Hoffman Vox AC30 build

[Big_Mike]

I am thinking of building a Vox AC30 clone using the Hoffman layout, and I have a couple of questions.  My goal is to make this amp as close to a JMI 64 or 65 Top Boost model.

I have a Top Boost chassis all ready to go.  All I need are components.  The only thing I am going to add to the Hoffman layout is a Type-2 PPIMV.

1.  As far as transformers, what is my best option to match the Woden transformers used on the JMI amps?

2.  With regard to board capacitors, should I buy vintage mustards, or would Soho or Mojo caps be fine?

Mike

[pdf64]

The Hoffman project isn’t an AC30 clone, ie the vib/ trem channel is missing.

I’m no expert but am pretty sure mustards weren’t used much until the late 60s.
Wima Tropyfol was the most common film cap type used in the early 60s.
As for any tone differences 
The rats nest wiring and layout that is an AC30, especially one with added top boost, probably has more effect on response than cap type.

[Big_Mike]

The Hoffman project isn’t an AC30 clone, ie the vib/ trem channel is missing.

I’m no expert but am pretty sure mustards weren’t used much until the late 60s.
Wima Tropyfol was the most common film cap type used in the early 60s.
As for any tone differences 
The rats nest wiring and layout that is an AC30, especially one with added top boost, probably has more effect on response than cap type.

Yes, I know the Hoffman board omits the vib/trem circuit.  I have no need for that anyway, but my understanding is the normal and top boost circuits are identical to JMI specs.

As far as transformers, does anyone have any recommendations to get to JMI specs?

[sluckey]

Go to Hammond's website and look in the guitar amp replacement transformer section. You'll find the PT and OT.

[pdf64]

… I know the Hoffman board omits the vib/trem circuit.  I have no need for that anyway …

Ok, but you’ll be missing a treat, several treats really, as aside from the effects themselves, the vib trem channel has a couple of unique tones and can be easily modified to provide another.

[/youtube]
https://el34world.com/Forum/index.php?topic=30510.msg336329#msg336329

my understanding is the normal and top boost circuits are identical to JMI specs…

AC30s were built in normal, treble and bass versions.
The differences were developed over time, so later versions were a little different to earlier versions, and then there were transitional versions 
It’s just a few different cap values and added resistors, but the basic schematic that gets used isn’t the be all and end all of the story. There’s a high likelihood that if you open up a vintage AC30, you’ll find it won’t exactly match the schematic people may be familiar with.

Regarding the iron, people might overlook the importance of using a choke with suitable spec; it needs to be high inductance and high resistance. A typical choke suitable for most Fenders or Marshalls will put more stress on an AC30s output valves.

[Big_Mike]

… I know the Hoffman board omits the vib/trem circuit.  I have no need for that anyway …

Ok, but you’ll be missing a treat, several treats really, as aside from the effects themselves, the vib trem channel has a couple of unique tones and can be easily modified to provide another.

[/youtube]
https://el34world.com/Forum/index.php?topic=30510.msg336329#msg336329

my understanding is the normal and top boost circuits are identical to JMI specs…

AC30s were built in normal, treble and bass versions.
The differences were developed over time, so later versions were a little different to earlier versions, and then there were transitional versions 
It’s just a few different cap values and added resistors, but the basic schematic that gets used isn’t the be all and end all of the story. There’s a high likelihood that if you open up a vintage AC30, you’ll find it won’t exactly match the schematic people may be familiar with.

Regarding the iron, people might overlook the importance of using a choke with suitable spec; it needs to be high inductance and high resistance. A typical choke suitable for most Fenders or Marshalls will put more stress on an AC30s output valves.

With regard to the vib/trem channel, I really don't use that effect much, and it makes the build much more complicated.  I want to stick to the Hoffman two-channel normal and top boost circuits for this build.

I hear you that there was a lot of deviation from specs in the course of the JMI years.  I guess the best we can do in making a clone is to go by the schematic, unless there are particular circuit tweaks that result in a better amp.  I might have to email Lyle Caldwell for a list of his mods that he uses.

I am still trying to figure out which transformers to use.  The Hammond ones Sluckey pointed to look like a good choice, not sure what others might fit the bill.

[acheld]

should I buy vintage mustards, or would Soho or Mojo caps be fine?

I am partial to Panasonic and Xicon film caps myself -- they are small, not expensive, sound great and are available in high voltage ratings.  What's not to like?

I confess that in my early days of amp building, I used the boutique style caps.  Not worth it -- especially in terms of space on a board.  And I can't hear the difference.

Interesting read in Merlin Blencowe's book "Designing High Fidelity Tube Preamps" -- p78-80 describes the differences in various film caps and the entire section on different types of capacitors is very good.   

