Vox AC-15 top boost channel project with reverb and tremolo

[tony321owen]

see below

[sluckey]

Let's see what you got.

[jamaio]

Let's see what you got.

Sluckey, he posted the schematic here:

http://el34world.com/Forum/index.php?topic=7957.msg261522#msg261522

[tony321owen]

I am building an amp for my buddy who's a pretty big time touring type fellow. He asked for a Ac-15 top boost type amp with reverb and tremolo in a princeton reverb sized chassis and combo cab. After searching through all the AC-15 schematics I could find and looking into Tubenits blues amp, muchle$$, and Sluckeys AC-15 schematics/November plus . I have come up with this circuit. This is my 4th amp build and by now I am pretty comfortable with the functional inner workings of a tube amp. However now that i'm delving into circuit design as opposed to strictly following the cook book, I have found myself in a few situations where my circuit designs didn't function properly(like the reverb being distorted or the tremolo wave being of an un pleasant shape and intensity.)   
I know that as far as this tube circuitry goes most all the building blocks have already been written and can be found with research on the internet, in forums, books, etc... yet the art of frankensteining circuits together seems to require a deeper understanding of the mathematics. Allowing the designer to choose the correct component values that will ultimately integrate those building blocks together for the desired sound and functionality of the circuit as a whole.

As for my project Im afraid that my understanding in these areas are lacking...
 Is there anyone more experienced than I that can take a quick look at my schematic and put their two cents in primarily about the correct component values for the integration of both the one tube reverb circuit, tremolo circuit, and power distribution design?
Any help is greatly appreciated!!
here are the schematics for my potential build. Preamp+reverb and poweramp+trem

[jamaio]

sorry if ive posted this more than once just learning about how to post/resize images etc

Tony, Sluckey is a master at tube amps, I am sure when he sees your design he will give you some excellent feedback.

[sluckey]

Looks workable to me. V3B needs a resistor from grid to ground.

[sluckey]

Phew! Finally, we're all in the same forum and reading the same thread. And you're not losing your mind. I'm the guilty one that's been moving your thread over to this forum. Your project will get a lot more exposure in this forum. Sorry for the confusion.

[tony321owen]

i fixed the trem circuit and reposted the schematic with the 1m grid leak resistor. Thanks again

[tubenit]

I redrew your schematic with some minor changes.  There is an editable ExpressSCH version that you can use to show whatever edits/mods/changes you make.  PLEASE make note of the differences from your schematic and this one such as:  one input,  330k off  reverb pot,  .01 post LTPI coupling caps,  I added an "enhance cap" to the LTPI entrance plate resistor,  the tremolo is a little different but similar …………… etc...….

Feel free to change it all to what you originally had.  The changes I made aren't there because they "should be", I simply added them cause they were easier for me to draw or made sense to me personally. 

I personally would NOT use that crossline master volume.  I think they sound awful.  My least favorite master volume of any.  I probably would some different values for the bass and mid in the tone stack also.

I am referring to this schematic as an   AC-RV-15

With respect, Tubenit

[tubenit]

DaGeezer used to be a forum moderator and he is the originator of the Tweed Overdrive Special, Tweed BluezMeister, HoSo56 and many other innovative designs. 

He drew up a cathode attenuation system that I used on a few amps like a master volume.  It worked great!  To the best of my memory, it did a wonderful job of retaining the tone of the amp at lower volumes.  This could be a great candidate for your amp idea for you to consider?

Again, I think the crossline master volume really sounds horrible compared to the other options out there. 

The original post is here and if you scroll to the bottom of the thread there is some info on retrieving links that currently don't open:   

http://el34world.com/Forum/index.php?topic=4437.msg38034#msg38034

With respect, Tubenit

[tony321owen]

Awesome! Thanks tubenit! I really appreciate you drawing up that schematic and applying your tweaks!

