A really big project, Vox AC30/4 clone

[pompeiisneaks]

So I've spent a LOT of hours on this, because the Vox AC30/4 (or AC34 0A/032 schematic) online is SO blurry/hard to read, but I think this schematic looks pretty close.  I'm thinking I'd like to build this hairy beast, but wanted the guru's eyes on the schematic, how does it look etc? 

First I'm looking at this schematic:
http://www.blueguitar.org/new/schem/vox/ac34colr.jpg (MAN IT SUCKS)
I also found the AC15 schematics a bit more readable, but can see that some values are different, so I've been comparing the two back and forth.  These are the two AC15 versions I've looked at:
http://el34world.com/charts/Schematics/files/Vox/Vox_ac151960.pdf, (or another like it) http://www.drtube.com/schematics/vox/ac151960.gif
also http://www.drtube.com/schematics/vox/ac15fact.gif

These all seem to be quite similar schematics with some slight changes.  I've read that the AC30/4 is pretty sought after with the EF86 preamp tube etc, and the vib/trem is supposed to be pretty damn nifty.  Therefore I'm wondering if I could build clones of these beautifully complex beasts. 

Any insight, comments, build suggestions, etc I'd greatly appreciate.  I'd also love any second set of eyes that can look for things I've miswired or mislabeled etc.  The main normal circuit seems pretty cool, straightforward and I like the top boost switch.  Just the EF86, into a PI into the 4 EL84's. 

Let me know your thoughts.

Edit: I'll be keeping the schematic updated as I go.

Schematic Change 1: Added the missing 220k resistors to ground for the power tubes.  Also added the missing top-cut post PI.
Schematic Change 2: put the cap on the output of the EF86 before the 1M resistor.
Schematic Change 3: fixed cap locations on C, D, fixed missing section for Tremolo Depth. 
Schematic Change 4: no longer updating, instead adding Sluckey's version and his board layout.  Keeping my old one just for giggles.

~Phil

[sluckey]

Several of the members have built this 1960 AC-15...

     http://sluckeyamps.com/VAC15/ac15.htm

There's a very legible schematic at the bottom of the page.

[pompeiisneaks]

Steve,

yeah I've looked over that page quite a few times, your detail on it is incredible.  I just was wanting to do the AC30/4 since its a pretty rare amp and is often talked about wistfully in some of the amp forums/facebook groups etc that I frequent.  For example, subtle differences exist like the cathode resistor on the first vib/trem has a 2.2k and 1.5k on each half in the AC15 but has a 2.2k on both in the AC30 etc.  Just a few locations here and there where things were different.  (I'm not sure why, but that's part of why I'm trying to rescue the old illegible schematic).  I'll have to go over the schematic on your page though with this one because that one is SO much easier to read than the others I linked!

I also definitely like the layout you've got there.  Id didn't realize there was a rectifier so small as that EZ81, pretty cool.  It made me realize I'm missing the name of the tube on the AC30/4, that ones a GZ34, I'll fix that right now :P

~Phil

[pompeiisneaks]

I've been looking over your layout and the AC15 schematic you posted and it does look almost identical minus a few minor value changes for components, so I think I def can use your layout.  What do you think would be wiser:

1. Modify the layout in the power section to support all 4 tubes
2. add a couple terminal strips for the two additional power tubes so the boards are pretty much 'interchangable'?  (I see only the need for the two 1.5k grid resistors and the 100 ohm screen resistor, so that's the only two changes needed... .

I've done a sloppy image mod to show how I'd pull lines to the different terminal strips that connect to the two additional tubes for the 4 I mentioned above.  of course the OT connections just feed from the one tube to the next for each half of the OT side. (I've done this on my AC30 I built from Doug's schematic, and my AC100 I did). 

For my image, basically Pin 2 for the new tubes would come off each red line but the resistor would be between the terminal strips and the socket and thne I'd connect the wire to the TS.  Similarly for the 100 ohms in blue, I'd wire in the new TS connnections from tube to TS to  wired connection on blue.

What sounds 'right' or 'best'?

Edit:  Of course I can also just jumper the grounds from each of the extra tubes to the associated tube before bringing those in as well.

~Phil

[sluckey]

What you suggest will work. That's not how I would do it though. I'd just add 1" to the board and mount those 4 resistors neatly and logically on the board. Very easy to do.

[pompeiisneaks]

The only problem is that I don't know if there is a way to do this on Doug's default layouts, I may have to manually redo your layout from scratch.  Maybe I'll ping Doug via email and ask if he can accomodate this and just make it Sluckey's AC30 :)  I still need to finish auditing what caps/resistors are different between the AC15 and the AC30, its probably only like 3, but I'm going for 'as close as possible' to the original.

