I found lots of feedback directed at other people who want to run 12AU7s at extremely low plate voltages, (e.g. 12V) but what about something more realistic and easily obtainable, which for me is probably somewhere between 150-180 volts of raw B+? I have two nice little identical toroidal power supply transformers which, with their 30V secondaries coupled, put out exactly what goes in, 120V, so I'm guessing post-bridge-rectifier-and-double-pi-filter, probably another 40 volts on top of that. All I'm attempting to do is get decent "warm" coloration at unity or maybe slightly more than unity gain from a homebrew preamp using a 12AU7 which I have lots of, or a 12AX7, of which I only have a few to play with. Would a little over half of the 300V rated maximum give enough S/N headroom for that application?
Thanks in advance for everyone's sage input on this. 🙂
-e
Thanks in advance for everyone's sage input on this. 🙂
-e
A cathode follower will give you less than unity direct coupled to a voltage gain stage should give you a gain of two to three times easily with 160v HT. Your only going to get a max gain of 20 under ideal condition which is more than enough, you could use an active load, IE a CCS to max gain.
Andy.
Andy.
Yes , 150V would be ok , but you need to chose right the schematic , operating point and components if you want something serious hi-fi not just a toy like the people who would run them at 12V 
This ultra-simple circuit is what I've been experimenting with, and have had decent luck. Its designer has mentioned an optional cathode bypass cap, but I could find no exact value for it.Yes , 150V would be ok , but you need to chose right the schematic , operating point and components if you want something serious hi-fi not just a toy like the people who would run them at 12V
Running a 12AU7 at B+ of 12VDC sounds like a fuzz box (Electric Guitars).
That is for the Instruments & Amps threads on this forum.
+100 to 300V B+, is for the Tubes / Valves threads (Hi Fi, and Stereo).
Warm sound is one thing.
Grunge is another thing.
Just my opinion.
Is this post in the correct portion of the diyAudio forum?
That is for the Instruments & Amps threads on this forum.
+100 to 300V B+, is for the Tubes / Valves threads (Hi Fi, and Stereo).
Warm sound is one thing.
Grunge is another thing.
Just my opinion.
Is this post in the correct portion of the diyAudio forum?
Seems ok in principle , not very sure about the operating point , you should measure the plate and cathode voltages .
Cathode bypass cap will increase the gain from let's say 10 to abut 13 . The value could be 100uF
My post is in regard to a hifi system, not a musical instrument amp. Isn't this the right department for that?
You used the right area to post your thread.
But +12V on a 12AU7 plate is not Hi Fi.
Two 12AU7s will not perform consistently the same at such a low plate voltage; even the two triodes in a single envelope.
The problem is not only the plates, but also the early grid current that does not occur at more normal plate voltages.
And, there will be very low dynamic range too.
I agree, we sometimes want 'warm' sound in Hi Fi, but let's use at least 90V on the plate (that is actually a data sheet suggested operating condition, along with the 250V other suggested operating condition).
However, the 90V plate quiescent state has 0V on the grid, and it already has some very low residual grid current, and that grid current is not the same from tube to tube.
When a 12AU7 is operated as a cathode follower, if the plate voltages and currents are reasonable, the stage is very linear, not 'warm'.
When used as a common cathode amplifier, and with reasonable plate voltages and currents, the 12AU7 does have some 2nd Harmonic distortion and 2nd Intermodulation distortion.
Use a real plate load resistor to get a little of the 'warm' that you desire.
A CCS in the plate circuit will make the 12AU7 more linear, and have less of the 'warm' that you want.
Just my opinions.
Have Fun!
But +12V on a 12AU7 plate is not Hi Fi.
Two 12AU7s will not perform consistently the same at such a low plate voltage; even the two triodes in a single envelope.
The problem is not only the plates, but also the early grid current that does not occur at more normal plate voltages.
And, there will be very low dynamic range too.
I agree, we sometimes want 'warm' sound in Hi Fi, but let's use at least 90V on the plate (that is actually a data sheet suggested operating condition, along with the 250V other suggested operating condition).
However, the 90V plate quiescent state has 0V on the grid, and it already has some very low residual grid current, and that grid current is not the same from tube to tube.
When a 12AU7 is operated as a cathode follower, if the plate voltages and currents are reasonable, the stage is very linear, not 'warm'.
When used as a common cathode amplifier, and with reasonable plate voltages and currents, the 12AU7 does have some 2nd Harmonic distortion and 2nd Intermodulation distortion.
Use a real plate load resistor to get a little of the 'warm' that you desire.
A CCS in the plate circuit will make the 12AU7 more linear, and have less of the 'warm' that you want.
Just my opinions.
Have Fun!
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Thanks.
Yeah, I had seen the stuff about obviously way-too-low plate voltages, and have been thinking much more realistically than that.