A few questions:

1: in my builds and modifications, mostly  fender based circuits, I have found bass and low mid response to be overwhelming for stage applications where the collective muddyness of the band is of chief concern. I have tried making coupling caps smaller, cathode bypass caps smaller in the preamp, and speaker swaps. That is the cause of my unusual cap values in the tone stack as opposed to the .1 .047 or the .02 .02 commonly found in many designs.
These values on schematic have on my previous builds allowed me to have a one knob control to tame those bass/low mid frequencies. Maybe there’s a better way to achieve this?Is there a reason why the post pi caps need to be bigger? I know that they are often .1 but that seems to let too much bass through in my experience..

2. Just curious about the the reason for biasing the first preamp stage cooler with the 3.3k as I have always went with the standard 1.5k. Also what does the 33k grid stopper do compared to 68k?

3.whats the enhance cap do?

4. I think I copied the term off one of your older schematics so I will probably go with your newer one if you have had success with it
5.not sure if I will use a master volume or not but thanks for the cool circuit idea:)

Thanks again for your input!

[tubenit]

I think you should put a higher priority of trust to your own ears and what your own experience has taught you.  AND that you should build this amp however you wish.

1)  DaGeezer used similar values in his tone stack in the mini-Bassman which used 6BM8 tubes.  So, you may be on
     to something there.  Typically with EL84 tubes or 6BM8 tubes,  I will use .01 or .02 post LTPI coupling caps, but
     perhaps the .0047 would sound better?

2)  Regarding plate/cathode,  I typically will use  220k/3.3k, 180k/2.7k, 150k/2.2k, 120k/1.8k, 100k/1.5k …. etc...
     Regarding the input grid resistor being 33k,  that is 68k hi & 68k low paralleled for one input jack.

3)  I found the enhance cap smooths out the higher frequencies withOUT eliminating them to my ears. It removes
     the harshness or hash without removing the highs, IME.

4)  Not sure what this refers to ……………

5)  The thing I like about this cathode attenuator that makes it a consideration for this specific build is that there
     is nothing about it that I think would interfere with the tremolo? 

With respect, Tubenit

[tony321owen]

4. Auto correct changed my trem to term.. lol
Aww 3.3k to make up for the 220k plate resistor. Now I see

Yeah every amp circuit behaves a little different in terms of frequency response so that tone stack/ coupling cap values are just a starting point. Then I usually just listen and tweak and listen and tweak....
I’ve never built an amp with el 84s so I don’t really know what to expect..
Mostly been a 6v6, 6l6 up until now

The enhance cap sounds intriguing and if I like it I may put one in my deluxe reverb which seems to have harsh treble no matter what I do to it..

[tubenit]

OK, I was interested enough in this idea to draw up a layout that can be edited/modified to match your schematic.  Partly, because it involves creating a layout board for Vox/Marshall/Bassman topology that has a one tube reverb and tremolo. 

You should presume that there are errors.  And that it is your responsibility to find them.  For example, I know a few things on the schematic don't match the layout such as I could not bring myself to draw a crossline master volume on the layout. 

CHECK for ERRORS!  Compare layout with schematic.  Where there is a discrepancy, go with the schematic.

One advantage of a paralleled turret board approach (like HiWatt uses) is that on "experimental" builds, it allows modifications to be easily made. 

There is both an ExpressSCH schematic and layout attached & they are both editable.

With respect, Tubenit

[tony321owen]

This is awesome and incredibly helpful! thanks for the interest in my project and i'm sure others will benefit from your work in future builds!
My next considerations will be fitting a board in my Princeton reverb chassis and working on how i will layout the filter caps and power distribution network. From my research I have found that filter caps work best as close as possible to the circuit block they are suppling power and filtration to and that cap cans, although useful in some layouts, are to be avoided when possible due to grounding limitations. I also have seen cap cans used in addition to a separate reservoir cap and sometimes in  addition to separate local preamp caps..

