Dennis -
You said it better than I did......
1. Arbitrarily changing an AB1 linear amplifier to Class C merely by adjusting the bias, is NOT a
good idea.
2. 'Considerably' more drive 'power' is required for Class C, vs NO 'power'
for Class AB1.
3. Grid dissipation is higher in Class C, that power has to go somewhere!
4. Yes, CW keying must be 'shaped' differently to soften it, usually in the
Driver.
5. My Viking II is much more 'efficient' _in the PA stage_ than the T-4X, and the keying is
pretty 'hard'. The Oscillator is Cathode keyed, and the signal is 'sharpened' quite a bit in the
PA. The PA and Driver are biased very near cutoff, so there is a definite 'switch' when keyed. The
PA is protected by a clamp tube, which actually softens the keying a bit as well. But this IS a 60
year old design!! (YIKES!) :-) The PA tank circuit is considerably higher 'Q' than the T-4X,
150% plus.
An interesting exercise might be to actually calculate the TOTAL efficiency of the T-4X vs the
Viking II, i.e., AC power in vs RF Power out. If you include the plate modulator in the Viking II,
it doesn't even take any calculations!! :-)
Bottom line is that there are considerable design differences between Class AB1 and Class C
amplifiers, and attempting to change one to another merely by adjusting the BIAS is generally not a
good idea. MANY 'internet' mods 'look good' on the surface, but are seldom vetted for those pesky
'unintended consequences'!!!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Dennis Monticelli wrote:
Guess I'll just toss in my 2 cents worth.
Converting a Drake to class C in CW mode only by simply changing the
bias of the PA may not be a good idea depending upon how it is driven
and to a lesser extent how much Q and harmonic filtering is in the
tank circuitry. I have not studied the schematic so I'll just leave
it at that. But I really don't think class C operation in general
should get a bad rap. It's a perfectly legit class of operation and
like any mode there is a right way and a wrong way to implement it.
It has a major advantage in terms of improving efficiency and thus
reduction of heat in the finals. I have measured my Viking II PA with
good instrumentation at 79 to 80% over the core bands on multiple
occasions. We're talking 144W out for 180W in. The final is parallel
6146's, fixed plus drive-induced bias, and a heavy duty tank with
continously optimized Q due to slaving a rollerducter to the plate
tuning cap. As for driving power, you generally need a little more
with class C, but not a lot more (assuming beam power tubes and
assuming we're talking CW, not plate mod AM). To control clicks one
should shape the drive signal shoulders softer than what one would
normally do when driving a linear amp because the turn-on and turn-off
characteristic of class C is more abrupt.
There is nothing fundamentally wrong with class C for CW. In fact
some cell phones use it. I think modern rigs avoid it simply because
the PA is already linear for other reasons and already has a fan to
get rid of heat. The maker would have to design the driver stage a
little differently and the antenna coupling circuitry as well. I
guess that is not worth adverstising additional 10's of watts of CW
output power on the brochure.
Dennis AE6C
On 1/20/12, Garey Barrell<[email protected]> wrote:
Gary -
Obviously the bias would remain in the Class AB1 area for SSB and AM.
Applying a keyed signal to a non-linear amplifier is a bad idea. Every time
the drive is started or
stopped there is the opportunity for additional distortion products to be
generated. Some SSB
distortion is caused by biasing the amplifier too high or too low, resulting
in 'compression' or
limiting the waveform to less than it's peak. More distortion.
Perhaps we have an amplifier expert on here who can definitively respond to
your question. There
are no doubt MANY reasons why all transmitters switched to linear operation
for CW, in spite of the
'power/efficiency savings. If nothing else than it takes large amounts of
drive POWER, more
trouble, including greatly increased Grid dissipation.
To me, turning an input signal on and off to a Class C amplifier is asking
for trouble. A Class C
amplifier is essentially a switch, ON or OFF, and relies on the Q of the
tank circuits to 'smooth
things out'. A lot of potential headaches for a few percent efficiency
increase.
There are LOTS of 'ideas' on the internet that 'sound' good.... :-)
Let us know what you find out!!
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
<www.k4oah.com>
Gary Winblad wrote:
Well yeahh Garey....
I am not suggesting we change the bias for SSB (or AM), just for CW (and
real FSK).
So, you would need a switch or relay added and USE it to change modes.
Yes,
bias it past cut-off, will result in no emission between code elements,
but also over
part of the cycle.
Key clicks could be delt with... The reason they had key clicks was
primarily because
they were cathode keying high voltages with no shaping as I recall. no?
Bias is only
on the finals, we don't key those or do we?
Junk? Isn't our driver pretty clean? If not, wouldn't we have the same
spurs and junk
with a linear final?
We DO have a tuned output tank circuit for the flywheel effect... no?
BUT, if we don't have enough (voltage) drive, that might be a problem, but
we don't
really need full power...
Don't ya think? Changing bias for CW wasn't my idea, but it did sound
good to me.
TIA, I am trying to learn here... There probably IS some reason nobody
did this
originally!
73,
Gary
WB6OGD
----- Original Message -----
From: Garey Barrell<[email protected]>
To: Gary Winblad<[email protected]>
Cc: drakelist<[email protected]>
Sent: Fri, 20 Jan 2012 15:31:44 -0000 (UTC)
Subject: Re: [Drakelist] AC-4 (bias mod)
Gary -
In a word.... NO.
The PA stage is operated as a LINEAR amplifier, Class AB1, meaning the
output signal is a reasonable
facsimile of the input signal. The BIAS voltage is carefully chosen to
'center' the operation of
the amplifier in the linear portion of the tube's operating region.
Increasing the BIAS voltage WILL reduce the IDLE current, (between code
elements,) but will shift
the PA to a non-linear region. This will result in distortion of the
signal, with the potential of
spurious signals, clicks, and other 'junk'. This is one reason clicks were
so prevalent in the older
transmitters. A true Class C amplifier requires considerably more driving
power, and relies on the
'flywheel effect' of it's output network to 'smooth' out the 'pulses' of
drive that exceed the bias
potential.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
and TR-4/C Service Supplement CDs
Gary Winblad wrote:
All this talk of rebuilding AC-4 supplies had me thinking...
I have seen a mod(?) that said to turn up(down?) the bias when running
CW
(and FSK?) to set the finals more to Class C operation to save power
dissipation.
Is this a good idea? Wouldn't it be easy to add a switch or relay and
another potentiometer while you are in there rebuilding?
73,
Gary
WB6OGD
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