Can I use a 12AU7 based preamp with my SS power amp?

[armstrom]

I'm new here but based on what I've read this seems to be the best place to find an answer to my question... I have built a nice little 12V solid state stereo power amp based on a tripath chip (www.41hz.com look for the AMP6-Basic). This amp is designed to accept a 1V line-level input. The inputs can take up to about a 4V swing. I am converting this into a small guitar amp head for a friend and would like to build a tube preamp for it.

Here's the rub... I'm worried about giving up a lot of volume from the amp by using the following design (one per channel) http://www.beavisaudio.com/Projects/...alveCaster.gif

I will run that ciruit at 12V also.

Other than swapping out the volume pot for something smaller (50K or so) is there anything else I should do to make sure I'm not giving up valuable gain from the preamp stage?

Any advice is much appreciated.

[6267]

I've always had better luck using a solid state front end, and a tube power amp, than vice versa. It's a pretty subjective art, and if you have a really nice SS power amp, and just want a bit of tube color, I'm sure it'll work out. I'd recommend the ecc86/6gm8, it's much more linear at that voltage and apparently sounds very good. I don't know what the appropriate values for the circuit you describe would be, but I definitely would use a wirewound pot for that cathode, or convert the 470k between stages into a pot, leave a fixed resistor in the cathode, and take the output to the next stage's grid from the pot's wiper (pots apparently don't much like DC, so it's generally not a great idea to put a pot in the cathode, although it's been done. You'd want a minimum resistance there too...). I can't imagine a 12au7 sounds great with only 9v on it's heater either. Altogether the circuit seems like it's a pretty hackneyed attempt to shoehorn the 12au7 into a role it's not great at. Google "space charge" tubes, I'm sure that preamps have been designed using these (ecc86 and ecc88/6dj8 come to mind).

[Enzo]

He's going to run this on 12v which puts the heater right where it wants to be. The rest of the tube will starve from the tiny voltage, but it won't hurt it.

The volume control is a simple voltage divider, changing its value will do nothing to the signal levels.

I wouldn't worry about that cathode pot, at 12v B+ and a 220k load, there is only going to be something like 0.05ma through the tube - at most. it might sit at 2-3 volts, but it will be under no stress.

What do you mean give up valuable gain? Do you mean the preamp won't drive the power amp fully? You mean the preamp can't be turned up fully without overdriving the power amp? Just what?

[armstrom]

Thank you both for excellent replies.

@6267:
Since this is going to be a guitar amp I really want to tube preamp to color the sound a lot. The power amp is VERY clean (at 60% output the THD+N is below 0.1%, details here http://www.41hz.com/main.aspx?pageID=118 ) so I'm relying entirely on the preamp to provide the "guitar tone".

@Enzo:
Reading my post back I realize I wasn't clear at all What I mean by giving up gain is that the output impedance of the pre combined with the input impedance of the power amp combining as a voltage divider and cutting any voltage gain from the preamp in half (or worse). What I would like is the ability to drive the power amp as hard as possible without clipping or overloading the inputs to the point of destroying the Tripath chip.

Dividing the output voltage of the preamp is a safe way of ensuring the voltage at the power amp inputs never exceeds the maximum safe voltage. However, it also shifts the entire "gain curve" down in the process, potentially giving up some volume out of the power amp. Would it be better to simply use some diodes to clip the output at the maximum safe voltage? I doubt I would ever reach that voltage under normal circumstances, but I don't want to dampen the gain out the preamp stage to make sure.

FYI, I have also purchased some 6111 tubes to use in a modified version of the same preamp circuit:
http://www.beavisaudio.com/Projects/.../SubCaster.gif These should be much easier to fit in to a small enclosure

-Matt

[Wakculloch]

A directly coupled emitter follower stage would allow you to lower the tone and volume pot values without causing impedance mismatch problems into the power amp.
I've used this when driving valve preamps into a computer, and it works well.