For this build  my aim is to create a small amp that is Princeton reverb size for easy touring purposes so space is of chief concern. Maybe i will modify the layout a bit to incorporate some of my filter caps on the board perhaps building the tone stack on a terminal strip next to the pots to make room for a big filter cap.. maybe use a multi can cap for a few of the caps.
i am thinking something like: reservoir cap close to power supply, multi cap can for 2 or three power nodes, and 1 or 2 separate caps for noise sensitive preamp stages..not sure if my logic here makes sense but these are my thoughts anyway..

I have about 11'' length available for the turret board or maybe 12'' if i don't use a cap can and about 2.5'' width space. I found a turret board on antique electronics that has dimensions of 11.75" length and 2.3" width with 29 rows of turrets which may work for me. Tubenits initial layout calls for 26 rows of turrets which leaves me with a few rows to play with..

my next consideration is where to mount power tube cathode resistors/caps and power rail resistors. I may use terminal strips or maybe the extra board space if there is any...

       
   

[tubenit]

Something you might want to consider in this build is using a mosfet CF.  They work great & I have used them with some decent success including one of my amps that I currently play regularly.  You can see an example of how it's used in a schematic, layout and photo here (note reply #21):

http://el34world.com/Forum/index.php?topic=23013.0

This would allow you some options:

1)  paralleled 12AX7 in V1 or a 5879 in V1
2)  more traditional "send" gain stage in the reverb with paralleled tube like a 12AT7.
3)  or a two triode tremolo/vibrato like in a 6G16

IF I were building it,  I would use a mosfet there in the cathode follower position. That does NOT mean that you should do it that way.  It simply has worked well for me.  IF this intrigues you & you can let me know what you might do with the extra triode, I can draw a schematic to show you what I am referring to.

Just a thought to consider ……………………

The other thing I'd say about this amp design is that I would use a stand up PT and a stand up OT to allow more room in the chassis.  The photos in "The Minimalist" link would show examples of that.

With respect, Tubenit

[tony321owen]

My friend recommended using mosfets as well but i dont know much about them. I am intrigued by the notation of a two triode tremolo if indeed it makes it more lush and deep. the 6g16 uses two triodes but is a fixed bias amp. is there any advantages/disadvantages beside freeing up tubes using mosfet followers?
is there a way to design a two triode tremolo for cathode biased amps that still injects into the grids of the power tubes?
also could a mosfet be used as the follower in a 2 triode tremolo?

[tubenit]

I don't see why you can't use a two triode tremolo with a cathode biased amp?

I have not used a mosfet CF with the tremolo but I think this would work?  However, I don't know for sure.

With respect, Tubenit

[tony321owen]

https://el34world.com/Forum/index.php?action=dlattach;topic=23601.0;attach=72459


An example of a mosfet cathode follower for a trem circuit in a cathode biased amp
Not sure how to match it to my circuit because there’s a few differences in injection point and what not

[sluckey]

It's easy to wiggle EL84s. You don't need a CF.

[tony321owen]

Sluckey Would you use the 1 triode tremolo circuit found in this ac 15 build for a similar cathode biased amp that uses 6v6s?
I built a similar wattage 6v6 amp with tremo that doesn’t stand up when compared to my fixed bias Princeton reverb build I did a few years back which has deep glorious trem..
I think I based the tremolo circuit of my less successful weaker sounding trem on a guitarmate traynor circuit ..
Never did sound right, probably didn’t use the right resistor values or something...
Any favorite circuits for this application?

[sluckey]

Sluckey Would you use the 1 triode tremolo circuit found in this ac 15 build for a similar cathode biased amp that uses 6v6s?

Sure. Here's one...

     http://sluckeyamps.com/RCA/Ampeg_J12B.pdf

[tony321owen]

Yes thats what im looking for!

a couple things:

1. is there a specific led to use over others?
2.from my recollection 6SL7  tubes are interchangeable with 12ax7s with minimal component changes?
are there any to be made?

[sluckey]

Use a 5mm red or yellow LED. Very common and produce the correct bias voltage.

Just use the component values shown on the Ampeg schematic.

