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Old 26th February 2004, 10:12 PM   #1
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Default 300b?

could someone explain the advantages of using an ancient designed directly heated triode such as the 300B or a PX25 when there are more available and recent (and cheaper) valves around?

Is it possibly a subjective thing, like the sound of one valve over another?

Speaking of DHT, I recently visited our local museum, where they have a Cinema equipment exhibition, one of the old (BTH I think) amps had 4 PX25 DHT in it, and the display case to the side, had NOS valves of PX4, PX25 and the much loved 300B, in sealed original boxes, dating from the 30's!
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Old 26th February 2004, 10:49 PM   #2
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Hi,

Quote:
could someone explain the advantages of using an ancient designed directly heated triode such as the 300B or a PX25 when there are more available and recent (and cheaper) valves around?
Have you ever had the chance to actually listen to a modern tube amp, I mean a well designed one, using these valves?

IMHO, once you have you'll stop wondering...
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Old 27th February 2004, 05:26 AM   #3
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...More Frank spamming...

Although anyone who knows curves knows that a 300B has nice and linear ones. But so doesn't a 6V6 (even in pentode class A2 no less ).

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Old 27th February 2004, 07:07 AM   #4
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Quote:
I recently visited our local museum, where they have a Cinema equipment exhibition, one of the old (BTH I think) amps had 4 PX25 DHT in it, and the display case to the side, had NOS valves of PX4, PX25 and the much loved 300B, in sealed original boxes, dating from the 30's!
You mind sharing the exact location of your local museum? I wish museums in Joburg were also stocked with pre-war PX25s
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Old 27th February 2004, 07:14 AM   #5
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But so doesn't a 6V6 (even in pentode class A2 no less ).
This is exactly the type of advice i've been looking for. You also have some good tips on how to replace my beloved Osram PX25s with something modern and cheap. And of course providing the same ambience and tone?
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Old 27th February 2004, 12:28 PM   #6
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Hi,

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...More Frank spamming...
Maybe you should stick to the words you actually know: pentode, perhaps?

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Although anyone who knows curves knows that a 300B has nice and linear ones. But so doesn't a 6V6 (even in pentode class A2 no less ).
Curves are only useful to people that can read them properly, moreover they are only just an indication of what you can expect under the conditions those curves were drawn.

Make a circuit with the same tube and plot again, you'll see what's left of those curves once a load is attached.

Cheers,
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Old 27th February 2004, 05:05 PM   #7
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Dear Alastair,
There seems to be alot of controversy, difference of opinion about these tubes.

There are two camps; either you like them or you don't, but seldom any middle ground.

May I suggest, if possible, audition like you would any audio component. Just seems prudent to me.
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Old 27th February 2004, 06:27 PM   #8
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Alastair: Can you you give an exact location for your museum (and nearest police station)? I've got my wrecking bar, striped jumper, woolly hat and bag marked "swag" at the ready...

Sadly (I hate the prices), 300B etc have lovely linear curves. We might like our women to be curvy, but valves need to be flat as a board. That said, there is a huge fashion element, and my personal opinion is that 300B is over-priced, over-sexed, and not over here!

Another point worth bearing in mind is that making valves was/is a high technology business ideally served by mass production. It is much harder to achieve consistent quality on small production runs. In other words, modern valves tend to be more problematic than NOS ones, and this is why you will find people desperately trying to find ways to use obscure NOS valves with a still reasonable price.
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Old 27th February 2004, 10:59 PM   #9
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Default PX25

I used to run Cinemas, and I visited the museum and found a fellow enthusiast of cinema equipment was holding the exhibition!
It was around 10 years ago that I had a cinema in a local town, where the equipment was all original--Carbon Arcs and changeovers!--The arc-lamps were fed by Nevlin Rectifiers, with Mercury bulbs--These were great!! A wonderful purple-blue glow!
Due to reliability reasons, ( and the expense of carbon rods!) I changed over to a solid state rectifier and Xenon lamps.

The Gaumont Kalee Amplifier was the size of your average room door, a large cabinet affair, using 4 KT66 in the output, 6SN7 pre-amp/phase splitter, and as I remember EF37 as gain amps It was only mono, and rated (according to the data) at 30W RMS--It was even Bi-Wired!--Two secondaries from the O/P Tx! Smoothing was done by huge chokes and 8uF caps, and the rec was a large DH double diode--Cant remember the number though....U52?...

The Cinema was 503 Seater, built on a nice generous size and during ownership I used to have great fun playing my music through this, --Amazing bass for such a low O/P--Probably due to the huge folded horn speaker--Im 6'6'' tall, and I couldnt reach the top of the thing!--It took three people to move it for the occasional stage show presented there!

I never had any problems whatsoever with either the amp or the valves It had been fitted sometime in the Forties!, and looked all original, all the caps and resistors were as the maker had made it, and covered with a nice thick layer of carbon arc dust, It worked so well, I didnt dare touch it!

Sound pick-up, was done by a Photo-cell, A cold cathode valve with around 90 volts bias--This is the only thing I had a little trouble with, a strange rushing popping sound A NOS cured it completely--(Found a whole bunch of valves carefully packed out in the battery/ rewind room)

I closed the Cinema some ten years ago, Its still there, and I guess the amp and projectors are too!
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Old 28th February 2004, 02:57 AM   #10
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Off the track but: "Diesel, Spark free since 1900" LOL, I love it Alastair.
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