Tube stage for acoustic guitar?

[MondyT]

Hi All
A while ago I put together an op-amp based preamp for my acoustic guitar pickup with some fantastic help from several forum members here. This works great and I am very pleased with the sound. I got to thinking about a simple tube preamp stage and wondering if this would work with an acoustic transducer. The only tube I have is a brand new, never been kissed ECC83 and so got searching for a single tube preamp . I found this preamp stage...

DIY 12AX7 Tube preamp

I know nothing of tube techniques and so wondered if you guys and girls could help me out. Is this circuit suitable for my needs? which are...

1. An input impedance of 1 Megohm (crucial!)

2. About 18dB gain maximum

3. An output suitable to drive a 20K to 40K amplifier input

4. Capable of handling around 250 to 300mV rms input max with very
low distortion and good fidelity

5. Using one ECC83

Can you good people tell me if the circuit is any good or can be modified to suit my needs. If not, do you know of any such circuit. I will design a build a custom linear power supply for the voltages and currents required for the unit but I would need a circuit that would be up and running with the minimum “fiddling” as my resources and time to devote is always limited!

One more thing, the components in the board photos in the above link do not seem to agree with the schematic. Is there anything I am missing here?

I am hoping you guys and girls can come to my aid!

Cheers
Ray

[kavermei]

Hi Ray,

it looks like that circuit fulfills all your requirements, except that it appears to have a gain of 51, which is about 34 dB while you only need 18 dB.

Of course, the ECC83 being a high-mu tube, most schematics you encounter will give more than that 18 dB gain unless some sort of feedback is incorporated.

Kenneth

[MondyT]

Hi Ken

Thanks for the reply bud! Looks promising, any idea how I can reduce the gain of the circuit, with NFB or other method?

Cheers
Ray

[kavermei]

Hi, from simplest to most involved, there's a number of things you could do:

- ignore the problem, maybe your main amp turns out to have enough headroom?
- put a 10 Kohms trimmer potentiometer across the output so you can trim the output level
- incorporate some form of NFB into the circuitry

Probably best to try it in that order, I think the last option would be overkill.

Best,
Kenneth

[OneyedK]

I'd go for the 6922 version of that pré-amp:
DIY 6922 / E88CC Tube preamp

Maybe include a tonestack between gain and follower stage...
(this would lower gain to the amount you want)

Feedback will not help "the cause" since you will alter the balance between even and odd harmonics, imho, not what an acoustic guitar needs...

[DF96]

If you put together a valve stage with "very low distortion and good fidelity" then it should sound exactly like your op-amp circuit. People often use valves, especially with guitars, because they want to avoid very low distortion (although they may not always admit to this!). I think you need to be clearer about what you are aiming at: fidelity or "tube sound". Either can be achieved using valves, but the design would be different.

[OneyedK]

Source = acoustic guitar pickup
--> fidelity is the key, but added 2nd order disto helps the cause...

[MondyT]

Thanks for all your replies!

Ken - seems like putting a volume pot on the output would be a solution, but wouldn't this raise the output impedance a little high for a 20K amp input. Also this output impedance would vary with the setting of the control?

Oneyed – Looks like feedback would not be good. Can you fill me in on what a Tone
Stack is and what it involves as I am a bit of a newb to all this (blush!)? The E8CC tube looks like it would be nicer but a little pricey (prices vary wildly and can go through the roof!!). I have an ECC83 to hand

DF96 – Good point! I think maybe I would like to try the tube sound on the acoustic and just settle for the best fidelity that a simple arrangement can give. Hopefully a little warmth (circuit distortion/compression etc) may just make for a good acoustic sound?

Thanks for your help

Ray

[kavermei]

Since the output of the circuit is around 600 ohms, the pot could be 5K, too.

A little 2nd order distortion (without overdoing it) is definitely pleasantly sounding for acoustic instrument amplification. The distortion can be raised a little by bypassing R9 with a large capacitor, but that will also raise the gain of the circuit, which is exactly the opposite of what you need

Kenneth

[DF96]

The ECC83 is not a suitable valve for your requirements. It is fairly linear, when used properly, but has high gain. The gain can be reduced by adding negative feedback, but then you are approaching fidelity rather than a little valve distortion.

You need to start with a much lower gain: ECC88 or ECC82. As a first rough stab, take the ECC83 circuit you started with. Leave the 1M resistors as they are, but divide all the others by a factor of 3 or 4. Use an ECC82 - NOS is quite cheap. Give it a try! If it needs more gain then bypass R9.