[tony321owen]

well I am finally getting around to building this amp! I have a question for anyone that knows about filter chokes. Currently I have a big 20 Hy, 100mA DC, D.C.R.= 419 Ohms choke sitting around designed for an ac 30.. will this work well in this smaller ac 15 amp? would love to save a few bucks and use it.
 better filtering, bigger voltage drop... any potential down sides?

thanks for the knowledge!

-Tony

[PRR]

It will work. It is twice the lump you need. But a choke in the hand beats shipping heavy iron.

[tony321owen]

Progress!! It wont be long until this design finally comes together:)

[tony321owen]

So I finally built this amp and it sounds glorious! Thanks Tubenit, Sluckey and everyone else who gave thier input! There are a few noise issues that I need to clear up and I have a feeling they are due to grounding layout! Maybe you guys can help?

I tried to follow Merlins bus bar grounding scheme and this meant that I tried to put filter caps as close as possible to the valve stage they were filtering. Since the reverb gets its B+ from node B which is the same node as the screens I was forced to run long power rail wires to feed the reverb transformer and reverb stage plates since node B is located next to the power tubes.. Im also trying to religiously run ground wires back to negative side of the Filter cap that feeds each stage meaning that If a preamp stage gets its power from a specific node then I made sure that all its grounds terminate at the negative side of of the filter cap connected to that specific node.. The tremolo circuit has the same issue with relatively long B+ supply and ground wires... im getting lots of tremolo ticking/ intstability at high reverb levels and fast tremolo speeds and if i set the amp to the brightest setting and play single coils then there is more hum than there should be!


My question is this.. would it be better ground each stage closer to where it is located on the bus bar instead of trying to be so strict about grounding it to the negative side of the filter cap that supplies its b+?
I know that long B+ . wires and ground wires aren't ideal but if b+ wires must be long is it better to keep ground wires short?

Thanks for the input! Below is a photo of the chassis. The reservoir cap is next to the rectifier tube and the cap that feeds the power tube screens is to the left of the PT.  Scroll up for a clean schematic and preamp layout!

[tubenit]

That looks to be a great build.  Congrats!

Other then the "ticking" with the tremolo, can you be more specific about what you are trying to resolve, please?   Also, it may be helpful to see a photo of the top of the chassis also?

I follow Hoffman's grounding scheme in my amps and I am happy with how well that has worked for me.

I noted not seeing much in the way of shielded wiring? 

With respect, Tubenit

[tony321owen]

I changed over to the hoffman grounding scheme and the hiss, hum and most of the ticking went away! This thing is dripping with tone! The One tube reverb sounds so nice and with the dwell control dialing in reverb is easy! I like it better than the reverb on my princeton reverb build which to me always felt over the top and too wet!

Too bad this build is for a friend because I would sure like to keep it. Im gonna have to build another one day!

Thanks again everyone who helped!

[tubenit]

Thanks for sharing your success story!   

With respect, Tubenit

[tony321owen]

After testing it Looks like I was wrong about being finished with my noise issues..

Changing the grounding scheme did a lot in terms of reducing hiss and hum but playing a single coil pickup with the volume at 5 is pretty un-bearable. There is also some ringing in the higher frequencies when amp is turned up. It sounds like there is a resonant frequency that is feeding back at high levels( might be part of high gain preamp design?)

does the noise that gets worse with single coils sound like RF interference and can I conclude that issue in the first stage if it is effected by the volume knob which follows the plate of the first stage?

The input grid wire is shielded are there other places to shield as well?

How can i turn this thing into a single coil friendly amp?

Also the reverb is so sensitive to shock that sometimes if I tap a knob it sends the reverb tank into a spasmic feedback of fluttery vibration...

Do people shield reverb leads?

[shooter]

The input grid wire is shielded

I'd double check it or just replace with wire for testing.

what happens with nothing plugged in and you start working to Vol10

[tony321owen]

With nothing plugged in, as I turn up the volume towards ten a loud hiss appears. From 5 to ten its Very audible.

