12AU7 - How absolute are maximum ratings?
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 5th January 2012, 04:09 AM #1 zedman diyAudio Member   Join Date: Aug 2011 12AU7 - How absolute are maximum ratings? Hi, My RCA datasheet gives the maximum plate voltage for this valve as 330V. But the characteristic curves plot valve behaviour for voltages up to 450V and higher. I also note that the GE datasheet gives the maximum plate voltage as being 300V, but it is defined as maximum 'DC plate voltage'. So do these maximum ratings only refer to quiescent voltages? I started worrying when I realised that a couple of valves in my design can exceed these voltages (peak voltages, not average), but now think maybe I shouldn't worry. Thanks, Eric
cerrem
diyAudio Member

Join Date: Jun 2005
Location: San Diego, CA
Quote:
 Originally Posted by zedman Hi, My RCA datasheet gives the maximum plate voltage for this valve as 330V. But the characteristic curves plot valve behaviour for voltages up to 450V and higher. I also note that the GE datasheet gives the maximum plate voltage as being 300V, but it is defined as maximum 'DC plate voltage'. So do these maximum ratings only refer to quiescent voltages? I started worrying when I realised that a couple of valves in my design can exceed these voltages (peak voltages, not average), but now think maybe I shouldn't worry. Thanks, Eric
The data sheet shows MAX for CLASS A1...
This typically means that the DC quescient is at this value but the AC can safely swing up to twice this value....
SO it's the sum of DC quescient + AC swing that really determines this upper max limit..
So if you operate at higher DC quescient then you must watch to limit the AC swing so the sum still remains the same...
Suppose you have 250V DC quescent and your AC swings another 250V higher on the load-line and reaches 500V..... this is all good according to data sheet max..
If you operated at 350V DC quescient, then you would limit the AC swing to 150V above this by selecting the proper load...
This is how class AB and B get away with higher quescient voltages...since the upper AC swing is limited due to cut-off and the PP tubes will spend most of the cycle swing in the negative side of AC ... A perfect example is the 12AX7 follower in the MC60 circuits....the quescient is operated at higher than spec but the AC cathode swing is safely limited...this tube is running right on the hairy edge ...this is why McIntosh later introduced a tri-filar winding to reduce the DC voltage to this tube...
Caution with heater to cathode voltage especially none bypassed cathodes and followers..again it's the DC + AC swing in this spot as well...

cerrem

Last edited by cerrem; 5th January 2012 at 06:24 AM.

 5th January 2012, 10:31 PM #3 zedman diyAudio Member   Join Date: Aug 2011 Thanks very much cerrem. That explains everything I wanted to know. Much appreciated.
 7th January 2012, 07:54 PM #4 v4lve lover   diyAudio Member   Join Date: Aug 2010 Location: Surrounded by glass you can cook mini's only will the cathode last a bit shorter

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