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.

[greekie]

For stuff like this I like to use a small step-up voltage converter, to get the B+ nice and high.

Check out this site Flyback Converters for Dummies

[Ronsonic]

If you search for Nixie tube power supplies you'll find a number of ready-made options. They'll want some extra filtering but the DC to DC step up is there.

[printer2]

I tried the 12AU7 preamp before at low voltage, not terrible but not great. The 12AU7 seems to work at low voltages because it has low gain and it does not go into clipping all that fast (as compared to a 12AX7). But there really is not much of a transition range where it can go from clean to dirty. The first stage does not add much distortion as it does not get overdriven, it mainly just provides a bit of gain. It would be much better if you had a simple SS booster to add a little bit of gain to hit the first stage a bit harder. Then adjust the level to the second stage to overdrive that stage somewhat. This will fatten up the distortion rather than just trying to get the second stage to give all the distortion.

Mind you, more voltage would be better, and then you could use a 12AX7 or other high gain tubes. Rather than a step-up converter I went the dumb route, just used low current back to back transformers to generate the high voltage (Search for Mctube).

[Bone Crusher]

OH BOY... Sorry to jump thread and this may be slightly off topic but very much in the vein of what I’ve been searching high and low for. The Line 6 Spider Valve (First version, not the MKII) has a very digital (solid state) input before its two 12ax7 preamp tubes. The engineers at Strymon once offered a tube input circuit mod called the SVpre to fix that issue... lookie here, at this>>> strymon » SVpre – tube preamp mod for Line 6 Spider Valve
But now this unit is no longer available and the only thing I don’t like about the Spider Valve is its lack of tube “feel”. My question to the Big Dogs here is this: Would a tube preamp pedal/driver give me that tube "sag" and articulation OR would putting something like this together from DIY kits (or whatever) a completly ridiculous thought? I think it relates to Armstrom’s Question… mostly…
But again… sorry again for jumping the thread.

[printer2]

No, it pretty much asks the same question. Can you run high voltage tubes at low voltages and get that 'tube' sound? Since I never heard it I can not say this manufacturer's product does a good job or not (well that is the one that they no longer sell). Some other manufacturer used a tube in the feedback circuit of an opamp to get a tube type sound, don't know if it was successful or if the same one here. All I can say is I have not heard tubes run at low voltages and sound like traditional overdriven tube circuits.

Not sure if this is what you are looking for but...


‪McTube II handmade guitar overdrive pedal test‬‏ - YouTube

[exclamationmark]

I've done a bit of testing, and found that a 12au7 basically sounds the same anywhere past 40ish volts. Starts to go weird at lower voltages. I think this may be because the bias point will have to be set fairly close to the point where the 12au7 will actually draw grid current (because of the lower voltage), which is somewhere around -0.5 volts. You probably need a minimum of 3 stages to get any sort of 'crunch' (which is where my clean channel is tapped off in my pre-amp). Alternatively, you could use an op-amp to boost voltage, and just use the 12au7 for clipping (which is what the Blackstar HT-5/blackstar tube pedals do, but with a 12ax7).

[defaced]

The stompbox guys have MANY HV and starved plate boost/overdrive/distortion tube designs. Diystompboxes.com would be a good place to start. Frequencycentral has many submini and 12AU7 designs up, including voltage multiplier and charge pump PSU circuits.