My avatar aside, don't use Astron Yellow.   

[sluckey]

I have a Top Boost chassis all ready to go.

Hoffman's board is designed for a straight line tube lineup as seen in his layout drawing. If your chassis looks like one of those crazy two piece Vox chassis his board may not fit well and would probably result in a rat's nest wiring. Can you post a pic of your chassis?

[Big_Mike]

I have the regular two-piece vox ac30 chassis.  It has six preamp tube sockets on the upper part, and the 4 el84 power tube sockets on the lower part. 

Based on the length of Doug's board, I should be able to fit it into the upper half, covering 3 of the six preamp tube sockets.  The remaining three will be used for the amp.

I have a Top Boost chassis all ready to go.

Hoffman's board is designed for a straight line tube lineup as seen in his layout drawing. If your chassis looks like one of those crazy two piece Vox chassis his board may not fit well and would probably result in a rat's nest wiring. Can you post a pic of your chassis?

[sluckey]

Might be worthwhile to cut a piece of cardboard to 10" x 3.125" and slide it around in your chassis. Don't overlook the fact that 4" of that board contains poweramp components.

[HotBluePlates]

I have a Top Boost chassis all ready to go.

Hoffman's board is designed for a straight line tube lineup as seen in his layout drawing. If your chassis looks like one of those crazy two piece Vox chassis his board may not fit well and would probably result in a rat's nest wiring. ...

I have the regular two-piece vox ac30 chassis.  It has six preamp tube sockets on the upper part, and the 4 el84 power tube sockets on the lower part. 

Based on the length of Doug's board, I should be able to fit it into the upper half, covering 3 of the six preamp tube sockets.  The remaining three will be used for the amp.

Then I would strongly, strongly recommend you stick with Vox' original layout, even if you delete the Vib/Trem channel.  Especially as the output tube wiring would be better in the lower chassis than having anything on the board in the upper chassis.

1.  As far as transformers, what is my best option to match the Woden transformers used on the JMI amps?

No idea.  But Pacific Transformers has models they claim to be like Woden transformers (ignore that the Mac used to build the site kept "correcting" the spelling to "Wooden"):
Power    Spec Sheet
Output   Spec Sheet
Choke    Spec Sheet

[Big_Mike]

Thanks for the recommendation, do you know of where I can find a tag-board layout for a JMI Vox AC30TB with the top boost on the upper control panel?  I have found schematics, but the only layout I found was for a "normal" JMI AC30 without top boost. 

I have a Top Boost chassis all ready to go.

Hoffman's board is designed for a straight line tube lineup as seen in his layout drawing. If your chassis looks like one of those crazy two piece Vox chassis his board may not fit well and would probably result in a rat's nest wiring. ...

I have the regular two-piece vox ac30 chassis.  It has six preamp tube sockets on the upper part, and the 4 el84 power tube sockets on the lower part. 

Based on the length of Doug's board, I should be able to fit it into the upper half, covering 3 of the six preamp tube sockets.  The remaining three will be used for the amp.

Then I would strongly, strongly recommend you stick with Vox' original layout, even if you delete the Vib/Trem channel.  Especially as the output tube wiring would be better in the lower chassis than having anything on the board in the user chassis.

1.  As far as transformers, what is my best option to match the Woden transformers used on the JMI amps?

No idea.  But Pacific Transformers has models they claim to be like Woden transformers (ignore that the Mac used to build the site kept "correcting" the spelling to "Wooden"):
Power    Spec Sheet
Output   Spec Sheet
Choke    Spec Sheet

[HotBluePlates]

... do you know of where I can find a tag-board layout for a JMI Vox AC30TB with the top boost on the upper control panel?  I have found schematics, but the only layout I found was for a "normal" JMI AC30 without top boost.

The Grosvenor book is usually the one people recommend for layouts.

I assume your other reference is to this page.

[Jalmeida]

Soursound Transformers are meticulously modeled after the Woden Transformers tone. It is insane how good they sound. Mojotone makes AC30 iron too. And even though I have no experience with the Mojotone AC30 iron, I like the other transformers from them. 

[SoundmasterG]

I built myself a clone of a Vox AC100 in a Sovtek Mig 100U chassis, and kept the Sovtek power transformer as the voltages were correct, but I used a Heyboer choke and OT and they are excellent and are cloned from originals. The amp does the AC100 sound extremely well as a result.


You may check with them about AC30 transformers. They may have clones of all the various types that were used (Woden, Albion, etc). You'll probably have to call or email them to find out but they are friendly.


I'd also strongly suggest to stick with their layout if using the 2 chassis amp. It is proven and works fine. If you can't get ready made boards, you could always use the tools Doug provides to draw up something and he can make it for you I believe.


Greg