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Old 24th June 2011, 05:45 PM   #1
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Default Why run the tubes so hot (over limits) ?

Hello again,

I said I wouldn't tinker with my three-month-old SSE until winter came - guess I don't score highly for 'willpower' - it's got the better of me!

I have read in other threads that some people, Tubelab.com in particular, run their power tubes above their spec'ed dissipation (e.g. 40-50W on a KT88 rated at 35W max). I am curious as why people do this...

1) More volume?
2) Better sound?

I am genuinely curious - I run my KT88s at 31W, for no other reason than it was a convenient value that was below limits - I know no better.

I can understand if it's to eek as much volume as possible...Most posters here seem to be from the US, where houses tend to be bigger than where I am in the UK and maybe you have a big room to fill with sound, so every last Watt is important.

I have two cathode bias points - one for EL34 and one for KT88. Both are conservative. I can run the EL34 at 21W or 26W (published max 25W) depending on where the switch is. I can't tell any difference in the sound at all between the two.

If the reason for running them hot is for improved sound quality (i.e. lower 2nd harmonic distortion?), isn't this 'pleasing' distortion what attracts us to singled-ended amps in the first place?
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Old 24th June 2011, 09:13 PM   #2
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Quote:
I can understand if it's to eek as much volume as possible...Most posters here seem to be from the US, where houses tend to be bigger than where I am in the UK and maybe you have a big room to fill with sound, so every last Watt is important.
Two things here. The 560 ohm cathode resistor value was chosen 5 years ago when I designed the amp. That value was the best compromise for using a 6L6GC or a EL34 or a KT88. All three ran below the published specs at that time. The line voltages have crept up, the typical Hammond sourced power transformer now produces 10 to 20% over voltage even at or above rated current, and many of todays current production tubes want more bias than NOS tubes due to a poorer vacuum resulting in some grid current. A 620 or 680 ohm resistor may be a wiser choice today.

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I can run the EL34 at 21W or 26W (published max 25W) depending on where the switch is. I can't tell any difference in the sound at all between the two.
Raising the current on an SE amplifier will not result in much if any power output increase. It will reduce the higher order harmonics and improve the damping factor. I have measured this effect, and a Tubecad simulation shows it.

On some speakers or music selection the improvement may not even be noticed. On my Silver Iris open baffles cranking up the current really cleans up the bass. I run my EH KT88's at 100 ma each when I use these speakers. I run less current on my small speakers. The original tubes are still in the amp and I bought them used for $20 each, so I have got my moneys worth from them.

Set your bias at whatever value that suits your taste. Higher current will reduce tube life. I have a cathode resistor switch and it makes a difference in some cases.
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