Thanks,

~Phil

[sluckey]

I can give you the vsd file and you can edit as you wish. Then you can either make the board yourself or give Doug the edited file so he can make it for you. Let me know.

[pompeiisneaks]

Welp I've gone over the whole thing and the AC15 is identical EXCEPT for that one cathode resistor that's for V1, second half of the triode.  The AC15 has 1.5k and the AC30 uses 2.2k. 

Other than that it's identical.  I think that the 1.5k vs 2.2k in the cathode resistor tends to bias the AC30 more clean right?  So if you like more drive doing it as 1.5k would likely be better no?

I think I'd like the vsd if you can pm it to me, thanks Steve, you rock! :)

 

~Phil

[sluckey]

This is what I had in mind for lengthening the board...

[pompeiisneaks]

See how you are, just go and do it for me ;)  Yeah that's what I'd expected as well.  Doug, want to make that change? :)

Don't think calling his name in posts does it though lol.

~Phil

[EL34]

I did not have time to read all the text above


A quick look at Steve's PDf looks like you just added two more resistors to the end of the board next to the big cathode resistor?
http://el34world.com/Forum/index.php?topic=17184.0

[sluckey]

A quick look at Steve's PDf looks like you just added two more resistors to the end of the board next to the big cathode resistor?

I also added two 1500Ω grid stoppers for a total of four. Follow the 4 green wires from the sockets back to the board. This required adding 1" to the length of the board and sliding some stuff around.

[EL34]

Ok, I see them now


I can make the board without too much trouble

[pompeiisneaks]

Excellent, I hope to get that project rolling pretty soon here, maybe first part of June.

~Phil

[sluckey]

Here's an updated drawing that includes the drill guide and jumper guide...

[G._Hoffman]

So I'm finishing up an AC-30 with an EF86 channel.  I'd be happy to share my schematic/layout, though I'm not sure how easy it is to read if you don't have a biggish printer (13"X19").  I've tried making it without the vibe/trem, and it isn't right.  This thing sounds right, even through my less than appropriate test bench speaker. 

This is not a simple amp, though.  Not even close.  With care, it is doable, but it sure ain't quick!


Gabriel

[pompeiisneaks]

Gabriel, thanks that would be great.  Any data gathering would really help.  I have built Doug's version with the 12ax7 and really dig the thing, but hope this one will sound interesting.

Phil

[G._Hoffman]

Gabriel, thanks that would be great.  Any data gathering would really help.  I have built Doug's version with the 12ax7 and really dig the thing, but hope this one will sound interesting.

Phil

Here you go.  I'm not sure if the layout will be all that much help, because the PDF printed out with very thin lines so it is kinda hard to see the colors, but with the schematic it should get you there.  The big thing you'll notice is I added a bunch of extra sections to the power supply, and I distributed the power supply through out the amp.  If you have any questions, feel free to ask.  And for what it's worth, the little bit I've heard the amp it sounds great, and is the quietest amp I've made.


Gabriel

[pompeiisneaks]

Gabriel, that's great thanks!,
Did vox ever make an ef86 version of the vox a ac30/6? Or did you take the two (ac30/4) and meld them?

Phil

[kagliostro]

As far as I can know only AC15 had a pentode in the preamp (1960 version)

the 1959 version of the AC15 had two, 1 with the ef86 and one with 1/2 of an ECF82

Franco

[pompeiisneaks]

No I know the AC30/4 had an EF86 Pentode in the normal channel input, that's what I'm planning on building here.  (I may even try a hybrid AC30/4/6 so that it has the top boost, or maybe a switch to enable top boost etc.  not sure yet. )   Due to vacation I'm going to hold off on the build for a bit, this month I'm going to recap my Blues Deluxe instead, but this is my next major project.

~Phil

[kagliostro]

You've put a flea in my ear

but I'm not able to find any trace of an AC30 with EF86 pentode in the preamp

Franco

EDIT: You are right, there was an AC30 with an EF86 pentode on the preamp, the AC30H2 / AC30H2L


http://forum.cxem.net/index.php?app=core&module=attach§ion=attach&attach_id=236890

[sluckey]

K, look at this rare bird...

     http://www.voxshowroom.com/uk/amp/ac30-4.html

[kagliostro]

Thanks Steve

I've never seen the schematic of that particoular AC30/4, those I've seen are without the ef86 tube (1960 -->)

Franco

[pompeiisneaks]

Franco,

It is here;

http://www.blueguitar.org/new/schem/vox/ac34colr.jpg

often listed as vox AC34 for 30/4.  it is REALLY hard to read and what I based my schematic on.  its called 4 due to having 4 input jacks the 30/6 has 6 and moved to the 12AX7.  The 30/4 was based upon the AC15, so Sluckey's amp is almost identical except for a few more power tubes.  (I found one other cathode resistor that's different as well, but that's it). 

