[Amps] SB-220 HV Transformer Wanted ; thread stolen for bias question

Fri Nov 1 12:49:17 EDT 2019

Yes! This is a clear and concise summary, Gary. The best I’ve read. Thanks!
73,
Jim w8zr

Sent from my iPad

> On Oct 31, 2019, at 8:57 PM, Leigh Turner <invertech at frontierisp.net.au> wrote:
>
>
> A most excellent concise summary here Gary....
>
> Leigh
> VK5KLT
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Gary Schafer
> Sent: Friday, 1 November 2019 3:31 PM
> To: 'Mark Bitterlich'; amps at contesting.com
> Subject: Re: [Amps] SB-220 HV Transformer Wanted ; thread stolen for bias
> question
>
> 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 at embarqmail.com]
>> Sent: Thursday, October 31, 2019 8:30 AM
>> To: 'Gary Schafer'; 'Joe Subich, W4TV'; 'Joe'; amps at 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 at largeriver.net]
>> Sent: Wednesday, October 30, 2019 10:13 PM
>> To: 'mark bitterlich'; 'Joe Subich, W4TV'; 'Joe'; amps at 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 at contesting.com] On Behalf Of mark
>>> bitterlich
>>> Sent: Wednesday, October 30, 2019 4:23 PM
>>> To: Joe Subich, W4TV; Joe; amps at 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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