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

[Carl]

As a First Radiotelephone holder since 1956 and currently keeping a vintage 
Bauer 707 running on the BCB I certainly agree.

That TX uses PP 4-400A's for RF and Audio and tetrodes/pentodes are NEVER 
used as Class B modulators. Instead they run AB1 for minimum distortion.

IMO there is no absolute cast in stone point for Class B or AB2 be it for RF 
or audio; grid driven or GG for RF.

I run GG triodes as dedicated,or bias switched, AM linears but biased closer 
to Class B in order to reduce idle current and increase efficiency a bit. A 
Spectrum Analyzer is used to set the point where distortion just begins to 
show.

A NCL-2000 is also used with low power AM exciters. It is swamped grid 
driven now in deep AB2 and bias can be set for a clean 1200W PEP thanks to a 
very efficient/strong forced air cooled blower system. Gotta love those high 
gain 8122 tetrodes!

Carl


----- Original Message ----- 
From: "Rob Atkinson" <ranchorobbo at gmail.com>
To: <amps at contesting.com>
Sent: Sunday, November 03, 2019 8:38 AM
Subject: Re: [Amps] SB-220 HV Transformer Wanted ; thread stolen for


> The RF PA is class C p.p.; When cathode emission dropped the RF final
> PA tubes were moved to modulator use, not the other way around.
>
> Vintage p.p. RF PA rigs are out there and still in use.  The Globe
> King 400 is one example.  These rigs usually had triodes and had to be
> neutralized with a pair of Cn caps.  The output network was balanced
> and used plug in coils.  There's nothing wrong with any of this except
> that 2nd harmonic attenuation isn't up to modern standards, but most
> operators modify the rigs to feed unbalanced line to an additional
> matching network such as a Johnson Matchbox or low pass L network
> which knocks down the 2nd harmonic.   What did in this design was
> television and TVI.  Manufacturers needed to make and sell rigs with a
> lot of shielding and they wanted operators to be able to operate the
> rigs  from the front panel controls which led to bandswitched pi
> output networks and unbalanced final PAs  with tetrodes and pentodes.
> Also, safety was starting to get some consideration.  But the p.p.
> final and balanced output network has some advantages, such as a
> relatively trivial plate choke, usually a dinky pie wound job on the
> cold center tap of the output coil.   Another nice thing is that
> there's no need for a DC blocking cap because B+ is stopped at the
> output coil gap where RF is coupled to a swing link that performs the
> PA loading.  You can have a single ended PA and still employ a
> balanced output with all of the advantages if you don't mind swapping
> plug in coils.
>
> 73
> Rob
> K5UJ
>
>
>> On 1 Nov 2019, at 20:05, Ron Youvan <ka4inm at gmail.com> wrote:
>
>>  I have seen plenty of push-pull "Class B" RF amplifiers, being a worn
>> out broadcast engineer.  The Raytheon Company use to make low power am
>> transmitters and they were superior to the common RCA and Gates units.
>> The 250. Watt transmitters use two RCA 810 tubes (triodes) in push-pull
>> "Class B" modulation amplifiers and two RCA 810 tubes in push-pull
>> "Class B" AF amplifiers.  For one kil-a-Watt transmitters they used two
>> RCA 833 (triodes) in push-pull "Class B" modulation amplifiers and  two
>> RCA 833 tubes in push-pull "Class B" RF amplifiers.
>>  In the Raytheon transmitter these tubes had a reputation of lasting
>> three years as the RF amplifier and when the emission was down they
>> moved the modulators to the RF stages and installed new tubes in the
>> modulator stage and got another three years.
>>
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