Gary,
Thanks for taking the time to write this!
Do you teach EE?
Jay, K2ZT
On Fri, Nov 1, 2019, 1:01 AM Gary Schafer <garyschafer@largeriver.net>
wrote:
> Mark,
>
> First let's define classes of operation:
> Class A, plate current flows over 100% of the cycle of the input signal.
>
> Class B, plate current flows over exactly only 50% of the cycle of the
> input.
>
> Class C, plate current flows over less than 50% of the cycle of the input.
>
> Class AB1, plate current flows over greater than 50% of the cycle of the
> input with no grid current ever drawn.
>
> Class AB2, plate current flows over greater than 50% of the cycle of the
> input WITH grid current drawn.
>
> True or (almost) class B amplifiers are only found in push pull audio
> amplifiers where one tube amplifies the positive half of the drive signal
> and the other tube amplifies the negative half. Even then often the tubes
> are biased for a small amount of idle plate current.
>
> Most grounded grid amplifiers are called class B amplifiers but are really
> class AB2 amplifiers as they draw grid current and the plate current
> conducts greater than 50% of the cycle.
>
> In AB2 the tubes bias is set to allow a certain amount of plate current to
> flow with no signal input. Less bias voltage the greater the plate current.
> In the case of the 3-500, a pair will idle at between 120 to 230 ma of
> plate
> current depending on the plate voltage and the amount of bias used. At
> lower
> plate voltages in the range of 2000 to 2500 volts these tubes are often run
> with zero bias as the idle current is in the desired range that way.
>
> The reason for operating the tubes with some idle plate current is to keep
> the tube within its desired plate current load line, where it is the most
> linear. At low plate current values the desired straight load line starts
> to
> bend or curve and that curve causes the amplification to go into a non
> linear range (distortion).
>
> A true class B amplifier has its bias set so that the plate current is just
> zero with no signal applied. When drive is applied plate current flows on
> 50% of the cycle.
> As discussed above, this allows operation in the non linear part of the
> plate current curve for part of the drive cycle which will cause some
> distortion.
> By raising the idle plate current up to some point above the non linear
> part
> of the plate current range the tube operates much cleaner.
>
> As a result of having some idle plate current the tubes are actually
> operating in class A at low levels of drive. This keeps distortion very low
> in the part of the load line that is otherwise non linear.
>
> Some tubes draw a substantial amount of power at idle. The 3-500 with 2500
> volts on the plates if I remember right calls for around 230 ma of idle
> current which is around 575 watts for a pair.
>
> Some people operate them at much lower idle current which runs them cooler
> but distortion products are greater.
>
> Back to your question of changing bias and class of operation.
> The heathkit SB220 changed the plate voltage (CW/SSB) to keep the amp legal
> during tune up.
>
> I don't have the voltages/currents in front of me but essentially this is
> how it works: Plate voltage of 1800 volts at 550 ma = 1000 watts input.
> That is a plate load impedance of around 1925 ohms. E/(IK) where K is 1.7
>
> At 2600 volts and 800 Ma that is 2080 watts input. This plate voltage and
> plate current supply the same plate load impedance as the lower plate
> voltage and plate current as the above so the amp is still tuned properly
> at
> the higher power level. 2600/(800 x K)
>
> Nowadays you can tune up legally at full power so this type of CW/SSB
> function was eliminated on modern amps. But some amps still had/have
> "tune/operate" voltages to allow tune up at lower power input as above but
> to save wear on the tubes and power supplies
>
> Some of the later amps that have a CW/SSB switch are there for an entirely
> different reason.
> As discussed above SSB operation requires a very linear amplifier and a
> rather high idle current to get that linearity.
> CW/FSK does not require the amp be linear.
> The switch on those amps change the bias to allow for much lower idle
> current, some go to zero idle current.
>
> Lowering the idle current on an amp allows for a lower plate dissipation as
> there is little to no plate current flowing between CW characters where as
> in SSB with the higher idle current there is a constant amount of plate
> dissipation even when you don't talk, until you un-key the PTT of course.
>
> The amp could be biased high enough to get it into class C operation for CW
> and efficiency could be increased some. However there are problems with
> transient voltages when driving an amp in a mode such as this that can
> generate undesirable artifacts (such as key clicks) that can cause
> interference.