With volume at 0 there are two types of hum though they are not terribly loud.. maybe 60cycle and 120cycle.. neither makes the amp unplayable but obviously they are not ideal!

reverb knob makes hum louder as do treble and cut controls...

Maybe I will try rewiring the input to see if the wire got compromised some how during construction..

[Williamblake]

I was looking for a similar oscillation recently and it was the reverb tank picking up mechanical vibration from the power transformer and would start like background rumble and would lead to a high pitched squeal. Also i had the tank pick up mostly tremolo noise when the tank to ground connection desintegrated.

[tubenit]

Hiss will be louder as the amp volume turns up.

Regarding the reverb ………    I use shielded wiring for reverb pot and dwell pot typically.  When you say you tap an amp knob and it sends sound into the reverb tank,  is that with the chassis and reverb tank both inside a cabinet at the same time  OR with the reverb tank and/or the chassis not in a cabinet?  You need to figure out whether this is a mechanical or electrical problem? Is the reverb tank covered?

Have you tried safely with a non-conductive stick like a bamboo chopstick to gently move wires and gently tap on components?   

Can you show us the exact schematic you're using, please?  Have you checked voltages and do they look reasonable?  I don't think of a Vox AC-15 as a high gain amp.

I think a guitar with humbuckers is a realistic and good test for how quiet an amp is.  A guitar with single coils may not be a realistic test? 

Look at the schematic (not layout) I posted in reply #13.  There is an "enchance" cap of 220p across the plate resistor going into the LTPI. I find this helps significantly in reducing high end hash/noise without losing any treble frequencies that I can tell. You might try that.  Sometimes it's a combination of small adjustment adding up that make for a quiet amp.

With respect, Tubenit

[tubenit]

Take a look at this chassis wiring.  It is not neat. It is somewhat messy. It look amateurish & I wired it.  There is a lot going on in a small space. 

The amp is amazingly quiet at idle at the volume I play it at. So quiet, it would be easy to accidently leave it on and leave the room.

IF you look closely, I used shielded wiring on 4 long runs from the front of the chassis back to the tube sockets.  Input, volume, treble pot to FX send jack and the PPIMV. 

The B+ wires are right under the edge of the layout board & the wires from the sockets are angled in a way to not be laying across the B+ wires.   

And the heater wires are floating up inside of against the chassis backside.  I honestly don't know if that makes any difference or not for you amp?  In fact, having them against the chassis might be "better" noise wise?  I will say I tried heater wires once against the chassis and changed it to having them over the sockets because that was quieter for me.

I don't know if these ideas will be useful for you or not?  However, I wanted to point out that my amp wiring is fairly messy but the amp is super super quiet at idle with volumes I play at.  The HoSo56 in a tweed chassis has some similar (not the same) topology from a gain standpoint.

with respect, Tubenit

[tony321owen]

Ok so I added shielded wire to the more noise sensitive parts of the circuit as Tubenit suggested, cleaned up some of my unnecessarily long ground wires and now the amp is very quiet!
The cabinet came last week and so I thought I would post some pics before I give it to my friend who I built it for.
Just gotta say this is the best sounding amp I have ever built and for documentation sake I will post the small modifications I made to the circuit while building and fine tuning.

