In the past few weeks I've been working on a Starving Student Hybrid version using 12AU7 tubes. Based on a post by Dsavitsk that a 12AU7 version could be 'done with literally one resistor change', I started by creating a simulation in TINA-TI and had a working virtual 12AU7 SSHM in a short time. Of course, having it work in simulation is one thing; making it work in real life could be quite another.
Having first built a P2P SSHM and then the outstanding PCB version by TomB and Dsavitsk I could use my P2P build for experimentation. It wasn't until this weekend that I could sit down and work on the project. Fortunately I can say it worked, as I'm listening to it as I type this. It's been running for almost three hours now, I've tested it with my iPod, my portable CD player and my laptop's headphone out with no problem at all
no clipping or noise. Unfortunately I don't have any testing equipment other than my DMM so I can't provide you with any distortion or frequency response figures.
Before the details of the circuit, a disclaimer: I'm not an electrical engineer, merely an electronics aficionado so While I understand some things about electronic circuits very well, I really don't understand others. In short: take what you're about to read with the required grain of salt.
The required resistor change is R2/R8 = 390 Kohms. This moves the voltage at the MOSFET gate to compensate for the heater voltage dropping from 19 to 12 volts. As with the 12SR7 version by Logistic, you need to have proper heatsinking since the MOSFET's will be running at ~35.4 volts @ 150 mA... let's see that gives us 5.31 watts of disipation! Compare to the 2.85 watts disipated by each MOSFET in the 19J6 version!
Other structural changes are also required since the 12AU7 is a 9-pin tube, thus requiring a larger socket. Additionaly, the 12AU7 pinout is different to the 19J6's and this requires rewiring of the connections to the tube.
An important detail to consider when building this version is that the 12AU7 has separate cathodes for each triode (pins 3 and 8), whereas the 19J6 uses a single cathode for both triodes (pin 7). This means you need to connect pins 3 and 8 together with a jumper, along with the 'standard' jumper to tie the two plates together, and the one to tie the two grids together.
Here's a pinout mapping for the two tubes:
pin 1=> pin 6
pin 2=> pin 1
pin 3=> pin 4
pin 4=> pin 5
pin 5=> pin 2
pin 6=> pin 7
pin 7=> pin 3, pin 8
Pin 9 for the 12AU7 is the heater center tap, used when connecting the heater filaments in parallel. In this circuit we want the heaters to be in series so pin 9 must remain unconnected. Here's a not-very-good picture of the socket wiring:
Still, there's something I'd like some help with. As described, I built the circuit using IRF610 MOSFET's and R2/R8 = 390 Kohms. According to the TINA-TI simulation this should put the gate of the MOSFET's at ~16 volts. But in the real circuit I'm measuring 14.6 volts !! I'm sure someone can explain why this is happening and if it's ok. My guess is it's not ok since the gate should be sitting at around 4 volts higher than the drain, which is sitting at 12.6 volts (the heater voltage).
UPDATE 3: Forget the above paragraph. Wrong voltage measurement was caused by a sh%&ty, cheap DMM. Measured with my trusty old Kyoritsu K-200 analog FET VOM, the MOSFET gate sits at 16 volts. So all is well with the circuit
I looked at user Logistic's post for his 12SR7 version and he's using R2/R8 = 330K and R4/R10 = 180K. If someone has built that version it'd be nice if he/she posted the voltage at the MOSFET's gate with respect to ground. Still, I think that lowering the value of R4/R10 is not a good idea since it should theoretically increase the bass rolloff in the RC coupling unless, that is, one incremented the value of C2/C4 accordingly.
As to the sound, I really don't consider myself and audiophile and thus I don't think I'd really be able to describe in audiophile jargon how does the thing sound; I can merely say it sounds quite good with both my AKG 701's and my Denon AH-D1001K
. I haven't compared it to the 19J6 version so I can't comment on that either... sorry !
About the only bad thing I can say about this version is that the 12AU7 heater glow is very subdued compared to the glow of the 19J6, so the amp doesn't look quite as nice IMHO. Here's a long exposure shot:
UPDATE: just for kicks I decided to try a couple of old 12AX7 tubes I had lying around. I expected to hear awful noise and distortion but guess what.... it WORKS ! beautiful sound out of the 12AX7 too. This most surely means that one could also plug a 12AT7 into this circuit and it'd work without a hitch.
UPDATE 2: I whipped out a couple of new production Russian made Mullard 12AX7's I have saved for use in the phono stage of my old tube amp (when I get around to buying a turntable) and plugged them into the SSHM. These tubes are low noise and microphonics and have long plates and enclosed heaters so they DO NOT glow at all; but the sound... I might just be fooling myself (very likely) but it seems to me these lend more depth and space to the sound. Try them!