Bottlehead Forum
Bottlehead Kits => BeePre => Topic started by: johnsonad on November 29, 2014, 11:23:33 AM
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BH team. A couple of questions that I should by now, be able to figure out myself ;)
The upcoming ELROG 300B has thoriated tungsten filaments. My HAM buddies tell me you need to be careful as if you apply voltage to quickly before the filaments are up to spec, it will strip them. Does the BeePre as stock build have enough of a soft start to allow this to happen? If not, could a soft start be integrated? If you can't tell, I'm seriously considering a pair as a Christmas gift to myself :)
Thanks,
Aaron
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Ham voltage and BeePre voltage are very, very different, in the BeePre, I wouldn't worry about this.
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Thanks Paul!
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From what I was told back in the day, thoriated filaments these days are not the thoriated filaments of old. I think it was Jack Strayer who explained that the techniques of old put the thorium deep into the filament alloy, and on the current stuff the thorium is has a much thinner penetration. On the old tubes you wanted to cook them well after sitting for any length of time, to get a new batch of thorium to migrate up to the surface. Not sure how that translates to newer thorium tubes.
As for calling this tube with a thoriated filament a 300B, it reminds me of the joke about the guy out walking his bulldog. A guy comes walking the other way with this really ugly, stubby legged dog with a huge head. As it passes by the first guy the ugly dog bites the head off the bulldog. The first guy says, s**t! what kind of dog is that?!
The second guy answers -
"Before I cut off its tail and painted it yellow it was an alligator."
RCA made a thoriated tube to compete with the WE300B in terms of linearity and sound quality in theater installations. But they didn't call it a 300B. They called it an 845.
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Thanks Dan. Funny, he makes an 845 and an 811. It may turn out to be a FOTM but if I ride the initial wave and don't like them, they will be an easy sell. The nice part is, they use a 1.2 Amp filament which none of the EML's do and have the potential to work well in the BeePre.
More to follow when I can get my hands on a pair.
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I'm very interested, but still waiting to learn the parameters. They will have a lower mu and a higher plate resistance, but nobody's saying how much. You will probably have to adjust the plate current in a BeePre, since it uses fixed (filament) bias.
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Alright PJ, this is what I received from Thomas: "preliminary data : mU about 3.3, rp about 1k"
Thoughts?
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Wow - much closer than I expected! gm=3300 vs. 5300 for the WE spec; that's a real accomplishment. Same transconductance as an 845, at 1/5 the filament power.
A quick look says in a Paramount, increase the cathode resistor to 1200 ohms from 1000 ohms - a really small difference. Gets 6 watts instead of 8 watts. Should work in BeePre; the plate voltage would go up 10v or so, no biggie.
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Thank you Paul! I report back some numbers once they arrive.
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These arrive Aaron? If so, any thoughts you can share?
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I decided to wait and read some reviews. I would only use them in the BeePre and It's sounding great with <$100 tubes.