12AE6 Millett "Starving Student"
Some months ago, I posted in this thread about a 'drawing board' version of the SSMH using a 30 volt HP printer supply and 12AE6 tubes. Today I finally got around to building the circuit and can happily report that it works
. I'm listening to it as I type and it sounds wonderful.
My original idea was to use the C2175A HP printer supply, since a friend in the US told me they were cheap and plentiful (and it uses the same connector as the Cisco 48 V supply). Unfortunately I couldn't manage to get a hold of one where I live and I was forced to use my own HP printer supply which provides 32 volts @ 350 mA and would be good to build the prototype in spite of it's weird proprietary connector.
Back in october I drew a load line for the 12AE6 with a 22 Kohm plate load and a 2 Kohm cathode resistor. According to my calculations these values would result in a quiescent point of 20 v plate voltage and .42 mA plate current.
Considering what Dsavitsk posted about load line drawing (try to choose a plate resistor that halves the supply voltage) today I redid the load line and the calculations resulted in 35 Kohm plate load resistor and 1.6 Kohm cathode resistor. These values would yield a 15 v plate voltage @ .42 mA plate current. Looking in my parts box I only managed to find 33 Kohms and 1.7 Kohms which I figured would do for the purposes of the prototype.
This is the first time I've actually 'designed' a common cathode triode amplifier of any kind and so the quiescent current I chose could probably be considered far from optimal for the 12AE6. I kindly ask the help of more experienced electronic designers here to choose a better quiescent current if that is the case.
The values measured in the actual circuit are 10 volts plate to cathode and -.97 volts grid to ground, which considering a 1784 ohm resistor results in a .54 mA plate current. The value for the plate voltage baffles me a bit... tube variance maybe ?... I guess using a 27 Kohm plate resistor would be a good idea.
So summarizing the only component changes in my prototype were R5, R11 = 1.7 Kohms, 12AE6 tubes and the HP 30 volts supply!! Incredible how adaptable this elegantly simple circuit is.
As with the 12AU7 version, please note that the 12AE6 while using the same 7-pin mini tube socket as the 19J6, has a different pinout. You must change the socket wiring accordingly. You can find the 12AE6 pinout in its datasheet
Some pics now
Going through the parts box
Proto-board wiring, one channel done, one to go
First time turn-on. The heaters took about 30 seconds to start glowing... but to me it felt like half an hour!!
After measuring DC offset at the inputs and outputs, (0 v at the input, 4~5 mV at the outputs); ready to plug the cheap earbuds and listen to Yes' "Drama"
The weird HP printer supply connector.