Vox AC15 grounding

[DLPublic]

Although I asked this question a week ago on my build thread it appears to have gone un-noticed so have posted on its own thread.

I can see that a ground bus is used across the control panel between the input jacks and the channel I & II volume controls.  But how are the many ground connections on the tag boards wired?  Are they grouped in stages with each stage wired back to a single grounding point (i.e star connections from each stage) or are they all connected together with just a single connection to a common grounding point?

I have not come across an image that shows this detail and Stephen Grosvenor's book does not cover this aspect, it simply labels each relevant tag as 'gnd'

Dave

[sluckey]

Could be that no one knows. I've never seen an AC-15. I don't know.

[Willabe]

There is more than 1 way to wire up grounds but this may help;

http://www.valvewizard.co.uk/Grounding.html

[DLPublic]

Willabe,

There is more than 1 way to wire up grounds but this may help;
http://www.valvewizard.co.uk/Grounding.html

I have seen that link and read the associated document, but what I am asking is how it was done on an original 1960 AC15.

From my brief time on this forum, it seems there are some very knowledgeable peeps (including your goodself Sluckey) so I was hoping someone would know.  As this is my first valve amp build, I just want to get it right.

Dave

[sluckey]

Here's a link to a discussion at the Amp Garage (TAG). Guys were building a replica '60s AC-15 (like you) and had lot's of good info. Someone there may be able to provide the exact grounding scheme you want.

http://ampgarage.com/forum/viewtopic.php?t=22846&sid=279b963dbc49abf3fed3b333203bbfc9

In the event you don't find the exact info you seek, just remember, there are many different grounding schemes. Just be logical about your approach and it will be fine.

[DLPublic]

OK, what I have done is...

a) connected the various grounds (i.e. grouped) of each stage together on the tag boards.
b) connected each group via a single wire to the ground bus behind the front panel.

all but two of these connections can be seen just right of the leftmost rotary switch (brilliance) in the attached image.
The remaining two - the Vib/Trem input stage (V5) and the oscillator stage (V7) which are both smoothed by the 32uF capacitor (C20) - are the ones connected between the two pots (Vib/Trem depth and speed) just left of the input jack sockets.

As can be seen, the ground bus extends to an earthing tag near the speaker output connection point, which will also be used to ground the common of the outputs.

Any comments on this most welcome.
Dave

[Willabe]

I have seen that link and read the associated document, but what I am asking is how it was done on an original 1960 AC15.

Ok but who says Vox got it correct?

If you read the grounding link you know Merlin goes through where the older big amp companies made mistakes in their grounding.

[HotBluePlates]

... how it was done on an original 1960 AC15. ...

If you ever find a definitive answer on this, please post it here! I'd love to know how it was done, as some things like this are often obscure knowledge.

[DLPublic]

Willabe

Ok but who says Vox got it correct?

Vox must have got it right given the number they sold and the famous guitarists who swear by them. 😁
Had hum been a problem I am sure they would not have so successful.

[pompeiisneaks]

I don't know that I would call that a definitive answer.  If their hum was better on average that others, it would be considered the 'best' even if you can make it better.  Also hum is very objective.  Some players don't care much because once they're jamming it pretty much is gone, others will go nuts at a very low level.  If you had some more evidence of proof that the original JMI AC15's had lower noise level by measurement than others, this becomes a more valid point... my 2c anyway :)  I'm always trying to find out how to reduce hum, and I loved Merlin's explanations of how to understand it and avoid it.

~Phil

[Ed_Chambley]

I owned a 60 Vox back years ago and the amps were noisy.  My last of experience at the time made me live with it.  I understand you are cloning a Vos AC15 as closely as possible, but if you were building a 5B3 Tweed Deluxe where the heaters were grounded to the chassis, I think you may consider using Modern advancements.


The reason I mention this is my current AC15 you cannot even tell it is on until you hit the strings.  I would run a bus wire above the pots to the input jacks, but I would make it look nicer than the photo that was posted.  This buss would be used for andy and all preamp grounds.


