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Old 7th April 2015, 11:46 AM   #1
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Default #27 + 6N8S Mu-Follower: Vhk Max Question

Just scored some #27 tubes and played around with some mu-follower design.
One problem: i can't find Vhk max specs for #27 tube. I'm not sure if the elevated heater voltage value on the attachment is safe for the #27. I really don't want to separate the heater supply for the 6N8S and #27 if possible. Note: Vhk max for 6N8S is 100V.

2x #27 Heater for stereo while 6N8S is double triode so only one heater is needed.

Also, can anyone comment on the use of LM317 CCS? What potential problem i could be facing? Limited voltage swing?
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Last edited by ballpencil; 7th April 2015 at 11:48 AM.
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Old 7th April 2015, 01:21 PM   #2
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I don't see a spec in the 27 data sheet, but 75V Vhk doesn't seem to severe.
I am more curious about your half-wave rectification of the 27 heater. That is not too friendly to either hum induction or the transformer, I would think. Promotes core saturation.
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Old 7th April 2015, 02:05 PM   #3
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I meant that as DC source. The two diodes drop the 6.3Vdc down to 5Vdc.

Yes could not find it on the datasheet as well.. but considering this is something that came out in 1920-1930, i think i should be more careful.
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Old 7th April 2015, 05:58 PM   #4
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Yes, it certainly is a DC supply to the 27 heater, by means of half-wave rectification. Did you intend rather to feed the 6.3 VAC into a full wave bridge (producing two diode drops), resulting in ~5.6 VDC ?
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Old 7th April 2015, 06:06 PM   #5
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No, the 6.3V source itself is already DC. You can see the "+" sign. I should have drawn it using battery symbol. 6.3VDC - 2 Vf diode = 5VDC.
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Old 7th April 2015, 06:30 PM   #6
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My experience with the 27 has been that you really shouldn't go over 50 volts if you are looking for longevity. Although all my 27's (about a dozen) have been purchased in different stages of 'used'.
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Old 7th April 2015, 06:45 PM   #7
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by MrCurwen View Post
My experience with the 27 has been that you really shouldn't go over 50 volts if you are looking for longevity.
What happens if you do go over 50V?
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Old 7th April 2015, 08:31 PM   #8
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Merlinb: Failure rate seems to increase. I cannot of course prove what causes the death of a tube that just failed, but at +20V or so I've not had any failures. Could be coincidence.
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Old 7th April 2015, 11:48 PM   #9
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Quote:
Originally Posted by Merlinb View Post
What happens if you do go over 50V?
I imagine us then having a spark plug instead of #27 tube sitting on the tube socket, arcing sparks all over since we have enough potential for electrons to jump over the gap between h and k. Perhaps since it's vacuum, it takes much less potential difference for electrons to jump the gap than in atmospheric conditions? AFAIK, spark plugs need something in kilovolts to jump 0.8mm of gap. I don't see how vacuum condition lowers it to sub-100v but i could be wrong. How much gap is there between h and k usually? What about heater insulation? Shouldn't that prevent arcing over?

I'd rather cause damage to the 6N8S so looks like i should lower the elevation voltage to something around 50V.
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Old 8th April 2015, 03:24 AM   #10
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Found it! Vhk max is 90V.
http://frank.pocnet.net/sheets/127/2/27.pdf
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