A question about valve RF PA output stage

[Malcolm T]

How many transmitters today use an push pull output stage in the final RF amplifier ?.

[lesmw0sec]

If you buy something commercial, I would say none - apart from linear amps.

[Malcolm T]

Well, when was the approximate years these types of output stages fell out of fashion then ?, and why?, to anyone up on valve technology,

[Radio Wrangler]

They haven't fallen out of fashion, they are still used where appropriate. Valves have fallen out of fashion for recent and new designs, but the push pull architecture lives on.

Every new amateur HF transceive you can buy will be push-pull, probably LDMOS.

Some VHF amplifiers were push-pull, but above that it becomes difficult matchingthe phase shifts of two paths racing against each other and power reduction happens.

David

[Cruisin Marine]

The last I can remember were the Pye F30 base stations which used a QQVO6-40 from recollection. They must have stopped making those in the mid-70's?

[lesmw0sec]

Originally Posted by Malcolm T

Well, when was the approximate years these types of output stages fell out of fashion then ?, and why?, to anyone up on valve technology,

One clear advantage of the semiconductor approach is the resulting size. I have an Alinco DX77 and a Yaesu FT101ZD. The Yaesu which has bottles in the output is around three times the size and more than three times the weight. Both are rated for 100W output. So an advantage in terms of desk space or fitting something in your car.

The disadvantage with modern design is that things get so small that a board populated with tiny surface-mount components makes it a challenge to put it mildly, if a repair is required. The Hybrid sets with valves are easily worked on.

[Joe_Lorenz]

To put it in my words: Most modern units do not reward any attempt to repair it, either you go hunting for strange chinese semiconductors or you dig hour by hour deeper into thick layers of white glue holding lots of components that need to be pulled out before you reach the components in doubt. Just to find out something else must be faulty.
In the end you bin the whole thing and look for a used but proven valve RF PA. No wonder some models fetch a good price!

Joe

[roadster541]

To get back to the OP's original question, I would say very few. I have a few transmitters and all of those with multiple PA valves have them in parallel. There were some push pull PA arrangements but I think that parallel operation was more commonly used; I am talking about amateur radio transmitters and as I have no experience of broadcast transmitters, cannot comment on their PA arrangements
73
Rod
M0LXY

[Malcolm T]

I probably phrased my question wrongly thinking people have esp .
I am looking from an historical point of view of HF, the question is more along the lines of a true balanced output into a balanced link coupler and two wire balanced transmission line and not coaxial cable or a toroid balun system. I believe the development of the transistor and the coaxial ( concentric) cable changed all that went before to a more convenient 50 ohm system.

[Cruisin Marine]

I am looking from an historical point of view of HF, the question is more along the lines of a true balanced output into a balanced link coupler and two wire balanced transmission line and not coaxial cable or a toroid balun system. I believe the development of the transistor and the coaxial ( concentric) cable changed all that went before to a more convenient 50 ohm system.

I suggest you look at ARRL handbooks, they will show the progression of all that you ask.
They are out there to download.

[dodgy-dxer]

To be fair the FT101 has a built in ac (and often dc) power supply while the DX77 runs from 12v so needs an additional external ac power supply to operate in the shack

Power supply will add weight and take up space

Fred

Originally Posted by lesmw0sec

One clear advantage of the semiconductor approach is the resulting size. I have an Alinco DX77 and a Yaesu FT101ZD. The Yaesu which has bottles in the output is around three times the size and more than three times the weight. Both are rated for 100W output. So an advantage in terms of desk space or fitting something in your car.

The disadvantage with modern design is that things get so small that a board populated with tiny surface-mount components makes it a challenge to put it mildly, if a repair is required. The Hybrid sets with valves are easily worked on.

[Radio Wrangler]

Link couplers and associated 2-wire lines make the lowpass filtering needed to meet today's expectations for harmonic suppression a lot more difficult, and they lose the freedom of routing that coax gives.

I use 2-wire line to a balanced antenna driven from an ATU with a link-coupled output. But the transmitter to ATU connection is coax.

One of the popular designs for high power amplifiers on 70cm uses a pair of 4CX250Bs in push-pull. A U-shaped metal plate is resonated and an adjacent U-shaped plate is a coupling link. It could be used with parallel lines for a balanced output, but all the ones I know of use the coupler unbalanced, into coax. It's popular with moonbouncers in the 400W class. Those with high power permits go for things like 8877 and all is single-ended.

Rotary antennae sort of force coax feeders.

The hybrid transceivers from the Japanese 'Big Three', KW, Collins, etc used 6146 or TV line output valves but ran them paralleled and single-ended with unbalanced outputs. Push-pull transistors and ferrite cores were the next generation. The Labgear LG300 used a single-ended 813 and an output intended for 75 Ohm TV coax, though 50 Ohms was within the range of the Pi-tank.

So, to find push-pull valves on HF, you need to look back befire the Pi output network took over the world. ARRL and RSGB handbooks of the 50s and 60s may be the last showing them.

David

[Malcolm T]

Radio Wrangler , thanks for that last sentence in your comment about the Pi network, that i think could be what i,m looking for, ie before coax and toroids entered the market en mass after perhaps WW2. before the advent of the transistor , coax and toroids. I will give those publications a look.

[murphyv310]

The Yaesu FTdx-10 uses a Push Pull RF output stage according to the Blurb and as a user have to say it's very good and extremely clean RF. MOSFET admittedly not valve.

[G6Tanuki]

Pueh-Pull valve PAs were the way to go for VHF amplifiers; the likes of the QQV06/40A even had integral neutralising capacitors so you could [generally] build a stable amplifier for 2M/70cm without the risk of it turning into an oscillator while your back was turned.

VHF, of course, implied coax output, all the moreso when you were talking about the likes of a Pye Vanguard [you'd have problems with installing open-wire or flat-twin feeder to a mobile antenna]

There were some push-pull valve HF transmitters around in times-past (1930s-1950s]; typically those using big triodes [811-A or similar] where, as for VHF, the push-pull configuration provided a built-in antiphase signal for neutralisation.

By the 60s, pretty much everything had gone single-ended and used pi-tank/coax output. Getting consistent neutralisation across all the HF bands was sometimes tricky!

'Single ended' also encompasses the grounded-grid or 'cathode driven' approach which removed the need for neutralisation at HF. I've got a 14MHz 811-A grounded-grid linear here; zero-bias, 1250V HT and it whacks the anode-meter over rather nicely.

When it comes to solid-state, from the late-60s onwards everything HF went broadband, using toroidal transformers; remember that the collector-impedance of a big transistor RF amp can be only an Ohm or two, so the output-matching network becomes a step-up to 50 Ohms. 'Traditional' L-C tuned-circuits working at such low impedances can have intriguingly- and inconveniently- sized component values. There are some excellent Motorola Application Notes detailing the design of such amplifiers and their matching-transformers.

Finally, [pun intended] the broadband-transformer-into-50-Ohms approach means that - by using band-switched low-pass-filters between the TX and the antenna - you do away with all that meddlesome tuning-and-loading which was the bane of pi-tanks etc.