Vox AC 100/2 schematic and some questions

[pompeiisneaks]

So I'm thinking I may repurpose the AC100CPH taht is a total tank and us the iron and sockets etc to build an old Voc AC100 from back in the '60's I've taken a few different ones and looked them over, and found the Vox AC100/2 that seems to be a second revision in about '65.  I've made a schematic of it with ExpressSCH and am going to attach the schematic.  I'd love for people to review it and see that it matches the schematic I copied from here: http://el34world.com/charts/Schematics/files/Vox/Vox_ac100.pdf

There are a couple questions though. 

1. I know what a Brimstor is, and the other schematic calls for one.  Do I need one?  Its supposed to slow down power up and reduce impact on the tubes at power up, but is that really needed?  Are they even available anymore?  If not and needed, what can I replace it with?  If so, where do I find them? 
2. In the original schematic, just below the OT is a note that I don't get that says "Anode to anode IMP"?  and has an arrow pointing up towards the center tab of the OT tube side... what is that?
3. On the OT speaker side, near the 8Ohm winding I swee L/S or something similar, but no clue what it means?

Otherwise, could people give the schematic a gander and tell me if it looks good? 

I had a few annoying things with this. I'm using the latest version, and Doug's custom things like tubes etc, and they don't line up to grid, so I'd ahve to turn off grid and manually keep moving to try and straighten out the lines, quite a PITA if you ask me :)

I also simplified the PT to just 120, I think the PT may have 120 and 240 so I could switch that in my case, but I thought I'd just leave that as is.

Thanks,

Edit: Updated the schematic to reflect actual build I've done, and including a layout diagram. 

~Phil

[sluckey]

I know what a Brimstor is, and the other schematic calls for one.  Do I need one?

I'd just leave it out.

just below the OT is a note that I don't get that says "Anode to anode IMP"?  and has an arrow pointing up towards the center tab of the OT tube side... what is that?

The note actually says "Anode to anode IMP.edance 1.75KΩ". The arrow points to the primary of the OT. So, the OT primary impedance is 1.75KΩ. Just about right for parallel P/P EL34s.

On the OT speaker side, near the 8Ohm winding I swee L/S or something similar, but no clue what it means?

L/S probably means Loud Speaker.

[sluckey]

You should put a dot everywhere that two lines intersect if there is a connection. Use no dot when lines intersect but there is no connection. Otherwise, how do you know if it should be connected or not?

Drawing schematics is a good way to begin understanding circuits. You get better and better the more you draw.

[pompeiisneaks]

Thanks, I thought I did that in the schematic, there was no dot where they didn't combine and a dot where they did, but maybe the dots in the application are a bit small/hard to see?  They don't seem to do the little 'u' when they cross like some seem to . 

Thanks for the info. 

[jazbo8]

So I'm thinking I may repurpose the AC100CPH taht is a total tank...

What does that mean? Does it not sound good?

[pompeiisneaks]

Oh it was damaged, so I was going to try and repair the pcb, but the traces are crap, everything is falling apart on it, I've tried sending for a replacement board, but everything I've read says the design is horrid and they are a nightmare to keep running.  I'm trying to find out if there are any other replacement PCB's for it so I can just swap in the one that's damaged, and then go from there, but so far, no luck, the schematic doesn't relate the parts list to actual images on the schematic.  The board that's gone out has power coming in and has a regular failure somehow where the input protection area blows things out left and right (LM337,LM317, etc.)  I have another thread on my attempts to repair that, but its not going well.   So I'm thinking of just using the Transformers, sockets and chassis to make the AC100 on a turret board like it should have been done :)

[jazbo8]

I see, PCB's and guitar amps often do not live well together... I'll have to look up the schematic of the CPH to see just what makes it a horrid design.

[pompeiisneaks]

Here's the thread I started on that, there's a link to the schematic for the ac100cph: http://el34world.com/Forum/index.php?topic=19283.0

[jazbo8]

Man, that is a complicated design with many bells and whistles, by comparison the 100/2 seems almost primitive, perhaps that is a good thing.

[pompeiisneaks]

yeah but a lot of the complexity doesn't make sense. at least per some of the forum responses I got (and get) on that thread.  If you look at some of it, it makes sense.  Making the reverb all solid state is a lot cheaper, and so they can save some money on a simple electronics pcb setup instead of tube and reverb transformer etc.  but then some of the other parts are just crazy. Read the comments in that thread about how odd it is.  I didn't realize it due to my lack of experience but now I think I'm getting it. 

One thing I've learned is I have a long way to go to be able to effectively desolder on some of these tiny component circuit boards without destroying traces.  I don't know if it is my bad skill or the horrible quality of the PCB, but I really screwed it up :P

[sluckey]

but maybe the dots in the application are a bit small/hard to see?

