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Old 18th Sep 2019, 12:40 pm   #1
mpegjohn
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Default Williamsons transformer workings and ac anode resistance

Hi all,

I would like some help please on trying to work out where Mr Williamson got his ac resistance of the KT66 as 2500 Ohms.

I do not see this figure in the datasheet. So I am guessing he obtains it from the graphs.

Can anyone take me through that please?

When the Mullard datasheet for an EL34 says Ra-a 6600 Ohms is that the ac resistance? So an output transformer with a 6600 ohm primary will halve this then. Or is this value what the load resistance should be, and you need to look at the graphs to get the ac anode resistance.

Regards,
John.
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Old 18th Sep 2019, 1:00 pm   #2
kalee20
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Default Re: Williamsons transformer workings and ac anode resistance

It won't be the AC r5esistance of the KT66; his 2500 ohms will be the load that he decided to put on his KT66's.

The Williamson amplifier uses KT66's as triodes. Putting an optimum load on a triode is dependent on circuit conditions, but without going into details it is usual to load triodes with a load of a few times ra. (Pentodes and tetrodes, far, far less than ra).
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Old 18th Sep 2019, 1:07 pm   #3
mpegjohn
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Default Re: Williamsons transformer workings and ac anode resistance

Thank you,
The output transformer has a primary of 10,000 ohms. He says the ac anode resistance is 2500 Ohms.

John.
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Old 18th Sep 2019, 5:43 pm   #4
G8HQP Dave
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Default Re: Williamsons transformer workings and ac anode resistance

Quote:
Originally Posted by mpegjohn
When the Mullard datasheet for an EL34 says Ra-a 6600 Ohms is that the ac resistance? So an output transformer with a 6600 ohm primary will halve this then. Or is this value what the load resistance should be, and you need to look at the graphs to get the ac anode resistance.
6.6k is a recommended load impedance, for a particular bias point. The actual anode impedance of an EL34 will be much higher, but this does not lead to a loss of signal - you cannot peer inside a Thevenin equivalent circuit and make deductions about losses in there. Most datasheets will give the anode impedance for a particular bias point too, but this will not necessarily be the same bias point as for recommended load. The graphs will tell you.
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Old 19th Sep 2019, 10:03 pm   #5
trobbins
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Default Re: Williamsons transformer workings and ac anode resistance

The 2.5kohm could arise from each KT66 having a nominal datasheet 1.45kohm plate resistance in triode mode, and the PP equivalent circuit where the drive signal passes through both plates, as per class A. Williamson may well have measured KT66 plate resistance - which changes with operating point - and determined that 1.25k was nominal, given that he developed the circuit in the mid 1940's, and the earliest datasheet I have is a Marconi dated 1956.

Reference books/articles often show this simplified equivalent circuit, and Partridge WWFB datasheet even shows it when used for their frequency response setup (although it does the test with 3k3ohm).
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Old 21st Sep 2019, 8:58 pm   #6
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Default Re: Williamsons transformer workings and ac anode resistance

The ultra-linear circuit puts the valve part way to acting as a triode. The data sheet gives figures for the anode impedance of tetrodes as tetrodes. So the impedance the anode presents to the transformer will be different and lower.

The impedance the transformer presents to the valves has nothing to do with the valve's anode impedance. The designer made plots of the anode voltage versus current and constructed a 'load line' on the plot, planned to swing the anode over a reasonable range considering the valve and the HT voltage, while taking a swing of current designed to take the chosen amount of power with that voltage swing.

Have a read around some classic valve-era design books like Terman and the radiotron design handbook. Look up load lines.

Get comfy with how plain triodes and plain tetrodes anodes are biassed-up and loaded. Look at ultra-linear once you're clear on the plain versions. You need to understand them before you can appreciate what the ultra-linear circuit does.

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Old 22nd Sep 2019, 12:48 am   #7
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Default Re: Williamsons transformer workings and ac anode resistance

What does UL mode have to do with this thread?

