VOX AC15C1 Schematics

[Bug2222]

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[inkomodo.dragon]

I don't have the actual values he used but if you keep reading one more sentence it will eventually make sence...

"I also replaced all the valve rectifiers (GZ34) with solid state rectifiers and upgraded the smoothing capacitors. This increased the amp power stage headroom giving a much clearer sound."

Let's start by saying that Pete Cornish doesn't look at tone as most guitar players do today. It is clear by looking at his work that he doesn't like Sag or any other "defect" even if we like how it sounds. Hi treats all signals as Hi-Fi audio.

Having that in mind, solid state rectification means higher B+ (higher headroom) and less heat (remember he was trying to make the amp more roadworthy) but not necessarily a "clearer sound".

As for the filtering caps, if you try a stock AC30 circuit with SS rectification a Treble Booster and a Red Special you'll hear that Brian's tone is there but there's something wrong with the low frequencies. Somehow they are muffled and lack focus.
Pete says he "upgraded" the caps and to me (considering his way of thinking) means that he raised their values so the low frequencies tighten up a bit ending up sounding more transparent and Hi-Fi-ish (¿?). So i just did that, raised the preamp filtering that was way too low for any kind of low end definition and the result was instant "Live Killers" tone.

The higher filtering will make it sound like it's somehow less distorted (at kind of is) so a higher gain booster (Rangemaster, TB-83) will work like a charm.

Hope that helps

[Bug2222]

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[inkomodo.dragon]

Yes, I did that to my AC15C1 and the result was perfect for Brian Queen-era tones using a BM Red Special with stock pickups and a Home made Treble Booster. I used this circuit for the TB:


I think the transistor I used was a 2N3904 or something like that.

When you speak about preamp filtering in the normal channel you can only mean the electrolytic filter capacitor C15 with cap C11, or? Which value(s) have you used?

Yes, was referring to C15 in particular. I don't remember the exact value but I'm almost sure I raised it to 22u and that was enough.

Or you tighten up the bass response with the cathode capacitor C7, but here is the TB channel also involved.

I was going for maximum originality so I didn't touch the cathode cap on that stage. Using the power supply filtering cap you can clear up the bass and leave the preamp EQ untouched.
That's not always possible but in this case it was because the filtering was really low to begin with.

One more thing, I found the AC15 sounds rounder and more pleasing if you set the impedance switch to 8 Ohms while using the 16 Ohm speaker. And keep in mind I installed the 1" piece of wood (high frequency diffuser) through the speaker hole like the old amps had. Both this changes make a HUGE difference in tone and response.

[Bug2222]

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[inkomodo.dragon]

I've changed both C15 and C22 to 22u but the last one only affects the TB channel so I think it's not necessary for Brian may applications. I didn't change C11 and I might have forgotten to draw the ceramic you mention but since I don't feel like opening the amp again I will have to check that info next time I change tubes.

I didn't change the input grid stoppers because the actual value on vintage amps (when plugging into input I) is 68k//68k=34k so 33k is closer.
As far as speakers, I know Brian used both Alnico Blues and Greenbacks throughout his Queen years so I feel the change in sound is not big enough to justify the cost.

There is a huge difference between an 8u and a 22u but if you feel you don't need it just don't do it. I felt the bass was loose and undefined, now I love it.

The speaker impedance "trick" is not ment to make the amp sound bigger but to shave some top end fizz. There's a hole argument going around about impedance mismatching and my point of view is that modern OTs are way sturdier than vintage ones so they can take the added stress without problems. Besides it makes this particular amp sound better and that's all I care for. I don't know if the C2 version needs any of this since I've never tried one.

My treble booster has the gain pot (10k) but I wouldn't recommend it. A 6k8 resistor hits the sweet spot of that circuit.

Since you've tried the AC15C2, could you tell me if it sounds a lot different than the 1 speaker version? I'm saving up for one right now and could use some info.

[Bug2222]

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[inkomodo.dragon]

Thanks for the info, I though the bigger cab and twin speakers should make a big difference but just wanted to make sure before I go out and buy another AC15.

Regarding the diffuser, it does all kind of strange phase cancellation stuff because the sound bounces off the wood and goes back to he speaker where it bounces again and comes out "delayed". If you ask me it is one of the main ingredients of theAC30/15 sound. Most people think it kills treble but it actually sweetens it.
If I where you I would just get a piece of wood and stick it in there with some super glue (as I did).

By the way, the filter caps in the preamp section are 10uF/400V not 8uF.

I know, the original circuit wich is the one Brian had on his amps was equipped with 8uf caps.

[Bug2222]

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[tbyrd]

Just got the AC15C1 and am very impressed with the mods on this site. Wondering if anyone can tell me a good place to put an effects loop (between the op amps U1A and U5A possibly?) I use the four cable method currently on my HRD and would like to keep that versatility. Thanks!

[tbyrd]

Anyone know how to add a loop to this amp? Looks like the area marked A on the schematic would be a good spot. Thoughts?

[Bug2222]

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[tbyrd]

Thanks , and I mis-stated the use of 4 cables, I'm using 3 as normal.

[brunomariano]

Hi!

It's really necessary remove the op-amps to perform the 220k plate resistor mod? I'm asking because I like to have a reverb and vibrato, and I use my ac15c1 mostly clean. If the OPAMP clipping starts at volume past half, I still have a lot of volume to play clean at recording or practicing levels, so I'm thinking to make the OPAMP bypass mod switchable. What do you guys think?

[brunomariano]

Bug2222:

You said you used a "Mercury Magnetics VXC Choke 10 Henry (rated at 250 mA and 280 Ohms DCR) ", but the Mercury Magnetics webside said the VSC 10H for AC15 is rated 100mA. Did you ordered a "custom choke" or something?

Another question: I intent to do the same mods as you. Now, do you recomend the exact same components? Can you tell me what's the best store to buy them? I'm from Brazil, and I can cut shipping costs by buing them all at the same place, if possible.

I'm also thinking about made the EQ mod, bright switches and "reverb (OPAMP bypass) switchable.

Thanks!