1. I used a standard 100k plate resistor and 1.5k cathode resistor instead of the 220k/3.3k on the schematic.

2. Changed the 1m vol pot to a 500k to reduce some front end gain which seemed a bit over the top for my taste.

3. Take a look at the layout that Tubenit graciously made for me and you will notice the way the tone stack is wired which varies from the classic designs of most TMB tone stacks.
I see now that it was my mistake on my original schematic which in turn lead me to an interesting discovery. One of my longstanding issues with some of the classic TMB tone stack designs is that I find often that they have too much bass which muddies up the mix while playing with a band or can cause blocking distortion/woofy sounds when the amp is pushed into clipping.
In this "mistake" modification, the Bass capacitor output is moved to the other side of the bass pot to where the middle capacitor meets the 10k resistor to ground.
  This point is also connected to the center lug on the Bass pot which puts 1meg+10k between the treble pot and ground with the "Bass" knob all the way down. this decreases tone-stack loss for the high frequencies which makes the amp break up easier in a chimey kind of way. The pot in this position simultaneously puts 1meg between the the bass/mid frequencies and the treble pot limiting the lows and keeping things tight.
With the bass knob at ten the control acts in an exactly opposite way. This does two things. First, the treble pot now only sees the 10k resistor to ground which makes the tone stack more lossy for high frequencies, and two it allows Bass/mid frequency to flow straight to the treble pot unimpeded giving a gentle bass boost.
I find it to be very subtle and usable alternative to a bass control and one that works well with amps like an ac15 that uses the more sensitive el84s and that often get their perceived midrange "fatness" from clipping the power tubes. This opposes the Fender blackface amps design with less sensitive 6V6/6L6 tubes. I find these tubes to be harder to clip and so BF Fender circuits rely on lots of low end to warm up the glassy highs and scooped Mids that they are known for.  There are many ways to tame bass and I have tried many. This Tone-stack mod in this particular amp really gives it a nice balance and addresses the above issues very successfully. I even "fixed" it when I noticed that it wasn't wired in the classic way but found that the "Mistake" gave the amp the voicing I preferred so I switched it back to the "wrong" way!
  Not sure how it would sound in other amps but I will be trying it. 
The bass knob seems to almost function like a tilt control now..??

4.I got rid of the master volume. After research it seems there are better master volume circuits and for this build it didn't make sense to use one at all.

5. I fine tuned the value on the cut control cap. with some testing I believe I found .0033 to be ideal. My goal was to set the cutoff frequency pretty high so that only the really harsh highs were to be cut while leaving most of the beloved sparkle even with the control fully engaged. This was achieved very successfully!
 I have done some tests sending a sine wave through my amp with a cut control and watched on my scope how the wave became distorted when the cap was too large and the cut was fully engaged. It can sounded like a wet blanket on the amps tone. Its better in my opinion to use the vox style cut control only for really high and harsh highs. Highs and high mids can then be adjusted with the treble knob. The 220pf cap across the phase inverter plate resistor seems to help keep the high end smooth though I still don't understand why I shouldn't put one across the other phase inverter plate resistor as well to balance things out..??

6. Put my power rail filter caps close to the gain stage in which they are filtering.

7. I changed one of the .01 caps in the tremolo circuit to .02 for slower speeds. The tremolo sound fantastic!! Its sooo deep and the speed control really goes as slow and fast as I would ever need. I believe the speed knob is wired backwards on the above layout. changed it and everything functioned fine!


Below are some chassis pics and a sketch of the alternate or "mistake" tonestack wiring I was talking about above.

         

[tony321owen]

more pics

[tony321owen]

Couldn't be happier with the results of this build! Since it was for my friend who tours all year I took more time in the planning stages than I usually do which resulted in a much cleaner build execution with less needed post-build modifying, better layout (thanks Tubenit) and ultimately a better sound! Above are Some pics of the chassis in its current state and what it looks like in its fancy new tweed cab. Now its ready for the road!

[Blackface5f6]

OK, I was interested enough in this idea to draw up a layout that can be edited/modified to match your schematic.  Partly, because it involves creating a layout board for Vox/Marshall/Bassman topology that has a one tube reverb and tremolo. 

You should presume that there are errors.  And that it is your responsibility to find them.  For example, I know a few things on the schematic don't match the layout such as I could not bring myself to draw a crossline master volume on the layout. 

CHECK for ERRORS!  Compare layout with schematic.  Where there is a discrepancy, go with the schematic.

One advantage of a paralleled turret board approach (like HiWatt uses) is that on "experimental" builds, it allows modifications to be easily made. 

There is both an ExpressSCH schematic and layout attached & they are both editable.

With respect, Tubenit

Hi,
are these the last schematics and Layout?