The rest I would simply attach to a Power Transformer bolt separating the preamp ground with the power amp section.  I can assure you that I have been inside quite a few Vox amps and have never seen one exactly as the next.  Speaking specifically to 64 and earlier models.  We accepted more floor noise years omg than we like today so implementing modern ground scheme will yield a quieter.  It may not be exact, but it will be better.


YMMV.

[DLPublic]

Phil

I don't know that I would call that a definitive answer.

Absolutely agree!  Let me reiterate where I am.  Four months ago my only experience building valve equipment was back in the late 60's/early 70's when I made a couple of valve transmitters (when I was into amateur radio).  So over the past 3 months, I have spent literally many, many hours researching the net and other sources to enable me to build my AC15 clone.  Unfortunately I have never seen or heard in isolation a Vox (vintage or modern) close up and personal.  So my actual knowledge of them is nil, nada and as such I cannot provide ANY definitive answers on the subject.

I am, though, willing to learn from those who do have the expertise and knowledge (for which I am immensely thankful) and through my own build experience.  However, I am also keen to keep my mistakes to a minimum, hence my many questions here.  I am a great believer in the five {six?} P's. [Proper Preparation Prevents {Piss} Poor Performance]   

Ed,

I would run a bus wire above the pots to the input jacks, but I would make it look nicer than the photo that was posted.  This buss would be used for andy and all preamp grounds.

In what way would you make my bus look nicer?  My bus was formed from some heavy gauge multi-core wire I had up in the loft which I striped, twisted even tighter then tinned with solder.  I chose it as it is stiff enough to bend and stay bent and it saved me buying something else.

Also what does 'andy' mean?

The connections to my bus are (each grouped by stage on the tag boards): input jacks, V5 (vib/trem preamp), V7 (osc), V6 (modulator), V2 (PI), the filter ladder connected to VR2 (CH II Vol), the junction of R19/R20 (both 220K) which connects to both volume controls, V1 (EF86), and the negative side of cap can C4/C8.  The last five (PI to C4/C8) are all connected adjacent to each other just beside the brilliance switch.

I am usure whether the R19/R20 junction should connect there or whether that should connect directly to R22/C12 (i.e the 130ohm/50uF) on the power chassis.

The rest I would simply attach to a Power Transformer bolt separating the preamp ground with the power amp section.

On the Power chassis, I have mostly followed the layout in Grosvenor's book which grounds R22/C12 at a 3 way tag strip near the EL84's, and C41/C42 (resevoir caps) to their own chassis connection (the cap can mounting nut).  He does not show a ground connection for the secondary centre tap (which is connected to primary 0v), but I have added a fuse between the centre tap and ground (on the PT nut) - i.e. as per AC15HTVH1 handwired schematic - at which point the common connection of two 100ohm resistors (heater virtual tap) also connects.

Hope that all makes sense!

Now would it be better, as you suggest, to connect C40/C41, R22/C12, PT CT, Heater CT to a single point, i.e. the PT nut ground?  I have a 4-way tag which I could use to isolate R22/C12, so easy to do.

Any other comments are most welcome.
Dave

[PRR]

>> This buss would be used for andy and all preamp grounds.
> what does 'andy' mean?

Finger-slip for "and". ....EDIT ......argh slippy fingers.... "any"

[Willabe]

Finger-slip for "and".

Finger-slip for "any"?

[Willabe]

Willabe

Ok but who says Vox got it correct?

Vox must have got it right given the number they sold and the famous guitarists who swear by them. 😁
Had hum been a problem I am sure they would not have so successful.

Back then their were 2 big amps in England, Vox came 1st then came Marshall. Others came later.

They were just happy to have an amp noise and all. 

[pompeiisneaks]

I think your current layout sounds like it will be great, and I agree with the 6 p's, my drill sergeant pounded that into me many years ago :).  I've only built 4 amps myself, so I'm no expert either, the pics of the amp progress look stellar.  I've built two voxes myself, an ac30, Hoffman's layout, adds an ac100 from JMI 1964 circuit.  They're a blast.

