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Old 27th December 2002, 04:37 PM   #11
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Default Re: JUST IN CASE

Quote:
Originally posted by fdegrove
Hi,

If you still experience hum problems and want to use DC on the heaters:

Use a full wave bridge (preferably a Schottky or hexfred) put a sizeable cap behind it (4700 microF or bigger) than use a LM317 or 7812 as a regulator and bypass the IC with about 10 microF.
Ground at the heater side.
Frank,
I more or less agree, except for two things. Use Schottkeys, not FREDs, and dont ground the negative of the DC heater. It's biassed up to +30V or so by the voltage divider connected to pin 8.

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Old 27th December 2002, 05:39 PM   #12
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Default IN THAT CASE...

Hi,

Quote:
It's biassed up to +30V or so by the voltage divider connected to pin 8
I see,one more reason to use DC on the heaters IMO.

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Old 27th December 2002, 06:12 PM   #13
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Frank,

Quote:
I see,one more reason to use DC on the heaters IMO.
Why is that ??

Jim
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Old 27th December 2002, 06:24 PM   #14
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Default DC

Hi,

Well,by using DC on the heaters you won't be injecting AC on the B+ since the cathodes are lifted by 30V with respect to ground in order not to surpass heater to cathode insulation.


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Old 27th December 2002, 06:24 PM   #15
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Default 5687

hi all,
frank: will be using tungsol/Raytheon 5687 D/O-getter.
as for the supply, i did build the DC supply using schottky diode with 3300uf b4 and 470uf cerafine after the 5R resistor..guess i need to put a regulator LM317/LM1085 and a trimpot to adjust to get 12.6volts or 6.3volts..am i in the right track?
thanks guys.

rgds,
tone
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Old 27th December 2002, 06:33 PM   #16
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Default RIGHT TRACK

Hi,

Quote:
LM317/LM1085 and a trimpot to adjust to get 12.6volts or 6.3volts
I don't see any reason to use the 6.3VDC,but yes,the LM 317 is a decent reg that is adjustable via its' ground pin (the middle one I think).
Also note that I recommend setting it a bit low,say 12VDC for longer tube life.
It won't hurt the sound at all since you provide ample current.

Check with the datasheet of the LM317/1085 for the maximum value of cap you can put behind it.
use a good quality cap such as the ELNA after that or if only a smaller value is allowed use a filmcap.

Actually you can ommit the 5R when you use a regulator.

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Old 27th December 2002, 07:10 PM   #17
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Default SLOPPY ANSWER

Hi,

Quote:
since the cathodes are lifted by 30V with respect to ground in order not to surpass heater to cathode insulation.
That should have been:since the heaters are lifted by 30V with respect to ground in order not to surpass heater to cathode insulation.

Sorry,
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Old 27th December 2002, 07:31 PM   #18
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Default 5687

hi frank,
i have read that by going AC, the tube will have a longer life span right?..i will go AC, since tubes was design for AC supply..so now i would have to connect the 12.6v to pin 4 and ground to pin5..right?
thanks frank for the advice..

cheers.

tone.
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Old 27th December 2002, 07:40 PM   #19
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Default NOPE.

Hi,

I guess it won't hurt to try AC.
Just don't connect either pin 4 nor 5 to ground,let that part float,it is already referenced to ground by the polarizing circuit.

Quote:
have read that by going AC, the tube will have a longer life span right?..
Yes,in theory.
However on a small signal tube used in a preamp I wouldn't make that big an issue out of it.

Anyway,if you do get hum you can always revert to DC operation so give it a shot.

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Old 27th December 2002, 11:33 PM   #20
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Default Re: SLOPPY ANSWER

Quote:
Originally posted by fdegrove
Hi, That should have been:since the heaters are lifted by 30V with respect to ground in order not to surpass heater to cathode insulation.
Actually, if the heater is positive wrt the cathode, then that would discourage the heater from emitting electrons too, something you don't want if the heater is running off AC.
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