[Amps] SB-200 input.

Sat Jul 26 10:28:38 EDT 2014

While the below is an ideal situation it certainly isnt mandatory in many 
cases.

Take the older amps with just a .01 coupling cap for the input that are 
regularly in use with a SS exciter. The various Dentrons, Hunters, Gonsets, 
etc are common examples and a lot of Alphas used only a broadband toroid and 
various resistive terminations on them to run GG 8874's and 3CX800's quite 
well.

I cant speak to most modern rigs but I can use the built in tuners in my 
TS-940 or TS-950SD to drive any of the customer amps I service as part of my 
business to rated power, even on 10M. The narrow pi-network and tuning range 
on 160 of the TS-830 6146B finals limits the drive somewhat but works just 
fine on other bands. Some years ago I used a Drake T4XC to do the same.

Ive also tried various MFJ and other tuners between a SS rig with the 
internal tuner off and get a good match on all bands, and again rated 
output.

IMD levels were down a bit compared to a similar tubed amp with a built in 
tuned input but nowhere near the 5-10dB of running a SS rig directly into a 
.01 to the cathode without the benefit of an internal or external tuner.

All these compromises take minimal or no extra power compared to using the 
TS-830 as the benchmark and many were tested when a certain self appointed 
expert on everything was pontificating on various forums a few to many years 
ago.

Carl
KM1H

----- Original Message ----- 
From: "Gary Schafer" <garyschafer at largeriver.net>
To: "'Manfred Mornhinweg'" <manfred at ludens.cl>; <amps at contesting.com>
Sent: Friday, July 25, 2014 10:57 PM
Subject: Re: [Amps] SB-200 input.