Ah, I see them when I really zoom in. They appear as small gray rectangles.

[pompeiisneaks]

Yup I'm not a huge fan of how the application does things, but all in all of the other schematic apps I've used, its been pretty simple.

[DummyLoad]

when in express sch.


you can change the work space size by changing page size.


click file
click page size...
the largest is "ANSI C": the smallest is "letter".
i usually work in "legal" or "tabloid" with a schematic as complex as this one. tabloid is also "ANSI B" - 11" x 17" 


i attached your draft with legal size page. you can un-clutter the PS. :-) 


the DOT represents a connection - if two lines cross with no DOT then there's no connection, in express SCH that's the convention. some CAD programs use a jump, some break the line to signify the nodes are not connected. what's a node? for example in your schematic: it's the wire or wires that connect C1 R6 R5; this is a node or net. the list of node(s)/net(s) is called a netlist. really only useful to know if you are going to layout a PCB.


suggestions:
click view
click options
in view tab - click inches, change the snap to grid and grid spacing to .025 - now make custom parts. 


--pete

[pompeiisneaks]

Cool thanks, I was trying to keep it sized for my printer which can only do 8 1/2" x 11" so yeah that would make it have more room I'm sure.  This is going to be my starting point, I plan on at least putting in reverb on this one though, using the  typical circuit that I've seen on the forum here.  (The only concern I have about that I'll bring up in another thread, the reverb is more for SS setup not Tube, so I don't know if I can pull that off easily or if I should try to go SS anyway for the reverb?, more on that later if I do go forward with this gutting/conversion)

[DummyLoad]

  > Cool thanks, I was trying to keep it sized for my printer which can only do 8 1/2" x 11"

you can scale down to fit page when you print.

tube reverb would be easier to implement with p2p wiring + IMO sounds better.

--pete

[pompeiisneaks]

Due to Pete's suggestion to go bigger, and my additiona need of adding reverb, I've got a bigger badder version with the typical one circuit reverb I've seen here before, but I don't know if this will work.  V1a is all that is used on this circuit of a ECC82, will that be fine for the first half of a valve on the reverb design?  The chassis I have has holes pre drilled for 4 preamp tubes so I can move them all down one and use half of a V3a on the reverb, output, would that be fine as an ECC83/12AX7?   Additionally, what would the impact be on the circuit in total if I just replaced all tubes with 12AX7's?  I know they have a lot more gain than the 12AU7, and I've seen most reverb circuits seem to lead off with either a 12AU7 or 12AY7 due to being a bit less gain, but I am just wondering...

Also, the circuit I've gotten came from this thread: http://el34world.com/Forum/index.php?topic=630.0 but I've seen several very similar variations.  Two other things to ask about related to it:

1. I see this circuit has a 100K which I've copied, but I saw another at 330k.  Does that value matter much?  It seems to be just to favor sending some of the signal through the reverb loop. 
2. I see the C and B connectors here but they connect in to a completely different circuit.  How do I choose the optimal points to link them into the B+ rail?

The reverb tank I have that is supposed to be a direct replacement is a Ruby Tubes 3EB2C1B which per the chart: http://el34world.com/charts/images/TANKCHART.gif doesn't exist, per the type '3' not sure where that sits, its a short 3 spring tank though, but with the other values that woudl give me:

E 600 Ohm Input Impedance,
C 2250 Ohm Output Impedance,
2 Medium Decay Time,
C Input Insulated/Output Grounded,
1 No Lock
B Horizontal Open Side Down ( which seems odd as this chassis obviously sits on the bottom, so I would guess they mount it horizontal with the open side up to the top of the cabinet?)

Anyway, any insight would be helpful.  It is possible I'll just get the right parts here from Vox/Korg and reassemble it and what not, but I'd love to build this amp if not.

[sluckey]

The reverb driver is typically supplied from the same B+ node that feeds the power tube screens (usually Node B). The reverb recovery is typically supplied from the same B+ node that feeds the preamp tubes (usually Node D).

That 600Ω input reverb tank will not work. The tank needs to have a 8-10Ω input impedance. 2.2KΩ is fine for the output.

Why do you want to regulate the bias voltage at -35VDC? I'd much rather have a pot to adjust the bias.

[pompeiisneaks]

sluckey,

Thanks, damn it was used,  Ithink, as a solid state part of the old circuit, thus why I have it.  I guess that means either no reverb, or I go for a new reverb tank. 

