100-120W plate
modulated ones that normally run barefoot or drive a big Class C plate
modulated amp. Even a Johnson Ranger/Ranger II at its typical 35W is
excessive.
The only use for a high quality tone generator is to check performance after
the rig is overhauled and to establish the distortion level before/after
mods are made. A decent distortion analyzer is needed for that and most just
wing it; I use a HP 334A that I cut my teeth on at National Radio in the
late 60's....yes it is solid state.
Be careful with modulating your old transmitter with a tone as you can
easily stress the modulation transformer if you overdrive it and /or its
final is not tuned correctly.
** Very true, even for Collins 32V series which are notorious for poor
quality transformers and chokes.
Keep the time short that you apply the full tone modulation as that amount
of audio continuously is hard on everything in them. They are designed for
voice service.
** Id rate them at IVS duty.
In order to not over modulate your transmitter a scope is almost essential
when operating AM. You do not want to over modulate on negative peaks at
any
time. With a tone for modulation you can easily see 100% as the carrier
starts to get pinched off. There will be a brighter spot on the waveform
at
the zero level on the scope as you hit 100% negative. You don't want to
flat
top on the positive peaks either.
The ranger is pretty good but some transmitters will flat top on positive
peaks before you get to 100% modulation.
** I never cared for the scope method (its not easy on the eyes after ashort
period) but used it for many decades from the 60's until I was able to
retire my repair bench HP-141T SA system and move it to betwen the modern
and vintage gear operating benches. It is now so easy to see the effect of
seemingly minor adjustments or component changes in both the audio and RF
sections.
The Johnson Viking is one of those. They have the modulation transformer
impedance such that the modulator goes into clipping before 100%
modulation
is achieved to provide some high level clipping. This happens when the
transmitter is tuned for full output.
**The Viking I and II are limited by the undersized driver transformer that
becomes saturated when the PP 807's attempt to transition from AB1 to AB2
plus the screen and modulator supply isnt even close to being regulated.
With a better driver transformer and some simple circuit changes at the low
levels it is capable of high 90's % modulation of a 95W carrier with low
distortion. A few more changes will allow a bit over 100% of 100W. In ALL
cases the modulation transformer is completely able to handle this but it is
not recommended as the insulation isnt the best and its easy to blow thru
it. Not surprising for a transformer going back 65 years.
Full output for AM is rated at 100W for both Vikings, not how much they can
be made to run at. Some hams still dont read manuals and tune for maximum
smoke. I run my V-I and V-II CDC at about 90W with several audio mods and
they never drive a linear, just a big Class C amp when Im in the mood or
want to heat the basement.
Watch your scope! Negative clipping (100% negative modulation) is easily
seen while positive peak clipping (flat topping) is much harder to see on
voice.
A good way of establishing the proper amount of drive from your old AM
transmitter to the amp is to first use your SSB transmitter to tune up the
amp, then drop the carrier level to 1/4 power and measure the transmitter
power out to see how much drive power is needed for your amp on AM.
** Thats one way with a TX that cant provide the drive for full amp output
such as a Ranger. For those without an accurate power meter such as
Ameritrons and some others tune up for maximum A0/A1 output at 100W or
whatever the maximum safe drive power is. As Ive said before go a bit beyond
1500W, for those amps that can and give yourself a bit of overhead. Increase
the loading a hair for a 50W power reduction,(even a relative indicator such
as the Ameritron is fine for this, or just watch the grid current drop a
small amount) If your trying to do this with a scope you dont need any more
potential variables thrown into the picture.
It is then a good idea to measure the output power of your AM transmitter
and match the amp drive level with it to what you had with the SSB
transmitter at 1/4 power.
** If the amp PS and cooling can handle this much. A SB-220 doesnt want much
over 250W out carrier even with a bias change. A Dentron Clipperton L can
run all night with short transmissions at 1200W PEP with more bias and a EBM
Papst 4600n fan on the tubes. Ive had good luck with Shuguang tubes bought
direct from China who has recently gone to factory direct. Recent anodes
appear a bit thicker than before and barely show color on voice peaks.
A MLA-2500 is happier at 900-1000W PEP unless you like spending over $1K for
a NOS pair of 8875's.
I can run my Alpha 76PA (3 holer) at 1200W with ease with just a fan on the
transformer, the tube air isnt even close to hot. That is usually driven by
a TS-940 at the second modern gear station. A TS-950SD and LK-500ZC with the
optional external Dahl transformer and cabinet runs an easy 1500W and keeps
the tubes gettered; it is the primary station.
If the AM transmitter can not be reduced by
reasonable loading adjustment then you may need an attenuator between it
and
the amp.
** I dont recommend messing around with screen voltage and grid current as
some are wont to do. Tetrodes (and triodes but those are pretty scarce at
low power) need as much grid current as is safe and fully loaded for proper
AM plus lots of bias to keep them deep into Class C which can best described
as a switching tube.....full on for one half cycle and hard off for the
other half.
The SA shows the effect of grid drive and loading changes and what you dont
want to see is what pisses others off on the band. AMers still have to live
with SSBers and each other.
You don't want to load the AM transmitter too lightly as the modulator
impedance will not properly match the final in it and the screen voltage
will not be proper anymore either. It will then be very easy to have your
modulation transformer arc or something in the final arc.
Once you establish the proper carrier level needed for the amp and your AM
transmitter can do that then you can apply the modulation tone and tune
the
amp for max PEP and you will have a clean signal if you don't over
modulate
the transmitter.
** Nix on a tone, use some good music or chatter from a good speaker (audio
quality not content (-; ) such as some talk show hosts or your favorite
windbag politician. Keep that PEP a bit below the max you tuned for and in
real life the final load line will be varying with speech. Here comes the
overhead hint again. This is where a SA really comes in handy and they are
included on some SDR's and others on receive, a friend with one can be a big
help in getting you set up.
Again, your scope pattern should double in size if you are getting full
100%
modulation on positive peaks.
Coupling a scope to your amp is easy. Often all you need is to have the
probe laying next to the feed line or maybe a little piece of wire
attached
to the end of the probe and you will get enough pick up to be seen. No
need
to have a direct connection. Just have a stable place for the probe to lie
so that the pattern doesn't change every time you move something.
Did I mention always use a scope!
73
Gary K4FMX
** Shake those Silver Certificates out from the mattress and get into the
21st century.
Carl
KM1H
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|