help with simplifying the AC15 circuit

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help with simplifying the AC15 circuit

Postby voxmusician on Tue Feb 25, 2014 1:56 am

Hello,
so long story short, I have an AC15CC1 from 2008ish that had a broken circuit board. Since I love the amp so much, I'm trying to rebuild it in a handwired turret board fashion using as many of the old parts that I can. However, since this is my first amp build, I think it would be a little too daunting to include reverb and tremolo, so I'd (regretfully) like to cut these out of the circuit to save chassis space and work. But I need help understanding where I can exclude the effects circuit.

Ive attached a picture of the pre-amp and power amp schematic I found online that seems to be exactly like my amp, and Ive added red lines indicating my guess as to where I can exclude the effects, but it really is just a guess.

Could It be as simple as what I thought it would be? Also, is there anything Id have to alter about the power amp stage?, specifically the area between the driver (V2) output and the inputs of the power tubes (V3 and V4) is where I'm most concerned with

Thanks for the help. I hope this is possible as the effects circuit pretty much doubles the component count. If needed I can attach the schematics for the power supply and effects sections, but the power supply seems intuitive (cap the taps on the power transformer that supply the effects circuit), and the effects circuit is what I'm trying to cut out (obviously)...

Please help me get my beloved AC15 running again! thanks again in advance for your input!
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Re: help with simplifying the AC15 circuit

Postby voxmusician on Tue Feb 25, 2014 2:08 am

ok so the forum isnt letting me attach the schematic (says "quota has been reached"???)

Ill try to attach again tomorrow but I can attach a link to the page found it on, and describe where I thought the "cuts" could be made in the circuit...
sorry if its confusing, but the forum should just let me post the picture...

http://www.tdpri.com/forum/amp-central-station/102167-vox-ac15cc-schematics-get-em-here.html

cut #1: Pre amp section- cut between the "reverb send" and the junction it attached to (so basically just cut off the reverb send)
cut #2: Power amp section- cut off the "reverb return" and R11
cut #3: Power amp section- cut off the "tremolo depth", but leave R28?

If this doesnt make sense I apologize, Ill try to get the picture up later.

So, Are my intuitions correct?
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Re: help with simplifying the AC15 circuit

Postby gusfinley on Tue Feb 25, 2014 6:59 am

Lets have a look at your questions:

voxmusician wrote:However, since this is my first amp build, I think it would be a little too daunting to include reverb and tremolo, so I'd (regretfully) like to cut these out of the circuit to save chassis space and work.


There was an AC30HW made in the marshall factory in the 90s that had just what you are looking for top boost channel, tremelo and reverb. I happen to have a schematic and other information if you want to take it all the way. It would require drilling some new holes in the chassis to get there using all tubes, and perhaps a new filament transformer. It could probably be done if you really wanted to do. I actually built a 15W non-reverb version of this into a AC15CC1 cabinet using a custom chassis, so it is definately possible. ( http://enginerdamps.blogspot.com/2008/08/vox-ac15.html )

voxmusician wrote:Could It be as simple as what I thought it would be?


Yup! Its that simple. The reverb and tremelo circuits work in "parallel", so you can just remove them. The reverb is fed from the 10K resistor, R9, in the tone stack (split to bass pot and reverb) and the trem is fed into the point where a fixed biased amp would be biased (junction of 220K resistors R33, R34). Just connect the R33, r34 junction to ground, and don't connect R9 to the reverb section.

voxmusician wrote:Also, is there anything Id have to alter about the power amp stage?, specifically the area between the driver (V2) output and the inputs of the power tubes (V3 and V4) is where I'm most concerned with


Yes, eliminate R30 and connect the junction if R33 + R34 to ground. eliminate all other components for the trem and reverb sections.

You should know that there where some short cuts Vox took when designing the AC15CC1:

1) Solid State Recifier
2) Top-boost only
3) no cathode follower in the tone stack
4) solid state driven effects

The traditional AC30/AC15 circuit has a buffered tone stack by way of a cathode follower. The problem with this in a low-budget amp is that it requires an additional tube stage and therefore an additional tube. This increases the cost. If the cathode follower is added, then there is a "wasted" triode that could be used for somethine else such as a normal channel (AC30CC and AC30C series) or a trem circuit (90s AC30HW). Of course adding more features means adding more components and now you're no longer a budget amp.

Since you are rebuilding the amp you could "correct" any of these cost cutting measures, again, at the expense of additional components. It can be done if you are up to it, you would just have to decide what it was you wanted.

If you decide to build it without effects I would certainly suggest added a 'Cut control' to the top panel. This would be done simply be replacing R23 with a 250K potentiometer.