~Phil

[kagliostro]

Thanks Pompeiisneaks

and ... yes, That schematic is really difficult to be read, I've found a copy of it on my archive, but my copy is also worse

so I bury it and didn't remember

Franco

[G._Hoffman]

Gabriel, that's great thanks!,
Did vox ever make an ef86 version of the vox a ac30/6? Or did you take the two (ac30/4) and meld them?

Phil

My starting point was the Weber schematic and layout, with a lot of references to old Vox schematics.  So, 12AX7's.  I added the EF86 when just about everything was done, because I had one laying around, and I thought it would be cool (and because it is the first tube in the amp, it was pretty easy to shoe horn into the board).  I just pulled up an AC15 schematic, and pulled pretty much everything for the EF86 from there (I might have used a larger screen resistor, I don't remember - I drew the thing about two years ago!!)


Gabriel

[pompeiisneaks]

Ahh gotcha, very nice.  Basically a hybrid of the two.  I'm wondering if doing the same may not be a good idea. 

~Phil

[G._Hoffman]

Ahh gotcha, very nice.  Basically a hybrid of the two.  I'm wondering if doing the same may not be a good idea. 

~Phil

I'm not sure any of this stuff is exactly a good idea.

Gabriel

[pompeiisneaks]

Lol no probably not, its an expensive hobby with no payout ... unless I somehow miraculously become the next Dumble (Unlikely hah)

~Phil

[pompeiisneaks]

The build begins, I'll show off the videos here, unless i should keep those in the video section. 

https://www.youtube.com/watch?v=EEcJ1nanz20&feature=youtu.be

~Phil

[kagliostro]

 


Franco

[VMS]

  ... looking forward to see more of these.

I did notice one mistake on the schematic on the video, the 0.1uf cap on the ef86 should be connected to the other side of the 1M resistor.

[pompeiisneaks]

Oh you're right, thanks!

~Phil

[sluckey]

The 8µF filter cap (node D) for the EF86 is connected wrong. It should be connected to BOTTOM of the 22K.

Same goes for the 8µF (node C) for the PI. It should be connected to BOTTOM of the 22K also.

Node E cap is fine.

And one more thing... On the board layout... Change all power supply node labeling. Node A becomes B, Node B becomes C, Node C becomes D, and node D becomes E. This is due to the slight difference between the AC-15 and AC-30 power supply. Sorry, I didn't catch that back when I modified my layout for you. You may want to make those changes on your vsd file.

[pompeiisneaks]

Will do, thanks.

~Phil

[pompeiisneaks]

Hmm, I don't see any difference on the power, remember this isn't the 30/6 but the 30/4.  Or could you explain to me what I'm missing?  Heres the 30/4: http://www.davidsonamp.com/sv/VX_34.gif

I see output from GZ34, into A which is a 16uF cap, then a choke, then a 16uF cap, then on to the rest of the amp.  That seems identical on the AC15 except the rectifier is a different type. 

Sorry if I'm missing something obvious (actually as crappy as that schematic is, it ISN'T obvious :P)

~Phil

[sluckey]

Your 30/4 feeds the OT CT from the first cap BEFORE the choke. You called that first cap node A.

The AC15 feeds the OT and screens from the second cap AFTER the choke. I called the second cap node A because the first cap was not used.

Did you notice that I only have 4 filter cap nodes but you have 5?

If you don't at least make pencil changes on my layout, you 'may' wire it wrong later on. Also remove the OT connection to my node  A when you change it to your node B.

[pompeiisneaks]

DOH!  I see it now, yup that's definitely different, the CT gets only that first phase of filtering.  Weird, is that going to have a major impact on the tone?  I've heard the power tubes don't need quite as clean power anyway, but I'm used to seeing two nodes with the choke in the middle before it gets to the power tubes. 

I'll update the visio.

~Phil

[sluckey]

Now for the biggie... Your trem oscillator and phase splitter are all tangled up. You don't even have a depth control. I don't see an easy way to fix all that. I suggest you just delete that section and draw it exactly like the orig AC-15 schematic, or even better, copy my visio schematic. This area will become troublesome when you start wiring IAW my layout and still try to keep the circuit as original as possible. BTW, the original circuit had a 3 position speed switch and it also had an internal depth preset trim pot.

[pompeiisneaks]

I've fixed the missing section for the depth, but I've spent so many hours that starting from 0 would be tough on my brain :(,

I don't see what you mean about the trem to the PI, I look that over and I don't see anything wrong on mine compared to the AC15 in yours? 

~Phil

[sluckey]

I've fixed the missing section for the depth

Still not right.

