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Old 16th June 2003, 04:11 AM   #1
bcherry is offline bcherry  Hong Kong
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Default PX25 operating points

Anyone have experience?
For SE Class A the data sheet lists 500V/50ma or 400V/62.5ma.
Is one preferable?
Brian
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Old 16th June 2003, 05:40 PM   #2
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Hi Brian

I run my Osrams at 350v/55mA or thereabouts into 5k with fixed bias. The sound is great but far from full power - can't get replacements easily


cheers
peter
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Old 17th June 2003, 12:13 AM   #3
bcherry is offline bcherry  Hong Kong
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Default grid stopper?

Thanks Peter,

Is it necessary to use a grid stopper?

I know what you mean about finding replacements. This one, at $691USD didn't make reserve:
http://cgi.ebay.com/ws/eBayISAPI.dll...tem=3029559482

Coming back to HK anytime soon? We have a new slogan "We love HKSARS", and our currency is SAR$.

Brian
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Old 17th June 2003, 07:44 AM   #4
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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.


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peter
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Old 18th June 2003, 12:21 PM   #5
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Default 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,
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Old 18th June 2003, 12:29 PM   #6
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Default RE:PX25.

Hi Peter,

This is where my old stock went to, roughly 10 years ago:

TELETONE.

I've no clue about the asking price but look for VR40.

Cheers,
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Old 18th June 2003, 12:46 PM   #7
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Default ... keep it in mind...

Welcome back Frank

Remember not to tell us what you really think...

or do satire...

We're not grown up enough for that...

ciao

James
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Old 18th June 2003, 05:58 PM   #8
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Hi,

Quote:
Welcome back Frank
Thank you sir.

Quote:
Remember not to tell us what you really think...
I'll do satire next time.

Cheers,
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Old 19th June 2003, 12:22 AM   #9
bcherry is offline bcherry  Hong Kong
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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
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Old 19th June 2003, 09:48 PM   #10
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Default RE:PX25

Hi,

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

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

Anyway, keep us posted on how things evolve,
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