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Old 16th Sep 2003, 8:13 pm   #1
high_vacuum_house
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Default Why are valves like PX4,s PX25,s so expensive

hello there,
I was wandering why certain valves are so expensive to buy apart from their rarity. Valves such as PX4's and PX25's seem to me to have ridiculous prices. when I have not seen a radio with one in before.
Any ideas why these types are so expensive?
Christopher Capener
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Old 16th Sep 2003, 10:07 pm   #2
Norm_Leal
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Default Re: Why are valves like PX4,s PX25,s so expensive

Hi Christopher

We also have valves that seem over priced. I can understand high prices for very old, rare types. Some, like 45 are not that scarce or special but expensive. These valves were very popular, used in most radios around 1930. Guys building audio ampliers are after them and willing to pay $80.00 (AES tubesandmore.com price).

Just about any tennis ball shaped valve by Western Electric goes for more than $100. Being scarce and quality are the main reason for these to be high priced.

Norm
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Old 17th Sep 2003, 9:11 am   #3
jjl
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Default Re: Why are valves like PX4,s PX25,s so expensive

The PX4 and PX25 go for daft prices as they are power output triodes that are in demand for " high-end " valve hi-fi amplifiers.
The folklore is that triodes provide a more pleasing sound than pentodes or tetrodes.
Power output triodes are not too common as most radio & amplifier manufacturers switched to using pentodes or tetrodes in the early '30s, these providing more gain and easier drive requrements than triodes. As a result, most power output triodes, like the PX4 & PX25 are directly heated as they are such old designs. The PX4 was used in quite a few pre-war radios made by EMI under the HMV name. The Mullard equivalent of the PX4, the AC044 was used by Bush in late '30s sets. Cossor also used output triodes in several of its '30s sets.
It is perfectly feasible to wire an output pentode or tetrode as a triode by connecting its screen grid to its anode through a low value resistor. This approach was used in the 1947 " Williamson " amplifier design to connect KT66 beam tetrodes as triodes and give similar characteristics to the by then outdated PX25.

John
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Old 17th Sep 2003, 10:44 am   #4
quantum
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Default Re: Why are valves like PX4,s PX25,s so expensive

Laws of supply and demand I suppose.

General production of these valves ceased or was done in lower quantities once efficient output tetrodes/pentodes became available, and as Norm says there is a healthy demand from audio enthusiasts for these valves, so prices go up.

I also have never come across a set which uses these valves but have experimented using indirectly heated triodes such as the 6B4, but generally speaking if I was looking for an output triode I too would consider a triode connected KT66 or EL34.
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Old 17th Sep 2003, 12:55 pm   #5
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Default Re: Why are valves like PX4,s PX25,s so expensive

Maddening isn't it. I've got a nice old 30's Philips that is missing its AC044 - the set was £20 ....

I think there is a link between hearing defects and the size of your wallet ; if you have an excess of money you think an antique directly heated triode sounds the best - or maybe they just look pretty when lit up ! Yep, looks have something to do with it as the old-style bottles are more sought after - or then again, perhaps an old PX4 that wore out in the 30's sounds better than a 50's manufactured PX4 that wore out in the 50's

I'm not for a moment saying they'd sound bad. Far from it. But as jjl pointed out you can always connect pentodes up like triodes. So, buy up all them PL509's pronto - higher gain, more power, lower impeadance and CHEAP !

TTFN,
Jon
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