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Old 24th September 2010, 01:11 AM   #1
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Default Good idle current for NOS 7868's

SE 4kpri opt
445v plate
270v on G2


Started testing today @ 25ma, after 8 hours of this, I bumped it to 32ma, and changed over to a speaker, and Music, minus a little hum from the poorly filtered G2 supply, The sound was quite a bit more open then my 807/7027a se I have @ home, and I really like the high frequency presence!


So I was wondering what a good safe idle current for these tubes would be? There copper plate tubes, so Id like to keep them around for a while, As finding copper plate 7868's has been hard for me.
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Old 24th September 2010, 02:17 AM   #2
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32mA is 75% of max dissipation, so that seems safe. Have you drawn some load lines yet? It may be more optimal to drop the voltage a bit and crank up the current.

What does a copper plate 7868 look like? I have never seen one.

Email me if you are interested in acquiring more 7868s. I have a pair of Sylvanias that I might be willing to trade...
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Old 24th September 2010, 02:20 AM   #3
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Plate dissapation for a 7868 is 19 watts per the RCA tube manual. So 32mA at 445 plate volts is 14 watts. I'd keep them at 35mA for just over 15 watts. 40 mA would be about 17 watts. Perhaps a little better sound quality at the expense of tube life. (more heat) A small low noise box fan would help if that's not objectionable.
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Old 24th September 2010, 02:45 AM   #4
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There is no sweet spot for SE at it's curves.
It's a typical P-P tube, with envelope temperature rated at 250 degrees Celsius. However, it will sound at such a current, but would produce large 2'nd order distortions. Deep feedback is needed to linearize it; I would use a combination od Schade type (local) feedback and a global one.
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Old 24th September 2010, 02:55 AM   #5
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Quote:
Originally Posted by 807tubenut View Post
SE 4kpri opt
445v plate
270v on G2

Started testing today @ 25ma, after 8 hours of this, I bumped it to 32ma, and changed over to a speaker, and Music, minus a little hum from the poorly filtered G2 supply,
So I was wondering what a good safe idle current for these tubes would be?
What is power rating on your OPT? If your tranny can take all the power the tube has, then since you are running it in SE, Class A, you can take the current up to the max plate dissipation. If you use cathode current numbers, you'll be safe because that is higher than the plate will see. But since you are running that at voltages higher than the data sheets give numbers on for Class A you may want to see if you can get more info about operating points above 300v.

Last edited by 20to20; 24th September 2010 at 03:02 AM.
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Old 24th September 2010, 06:29 AM   #6
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Ill snap pics in the AM,


OPT's are Foster SE, rated for a conservative (came from Fosters engineers) 50w continious, with no active cooling, and spaced in close proxcimity to the power tubes, so out in the open, with no hot parts near them, I make 3.5w daily on them from the time I open the office, till I leave, and they ever get a single degree past ambient (75*f) I was told by foster, the same OPT's were being used back in the day for 60+ watts by the guys they were selling them to (CONN was one of them)



My design right now employes no feedback of any sort.



NOS tubes, rare ones at that, I don't want to run them hard, so that's why I called it quits at 32ma until I secure a few more sets
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Old 24th September 2010, 11:15 PM   #7
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Is this what a copper plate 7868 looks like?

I was looking at characteristics and doing a rough approximation and it seems that for an SE amp, your voltage may be a bit high and current may be a bit low. It seems that clipping would occur far sooner on positive plate swing than on negative plate swing. Am I wrong?
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Old 25th September 2010, 02:23 AM   #8
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I think you have the right idea, and yes, that's a copper plate 7868.
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Old 25th September 2010, 04:12 AM   #9
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I think ill back my voltages down to 300/250 respectively, and Ill use a gas regulated V reg arrangement for the grid, I'm just running a bridge rectifier right now, on the grid and B+ supply.


I <3 vacuum tubes.
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