# 1.2k vs 2.5k TC on a 6550 amp?

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#### killerfishes

in the attached there is a graph i made, what would te the advantage of one vs the other? they look like they are both equally linear, am i not reading it right or would one have better power output, would the 2.5k help the tube run cooler? or is it just a saving of power draw from the PS?
the specs are 250Va and -14v bias

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#### PRR

Paid Member
> they look like they are both equally linear

They better both be linear: you are showing "only" 2 Watts of audio for 37 Watts of dissipation.

The 1K2 line is a little more power and surely a little more THD due to the large swing of current. Neither the power difference or the THD difference would be very audible.

One thing that may matter: speaker damping is less with the 1K2 line. At 1K2, DF~=2, at 2K5, DF~=4. For a speaker tuned for dead-flat with zero-Z drive, and an infinite impedance peak at bass resonance(s), DF=4 gives a 2dB bump, Df=2 gives a 3.5dB bump. Not big, but maybe enough to notice.

> would the 2.5k help the tube run cooler?

No. The power draw is fixed by your plate and grid voltages: 250V and 150mA or 37.5 Watts. Because you are staying way up in the linear range, effectively running "small signal", you don't even have any "rectification" which would tend to reduce cathode current under large-signal conditions. So plate dissipation is 37.5 Watts minus output power. At full design power sine-wave, you have about 1.5 Watts in 2K5, about 2.2 Watts in 1K2. So the 1K2 condition actually runs cooler, 35.3 Watts at full power instead of 37.5 Watts at idle. For 2K5, full-power gives 36 Watts of plate heat. No real difference. Anyway we don't listen to a lot of full-power sine-waves: for typical speech/music audio, played to the edge of clipping (which may be the only way to play a 1.5W-2.2W amp), the average output power is less than 1/10th of the maximum power available. So if you idle at 37.5 Watts plate heat, you listen at 37.3 Watts plate heat. You will need a very sharp thermometer to tell the difference.

If you want some POWER, stick with 1K2 but shift your bias point down to 90mA-100mA. There's 4 Watts around there, but the THD will rise past 5% at full power, over 1% above 1 Watt. Nearly pure 2nd, very listenable. The difference between 1-2W and 3-4W or 1% or 5% THD may not be a big deal, but the plate heat reduces to 25 Watts, which to my mind is more comfortable than running right AT the rating.

#### killerfishes

how did you figure wattage output? also is that rms or max. the specs i was using show this should be around 12.5Wmax and 7% THD unless i goofed "vary possible". I know out put has to do with the area under the lines but how is it derived?

#### SY

killerfishes said:
how did you figure wattage output? also is that rms or max. the specs i was using show this should be around 12.5Wmax and 7% THD unless i goofed "vary possible". I know out put has to do with the area under the lines but how is it derived?

P = (Imax-Imin)(Vmax-Vmin)/8

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