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Old 19th Sep 2020, 10:42 pm   #21
Radio Wrangler
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Default Re: PX4 substitutions

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Originally Posted by Cobaltblue View Post
Unfortunately its not just PX4's the humble 2P are being ripped out along with 41MP's the thirst for the "Power triode" sound knows no bounds.
I think they only just stop short of Silicon Triodes

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Old 19th Sep 2020, 11:40 pm   #22
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Default Re: PX4 substitutions

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In the original Williamson amplifier book there was a version for PX4s as well as the more familiar KT66 version.

They were to all intents and purposes identical. I changed my PX4 version to the KT66 type after an unfortunate accident involving a leaky garage roof and the mains transformer, the replacement had no 4 volt windings.
What do you mean by "the original Williamson amplifier book"? DTN Williamson wrote several articles for "Wireless World" which were later gathered together as a pamphlet entitled "The Williamson amplifier" but nowhere in it is there a design for his amplifier using PX4s. He does mention the PX25 but dismisses it in favour of the KT66. There was a design for a push-pull PX4 amplifier in a "Wireless World" article in 1943 but that wasn't by Williamson.
There were quite a few variations on the PX4 by other British valve manufacturers but I doubt if they are any more available or cheaper than the original.
There is an earlier book than the one usually seen - I have a copy in my loft somewhere but I can`t find it - I seem to remember it was called something like "High quality sound reproduction" by DTN Williamson and published by GEC.
It contains , from memory, PX4 and KT66 versions of the classic amplifer, an AC/DC unit using KT33c, Tuners for both the AM and FM experimental VHF broadcasts, a preamp with a huge number of different disc EQs and a puff piece about the merits of the GEC matal cone loudspeaker.

I was given the book and amplifier by my Uncle, who used it to build the PX4 amplifier in the early fifties, possibly because of better availibility of PX4s at the GPO.
GEC published a booklet in 1954 on "High quality sound reproduction" but that was a design for a push-pull ultralinear EL84 amplifier. In 1957 they published a book called "An approach to audio frequency amplifier design" which contains a short chapter on "The Williamson KT66 amplifier" but it is exactly that and there is no circuit containing PX4s in it. The book is attributed to Heath, Leakey and Woodville in the editor's preface. GEC never published a book by DTN Williamson probably because he had left their employ in 1946 before the publication of his circuit in "Wireless World". I think your uncle built the "Quality push-pull amplifier" designed by W.T. Cocking and published in "WW" in 1943, which is seen as a predecessor of the Williamson amplifier.
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Old 20th Sep 2020, 12:18 am   #23
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Default Re: PX4 substitutions

"Art and Science in Sound Reproduction" by F H Brittain, published by GEC in 1953, contains designs for three "basically similar" push-pull amplifiers. A 14W version using PX4, a 12W using triode connected KT66, and a 20/30W (from different HT voltages) with pairs of KT66.
There is a picture of the 30W amplifier, captioned as having been designed by Williamson for the MO Valve Co.
I wonder if this is the publication being referred to?
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Old 20th Sep 2020, 6:38 am   #24
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Default Re: PX4 substitutions

That is the book - my memory failed me, as I said I can't find it now, haven't seen it for 30 odd years.

I remember the 30 watt amplifier, that must have been why I thought the whole thing was by Williamson.

As I say, my Uncle built the PX4 version (and the pre-amp) which I later converted to the classic KT66 type.
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Old 20th Sep 2020, 8:22 am   #25
Robert Gribnau
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Default Re: PX4 substitutions

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Originally Posted by Joe_Lorenz View Post
Hello,

my favourite substitute would be the AL 4 valve in triode hookup. It has Uf=4 V, all you need to make is a base adaptor. AL4 is easily obtained via ebay

Regards, Joe
I see two problems with your suggestion:

1) The AL4 takes 1.75 A filament current against the PX4 taking 1 A.
2) The maximum anode dissipation of the AL4 is 9 W, that of the PX4 is 15 W.

Problem 2) could be adressed by using the AL5 (Pa max. = 18 W), with the extra advantage that Ri of the AL5 in triode connection is about half of that of the AL4 in triode connection. But problem 1) would just get bigger because the filament current of the AL5 is 2 A. Beside that, the AL5 is harder to get than the AL4.
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