Would like some feedback on clean preamp idea I had

[MakerDP]

If anyone would like to take the time to look at this and offer your thoughts, I would really appreciate it. I'd like to know if people think it's actually even worth the effort before I attempt to prototype it.

Thanks!

http://www.1darren1.com/2015/03/idea-for-clean-preamp.html

[Ken Moon]

Hey Darren,

Hard to tell what your design goals are from the schematic, but in general:

1) Why use 3 triodes in the preamp? Do you have plans for the second half of V2? If not, you could do this circuit with one tube or parallel the 2 halves of of V1, or use V2B for a cathode follower (like a Marshall). Personally, I like to put the tone stack right after the input gain stage rather than after 3 gain stages like you have it.

2) Any particular reason for using a 12AU7 for V1 vs. a 12AX7?

When I want a clean preamp  with high headroom I usually just use a high B+ (your 270V to 300V is OK. but 350V is even better), use typical 100K/1K5 for the anode/cathode resistors (center biased) and 22uF to 33uF for the cathode bypass cap (fully bypassed). 

If you parallel both halves of V1 for less noise,  lower the anode resistor to 51K to maintain center bias (12AX7 values).

3) Any reason for that particular tone stack?  Looks pretty lossy. There are lots of tone stacks available, like the Bandmaster and James tone stacks, that have less than 10dB loss, about half of the typical FMV stack. I don't know what yours is.

4) Coupling caps/interstage attenuation - lots of 22nF coupling caps there (pretty small for an input stage), and a voltage divider between V1A and V1B, plus the middle control - all this stuff can add noise to a clean preamp - sometimes I like to use a spilt anode load for V1A, using a 100k pot so I can "dial in" the voltage divider values, then replace with nearest fixed resistors.

Of course you may have considered all of this (and more) when you designed the circuit, but I can't know from just the schematic.

Ken

[MakerDP]

Thanks for your thoughts.

1) Why use 3 triodes in the preamp? Do you have plans for the second half of V2? If not, you could do this circuit with one tube or parallel the 2 halves of of V1, or use V2B for a cathode follower (like a Marshall). Personally, I like to put the tone stack right after the input gain stage rather than after 3 gain stages like you have it.

I addressed that in my "Background" text on the page. I copied it at the end of this post. The unused triode can be used for a crunch channel or for a CF if it needs it or for reverb recovery. Lots of things I could do with it but I haven't thought that far ahead yet. Since it's a home-build I could just leave it unused too.

2) Any particular reason for using a 12AU7 for V1 vs. a 12AX7?

Just to tame the gain at each stage to allow for more stages really. And to try and see if I can get something "different" sounding.

3) Any reason for that particular tone stack?  Looks pretty lossy. There are lots of tone stacks available, like the Bandmaster and James tone stacks, that have less than 10dB loss, about half of the typical FMV stack. I don't know what yours is.

The treble/bass stack is a Bandmaster. The mid control is an "Alternate Bridged-T."  Wanted to try it because Merlin mentioned it is a very powerful and flexible combination. Again, in the background info I copied below.

4) Coupling caps/interstage attenuation - lots of 22nF coupling caps there (pretty small for an input stage), and a voltage divider between V1A and V1B, plus the middle control - all this stuff can add noise to a clean preamp - sometimes I like to use a spilt anode load for V1A, using a 100k pot so I can "dial in" the voltage divider values, then replace with nearest fixed resistors.

The 22n couplers were chosen just because it is the value I used in another amp and that is what KOC used in his rough sketch designs. KOC mentioned the possibility for added noise from the stage attenuations but then goes into a section on how to combat that if it arises in your design.


I typed-up a little background on my thinking behind this design on the blog entry. I'll paste it here for you to read too...

Background:
I was reading through "The Ultimate Tone Volume 5" by Kevin O'Connor and was intrigued by some of his ideas for three-stage clean preamps. Build it up, tear it down, build it up, tear it down, build it up, tear it down. Each build-up adds harmonics, each stage has different values for anode and cathode resistors, the third stage is a different tube than the first two. Each stage should add it's own unique character to the tone.

I was also reading a thread about tonestacks on AX84.com and Merlin suggested a Bandmaster for treble and bass coupled with a Bridged-T for mids. So I threw those in there too to try out something different.