Also, don't be afraid to buy quality components for your build. It doesn't take much to get better components that the stock ones. Some Mallory 150 series caps will go a long way. Don't be afraid to spend an extra $10 on a few hundred dollar amp!
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Re: help with simplifying the AC15 circuit

Postby voxmusician on Wed Feb 26, 2014 7:10 pm

Thanks for your help! Very informative reply, I appreciate it.

only thing i don't get is when you say eliminate r30, do you possibly mean r28? b/c r30 is in the heart of the tremolo circuit to my knowledge. just wana make sure.

everything else makes sense tho. I think Im going to go for the effects circuit using the ss components it came with unless it becomes too bulky to hand-wire. I will most likely be replacing the caps tho, as many of them are radial.

but If i do exclude the effects i was defiantly looking for something like a tone cut to replace the vacancies in the top, but didn't know how to do it, so thanks for that suggestion as well. Didn't know it was that easy.

btw your custom built is awesome! unfortunately I think chassis modifications are a little out of my reach for now. Your quite lucky you got a chassis built for free!
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Re: help with simplifying the AC15 circuit

Postby gusfinley on Thu Feb 27, 2014 1:05 am

voxmusician wrote:only thing i don't get is when you say eliminate r30, do you possibly mean r28? b/c r30 is in the heart of the tremolo circuit to my knowledge. just wana make sure.


Glad to help, if i can.

According the AC15CC1 service manual Issue ISS3C, R30 (33K) is the dropping resistor supplying power to the trem section. R28 supplies the wiggle to the power amp section.


voxmusician wrote:btw your custom built is awesome! unfortunately I think chassis modifications are a little out of my reach for now. Your quite lucky you got a chassis built for free!


Thanks for the compliments. I'd love to redo this amp, actually, but haven't find a custom chassis source in my new location, or online- and for the record, I bought the steel for the chassis for about $10 from a scrap pile and the guy bent it up and welded it for 'free', though I recall forcing him to accept a 20 dollar bill for his time.
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Re: help with simplifying the AC15 circuit

Postby voxmusician on Thu Feb 27, 2014 2:08 am

ok I see. I was confused cuz I was planning on dropping the entire effects circuit which includes r30 anyways so I wasn't sure why you specified that particular resistor.

edit:wait, i think i see why you specified that one, I didnt realize the trem circuit was also powered by the HT supply, makes sense now.

Well after trying to de-solder the IC's from the circuit board (borderline IMPOSSIBLE) I think Im guna just do the basic amp for now and add in the effects circuit once I get the thing running. And since components are so cheap Im definitely guna buy fresh caps/res's.

I know its a whole long debate but Id also like to ask you if you have an opinion on carbon comp resistors? by the looks of the layout pic in that link you posted you used carbon comps but there too small to be sure. Can you really detect a "sweeter" tone from them? (assuming you use them that is)

you dont have to explain the measurable differences Ive read up on that a bit, just wondering if you have in person experience with them
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Re: help with simplifying the AC15 circuit

Postby voxmusician on Thu Feb 27, 2014 3:12 am

oh and one last question if its not too much trouble...

In my amp, the circuit board's star ground passes through two opposite diodes (D3 and D4), a .22 uF/63V cap (C56) and a 10 ohm/1W resistor (R68), all in parallel before connecting to the chassis ground. My schematic however doesnt have this but ive seem other schematics that do have this. What does this do (a filter of some sort???) and do I need to include it?
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Re: help with simplifying the AC15 circuit

Postby Unit_1 on Thu Feb 27, 2014 5:03 am

voxmusician wrote:ok so the forum isnt letting me attach the schematic (says "quota has been reached"???)



best to post pics like this (just click on QUOTE to see the source code I used to display your pic)

Image
Image
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Re: help with simplifying the AC15 circuit

Postby gusfinley on Thu Feb 27, 2014 5:56 am

voxmusician wrote:In my amp, the circuit board's star ground passes through two opposite diodes (D3 and D4), a .22 uF/63V cap (C56) and a 10 ohm/1W resistor (R68), all in parallel before connecting to the chassis ground. My schematic however doesnt have this but ive seem other schematics that do have this. What does this do (a filter of some sort???) and do I need to include it?


It looks like they are trying to isolate the chassis ground from the signal ground. Why is the question. Was it prone to ground loops? Is this required to meet european CE requirements (like the US's UL tests). In recent years vox has done quite a bit with ferrite beads and and such as if they are trying to qualify the amps for EMI specs.

They are doing some wierd stuff in their low voltage solid state power supply. There's no bridge rectifier there (looks like a positive and negative voltage doubler) and no center tap to the transformer winding, perhaps this is why they are isolating the 'ground reference' for this circuit. You'll notice one of the windings on this power transformer taps (W21) is connected directly to ground. Not the way I would have done it for sure.

I don't think you'll need this in your rebuilt amp.
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