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Hi All. I am trying to design a two stage 12AU7 preamp similar to the Foreplay but using a IXYS current regulator as a plate load (IXYS regulator, 600 ohm resistor K to G @ 5mA) and a cathode sink (IXYS regulator, 600 ohm resistor K to G @ 5mA). I would like to use around 265V B+ with a 3v LED on the cathode of the first stage. The cathode follower is direct coupled to the input anode. I would like to use ac on the filaments with the heaters raised @ 10% of B+. Does anyone see any obvious mistakes of have any suggestions? This pre will be used with a Gainclone.
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If all the 12AU7s are on a common winding, it's probably best to bias that winding halfway between the cathodes. Hopefully that's around 1/4 of the HV supply voltage, for best headroom.
If there are two heater windings available, then one can supply the voltage amp, and the other the cathode follower; they can then be biased separately.
For a preamp like this, linearity is not really an issue, since the output voltage has really enormous headroom. Using current sources greatly improves the circuit linearity, as well.
Hi Paul. Would the filaments on the CF stage need to be raised at all?
You can look up the data sheet, but most tubes are spec'd at +/-100v heater to cathode. You don't want to get too close to either end.
Paul
I found that !2AU7s should be run at about 8ma to be their most linear.What puzzles me is a 6CG7 and a 6SN7 are much more linear but the mass availability isn't there with the 6CG7 as it is for the 12AU7.
A friend you get for nothing,an enemy has to be bought
At what plate-cathode voltage did 8mA seem best?
About 300v as I recall.
A friend you get for nothing,an enemy has to be bought
A preamp is a tool to solve a problem. What is the problem you want to solve?
Gain? What signal drives your Gainclone to full output? 0.25mV? 0.5mV? You are likely NEVER going to use gain on the original signal to drive your amp. So what you have is a 1 to 2 Volt input signal, run through a Pot to throw away 95% of the signal only to re-amplify it back up to 0.25 mV.
Driving LONG cables? A preamp is part of a system. A buffer/ transformer attenuator will help. One of Dave Slagle's autoformers will be a major sonic improvement in this case over a 12AU7..
If you are bent on a preamp with loads of gain, at least use a 6SN7 or 6CG7. A little gain?, why not a 45 or a 2A3 through a output transformer.
A single stage 12B4A works really well too.
Or I could use a AD841 as a buffer. I could use a lot of things. Actually you just relegated most tube preamps that have ever been made to the trash heap. I'm asking whether you see any problems with the circuit as described and have any suggestions on how to make it work better. Not whether IYO it has too much gain. I may want to use it with a tube amp one day. Where I use it is irrelevant. Also I have tried many preamps with a 6sn7, 6sl7. 5687........ etc. The 12au7 is readily available and has low input capacitance which is why I chose it. Plus I like tubes and the way they sound.
Edits: 04/15/16 04/15/16
You can't listen to a preamp by itself, so it is kind of important what you are planning on using it with. You stated pretty clearly that you want to use it with a Gainclone, thus my comment that it is not the best fit..
It comes down to understanding a "system". Tube amps also have typically sub 1V input sensitivity for full power. Most preamps serve as source switchers and attenuators with way too much gain. When you build your own system you are freed from the constraints of manufacturers having to make their preamps compatible with any any amp and any source. You know yours so design something that is specific and optimized to your system. That is the benefit of DIY.
If you are just looking to build something, yes an active load on a triode will make it function. A cathode follower with an active load will give you lower output impedance.
The 12AU7 has few fans, try a 6CG7 for a 9 pin or one of the other tubes people suggested. Your desire to use AC on an indirectly heated tube is OK to save cost, though it will add noise. People use AC on directly heated filaments for the sound impact, there is no sound impact on indirect heaters with AC except noise.
Heating an indirectly heated tube with AC does sound different from using a DC supply. Some prefer the sound with DC on the heaters; I prefer AC. But whatever your preference, there is clearly a difference.
I'm referring only to IDHTs, of course.
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Buy Chinese. Bury freedom.
The 12AU7 triode is non-linear and makes a poor voltage amplifier. Use a linear 12BH7 under the conditions you envisaged.
Eli D.
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