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    Building, troubleshooting and testing of these amplifiers should only be
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PX25 operating points

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#4
Hi Brian

I use a grid stopper but haven't tried without it. You're probably going to try it with one of your lovely grid chokes; please tell us how it comes. I am no big fan of hot weather but my wife is so set to get back to HK and finish shopping 🙂 that she'll probably drag me back some time soon.
SARS, btw stands for South Africa Revenue Service here and finally we have a good excuse for an 'Avoid SARS' car sticker.


cheers
peter
 
#5
FAMOUS PX25.

Hi,

Typical operating conditions for the MOV PX25 are very close to what Peter is currently using for his amp.

At 500V you'll quite likely approach maximum ratings foreshortening life expectancy of the valve in the process of doing so.

Plate Voltage 350V
Plate Dissipation 20Watt
Plate Current 57mA
Grid Voltage -26Volt
Plate Impedance 1140 Ohm
Amplification Factor 8,8
Transconductance 7,8 mA/V
Load Impedance 3000 - 5500Ohm
Grid Resistor max. 50k Ohm
Operating Lifetime 40.000 hours

From what I gathered you're looking at using a chines PX25, so in that case data may differ depending on how close they intend to approach the original MOV design.

Happy tunes,😉
 
#9
Thanks Frank, Peter,

Thanks for that link to Teletone.

Yes, I've got a pr of China-made pX25.
When I was in Tianjin in April they were just putting the finishing touches to a meshplate PX25. Mr Liu had the original data sheet at his console where he develops tubes. He makes a prototype, inputs the operating voltages and compares the curves with the ones from his Tektronics 570. He keeps making prototypes until he gets the curves to match, then finalizes the structural schema. I got one pr of the protoypes which I'm running at 500V/42ma which is a touch above the recommended 20W on the plate. But I guess at that voltage nobody will loan me their Osrams to compare!
I been playing them in my office hidden under cages. Many peer into the cages to discover what's making that non-300b sound.

Is there any advantage to running low voltage/higher current compared to high voltage/low current assuming we keep to ~20W plate dissipation?

Brian
 
#10
RE😛X25

Hi,

Is there any advantage to running low voltage/higher current compared to high voltage/low current assuming we keep to ~20W plate dissipation?
Click to expand...

I'd not make sweeping generalisations on this, let's just say that it's a matter of finding the "sweet spot" for that particular valve under the conditions you intend to use it with and your particular OPT.

A few experienced contributors on the forum often report best results with higher current through the valves but YMMV.

Many peer into the cages to discover what's making that non-300b sound.
Click to expand...

I've never been a big 300B fan, I like my 2A3s better...but let me tell you upfront: if I could lay my hands on some NOS AD1s or PX25s for a reasonable amount of money I wouldn't hesitate for a second and buy them to build myself an amp with them.

You never heard real music untill you heard some of those, their magic is truly scary.

Re: Meshplates, I heard some old samples and they were excellent sounding but I don't have a clue as to why meshplates could be superior to the regular ones.

Technically it doesn't make any sense at all to me, not that I'm such a great expert on this but it's analogous to trying to catch small fish with big holes in your nets....:scratch:

Anyway, keep us posted on how things evolve,😉
 
#11
Thanks Frank


Re: Meshplates, I heard some old samples and they were excellent sounding but I don't have a clue as to why meshplates could be superior to the regular ones.
Click to expand...

Is it akin to why a matte surface is a better collector of thermal energy than a shiny one - more surface area? Too bad we can't actually see what those electrons are doing on their journey from cathode to anode. I have a solid plate prototype of the PX25 before it went into production with meshplate, and the two sound nothing alike.

The Osram PX25 data sheet shows the original valve as having an ST bottle, but all the pictures and samples I've seen show balloon shape with different dimensions. Was there ever an ST PX25 Osram?

Brian
 
#13
Re😛X25 ST

Actually the data sheet shows GT, not ST; my mistake.

But I posted the question in HKtubeaudio forum and got this interesting reply.

The data sheet show a "GT" type (with straight side) rather than "ST". To my knowledge, there is only globe and GT type PX25 from MOV/GEC and no ST type. And some amp (like some from Decca) can only use GT type PX25 as space allowed is very small. GT type PX25 is much cheaper compare to globe PX25.

On the contrary, PX4 has globe, ST, and GT types.

Tim
Click to expand...

So, will be on the lookout for some "much cheaper" PX25 GT.

Brian
 
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