Preamp project - opinions welcome

Carl Bergman · From: NÃ¤rpes, Finland

Hi!

My friend is planning a preamp suitable for pedal steel guitar. We would love to hear your opinions. You can read his description of the project below.

Thanks in advance. Carl

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The main part of the circuit is pretty much based on the pre-stage in an Ampeg B15.

The curves in the pictures show how the EQ knobs work when you sweep the respective knob from zero to max.

The Mid control is in the circuit separated from the bass-treble and works "independently", so it therefore has its own image.

As can be seen on the curves, the mid control loses a bit of treble when it is set to max, which is due to the output impedance from the first tube stage before it, but if it is at all noticeable in practice, it is easy to compensate either with the treble control, Bright -switch or somewhere else in the circuit.

As a starting point, I have limited the Bright switch a bit with a resistor so that the treble increase is not so strong, that's something we can tweak according to how it sounds later.

An advantage (as I see it) of this so-called James EQ plus the mid control is precisely that all three knobs work independently.

In many guitar amplifiers, the knobs are more interactive, for example when you turn the knobs on the bass, the cutoff frequency of the mid range is moved, etc.

Here, the mid-range around 1kHz is kept fairly constant at the same level and in the same frequency range, regardless of how you turn the bass and treble knobs, and the idea is then that you can reduce the mid-range in just those frequencies with the independent Mid-knob.

A problem with Fender's EQ in particular, both those with 2 and 3 knobs, is that the more middle register you pull on (with the middle knob on a 3-band or with a fixed resistor on a 2-band), the more ineffective the bass becomes - and the tweeters.

To get a straight frequency response in a Fender, you instead have to turn off all the bass and treble, but then you end up with no volume.

If you want roughly the same frequency response as in a Fender EQ in this preamp, you set the bass and treble to 12 o'clock and turn down the Mid knob.

The biggest question mark for me in the basic circuit itself here is the first half of the 12AU7 tube (labeled V2A on the schematic).

If it turns out that it does not add anything other than an unnecessary increase in volume, you simply have to bypass it. Otherwise, the idea with it is that it should to some extent simulate what happens with the sound in the so-called phase inverter and the output stage in an amplifier.

The second half (V2B) is mainly for lowering the output impedance, which is good when connecting to mixers, power stages, etc. It is a so-called cathode follower that does not add any volume.

//Tom

Larry Dering · From: Missouri, USA

Interesting project. Like to see the prototype once you get one together and tweaked.

J Fletcher · From: London,Ont,Canada

So the "James" type of EQ seems to be showing up in tube circuits these days . Wonder what's going on with that ?
Since you did invite comments , perhaps a variable Bright control instead of an on , off Bright switch .
Also , the last triode stage looks a bit unusual as it is not really a cathode follower , is it ? Isn't it a common cathode amp , with the signal taken from the cathode instead of the plate . Why not use a cathode follower instead ? Just curious . So , have you built this yet ?
Looks interesting though , appreciate you posting it.
Jerry

Scott Swartz · From: St. Louis, MO

Most of it looks like it should work well except for that last stage.

It will not operate as a cathode follower with respect to linearity and voltage swing capability, tapping off the cathode of a gain stage with a plate resistor is not the same thing.

Think about about - the cathode will bias out to maybe 5 to 7 volts DC on a 12AU7 so the signal can only swing downward that far toward zero. It might sound like enough but the linearity will be poor. A correct cathode follower for a 200V supply would sit at 120-130 V and give probably 60 volt swing in both directions.

It was probably chosen to allow the 1 meg volume pot to connect to ground for biasing of the stage and no DC on the pot.

The better way to do it would be to use a tapped cathode bootstrap bias arrangement after the volume pot, as seen on the phase inverter of 5e3 deluxe for example and many others. Or you could do two resistors, as often seen to bias a transistor stage. Obviously you have to add a cap after the volume pot for these ideas.

Also I would use a small choke instead of the 2.2K resistor for power supply to ensure no hum.
_________________
Scott Swartz
Steeltronics - Steel Guitar Pickups
www.steeltronics.com

Carl Bergman · From: NÃ¤rpes, Finland

Preamp is under construction. Updates on progress and testing experience will follow.

Carl

Brad Sarno · From: St. Louis, MO USA

Good looking design. Should have plenty of gain and steel friendly tone shaping, even with insertion loss of the EQ sections.

And exactly what Scott said regarding final follower stage and bootstrapping the bias.

B

David Burns · From: California, USA

Here's a design I drew up about 10 years ago. I built it but with a 12AX7 / 6CG7 tube set. The Black Box design has been floating around the internet for years and from what I remember it was made to warm up a steel guitar going into a solidstate amp. Anyhow, there's a few fun ideas here...

Dave

Carl Bergman · From: NÃ¤rpes, Finland

Thanks so much for the responses and for sharing the schematic.

Carl

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