Phil

[Ed_Chambley]

Phil

I don't know that I would call that a definitive answer.

Absolutely agree!  Let me reiterate where I am.  Four months ago my only experience building valve equipment was back in the late 60's/early 70's when I made a couple of valve transmitters (when I was into amateur radio).  So over the past 3 months, I have spent literally many, many hours researching the net and other sources to enable me to build my AC15 clone.  Unfortunately I have never seen or heard in isolation a Vox (vintage or modern) close up and personal.  So my actual knowledge of them is nil, nada and as such I cannot provide ANY definitive answers on the subject.

I am, though, willing to learn from those who do have the expertise and knowledge (for which I am immensely thankful) and through my own build experience.  However, I am also keen to keep my mistakes to a minimum, hence my many questions here.  I am a great believer in the five {six?} P's. [Proper Preparation Prevents {Piss} Poor Performance]   

Ed,

I would run a bus wire above the pots to the input jacks, but I would make it look nicer than the photo that was posted.  This buss would be used for andy and all preamp grounds.

In what way would you make my bus look nicer?  My bus was formed from some heavy gauge multi-core wire I had up in the loft which I striped, twisted even tighter then tinned with solder.  I chose it as it is stiff enough to bend and stay bent and it saved me buying something else.

Also what does 'andy' mean?

The connections to my bus are (each grouped by stage on the tag boards): input jacks, V5 (vib/trem preamp), V7 (osc), V6 (modulator), V2 (PI), the filter ladder connected to VR2 (CH II Vol), the junction of R19/R20 (both 220K) which connects to both volume controls, V1 (EF86), and the negative side of cap can C4/C8.  The last five (PI to C4/C8) are all connected adjacent to each other just beside the brilliance switch.

I am usure whether the R19/R20 junction should connect there or whether that should connect directly to R22/C12 (i.e the 130ohm/50uF) on the power chassis.

The rest I would simply attach to a Power Transformer bolt separating the preamp ground with the power amp section.

On the Power chassis, I have mostly followed the layout in Grosvenor's book which grounds R22/C12 at a 3 way tag strip near the EL84's, and C41/C42 (resevoir caps) to their own chassis connection (the cap can mounting nut).  He does not show a ground connection for the secondary centre tap (which is connected to primary 0v), but I have added a fuse between the centre tap and ground (on the PT nut) - i.e. as per AC15HTVH1 handwired schematic - at which point the common connection of two 100ohm resistors (heater virtual tap) also connects.

Hope that all makes sense!

Now would it be better, as you suggest, to connect C40/C41, R22/C12, PT CT, Heater CT to a single point, i.e. the PT nut ground?  I have a 4-way tag which I could use to isolate R22/C12, so easy to do.

Any other comments are most welcome.
Dave

Dave, andy is suppose to be any as PRR suggested.  I was not referring to your build when I said I would make it look nicer.


I use electric fence wire for buss wire.  It is more rigid.  I attach it to a single insulated Turret.  I will run a ground to the inputs which is my single connection.  Then the buss is constructed with using the fence wire to all ground in the preamp.


If you have some Teflon tubing you can cover it as you work and makes it look nice.  I don't, but I also don't worry if all my leads are perfectly straight either.  This takes care of the preamp.

[Ed_Chambley]

Now would it be better, as you suggest, to connect C40/C41, R22/C12, PT CT, Heater CT to a single point, i.e. the PT nut ground?  I have a 4-way tag which I could use to isolate R22/C12, so easy to do.

Any other comments are most welcome.
Dave

Dave, I don't know if it is better, but my amps are quiet.  I don't solder to pots and ground all preamp to the buss and use buss wire if I need to ground a lug on a pot.  Then Center Taps and Filter Cap grounds all go to a transformer bolt.  The only other thing I do is add a earth using a solder eyelet and a bolt and locknut.


I also use blue thread locker on every bolt and use Hex head bolts as I find it much easier to tighten things since a philips screwdriver on the blind side is a pain to keep in place.


That is the way I do all small amps.