The -35V bias was part of the original schematic, I definitely could add a bias adjustment mod to the circuit, I'll have to look at that.  What about as far as the mention of using a 12AX7 instead of the 12AU7 in the schematic everwhere?  Would that be fine?  Do I want a different specific reverb pan taht would fit within those parameters?  I've not seeen a good write up on how to determine the right reverb transformer, tube and pan combo, or I just didn't look hard enough.

~Phil

[sluckey]

Short answer... Just get a 4AB3C1B tank.

Download the attachment and save it on your HD, your phone, your thumb drive, etc. It contains just about everything you'll ever want to know about Accutronics tanks. Easy to read also.

As for the tubes, just try it and see. Use the ones you like best. You won't hurt anything.

[pompeiisneaks]

Excellent, I'll read that, thanks.

I've just finished a mod with a bias pot (I still left the -35v there for reference) but I think that I may need to lower the R21/R22 now with the bias pot in line right?  Maybe even a higher bias pot of 50kl instead?  Let me know if the modification makes sense.  (I also added 4 1ohm resistors to the pin 1-8 lines for obvious reasons).

Let me know if this makes sense or what I'm doing way wrong. 

Thanks for the update on the tubes.  I'll give that a try.  Also, it looks like I'm going to be building this circuit after all, if the Transformers in the Vox AC100cph aren't too hot, (It seems they are 497V right after the bridge rectifier and 488V after the choke and first stage caps, but the schematic for this wants 450 volts.  I guess I can just add in a resistor shortly thereafter to lower voltage a bit no? Or do I just leave them as is and adjust the two resistors like I've done on the Vox I have now to make sure the plate current isn't too hot and I'm fine with whatever voltages hit the tubes?

[pompeiisneaks]

Helps if I actually attach the new schematic :)

[sluckey]

That bias still won't work as you expect. Just use this similar circuit...

[pompeiisneaks]

So basically everything on that bottom section out, and reverse this because mines 180 opposite, right?  I'll get on that, thanks.

~Phil

[pompeiisneaks]

I "think" I got this right.  By the way, the board I have now has odd 2x20k types, one for each pair, seems a bit odd, is that a balance instead of bias pot? Could I use one of those here, or would it just be best to get a 50k. 

[sluckey]

Move C15 to the junction of the diode and the 15K resistor. And flip both caps because polarity is wrong.

[pompeiisneaks]

Ok done, I also think the right most C16 was also off, so I reversed that as well.  Added in my A-D power supply points.  I think those should be right. 

Thanks for the help!

~Phil

[sluckey]

C16 is not polarized. But notice it is a special Class X cap.

Connect the wiper of the bias pot to one of the end lugs. Then you're good to go.

[pompeiisneaks]

Not sure I know what you mean by 'one of the end lugs' does this look right?

[sluckey]

this...

[pompeiisneaks]

Ok cool, thanks, I'll fix that later.  I don't see any caps on Hoffman's site that say Class X.  I'm also having no luck finding them online.  I usually use Digikey if I can't get it from Doug, any pointers there?

[DummyLoad]

http://www.mouser.com/new/Kemet-Electronics/KEMET_XY_Film_Caps/


click buy X caps.


--pete

[pompeiisneaks]

Thanks!

[pompeiisneaks]

Okay, I've got quite a few more pics here of my layout diagrams, Please if someone has some time to review them, I'd greatly appreciate it :)  Let me know if anything seems odd, wrong or just not optimal.  Also, I'm breaking it out into three boards because I have the area a bit tight with the socket board kinda filling up a lot of the thing.  I've debated removing the existing one and just installing new sockets, but I'm trying to reuse what I can here.  At any rate, It currently has three boards, so I figured I could do three as well.  That way I can fit it better.  I may make the bias/power section one board, not sure yet, but that won't change the layout much there.

[pompeiisneaks]

ahhh bummer, no takers :(  I know its a bit of a mess heh. 

[Willabe]

Well, you don't have any screen R's on the power tubes.

I check my layout drawing against my schematic like most/all the guys do.

Start at the input jack and 1 by 1, I use a marker/highlighter and as I confirm the part/wire on the layout it is as on the schematic, I highlight that part/wire on the schematic.

You haven't shown where the PT and OT will be placed on the chassis and where you'll bring their wires through. The better CAD programs let you do layers that can be seen 1 at a time or super imposed on top of each other. That's very helpful for seeing where the iron will go on the outside of the chassis. 

I do my layout drawings to scale so I am sure things will fit. I measure every part and then make that part in the CAD program. It's a lot of work but it's the only way I know of that will make sure it will all fit/work.

That's a big amp and there's lots that could go wrong. 

