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Old 3rd July 2008, 04:30 PM   #1
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Default interpreting max. values listed in datasheet

Lately i came across a (commercial) 2a3 SE amplifier, running at 450Vdc.


Datsheets state 300vdc maximum.
(older ones 250V)

Though, even old schematics run them at 350V.

???
are (old) manufacturers stating the values on datasheets too carefully?

Is there a 'gray' area above stated dissipation with more tubes, that (after testing thoroughly) keep operating without harm??
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Old 3rd July 2008, 04:43 PM   #2
DougL is offline DougL  United States
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Just a couple of quick thoughts.

The tube data is anode to cathode, not B+ to Common.
For a 2A3, this could account for over 50V between the cathode bias and transformer IR losses.

IIRC, The Sovtek 2A3 is specified for higher voltage and power than a "real" 2A3, and the amp may be designed to take advantage of that.

Doug
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Old 3rd July 2008, 05:27 PM   #3
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Default Tube ratings

Most maximum ratings for power tubes are "soft" in the sense that if you exceed them, the tube will still run, but its lifetime will be shortened, as compared to transistors, where they can blow up in a millisecond. This is what makes tubes so forgiving for us clumsy experimenters!

The key factor in tube lifetime is to preserve the vacuum, and heat is the thing that most directly affects the vacuum. Running the tube too hot (too much plate dissipation or poor cooling) drives trapped gas out of the glass and metal in the tube. It also hastens a thing called electrolysis that happens when the glass starts to conduct current. Electrolysis causes the glass to degrade around the points where high-voltage leads pass through the glass, eventually causing a vacuum leak. Electrolysis is also hastened by high voltage. Most power tubes can tolerate an excess of plate voltage, but the total plate dissipation (plate-to-cathode voltage times plate current) should not exceed the rated spec.

There are some new 2A3s that are basically 300Bs with a 2.5V filament. The larger plate structure can dissipate more heat than a standard 2A3. If the amp is designed around this kind of 2A3, don't use conventional or NOS tubes, since they will fail prematurely.

Some amplifiers are designed with a disregard for maximum ratings. These amps tend to be the ones with a reputation for "eating" tubes. Beware of these amps!

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Old 3rd July 2008, 08:51 PM   #4
fred76 is offline fred76  Philippines
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Quote:
Originally posted by DougL
Just a couple of quick thoughts.

The tube data is anode to cathode, not B+ to Common.
For a 2A3, this could account for over 50V between the cathode bias and transformer IR losses.
Hi,

Is the "Plate Voltage max." stated in tube datasheets always mean Plate to Cathode Voltage?

Thanks for sharing your invaluable knowledge guys....
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Old 3rd July 2008, 09:54 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by fred76


Hi,

Is the "Plate Voltage max." stated in tube datasheets always mean Plate to Cathode Voltage?

Thanks for sharing your invaluable knowledge guys....
Yes
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Old 4th July 2008, 09:41 AM   #6
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Hello again,

I'm aware of Va-k.
Ofcourse i know that there will be many adverse effects when running too high a voltage (without going into sparking etc.)

What strikes me, is that although max va-k for a normal 2a3 is stated as 300V (older datasheets say: 250V for single ended, 300V for push-pull), even old schematics run them higher......

I'm not sure about the commercial amp's tubes, they were double plate types (2 anodes, internally parrallel connected), maybe the voltage tolerance goes up then.
Although they were double plates, they had written 2a3 on them, nothing else....(chinese tubes).
Surely they will have had a higher voltage tolerance, i cannot grasp that any designer would run their tubes 150V over maximum....

Ah, well, i will stop worrying about it, and use the stated voltages.......
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Old 4th July 2008, 04:53 PM   #7
fred76 is offline fred76  Philippines
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Thanks Kevin for confirming that...

Some datasheets of the same tube type differ a bit as kathodyne noted, but I guess it's manufacturer dependent etc.


kathodyne,

FWIW, here is a thread that talks about modern JJ 2A3-40 and how they are similar to JJ 300B's in construction w/ some minor differences... That may explain why some commercial amp manufacturers run them at higher voltages. OTOH, I'm not sure if it's the same case with Sino 2A3 double plates:


http://www.audioasylum.com/forums/tu...14/142976.html
http://www.audioasylum.com/forums/tu...14/142993.html
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Old 4th July 2008, 06:49 PM   #8
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gheghe, internally jumpered.....

I'd love to find out if these old double-plated 2a3's were also rated for higher voltage...that would be it then.
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