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In Reply to: RE: Tube heat posted by saki70 on March 03, 2014 at 12:06:38
There are two parts (95% of the time) that determine how hot a tube actually gets.
1. Heater/filament dissipation
2. Plate dissipation
6CA7 - 6.3V @ 1.5A is about 9.5 Watts of heater dissipation per tube.
300B - 5V @ 1.25A is 6.25 Watts of filament dissipation per tube.
Nearly the rest of the heat will come from plate dissipation. We can just assume that each tube is run at 75% of maximums for the sake of comparison.
6CA7 - 25 Watts x 0.75 = 18.75 Watts (I'm ignoring screen dissipation)
300B - 36 Watts x 0.75 = 27 Watts
As you can see, a single 300B and a single EL-34 aren't all that far off, but it would be reasonable to expect the 300B to put off a little more heat.
Something like an 845 will have 32 Watts of filament dissipation and 70-100 Watts of plate dissipation.
Follow Ups:
Can I assume then that the 211 & 845 are about the same in this respect ?
Thank you
saki70
The 211 can't quite cook off as much power on its anode (75W), but it's pretty close.
Well thank you all for the schooling .
I have my question answered and picked up some knowledge too .
Very much appreciated .
saki70
Well, again, I'm drawing on knowledge I've gathered elsewhere, but here's my take on this:
- They draw the same heater current, so given the 845 can output more wattage, economies seem better with the 845.
- Lower plate resistance is apparently better for audio. The 845 has a plate resistance of 1.7k versus the 211 with 3.6-4.4K. If the assumption is correct, the 845 should be the winner here.
- Control grid-wise, they're very differently spec'ed. (80V peak versus 150-190V to the grid)
http://frank.pocnet.net/sheets/049/8/845.pdf
http://www.hifitubes.nl/weblog/wp-content/rca-211.pdf
I know nothing about SET circuit designs, but to me, driving an 845 would be a little more "involved" tube-wise than a 211. A lot of 211-based amps I'm seeing on the net can get away with using small and mid-sized signal tubes to the drive the 211's grid. As seen in the supplied photo, this 845 tube amp is using KT66s!!!
I hope others can chime in, because that's pretty much the extent of what I know.
God, all this talk of big triodes is making me moist for one now hahahahahaha
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May your tubes be lively and long-lasting. Holy be thy heater.
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