Is there a favored schematic for an AC15?

[mort]

I'm not at all familiar with the progression of Vox and all the little model number changes. I need a good legible drawing for a straight up vintage style AC15. It's gonna get a 1x12 probably Celestion Blue. Can anyone point me in the right direction?

[mort]

After doing some snooping around, I think I'd like to find a schematic for this version of the amp: http://www.voxshowroom.com/uk/amp/ac15.html

[mort]

This looks like the drawing. Not a great one but the drawing none the less. So does the brilliance switch get replaced with a pot? Value?

And I can't seem to find the vibrato speed control. Is there a mistake on the drawing or am I just blind?

http://www.blueguitar.org/new/schem/vox/ac15fact.jpg

[bob p]

I went through the same search a year or two ago. I never found an original Vox AC-15 schematic that I really liked, though this one seems to be about the best I could find:



After doing a lot of schematic review I ended up focusing on the "simplified" designs of the Matchless Spitfire and Lightning, which I think are two great sounding derivative works. Then I ended up following Chuck H's recommendations for modding the output stage on a 2xEL84 amp. IMO Chuck would be doing everyone a great favor if he'd draw up a schematic for his EL84 amp.

The funny thing that I learned about all of this is that a used OTT is a pretty good place to start as a modding platform.

[Justin Thomas]

Mort, Bob,

I think I got Chuck's tweaked Spitfire, unless he's changed it. It was in a Spitfire-too-bright thread, cuz I had the same problem. So I know it's out there somewhere. Chuck? Care to repost? Meanwhile I'll search for the thread...

Mort, There's a 3-way switch on that schemo. I tried a vintage AC30 and was disappointed to find the same setup - a switch for speed. The 30 did have a depth control, though. And I couldn't get that EF86 channel to work for anything from that schematic, though I heard an EF86 in a Matchless and I know it works! It' VERY revealing about how good (or bad) of a player you are, amd very pedal-friendly. I'll revisit it someday, as I got 2 Mullard EF86s out of an old projector. Good luck with it!

Update - seems Chuck' schemo got wiped in the data crash a while back, as I found the thread but his schem was gone. If I magicakly get lots of time, I'll pull it up on my laptop & see if I can make the changes... I'mnot exactly tech-savvy.

Justin


Justin

[mort]

I went through the same search a year or two ago. I never found an original Vox AC-15 schematic that I really liked, though this one seems to be about the best I could find:

[ATTACH=CONFIG]29151[/ATTACH]

After doing a lot of schematic review I ended up focusing on the "simplified" designs of the Matchless Spitfire and Lightning, which I think are two great sounding derivative works. Then I ended up following Chuck H's recommendations for modding the output stage on a 2xEL84 amp. IMO Chuck would be doing everyone a great favor if he'd draw up a schematic for his EL84 amp.

The funny thing that I learned about all of this is that a used OTT is a pretty good place to start as a modding platform.

My eyes wanna say thanks. That's tons better. Looks like one of the same drawings I was looking at but mine is poorly defined.

OK so I see the switch. So I could just plop in a 1M pot and solve that. And for the brilliance switch I should be able to sub in a pot but I'm not sure how to select a size. Maybe I could just do a 1M with a switch open at full CCW... (??)

[mort]

And on that speed control, does the speed decrease as the resistance increases? As in, will it go super slow if I put in a 2M pot?

[Gingertube]

See this AC30 Schematic
http://www.freeinfosociety.com/elect...wPmOqks7zD3-sc
The 3 way speed switch has been replaced by a pot - 2M as you suggest.
Cheers,
Ian

[jazbo8]

I think the best documented build is from Steve Luckey, absolutely meticulous like all his builds.

[Chuck H]

Whatever I posted to that thread would have been specific to the Spitfire and the poster wanting to tame the brightness. Unfortunately lost when MEF files disappeared. Not much to do with the AC15. But I may have included my additions to the Paul Ruby mod. Basically...

The Paul Ruby mod clamps the negative PI swing to avoid driving the power tubes into grid conduction and therefor avoiding the bias shift that so commonly causes heavy crossover distortion in EL84 amps. I fine tuned the mod a bit to improve tube performance and insure against any audible diode clipping (Paul does mention this in his theory but doesn't clarify procedure). The real credit should go to Paul for his implementation of the zener clamping. It's rare that such an eloquent circuit is "discovered" for use in tube technology because there's very, very little new under the sun. So, for starters, kudos to Paul.



The component values in the above schematic are for MY amp. not any other. Here's how I arrived at my values. The zener voltage for the diode across the cathode resistor is set by putting the amp on a scope and bringing it to where it just begins to clip. Set the that zener voltage one or two volts higher than the voltage on the cathode pin in the above test. This makes it so the amp operates as cathode biased for all clean signal applications. Providing a small amount of compression. But the bias is fixed when the amp begins to clip. This tightens things up and eliminates further cooling of the bias because of voltage rise across the cathode resistor. For the Paul Ruby mod use a zener voltage one or two volts higher (whatever standard values allow). This guarantees that there will be no audible clamping from the diodes because the tubes will be in cutoff. Crossover distortion may not be eliminated completely, but a little can be a good thing and this arrangement guarantees the amp won't buzz and fizz unpleasantly. My last addition is the "conjunctive filter". Which is just a fancy name for the shunt across the output transformer primary winding. As per most OT shunt filters I used a resistance of 1.4X the primary impedance. The capacitance of 1500pf was chosen while looking at the scope. This value best eliminated a voltage spike without much rounding of the leading edge of the clipped wave form. So it doesn't sound like a blanket over the cabinet like so many shunt filters do. It does eliminate peaky and spikey unpleasantness that EL84's can be prone to. These mods combined really take the guess work out of choosing plate voltage, OT, speaker, etc. It pretty much forces the tubes to behave.

Again, if your amp uses different plate voltage, different tubes, other component values, etc. the above values must be calculated on a scope. For four EL84 amps the capacitance of the shunt filter would be roughly doubled as a staring point.

Not sure how much this applies to the OP but there you go

[Gingertube]

Chuck,
Thanks for posting your schematic. I have a question related to it.
For a 47K tail resistance then the best THEORETICAL balance would be with anode resistors of 91K and 94K rather than 91K and 100K. Did you try your amp with both anode resistors ar 91K or both at 100K? Probably wouldn't make a huge difference but was wondering.
Cheers,
Ian

[Chuck H]

I didn't try any additional balancing on the PI. This circuit is actually a leftover from a previous incarnation of the amp. Since I'm not convinced that balancing the PI has either a positive or negative affect for guitar amps I decided to just leave it be. BUT, before you go too far with your calculations... Notice that the plate circuit for the input side of the PI is a split load. I actually lifted this circuit from a Bruce Zinkey design. I think the idea was simply to create equal plate voltages while still reducing output for the input side of the PI. Though, as you noted, it's almost moot with a large tail resistor. The amp sounds great so I'm not in a hurry to change it just for the education.

[Gingertube]

.... BUT, before you go too far with your calculations... Notice that the plate circuit for the input side of the PI is a split load. .

Thanks - I missed that with a quick look.
Cheers,
Ian

P.S. Long Weekend here in South Oz. so I have a 4 day break - 6GW8 (ECL86) version of a "Tiny Terror" to finish off for the IT Manager at work. Since its a little lower in power than a TT I've called it the "Ankle Biter". I'm running equal 100K anode loads resistors with 47K tail on that.