I don't see what you mean about the trem to the PI

Not talking about the PI in the power amp. Talking about the PI that's in the same 12AX7 tube the trem oscillator is in. It's hard to see on the original AC-15 schematic and even harder to see on your sch schematic, mostly because that part of your schematic still has errors.

Look at my Visio schematic. It's drawn out in a logical manner where you can easily see the oscillator circuit and how it connects thru the depth control to the grid of the phase inverter/splitter.

[pompeiisneaks]

okay I'll do so.  I think I may just bite the bullet and try this in visio, I have so many issues with ExpressSch or whatever it is, due to it not locking well to grid on most of the tubes etc, taht it just starts looking hokey anyway.  I just didn't find any valve images in Visio that had the surrounding circles, maybe I'm missing something, I'll give it a try, though.

~Phil

[PRR]

> valve images in Visio that had the surrounding circles

Circles are for mass publication (Sams FotoFacts, etc).

Much small in-house tube documentation omitted circles.

[sluckey]

It's not really important which program you use to draw the schematic. But it is very important that the schematic and layout agree. Otherwise, there will be confusion when you go to actually build the amp, especially when using my layout. My layouts are mainly used to build a board and show how to connect the board to chassis components. IT IS NOT A COMPLETE LAYOUT! You must refer to the schematic to build a complete amp with any of my drawings.

I just didn't find any valve images in Visio that had the surrounding circles, maybe I'm missing something, I'll give it a try, though.

I have circles on all my tubes. They are included in the Visio Professional version. Look under More shapes -> Engineering -> Electrical Engineering -> Semiconductors and Electron tubes (US units). If you don't have Visio Professional, you may not have any of the Engineering shapes. In that case, just open one of my schematics and copy/paste a tube shape to your drawing. Then right click on the tube shape and you can show/hide the envelop and can also set to direct/indirect heaters.

[pompeiisneaks]

I have circles on all my tubes. They are included in the Visio Professional version. Look under More shapes -> Engineering -> Electrical Engineering -> Semiconductors and Electron tubes (US units). If you don't have Visio Professional, you may not have any of the Engineering shapes. In that case, just open one of my schematics and copy/paste a tube shape to your drawing. Then right click on the tube shape and you can show/hide the envelop and can also set to direct/indirect heaters.

I have professional, and I have that section, I get, as mentioned, only the anode, grids and cathode, no envelope.  Per PRR, maybe I should just live with it that way, or copy/paste yours and see what happens there. 

It's definitely odd.  I could use the other app and start from scratch, because I know mines a bit ugly too, but the damn thing's a beast.  I'm not sure I understand why you say your's isn't a complete layout?  I see all the connections from inputs to tubes, back to pots, etc.  What specifically is omitted (otehr than power supply part which is pretty simple for me to gather from mine and isn't 'wrong'.

I still don't see what's wrong with my PI that you're referring to, I compared mine to the AC15 schematic you have in yours, and I only saw the missing section for the depth that I added and was fairly sure was right, but you've indicated isn't.  (sorry, this is probably my biggest schematic ever, so I'm learning... be patient lol).l

Thanks,

~Phil

[sluckey]

I have professional, and I have that section, I get, as mentioned, only the anode, grids and cathode, no envelope.  Per PRR, maybe I should just live with it that way, or copy/paste yours and see what happens there. 

You don't have to live with it. The circles are available just by right clicking on the tube shape.

When I drag a triode onto a drawing it initially has no envelop and doesn't even have a cathode. All that shows is a plate, grid, and heater. Then I right click on the tube shape and select "Show Envelop" and the circle appears. Right click again and select "Set to Indirectly Heated". Now my tube has a separate cathode and heater and also has an envelop (circle). See the attached pic.

Please tell me your Visio does the same. I'm using Visio 2010. Which version are you using?

I still don't see what's wrong with my PI that you're referring to

I hate using ExpressSCH. If I didn't have Visio and some experience with it, I might feel differently. After all, it is free! Attached is a pic of the corrected trem section. There may still be other issues. I quit looking when I spotted this one. That section of your schematic is so twisted around that it makes my head hurt trying to sort it. You really need to swap positions with the triodes or swap positions with all the components that connect to the triodes.

[pompeiisneaks]

I've got 2013 I think.  I didn't realize that option was there, I'll see if I can get it.

I may redo it basically cloning the AC15 schematic, its so much more legible.  (I like yours breaking them out into segments, but I also like one unified schematic if possible)

~Phil

[sluckey]

(I like yours breaking them out into segments, but I also like one unified schematic if possible)

Be patient.

[sluckey]

OK. Here's a one page schematic that agrees with your (hard to read) schematic and my layout. LMK if you find any errors. I can provide the Visio file if you want it.