# [Amps] SB-220 bias question

Gary Schafer garyschafer at comcast.net
Sun Aug 30 19:11:54 PDT 2009

```Everyone seems to be using different terms in this discussion so no one
agrees. Most are saying the right thing but referring to the wrong thing.

First, as to the SSB amplifier being tuned properly at only ONE power level,
that is true and not true, depending what you are looking at.

As far as efficiency goes it is true. When you tune the amp for maximum
output at a given drive level for SSB operation you are setting the tank
load impedance to match the tube impedance ONLY at the maximum power output
point.
As you reduce drive without changing the load network tuning the power
output drops and the efficiency drops too. The circuit is at maximum
efficiency at only the ONE power level.

So how does this work with voice on SSB where the power level is constantly
changing?
We don't really care that the efficiency is not the same at lower voice
levels as it is at maximum level.
The thing we care about is the linearity in the output with respect to
drive. In other words if our amp has a gain of 10 db at maximum output level
we still want it to have a gain of 10 db at low output levels too. If we
drive it with 100 watts and get 1000 out then when we drive it with 10 watts
we should get 100 watts out. 1 watt of drive should give 10 watts out.
Always 10 db of gain.

But the input power of the amplifier is NOT linear with respect to drive
power or output power. As an example at maximum power output say it has 60%
efficiency. For 1000 watts out it would have to run 1666 watts input. At 250
watts out the efficiency will be 1/2 that or 30%. So to get 250 watts out it
would take 833 watts input. We have dropped the input power in half but the
output power has dropped to 1/4. We have not touched any tuning controls. We
have only reduced drive to 1/4 of what it took to produce 1000 watts output.

Is the amplifier then mis-tuned at all power levels but the maximum power
level? NO it is not.

Is there a mismatch in the tube plate load impedance and the tank network
impedance at lower voice power levels from what it is at maximum power
levels?
YES there is! But that is the way it must operate in order for the amplifier
to remain linear between input drive and output power.

REDUCING POWER OUT FOR CW OR RTTY:

Let's say that we only want to run the above amplifier at 250 watts output
for RTTY. We can simply apply our 100 watts of drive and tune the amp for
maximum output of 1000 watts and then reduce the drive to 25 watts and it
should produce 250 watts output. But our efficiency will now be 30% rather
than 60% that it achieves at 1000 watts out. With 250 watts out the input
power will be 833 watts.

If we retune the output network for maximum output to get our efficiency
back up to 60% at 250 watts output, our input power will drop down to 416
watts.

TANK CIRCUIT STRESS:
Leaving plate voltage the same.

If we retune for maximum output and efficiency at the lower power output
(250 watts), we are now matching a much higher plate load impedance than
before. Less plate current same plate voltage.
We have stepped up the impedance of the input of the tank circuit by
retuning its capacitors to match the new plate load impedance presented by
the tube rather than leaving it at the mismatched load impedance when we
first reduced power from 1000 watts.

Since we are not changing the tank inductance the tank Q will be increased.
Increased tank Q means higher circulating currents in all components and
greater phase shift across the tank coil providing for higher voltages
across it and the band switch.

BUT we are now operating at a lower power level than the original 1000 watts
power output that the amp was designed for so all currents and voltages will
be lower than at the 1000 watt level. How much lower, I haven't done the
math to see if the increased tank Q will bring them back up to as high or
higher than operating at the 1000 watt level. Someone may want to do that
for comparison. It shouldn't be too hard with the on line calculators.

Dropping plate voltage and plate current to maintain the same load impedance
at lower power as at full power will maintain the same tank Q which would be
the most desirable. Then all currents and voltages in the tank will be sure
to be lower.

73
Gary  K4FMX

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
> Behalf Of Joe Subich, W4TV
> Sent: Saturday, August 29, 2009 11:18 PM
> To: 'jerome schatten'
> Cc: amps at contesting.com
> Subject: Re: [Amps] SB-220 bias question
>
>
>
> > If the amplifier was then used on SSB driven by a human
> > voice, the only time the amp would _not_ be mis-tuned is
> > when the human voice peak drove the amp to exactly the
> > same level as the peak of the tone used for tun-up.
> >
> > That seems wacky to me.
>
> No ... with SSB the plate load impedance is dynamic as
> opposed to static (a single value) with CW or RTTY.  For
> SSB or any other amplitude modulated emission, the
> calculation of plate load impedance includes the plate
> conduction angle and average (dynamic) Ep/Ip ratio.
>
> The tank (flywheel) effect of the output network smoothes
> the impedance swings.
>
> With On/Off keying or FSK, Ep/Ip is constant there is no
> dynamic variation.
>
>
> > -----Original Message-----
> > From: jerome schatten [mailto:romers at shaw.ca]
> > Sent: Saturday, August 29, 2009 9:33 PM
> > To: Gudguyham at aol.com; lists at subich.com; sub1 at rogerhalstead.com
> > Cc: amps at contesting.com
> > Subject: Re: [Amps] SB-220 bias question
> >
> >
> > And by extension, consider an amplifier tuned with a steady tone for
> > maximum power out when the i/p side of the pi network matches
> > the tube
> > plate Z and the output of the pi matches the load Z. If the
> > amplifier
> > was then used on SSB driven by a human voice, the only time the amp
> > would _not_ be mis-tuned is when the human voice peak drove
> > the amp to
> > exactly the same level as the peak of the tone used for tun-up.
> >
> > That seems wacky to me.
> >
> > jerome - va7vv
> >
> > On Sat, 29 Aug 2009 17:56:36 -0700, <Gudguyham at aol.com> wrote:
> >
> > >
> >
> > >
> > > Fine, but none of those amps change the plate voltage in the
> > > different
> > > modes, so you mean to tell me they are designed wrong?
> > >
> >
>
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