Williamson used a triode connected KT66. The article was made public about 4-5 years before UL began to be discussed.
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Old 22nd Sep 2019, 1:36 am   #8
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Default Re: Williamsons transformer workings and ac anode resistance

Quote:
Originally Posted by mpegjohn View Post
When the Mullard datasheet for an EL34 says Ra-a 6600 Ohms is that the ac resistance? So an output transformer with a 6600 ohm primary will halve this then. Or is this value what the load resistance should be, and you need to look at the graphs to get the ac anode resistance.
John, Ra-a is the speaker load as seen by the output stage valve anodes operating in push-pull arrangement. The speaker impedance is transformed by the OT to a plate-to-plate impedance that the valves pull their operating current through. The output transformer is usually defined by its PP primary impedance, and its speaker impedance - as that is just a ratio determined by the turns ratio of the transformer, the designer can use it in any way they want, and the variation in speaker impedance means you have to be mindful not to pedantically assume that only that constant impedance has to be designed for.

A PP loadline refers to just one valve in a push-pull configuration, so the mode of operation dictates the slope of the line being overlaid on a valves plate characteristics. As indicated, many books like RDH4 go through the process and understanding. Modern links sometimes go through the design process with actual voltage and current levels like:
https://robrobinette.com/Drawing_Tube_Load_Lines.htm

Note that loadline design typically doesn't concern itself with the inherent anode resistance parameter Ra, as the design process starts with the assumption of a target Raa. Some datasheets just show a single Raa value in amongst tables of operating valves - that Raa value is just a recommended value (a benchmark value that is known to be a good compromise). Some datasheets show characteristic plots of the influence in choosing a particular value of Raa for a PP output stage - often the distortion and max output power curves of the stage are plotted, and they often show that the typical Raa value used for that valve aligns with the output stage achieving its maximum capable output power, and having near its minimum distortion level.
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Old 22nd Sep 2019, 4:37 am   #9
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Default Re: Williamsons transformer workings and ac anode resistance

It is a very interesting topic what the ideal load might be for triodes in push pull. The better explanation I could find for this when I looked at the topic some years ago, was in a book from the staff at MIT. If you look at this article from about page 7 on, there is an explanation of it based on their reasoning.

One interesting thing is that the ideal plate to plate load resistance in push pull for the particular valve depends on its bias conditions, whether it be push pull class A (a rare configuration these days but not in the 1920's) push pull class AB or push pull class B because in those cases one valve has a significant to little effect on the operating load seen by the other valve over the range of those bias conditions. In any case it makes for an interesting read:

http://worldphaco.com/uploads/UX-171...amplifier..pdf


Also, an Ultra-linear connection pushes a pentode part way towards a triode in its operating characteristics as David pointed out.

Designing amplifiers with triodes there was always more of a concern to use the anode resistance specification of the valve as a parameter to help calculate the ideal load resistance (as pointed out by the staff at MIT) however, with pentodes they tend to act more as an ideal transconductance device, so other factors can dominate in the calculation of the ideal load resistance.

As always though, the same for triodes too, it is largely determined by the required maximum power output from the available supply, while still keeping the valves inside their maximum ratings, combined with how much distortion one is prepared to tolerate.
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Old 22nd Sep 2019, 9:27 am   #10
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Default Re: Williamsons transformer workings and ac anode resistance

In 1944, Williamson was lucky enough to get a job in the applications laboratory at Marconi-Osram, and it's apparent that he had access to the specialist (at that time) equipment to make the frequency and distortion performance results that ended up in the 1947 WW articles. By that time he apparently had had a keen interest in amplifier design and had become adept at prototyping over the previous 5 years. So one could assume that he had the time to tweak the class A bias point for the KT66's, and arrange the output transformer and its performance and impedance characteristics to get a good outcome.

Holden's comment in that UX-171 link about purposefully rolling off the treble to make a more pleasing outcome was also a common response to audiophiles setting up their first Williamson - its flat high frequency response along with the best and latest wide range speaker made all the radio borne noise and glitches almost intolerable to listen to.
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Old 22nd Sep 2019, 10:39 am   #11
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Default Re: Williamsons transformer workings and ac anode resistance

Most people in the UK associate Williamson and ultra-linear. Though his first published amplifier was a triode-strapped design, and someone else invented ultra-linear. Still, the association is strong if erroneous. So I wrote to cover tetrode, triode and ultra-linear operation.

Valve manufacturers want to show their products in their best light, so datasheet entries normally cover the conditions giving the most impressive power and gain figures, so the data sheets for tetrodes showed them in fixed Vg2 mode for the largest power and gain figures. Back in the day, the beam/kinkless tetrode was seen as a massive improvement over those old triodes.... they were so 1920s!

Only later did some devices get data entries for other modes of use.

David
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