Vox ac15 phase inverter inputs

[jyrki3101]

Hi, I think I've reached the point of putting a switch between the two : two preamp channels going into one PI input(w/separating resistors) or, one going to the inverting input and the other going to the non inverting input of the PI.
I can't decide between the two probably 'cause I haven't A/B:d them !
If someone has better experience on this , I would greatly appreciate your help !
kind Regards,
Mr.Jyrki T.

[pompeiisneaks]

Hi, I think I've reached the point of putting a switch between the two : two preamp channels going into one PI input(w/separating resistors) or, one going to the inverting input and the other going to the non inverting input of the PI.
I can't decide between the two probably 'cause I haven't A/B:d them !
If someone has better experience on this , I would greatly appreciate your help !
kind Regards,
Mr.Jyrki T.

What are you trying to accomplish? Which build is this? Basically one way will keep the channels non inverted from one another the other puts them inverted. Inverted basically ends up losing tone, so is not preferred. In the vox I've seen the Normal channel and the Top Boost have one different number of gain stages, so you would want them into opposing sides of the PI to avoid the negative effects. (if jumpering that is).

~Phil

[jyrki3101]

In a vintage AC30 the normal and top boost channels are going to the same side of PI. each one through a 220k resistor,The vibrato channel alone is going to the other side of PI !
Vibrato channel has also that "notch filter" before the PI input (a number of 4n7 in series->1m to ground and again 4n7 cap in series->1m to ground)
With a switch I could really hear the difference of feeding just one side of PI or just one with two channels
My channel one is an EF86->PI and the other is a half of 12ax7-> PI

[pompeiisneaks]

In a vintage AC30 the normal and top boost channels are going to the same side of PI. each one through a 220k resistor,The vibrato channel alone is going to the other side of PI !
Vibrato channel has also that "notch filter" before the PI input (a number of 4n7 in series->1m to ground and again 4n7 cap in series->1m to ground)
With a switch I could really hear the difference of feeding just one side of PI or just one with two channels
My channel one is an EF86->PI and the other is a half of 12ax7-> PI

So your'e referring to an AC30/4 if you're talking about EF86, and vibrato. I've built one, have it still, love it. The schematic can be seen here: http://el34world.com/Forum/index.php?to ... #msg213400 (This was my build thread).

The Normal channel goes into one half of the PI, yes. The other channel is the vibrato channel. If you're talking about the Vox AC30/6 it has no EF86, just a normal, bright (or top boost) and vib/trem channels. I'd have to find that schematic to be sure, but basically the only Vox AC30 with EF86 per my knowledge was the AC30/4. You can shut off the trem and use the trem channel as a simple 12AX7 channel of course. Not sure that it would quailfy as a normal, it has some component differences.

Also that part you're referring to as a 'notch filter' is what's actually called a 'shift network' and handles phase shifting the signal.

Thus why i was asking for the exact amp details.

I'm a bit of a vox nerd, but still have a lot to learn about them, by the same token. My first ever build was an AC30/6 style Hoffman AC30 minus the vib/trem, it just had a Normal and Top Boost, then I built a Vox AC100/2 and finally ended up building the AC30/4 due to that lovely EF86 pre. It's noisy still, I need to shock mount the socket somehow to make it less microphonic though.

~Phil

[pdf64]

Generically, both arrangements should work and sound fine, no glaring reason they should differ much.
Specifically, details matter, and how these ideas are actually implemented may cause significant differences.
Really, it's kinda pointless asking such questions without a schematic to reference.
As your amp seems to differ to anything in the public domain, that will require a little effort on your part.
But not much, all you need to do is just sketch it out, take a decent photo of that drawing (eg on a phone), upload that to a hosting site and provide a link to that here.
Please don't try to describe your circuit

[M Fowler]

I would just use two 220k mixer resistors at the PI entrance.

Some interesting read is Dana Hall's Rocket/Liverpool amp something I've wanted to build since he introduced it years ago.

http://ampgarage.com/forum/viewtopic.php?f=1&t=3026