On Friday, 19 July 2013 16:26:55 UTC+10, ZeeTso wrote:
hi there! first to state, i got no real expirience with vacuum tubes, but i understand basic electric/electronic principles, and have some soldering skills 
that said, i have a question regarding voltage and current for 12AU7 (ECC82) heater in cheap and simple headphone amp. tube is in input stage, driving an op amp. now, design says that amp needs 12V DC power supply. there is an LM317 current regulator in series with heaters, and it has 5ohm resistor giving max 250mA load (calculated value). with 12V DC supply voltage there is only ~10,5V between pins 4 and 5 (heaters in series). what is confusing me is the fact that every 12AU7 datasheet says it needs 12,6V for heaters (in series). is this value absolutely necessary, or it might vary?
The simplest way to get +12.6Vdc at 0.15Adc for 12AU7 heaters is to use a 7812 12V regulator with a single diode 1N4007 in the leg to 0V, and then you get very close to 12.6Vdc without anything to adjust. You must have +15.6V at least for input voltage.
how is that affecting tube performance and sound?
The tube heater gets the cathode to the correct temperature for optimal electron emission and tube life. In fact, it is not critical and the tube will work OK with heater volts between 11V and 14V as Phil says. ( ie, 12.6V +/- 10 )
i connected the amp to regulated power supply and noticed that it sounds better with higher than 12V supply voltage. to get 12,6V for heaters i need ~15V DC power supply, so i guess that would be best option. what says you? p..s. original schematic is in pdf file he
http://www.fredsamplifiers.com.au/do..._Valve_KIT.pdf p.p.s. sorry for gramatics, english is not my native language...
Operating the 12AU7 with 12V B+ anode supply is not good, and your cheap headphone amp kit has garbage quality. The poorly presented schematic shows no good details for the power supply and has +12Vdc B+ rail, and a 47k anode R and a direct coupled chip of some sort which actually powers the phones. This sort of kit what I like to put on the concrete floor and smash with a sledge hammer because its such low quality, like a cockroach. :-) !!
The 12AU7 probably makes the sound a whole lot worse because of how it is being used.
I think a cheap headphone amp could be made much better with a chip placed after a volume control with supply rails of +/- 12V and no tube. 12AU7 works best with B+ of about +300V, and anode voltage of about +130V, with Ia = 5mA, and you get voltage gain of about 13 with THD 1% at 5Vrms output from anode. But you probably need much less signal for your phones, and trying to use a 12AU7 like it is a small signal transistor seems quite stupid IMHO.
I have made headphone amps using nothing but tubes and then ppl have told me they sound better than anything else. But it is far more difficult to do this, and it will cost more, because you need 1/2 12AU7, 2 x 6BQ5, 2 x OPTs, and a decent PSU and a lot of applied knowledge and experience to keep the noise level below 0.05mV at the heaphone output terminals.
Patrick Turner.