> Even lowering the idle plate current to close to zero as do some CW/SSB
> modes, can cause undesirable artifacts.
>
> A note on amplifier efficiency:
> If an AB2 amplifier has an efficiency of say 60%, it is only 60% at FULL
> OUTPUT POWER when tuned properly for SSB. As input (drive) level is reduced
> efficiency falls off. As an example for AM operation amp output level has
> to
> be reduced to 25% of full output power (so that the modulation peaks have
> room) that the amp was tuned for. At 25% output level the amp will have an
> efficiency of about30% or around one half of full power efficiency.
> This is normal amplifier operation.
>
> This also means that with low drive levels in SSB operation where you are
> operating under the plate idle current range the amp is near the class A
> range and yes efficiency is lower also.
>
> 73
> Gary K4FMX
>
>
> > -----Original Message-----
> > From: Mark Bitterlich [mailto:markbitterlich@embarqmail.com]
> > Sent: Thursday, October 31, 2019 8:30 AM
> > To: 'Gary Schafer'; 'Joe Subich, W4TV'; 'Joe'; amps@contesting.com
> > Subject: RE: [Amps] SB-220 HV Transformer Wanted ; thread stolen for
> > bias question
> >
> > Hi Gary, thanks for your reply.
> >
> > Yes, I was around when regulations were based on input power (1968) and
> > have
> > heard your explanation before but I tend to question it simply because
> > the
> > Henry 3K and 8K series of amps also came stock with the CW/SSB selection
> > and
> > either of those two could easily exceed 1000 watts DC input in the CW
> > position. FSK and AM modes were also advised to use CW and not SSB
> > modes
> > possibly with plate dissipation limits in mind as well as legal issues
> > of
> > input power.
> >
> > But based on the typical pair of 3-500's say in the 3K original designs,
> > in
> > CW mode idling current was 90-120 ma, and in the higher plate voltage
> > SSB
> > position 170-220 ma.
> >
> > My logic was simply:
> >
> > Bias controls the class of amplifier.
> > A change in bias means the class of amplification must have also
> > changed.
> > Switching from CW to SSB causes an indicated bias change from 120 ma to
> > 220
> > ma.
> > The impact of a bias change of 120 to 220 ma would lead me to believe
> > that
> > the class of amplifier shifted slightly in a more linear direction with
> > also
> > the result of slightly lower efficiency.
> >
> > Right or wrong or maybe just trivial ... if wrong could you please
> > explain
> > your logic?
> >
> > Mark
> > Wa3jpy
> >
> > -----Original Message-----
> > From: Gary Schafer [mailto:garyschafer@largeriver.net]
> > Sent: Wednesday, October 30, 2019 10:13 PM
> > To: 'mark bitterlich'; 'Joe Subich, W4TV'; 'Joe'; amps@contesting.com
> > Subject: RE: [Amps] SB-220 HV Transformer Wanted
> >
> > No. The amp runs AB2 meaning it draws grid current.
> >
> > The reason for the CW/SSB switch was to allow tune up at 1 KW DC input
> > on
> > CW.
> > This was the regulation at the time the amp was designed. You were not
> > allowed to ever exceed 1 KW DC input, even during tune up.
> > Switching to SSB after CW tune up allowed higher PEP input by
> > maintaining
> > the same plate load impedance (where the amp was tuned)for the higher
> > power.
> > In other words the voltage to current ratio remained the same when the
> > higher voltage was selected.
> >
> > 73
> > Gary K4FMX
> >
> > > -----Original Message-----
> > > From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of mark
> > > bitterlich
> > > Sent: Wednesday, October 30, 2019 4:23 PM
> > > To: Joe Subich, W4TV; Joe; amps@contesting.com
> > > Subject: Re: [Amps] SB-220 HV Transformer Wanted
> > >
> > > Ok, this brings up a technical question that pertains to the topic
> > > amazingly enough. When you shift from single sideband to CW on this
> > amp
> > > are you or are you not moving the class of operation from AB1 towards
> > > AB2 and thus improving efficiency?MarkWA3JPY
> >
> >
> >
>
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