Re: AC vs DC
When built, measure the filament voltage under load. It might be a bit high with only one 450mA filament in use. In that case, add a small series resistor to each leg to get it to the correct voltage.
The only place I use DC filaments are my phono stages, where I use a series reg, shunt and current source.
Cheers
Agreed. As you've already biassed the centre of the filament up 30V or so, hum shouldn't be a problem in a linestage, even with very high efficiency speakers. If you find hum is an issue, I'd check all your wiring, and maybe reverse the filament wires on the heater pins.jim said:
When built, measure the filament voltage under load. It might be a bit high with only one 450mA filament in use. In that case, add a small series resistor to each leg to get it to the correct voltage.
The only place I use DC filaments are my phono stages, where I use a series reg, shunt and current source.
Cheers
You might want to consider this circuit also. http://www.netaxs.com/~vkalia/5687.html
There seemed to be a problem with this tube with respect to AC hum that was solved by using a battery supply. I realize I haven't built any tube gear yet but I think this is worth your consideration .
Joe
There seemed to be a problem with this tube with respect to AC hum that was solved by using a battery supply. I realize I haven't built any tube gear yet but I think this is worth your consideration .
Joe
JUST IN CASE
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.
Remember the reg. needs at least a couple of volts over its' regulation voltage to operate properly.
You will have this automatically no matter what diodes you use in full wave bridge:
12VAC *1.4 =16.8 VDC.
This may also improve the SN/R of the preamp.
Cheers,
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.
Remember the reg. needs at least a couple of volts over its' regulation voltage to operate properly.
You will have this automatically no matter what diodes you use in full wave bridge:
12VAC *1.4 =16.8 VDC.
This may also improve the SN/R of the preamp.
Cheers,
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
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
RIGHT TRACK
Hi,
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.
Cheers,
Hi,
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.
Cheers,
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.
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.
Cheers,
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.
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.
Cheers,
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