Television line deflection valves or "Sweep Tubes"

Much has been written about television line output driver valves, more commonly known as "Sweep Tubes" and their use in applications other than TV sets. Although, with the passing of time these tubes have become less numerous, there is still some interest in Ham Radio and tube 'audiophile' circles in these devices and a lot of sweep tube gear is still in operation. A few sweep tube types are being manufactured today although some are of variable quality.

A number of scanned articles from the 1970/80 period were forwarded to the webmaster with the idea they be put on the internet. Thanks to Ray W5XE, his XYL and Ron K5BDJ for scanning these and passing them on.

Permission to reproduce these articles here was kindly given by the American Radio Relay League, 73 Magazine and in the later case the author(s) of the original articles as well. You can find the ARRL website here.

If you have a fairly fast internet link (or are patient :) you may want to click here to go to a page where all of the articles will be displayed without any further clicks.

If you are on a slower link you can display or download each page individually in the section below.

Further down the page I have added some commentary relating to sweep tube use in ham radio applications. You can scroll down or click here to go there. I will add some pictures (just as soon as my digital camera is fixed) and data to this section fairly soon.

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"New Beam Power Sweep Tube" (Write up of the then new 6MJ6 tube)

one page   

Used with permission from October 1974 QST; copyright ARRL.

"Some Thoughts About TV Sweep Tubes"

Part 1    Part 2    Part 3    Part 4    Part 5   

Used with permission from August 1975 73 Magazine; copyright 73 Magazine and the article writer.

"Better Results From Those Sweep Tubes"

Part 1    Part 2   

Used with permission from February 1980 QST; copyright ARRL.

Ham radio applications

The main commercial ham radio application of sweep tubes was in the manufacture of compact HF SSB transmitters and transceivers in the 100-300 watt class. A number of commercial linear amplifiers were also manufactured using multiple tubes with outputs of up to 1 kW.

Many hams made their own linear amplifiers using these tubes because there were tough and at the time both cheap and plentiful. The most popular was the circuit for these linear amplifiers was the 'grounded grid' configuration which offered good stability and simple circuitry.

When you look at a specification for 33 watt tubes and then a commercial ham radio specification that stated over 500 watt peak input for a pair of these you wonder how the sums add up! My rough calculation for a Yaesu FTdx401/560/570 is this:

The Yaesu Musen FT401B HF transceiver, made in 1974
Stated max SSB PEP input:   560 watts
Estimated efficiency of PA: 60%
Output power (PEP):         336 watts  [single tone]
Dissipation, both tubes:    224 watts
Dissipation, per tube:      112 watts 
In other words more three times the stated rating the "33 watt" 6KD6!

The key to this apparently drastic over rating of the tubes is that speech has a relatively low duty cycle compared with the "single tone" input used in the above example and so the average dissipation of the tubes is still (hopefully) around the official figures. Valves, unlike semiconductor devices valves have appreciable thermal mass (consider it as inertia) and so will withstand short overloads without excessive heating.

Ratings for AM on the sweep tube sets were always much lower because the PA is producing a continuous carrier output but is still linearly biased.

The high peak current rating of sweep tubes enables them to produce the power on speech peaks without "flat topping" which will ultimately result in lower speech quality and a wider transmit signal bandwidth due to intermodulation.

The big cathodes used in sweep tubes also help to produce power with relatively low values of HT, normally in the 600-800 volt range in transceivers but often between 800 & 1200 volts in linear amplifier circuits. These large and highly emissive sweep tube cathodes are said to have a high perveance but will 'wear out' over time as their emissivity falls.

Most alternative power tubes in this output power class would require a supply of over 1 kV with consequential larger (and more expensive) components in the output tank circuit and power supply. It is doubtful if any other glass bodied tubes would have allowed the compact design and output power that these series of radios had and certainly not at the same cost.

