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Old 11th September 2005, 10:39 PM   #1
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Default Millett Low mu Preamp help

I am looking to build a slightly modified version of Pete Millett's low mu preamp. (originally here http://www.pmillett.com/lowmu_preamp.htm)

The primary change I am making is to use the Bottlehead C4S constant current sourse rather than the pentode based version Pete uses. As far as I can tell, the voltage drop across the C4S is just a few volts. Pete suggests running the plate voltage at about 150V, so I am planning to lower the B+ to about that.

Also, Pete biased the heaters up by 45V due to requirements of the el34 based CCS, but since I am not using this CCS, I am planning to not do this. In fact, I am intending to use a toriod (perhaps with SS rectification) for the B+, and a second small transformer for the heater.

Here is the basic schematic I am looking at. Can anyone offer any comments or suggestions before I get started? This will be one of my first tube builds, so even obvious comments and suggestions are appreciated.

Thanks,

-d
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Old 11th September 2005, 10:48 PM   #2
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put some 470K from pot wiper to ground-tube just don't like that grid loose ground-in case that your pot make some weird thing

elevate Uf to some DC potencial ,because you'll get less hiss and whistle from your tube ; tip-there is invisible diode between heater and cathode in every indirect heated tube and that diode is just waiting to rectify every AC gremlin near those preciouss electrodes
when you put some DC distance between Cathode and heater ,lets say that mentioned diode is removed from circuit.
if you don't believe-try......then choose
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Old 11th September 2005, 10:57 PM   #3
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Quote:
Originally posted by choky
put some 470K from pot wiper to ground-tube just don't like that grid loose ground-in case that your pot make some weird thing
Good Suggestion. I'll definitely do that. I have also seen others using input transformers, and I may look into that as well.

Quote:
elevate Uf to some DC potencial ,because you'll get less hiss and whistle from your tube ; tip-there is invisible diode between heater and cathode in every indirect heated tube and that diode is just waiting to rectify every AC gremlin near those preciouss electrodes
when you put some DC distance between Cathode and heater ,lets say that mentioned diode is removed from circuit.
if you don't believe-try......then choose
I'm not sure I understand what you are saying here. Could you explain more fully
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