Looking for ideas for a tube instrument preamp

[CurtZHP]

It's an itch I just have to scratch.

I want to build a simple, effective tube instrument preamp, sort of a tube direct box. Basically, plug the guitar or bass in, turn the knob, and get a balanced line level output. No EQ, no high-pass, no low-pass, no phase flip, no mess, no mistress, no mas.

I've done all the usual web searches, and the closest thing I came across was the input stage of a guitar amp, and the circuit had a lot of things I didn't want.

Anyone have any sources?

 

[RAM]

I don't know enough to come up with an original idea: but why not the preamp from a Fender 5e1? It'd be just one ECC83 with a volume knob. One side of the ECC83 acts as the Gain stage and the other is the PI. Then you'd just need to configure an output stage for it to bring it up to line level.

 

[emrr]

Build any of the many tube mic preamps, but leave out the input transformer and just go straight to tube. Most of the designs posted have DI inputs also already.

 

[pstamler]

If you don't want to add distortion and you're okay with including a transformer, it's not hard at all. A 6SN7 can be your tube, with a 100k log-taper volume control between the two sections; with bypassed cathode resistors, you can get about 38-40dB of gain out of the pair. Run the second stage a bit hot to drive the input (say 4mA) and capacitatively couple it to a good-quality 1:1 output transformer to drive a 10k balanced line input. Or use a Jensen DI transformer to make the signal mic-level, if you need that.

If you want to overload the input tube, then use something like a 12AX7 or 6SL7, but you'll still need something juicier for the output, like a 6SN7 or a 12AU7.

Peace,
Paul

 

[CurtZHP]

PAUL STAMLER!!

How've ya been?? It's been a couple years, or more. The Project-R you helped me build is still working like a champ. It's pretty much become my "go-to" preamp. But I digress.....

You mentioned putting the volume control between tube stages. Most of the designs I've seen so far have pot right before the first stage. I assume I'd do that if I wanted separate gain and volume?

I'd like to use a 12A(whatever)7, only because I've got a pile of them.

I don't mind if it adds a little flavor. That's the point of using a tube.

 

[Curtis]

Check out the preamp sections from the AX84 website. Lots of one bottle ideas there.

 

[pucho812]

So you want a tube DI that has line level on the end to go right into the tape machine or daw. Have you considered building the MILA by or buddy New York Dave? Just skip the input transformer and XLR input jack. Wire up the 1/4 input jack and the rest. Plus the output transformer is inexpensive from edcor.

 

[pstamler]

[quote author="CurtZHP"]PAUL STAMLER!!

How've ya been?? It's been a couple years, or more. The Project-R you helped me build is still working like a champ. It's pretty much become my "go-to" preamp. But I digress.....[/quote]

I've been well -- thanks!

You mentioned putting the volume control between tube stages. Most of the designs I've seen so far have pot right before the first stage. I assume I'd do that if I wanted separate gain and volume?

Well, it depends. A few cheap guitar amps used pots at the input, but they almost inevitably had pentodes after the pot. And the pot makes for a much noisier preamp, particularly when set to about 6dB of attenuation.

I'd like to use a 12A(whatever)7, only because I've got a pile of them.

I don't mind if it adds a little flavor. That's the point of using a tube.

True. Well, try it first with a 12AU7. You'll need one anyway to drive the output transformer. Bias the output up to, say, 4mA, maybe 6mA. Run the output stage from, oh, about 300V, the input stage maybe 180V. Bias up the filaments to about +75V dc to cut down hum. Use a good transformer (Jensen) or, maybe, a crappy one (Edcor), since this is a guitar preamp and you want some color. Or, maybe, use one of each, with a switch.

Peace,
Paul

 

[JCraft]

This is strange, I haven't been to the board in a while and this was the first post on the forum. I have been working on this idea for a week or so now (had the idea a year or more ago really, just didn't have the time), parts I know I needed are coming in like tube sockets and a NOS Triad A68J output transformer I just got from Ebay. I originally was just going to start with the preamp stage from the Fender Deluxe 5E3 and go from there, but my buddy Ron Laury thought we should go from scratch. The 5E3 has a 12AY7 for the first stage and a 12AX7 driving the 6V6GT output tube, we're gonna change this up and use the 12AX7 for the first stage (30+ db of gain) and the 12AY7 to drive the 15k primary of the output transformer (we are also toying with the idea of a 12BH7 for the output). We will have an input gain in the 12AX7, a Master Volume to the 12AY7, also adding a Hi & Lo (5K & 100Hz) shelving EQ to the unit. This unit is being designed for the above "Direct Box" application and also to be used in conjunction with a fairly modern solid-state power amp/single 15" cabinet as a clean/quiet bass amp that has some tone versatility to it. I am not a tube/electronics "Guru" like some around here (although I have been severely shocked poking around inside tube circuits on many occassions and have enough knowledge to accomplish this), I do have 30+ year recording experience which leads me to know what works and what I want from gear. When Ron gets the design done I will get him to post the schematics/therory/results to this thread if he can.

 

[PRR]

> No EQ, no high-pass, no low-pass, no phase flip, no mess, no mistress, no mas.

No mistress????

"Gain stage ...PI ...output stage ...to line level"

Uh, but guitar is nearly line voltage already. We need more current gain than voltage gain.

"tube mic preamps" "MILA by our buddy New York Dave?"

But even without input iron the gain is plenty and the complexity is high. Plate supply, more plate suply, heater supply... If you need a mike-amp anyway, sure.... but what if you don't?

This is an appropriate solution.

You want a transformer anyway, to transform low tube current for potentially 600R loads, and to float to give "balanced" line and break ground loops.

But it is a narrow-band instrument, and dead-clean can be got cheap with chips, so we don't need gold-plate parts.

The 60FX5 is a forgotten cheap-stereo power amp. Over 1 Watt with 115VDC supply, we want over 0.1W. If we give it half of nominal voltage it will beat requirements (even mismatched) AND the 60V heater can eat plate power. One power rail.
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/6/60FX5.pdf

Assume large cathode bypass.

Voltage gain in Triode mode is not quite unity, but most line inputs can take that and bring it well up on the meters. Frequency response with the low-cost 125ASE may extend to 30Hz, and far past the guitar range.

Voltage gain in Pentode mode is 3 or more; but bass response may droop below 60Hz. You can trim the output resistor for less midrange and therefore more level bass. Or you can apply a 40Hz bump in ProTools. It's not like "flat" is sacred.

The 125ASE does not have a UL tap, but you may have snazzy iron on hand, or a 10W p-p OT. UL mode splits the difference between triode and pentode.

Max input voltage is 1.3V peak. This should be ample for most pickups and picks. THD may approach 4%, which does no harm. I've shown the hi/lo jacks, the top jack can go 2.6V at 4% THD or 1% THD at more normal levels. Un-bypassing the cathode resistor allows higher inputs at lower THD. "Small" cap like 10uFd will give a treble shelf boost, very popular on guitar.

The power supply is simple, not elegant. I assume if you buy more than a few parts, you may as well get the price-break on a 10-bag. Hum should be low, may not be zero. If it is a problem, feed the heater as shown, then add 470R and 470uFd 100V filter for the plate power.