Can someone explain this PI in this amp.

[leadfootdriver]

It's an AC100. The PI isn't quite a LTP, so I don't quite understand it. This amp might be good for a LTP mod and substituting the AU7's for AT7's. The tone stack looks like a Top Boost setup.

Here's the schematic.

http://www.webphix.com/schematic%20h...amps/ac100.pdf

[soundguruman]

It's an AC100. The PI isn't quite a LTP, so I don't quite understand it. This amp might be good for a LTP mod and substituting the AU7's for AT7's. The tone stack looks like a Top Boost setup.

Here's the schematic.

http://www.webphix.com/schematic%20h...amps/ac100.pdf

Vox did have a lot of unusual phase inverters.
But they also had a lot of schematics that did not really match the amp. Looking closely at the actual construction of the amp, you may find that the schematic is wrong.
The actual schematic, a lot of times, was secret.
I know that the schematic for AC120 was largely wrong....having worked on them before.
Vox did not want the amp copied. And published misleading schematic drawings.
Before, I have just gone thru and drawn my own, with all the correct details that the schematic did not show.

No, I wouldn't modify it.
Vox amps were great, just as is. Draw yourself a schematic, and see what's really happening.

No correct schematic, but when I got the AC120 working, it sounded unbelievably killer. One of the best sounding amps I ever tried, no question.
I wish I owned one. sweeeeeeeeeeet!

[Chuck H]

As SGM said, do check the schem against the actual amp. But if the schem IS correct it looks to be a sort of hybrid paraphase/LTP. Some might say it's just a LTP, but I don't think so. There's a small couple to the non input of either side via the non bypassed shared cathode R (LTP, also called a cathode coupled amplifier). And there is an input side favor to the equalized grid coupling resistors (like a poorly balanced paraphase). Perhaps the two coupling methods equalize the sides. But I doubt it. That's not always a bad thing. If you have a meter and some way to inject signal you could test this. How does it sound?

[pdf64]

It's the regular later AC50 'floating paraphase' The Valve Wizard unity gain / virtual earth nfb inverting stage. Gain set by R20/R19, node is the virtual earth.
The unbypassed, shared cathode resistor can cause horrid 'blips' of positive feedback when overdriven, but that generally get masked by power tube grid clipping; not good for a LarMar master vol though. Simple remedy is to split and bypass the cathodes.
Earlier AC50s, with a tube rectifier, used a LTP phase splitter.
Pete

[Tage]

This is pretty well identical to the PI's you find in bigger Ampegs, V4's from my experience. it is indeed referred to as a floating paraphase, I have never noted any nasty distortion "blips" but it does make sense that it could occur.

[EFK]

I build a lot of Valco-based amps with the earliest version of the Valco paraphase, which utilizes a shared unbypassed cathode R. I always thought the purpose of the unbypassed shared resistor was an attempt to equalize the two sides via some form of slight feedback cancellation. I've experimented with varying values of shared R as well as split resistors, bypassed shared R and bypassed split R, etc. in the same circuit, and invariably I always leave it alone as an unbypassed shared resistor. This seems to provide the closest AC drive balance to the two power tubes across the volume knob with test signals. Apples and oranges here, but since the issue of the paraphase cathode R came up....

[soundmasterg]

I built myself an AC100 clone using that exact PI and it sounds great and works fine. I personally don't see any reason to change it up as it drives the power tubes with no problem and AC100's sound great, though they are very clean amps all the way up to their max power. The most overdrive you'll get out of a stock one is the same as John Lennon's tone on "I Feel Fine". I added a second, higher gain channel to my clone and I can get fantastic cleans and Marshall JCM800 style overdrives. The LTP introduces it's own characteristic tone and overdrive, and if that is what you are after, that is the only way to get it, but as I said the amp sounds and works just fine with the floating paraphase.

Greg

[trem]

I built myself an AC100 clone using that exact PI and it sounds great and works fine. I personally don't see any reason to change it up as it drives the power tubes with no problem and AC100's sound great, though they are very clean amps all the way up to their max power. The most overdrive you'll get out of a stock one is the same as John Lennon's tone on "I Feel Fine". I added a second, higher gain channel to my clone and I can get fantastic cleans and Marshall JCM800 style overdrives. The LTP introduces it's own characteristic tone and overdrive, and if that is what you are after, that is the only way to get it, but as I said the amp sounds and works just fine with the floating paraphase.


Greg

I do not believe I have eve seen an AC-100... other than in pictures. Those were/are a rather "rare" head were they not.?
best

[leadfootdriver]

Thanks for the replies fellas.

I have not actually got my hands on this amp. See, I just joined this band, and we practice at the singers house. SHE said that it used to be her X-husbands amp, along with a lot of other analog gear! But I don't think she knows anything about amps, because I asked her if it was an AC50, and she said "actually, I think it is". But I couldn't find any pics that resemble the amp she has.

The amp sits on the Vox trolley stand with the matching refrigerator 4x12 cab. The amp has a badge on front of it, but I forget what it said. I wanna' say that it said tremolo something, but in the heat of the moment of talking to her I can't really remember. I just joined this group, so it's to early for me to pressure her into letting me get my hooks all over her gear! In my searches, I didn't find any pics of amps that had the small badge on the right side front of the amp tho.

I'm thinking that she has a real jem with the original tubes it it, as it has been sitting there in the corner of her basement. I'd like to swap out the AU's for AT's , and put it through my Rockcrusher attenuator. But I haven't taken a great look at it. It might have that 4 prong XRL type speaker cable on it. But my wheels are turning!

[soundguruman]

Thanks for the replies fellas.

I have not actually got my hands on this amp. See, I just joined this band, and we practice at the singers house. SHE said that it used to be her X-husbands amp, along with a lot of other analog gear! But I don't think she knows anything about amps, because I asked her if it was an AC50, and she said "actually, I think it is". But I couldn't find any pics that resemble the amp she has.

The amp sits on the Vox trolley stand with the matching refrigerator 4x12 cab. The amp has a badge on front of it, but I forget what it said. I wanna' say that it said tremolo something, but in the heat of the moment of talking to her I can't really remember. I just joined this group, so it's to early for me to pressure her into letting me get my hooks all over her gear! In my searches, I didn't find any pics of amps that had the small badge on the right side front of the amp tho.

I'm thinking that she has a real jem with the original tubes it it, as it has been sitting there in the corner of her basement. I'd like to swap out the AU's for AT's , and put it through my Rockcrusher attenuator. But I haven't taken a great look at it. It might have that 4 prong XRL type speaker cable on it. But my wheels are turning!

I think I built one of those PI before, and it was not as smooth as a Fender PI, but it worked just dandy.. It had kind of a hair trigger. That's the characteristic of Vox.
It is better to restore it than change it. Afterall it is an old original. It is an unusual model, very difficult to find one. I have only seen a very few. They do sound killer when operational.

[soundmasterg]

I do not believe I have eve seen an AC-100... other than in pictures. Those were/are a rather "rare" head were they not.?
best

Yes they are very rare. I've never seen an original one myself or played one, but the clone I built sounds very close to recordings that I know used them. I have played a JMI AC50 which is a little different than the AC100 and probably has a little more drive and grit, and it sounded fantastic! It had the feel of an AC30 with a cleaner but not quite as bright of a sound. I built the AC100 clone into a Sovtek MIG 100U chassis and used the Sovtek power transformer. For the OT and choke I sourced those from Heyboer. I made mine switchable between cathode bias and fixed bias to emulate the two early versions of the AC100, though if I was going to do it again I would just build the fixed bias version as they sound very close to each other, and the fixed bias version is much easier on the power tubes.

Greg