The heyday of Ham sweep tube applications was probably from the 60's to the late 70's although these parts have been around since the advent of electronic television. The most popular radios of this 15 or 20 year period were probably the Yaesu FT200/B, FT101/B/E, FT401/B, Swan 350/400/500, Drake T4A/B/C, TR4 and the Trio/Kenwood TS520 & 820. All but the last 2 radios mentioned used sweep tubes in the output, the Trio sets used the 6146B.

One can correctly argue that a 6146B is a much "better" RF tube than the sweep tube types but as a rule they don't give as much power because of the smaller anode dissipation and cathode sizes. The FT102 used three 6146B tubes but was no more powerful than the FT101/B/E which used only two 6JS6Cs. Post WARC '79 radios with the "new" bands all were all designed with 6146B tubes, the writing was on the wall for sweep tubes by then.

More recently there has been much interest in converting sweep tube radios to use alternative parts - in most cases the 6146B will fit the bill in size terms but, as noted, you will get less power from these. Normally you will have to change the base(s) in the chassis from 9 or 12 pin types to octal and adjust the biasing, neutralization and possibly driver tuning; the modification is not a trivial one but maybe be required to keep these radios on the air.

Sweep tube background

Driving the line deflection coils in a TV set is an operation more akin to switching than conventional amplification. During the active portion of the line scan the line output device must pass a high current from a relatively low HT voltage; during the 'flyback' period the output device is switched off and must withstand the very high voltages generated due to the inductive nature of the coils and the energy stored in them.

The line output function thus required a class of tube with characteristics specially shaped for such an application and these characteristics are quite different from regular amplifying power tubes of the same rated dissipation.

Because of the big cathodes and high gain sweep tubes are ideally suited for the peaky power delivery required in a SSB amplifier in class AB1 or AB2. In this class of operation the tube is biased to continuously pass a fairly low idling current with the intention that it linearly amplify one half the input waveform when driven.

During the half of the RF cycle when the grid is driven more negative the tube will be completely cut off but the amplification can still be made 'linear' because the resonant action of the tuned circuit will restore the un-driven portion of the waveform. In class AB audio amplifiers a pair (or multiple pairs) of valves must be used in 'push-pull' so as to provide a faithful amplification of the input waveform, there is no resonant action to rely on because the output transformer is a broadband device (or so we hope :)

In the bigger sweep tube specifications (around 35-40 watt anode dissipation) you will find maximum anode voltages of perhaps 7 kV and peak cathode currents of more than 1000 mA! Obviously both conditions can't exist simultaneously or the device would be destroyed but even so these are pretty large values for small consumer tubes.

Driving the line deflection coils in a TV set is a tough challenge for semiconductor devices (because of the very high voltages during the flyback period) and before semiconductor technology had evolved to permit all solid state TVs many 'hybrid' sets were built. These hybrids used solid state devices in the low voltage portions of the set but with tubes around the deflection coils. This approach was also followed in Ham radio transmitters during the same period with the output and driver section being thermionic in an otherwise all solid state radio.

The horizontal stage required greater drive power than the vertical section because of higher frequency operation. In addition, on all but the very early sets the EHT voltage for the picture tube was usually derived from a special 'over wind' winding on the line output transformer. In the days before reliable high voltage semiconductor rectifier assemblies the rectifier tube filament(s) would also powered from the line output transformer; this was the reason the sound on a set would normally come on well before the picture, the EHT rectifiers would not even start to warm up before the line output stage itself warmed up and running.

Of the data sheets and specifications I've found on line there is sometimes a small variance between the published figures for the same types, mainly because of the various different manufacturers.

As televisions grew larger, the circuitry to drive the deflection coils got larger too. Early sweep tubes like the PL36, 6BQ6, PL504 and 6DQ5 had rated anode dissipations of 11, 11, 17 and 24 watts respectively. The end of the line parts such as the 6MH6 and the European PL509/519 were rated around 40 watts anode dissipation.

With the intermittent speach waveform it's customary to expect a PEP output of perhaps 3 times the continuous rated anode disipation although this is a rule of thumb and is not supported by the official figures - try at your own risk, and with care.

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