[Willabe]

Here's a link for a couple of my layout drawings that I made with Sch.


http://el34world.com/Forum/index.php?topic=14835.msg146807#msg146807

You might need to down load this program to see them?

http://el34world.com/Forum/index.php?topic=590.0

[pompeiisneaks]

Thanks for the pointers, I was trying mostly to make sure the boards themselves were laid out right with respect to the turret positions etc.

which 'screen' do you mean, the pin 1 or pin 7?  There are definitely some on the board for 1, and 7 was done that way on the above linked schematic I've done in EasySCH.  Not sure I follow what you mean there?  (I am correct that there are two grids in an EL84, Screen grid and control grid?)  Maybe I've got the wrong kind of tube diagram here and I'm missing what you mean in that way? (note on the diagram and on the schematic, I have R23 to ground off the point where the pin 7 ties into B and the choke output side, is that what you mean?  Did I draw that wrong or in the wrong place?)

I'll give it a go with the highlighter though and see if I see I'm missing anything.

~Phil

[Willabe]

AC100 uses 4xEL34 not EL84.

They both are pentodes and both have 3 grids as all pentodes do.

Grid 1 = control/input grid, grid 2 = screen grid, grid 3 = suppressor grid.

 

[pompeiisneaks]

Doh sorry I have a Vox AC30 that uses the el84's and I keep getting them confused hehe, sorry, as for the third grid, looking at the schematic, it seems that grids 1 and 3 (pin 1(grid 3) is tied into pin 8 and down to earth with a 1 ohm per the schematic I copied, and I have those on the schematic and layout) have resistors, and the other grid goes directly into B+ line with nothing (per the original schematic as well).  Is there something else I'm missing?

I still don't intuitively know the pinouts and grid assignments etc, if you look at the pdf in my first post, that's what I used to create the schematic I did in expresssch.

[Willabe]

Is there something else I'm missing?

Yes. Power tubes need screen grid (grid 2) resistors to limit the screens current.

The control grid (grid 1) should have a 'grid stopper' on it.

Suppressor grid (grid 3) is usually internally tied to the K.   

[pompeiisneaks]

ok now I'm really confused, the mullard EL34 you showed has 8 pins, the one you just showed has 9 (like my el84's in the AC30).  I'm attaching a pic from the original schematic, pins 1-8 connect to ground with no resistor (but I added the 1 ohm so I can measure current better) pin 4 has resistors coming in and pins 5 on either side tie directly into the B+ with no resistor.  Are you saying the schematic isn't right and hte pin 5 from the image should have resistors?  If so, how do I determine their rating?

~Phil

[pompeiisneaks]

Sorry I think I just crossed pins 4 and 5, ping 5 is the input right... 4 is the grid that goes to the B+

[sluckey]

Yes. Power tubes need screen grid (grid 2) resistors to limit the screens current.

Many old design amps did not use a screen grid resistor. The Vox AC 100 is one such amp. Fender 5E3 is another example.

[pompeiisneaks]

Should I add one?  Or will it be okay without it?  If so what is a good value?

[pompeiisneaks]

Oh one other question, I've seen other builds, including my AC30 that have resistors to ground for the heaters, is that also a good idea?  Required?

[Willabe]

ok now I'm really confused,......

So am I.

the mullard EL34 you showed has 8 pins, the one you just showed has 9 (like my el84's in the AC30).  I'm attaching a pic from the original schematic,.......

Are you saying the schematic isn't right........

The schematic you just posted labels the 2 power tubes as EL34's. They show 8 pins.

That can't be from an AC30, has to be from an AC50 (2xEL34) or AC100 (2xEL34).

[Willabe]

Oh one other question, I've seen other builds, including my AC30 that have resistors to ground for the heaters, is that also a good idea?  Required?

The heaters need either a CT to ground from the heater wind OR a faux CT/2x100 ohm to ground so the amp doesn't buzz.

[Willabe]

Yes. Power tubes need screen grid (grid 2) resistors to limit the screens current.

Many old design amps did not use a screen grid resistor. The Vox AC 100 is one such amp. Fender 5E3 is another example.

Yes, I should have explained that.

[Willabe]

The schamtic you posted in the 1st post of this thread is a Vox AC100.

And I thought the amp you bought and gutted for this rebuild was a Vox AC100 with 4xEL34's?   

[pompeiisneaks]

Both are absolutely correct statements.  The vox ac100cph uses 4xEL34's the '65 vox ac100 uses 4xEL34's ... :)  the both use the same output tubes.  I'm glad I'm not the only one that is confuse heheheh... I do appreciate the help, just reassure me that as a noob I'm not totally lost :D  I probably didn't make things very clear.  I accidentally mentioned EL84's which are the tubes in my previously built AC30 clone.  It has nothing to do with this thread whatsoever, so I apologize for getting my EL's crossed