>
> It is always best to have the GG amplifiers tuned input circuit as close 
> to
> the cathode as possible.
>
> There needs to be capacitance directly from the cathode to ground. A pi
> network input is ideal and easiest to deal with. A parallel tuned input
> circuit can also be used as the tune capacitor is directly from the 
> cathode
> to ground but it also needs to be very close to the cathode.
>
> The rule of thumb as I recall, there needs to be about 13 pf per meter of
> capacitance from cathode to ground. The 10 meter band would require about
> 130 pf of capacitance at the cathode side of a pi network.
>
> Without this capacitance directly at the cathode the efficiency also 
> suffers
> as there is not a direct path to ground for the harmonic currents in the
> cathode.
>
> A tube type exciter with pi network output is better than nothing but is 
> not
> ideal as the output capacitor of the pi network is too far away from the
> cathode of the driven amplifier.
> Also using an external antenna tuner such as is found used with most
> transistor type radios is usually a T type network. This doesn't provide 
> the
> required capacitance to ground for the cathode unless the amplifier has 
> its
> own built in pi network input matching system.
>
> 73
> Gary  K4FMX
>
>
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Manfred
>> Mornhinweg
>> Sent: Friday, July 25, 2014 11:54 AM
>> To: amps at contesting.com
>> Subject: Re: [Amps] SB-200 input.
>>
>> Hi all,
>>
>> adding to Carl's comment:
>>
>> > Tuned inputs provide the flywheel effect that makes the tubes easier
>> to
>> > drive and improves IMD 5-10dB.
>>
>> That's indeed to biggest reason to use a tuned input. The impedance of a
>> class
>> AB grounded grid tube's cathode varies dramatically throughout the RF
>> cycle.
>> During a good part of the cycle, the tube is cut off entirely, and the
>> cathode
>> impedance is essentially infinite. During the rest of the cycle, the
>> tube
>> conducts, and the cathode has an impedance that changes depending on the
>> instantaneous current. The curves of triodes are pretty crooked, and
>> this
>> reflects on the cathode. Having a big flywheel there, meaning a
>> relatively high
>> Q network of some sort, helps a lot in keeping the waveform clean, and
>> in
>> providing a huge drive current during that part of the cycle when the
>> cathode
>> needs it, while accumulating energy from the driver not needed by the
>> cathode
>> when it's in cut-off.
>>
>> The point is that with any grounded grid amplifier, you need a
>> reasonably high Q
>> network between the driver's output tube or transistors, and the
>> amplifer's
>> cathode. If the driver radio has a tuned output network, such as the PI
>> tank of
>> a tube rig, that's really quite enough, and a WELL DESIGNED broadband
>> drive
>> matching circuit can work OK. Same thing if you have a solid state radio
>> with an
>> antenna tuner inserted between the radio and the amplifier. In that case
>> the
>> tuner provides the required Q and flywheel effect - or at least we can
>> hope the
>> Q is high enough! But if you have a bare bones solid state radio, whose
>> only
>> flywheel effect comes from a 5 pole lowpass filter having a Q of unity,
>> you will
>> see trouble with a broadband cathode driving circuit. In that case, a
>> tuned
>> drive circuit works wonders.
>>
>> We can also see it this way: When you load a radio with a correctly
>> tuned
>> antenna, or with a plain resistor (dummy load), it sees a constant load
>> all
>> through the RF cycle, and that load is ideally 50 ohm, making an SWR of
>> 1:1. But
>> if you connect it to the cathode of a grounded grid tube, it sees a
>> terribly
>> varying impedance, with the SWR being far above 1:1 most of the time,
>> and at
>> infinity for a part of the RF cycle. No amount of broadband matching can
>> fix
>> that! Any matching transformer can only change an impedance by a fixed
>> ratio, it
>> cannot match to a wildly varying impedance. But if you insert a tuned
>> circuit,
>> which acts as a flywheel, this tuned circuit evens out the varying
>> impedance of
>> the cathode, and presents the average impedance to the driving radio,
>> smoothly
>> and cleanly. This impedance may still be wrong, but at least it's
>> constant
>> throughout the RF cycle, and that allows matching it to a nice 1:1 SWR
>> with
>> either a broadband transformer or a resonant network. Usually, when you
>> go to
>> the trouble to install such a resonant circuit, it's very easy to tap it
>> for 50
>> ohm, or to elaborate it into a PI section having 50 ohm input impedance.
>> So
>> that's what's usually done, rather than combining a broadband
>> transformer with a
>> set of simple tuned circuits.
>>
>> Please note that there is yet another problem: The tuned cathode
>> matching
>> circuit can very well even out the cathode impedance over the RF cycle,
>> but it
>> cannot do anything when the average impedance over the RF cycle varies
>> according
>> to the amplitude of the signal! So, even with a tuned matching network,
>> the
>> input SWR can change according to the exact instantaneous drive level.
>> That is,
>> an amplifier might present a 1:1 SWR to the radio at 50 watts CW drive
>> level,
>> but at 100W or at 20W the SWR will be higher. During SSB transmission,
>> the SWR
>> will be varying all the time, according to the signal's envelope. Some
>> radios
>> cope better with this than others. If an amplifier needs far less drive
>> power
>> than the radio can deliver, it's a good idea to insert an attenuator, so
>> that
>> the radio runs at full power, and the attenuator dampens the SWR changes
>> of the amp.
>>
>> But the best way to avoid all this trouble is to avoid grounded grid
>> amplifiers!
>> If you use a grid-driven amplifier in class AB1, the tubes almost don't
>> load the
>> driver at all. A dummy load inside the amplifier loads the driver
>> smoothly and
>> cleanly. No tuned drive circuits are required, no drive distortion
>> happens, and
>> many tubes can be driven without needing any impedance transformation,
>> and just
>> a low drive power at 50 ohm. The disadvantage, of course, is that the
>> tubes have
>> to be tetrodes, at least, with an additional screen power supply.
>> Triodes aren't
>> linear enough to use them in this way.
>>
>> When using MOSFETs rather than tubes, the situation in this regard is
>> way better
>> than with grounded grid tubes , but not as good as with grid-driven
>> tubes in
>> class AB1. By using proper circuit design, with enough negative feedback
>> and
>> resistive gate swamping, normally the performance is pretty good without
>> needing
>> any tuned input circuit. But in extreme cases, when milking a marginal
>> MOSFET
>> for all the gain it can provide, leaving no room for gate swamping nor
>> negative
>> feedback, a tuned input circuit can be required!
>>
>> Many of you of course knew most or all of this, which is all pretty old
>> technology, but perhaps somebody learned something new to him. And I had
>> fun
>> writing it...
>>
>> Manfred
>>
>>
>> ========================
>> Visit my hobby homepage!
>> http://ludens.cl
>> ========================
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>
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