Acoustic Guitar tube PreAmp & FX

[10thTx]

My son-in-law asked me to build him a tube PreAmp for his acoustic guitar to run into a PA system at his church.

I thought I'd share what I've ended up with as I am very happy with the sound/tone I'm getting with my Larrivee thru it.

And flipping a couple of switches and turning the FX level and volume pot, it works quite well for an electric also. I bypass the frequency control and engage the mid-boost. Sounds great with a Zenith Drive pedal.

The active FX works great with delay or reverb pedal. The frequency control is for the acoustic to eliminate/dial out any feedback problems.

I used a $25 Weber WRVBPT trannie for this:
https://taweber.powweb.com/store/wrvbptsch.jpg

And a $13 Edcor XSM10k/600 matching transformer:
https://www.edcorusa.com/search?q=XSM10k%2F600

Guys on the Hoffman forum helped a great deal on the frequency control idea. The lengthy thread is here:
http://el34world.com/Forum/index.php?topic=17920.0

With respect, 10thtx

[10thTx]

The amp is very quiet with no unwanted hum. Only typical slight white noise that a tube amp would have.

Total cost was around $200 because I already had caps (excluding filter caps), resistors and an inexpensive tag board.

With respect, Tubenit

[cbass]

Very cool design . you're an amp building machine!

[10thTx]

I modified some of the values on the Low Pass Filter and added a notch filter switch. Works great! Does an excellent job eliminating feedback on my acoustic.

This has an incredibly warm and super super transparent sound, IMO. Acoustic sound superb to me. Electric sounds reasonably good thru it.

With respect, 10thtx

[Masco]

Is this used w/ a microphone, pickup or both?

[10thTx]

It's used with an acoustic guitar that has a pickup like a Western K& K mini.

http://www.kksound.com/products/puremini.php

I think you could use it with a microphone, but the idea behind it was to have it for an acoustic guitar that has a pickup primarily & also have enough features to be a reasonable choice for an electric guitar.

With respect, 10thtx

[pops]

Nice amp, have you thought about switching the bypass caps on and off, with no bypass cap you will have less gain, but a flatter frequency response. I use this in amps all the time makes it warmer and with a switch you have the best of both worlds. Just curious.

[10thTx]

have you thought about switching the bypass caps on and off, with no bypass cap you will have less gain

Yes, I've used that approach on quite a few amps.

Given that this is using a 12AY7 and the cathode caps in the PreAmp are small 5uf and 2.2uf, I don't think that the gain is much of an issue at all. Currently, I have a 12AV7 in V2 and that has even less gain then the 12AY7. Note that I'm not using the typical Fenderish values of 22uf or 25uf.

Typically, when I've switched cathode caps in and out, it's on 12AX7 or 5751 tubes.

And the PreAmp already has 8 mini-toggle switches on it.

With respect, 10thtx

[pops]

Yea i use 12AY7's for the sound and rarely use a 12AX7. I noticed the smaller caps and have done that too, it still makes a difference in gain tho less noticeable as you are not boosting the bass as much. Just thought as it was for an acoustic guitar it might worth a try to just lift the grounds and see if it makes a difference. I am mostly curious as electrifying any acoustic instrument is not an easy thing to do to sound acoustic. If you decide to try please post what you find as i would be interested in knowing if you think it was flatter and warmer, better or worse, etc..
Thanks, pops

[10thTx]

Acoustic PreAmp now has a cab head. Sapele, flame maple trim, black walnut front panel.

With respect, 10thtx

I made a quick sound clip to show how this PreAmp tone is:

http://www.soundclick.com/player/single ... i&newref=1

EDIT: This amp was not successful with the PA system it was used with using the Edcor trannie. The trannie was removed and the signal was run into a direct box and then into the PA system. Reportedly, this sounded excellent and was DEAD QUIET.

2/3/2015 update