The VOX AC3 Top Boost

[Kagliostro]

Kagliostro,

Here are some other pictures taken during construction :

Many thanks Tubelectron

Happy new Year to all

Kagliostro

[tubelectron]

Kagliostro,

Here in Europe (France, Italy) we are still in 2011 for circa 1H30mn at the time of writing... And USA should follow circa 9 hours later, if I am right.

Avec un peu d'avance, Meilleurs Voeux à tous pour cette nouvelle année 2012 à venir !

A+!

[Thisch]

Hi Tubelectron,
Some help would be appreciated.
I cannibalized an old Peavy and made everything fit. I added a choke. Lots of real estate taken from the bottom of the chassis indeed.
Problem is... No sound at all. I haven't checked the voltages, but I suspect I messed up the opt wiring. Instead of the long time defunct audax, I used a deluxe top OPT from classic tone (http://site.tubedepot.com/pdf/tr-ot-ct-40-18087.pdf). I used brown and blue for primary, and I might have needed to use brown+blue and red.
Any advice?

God news is that the amp looks stunning already. I can't wait to hear something outta it!

[M Fowler]

http://www.classictone.net/40-18087.pdf

Shows the wiring diagram and red is your CT connect to B+. Brown and Blue to plates.

Mark

[Thisch]

Thanks a lot Mark. I try that tonight.

[Thisch]

I did some further mods: changed the OT using a champ one to fit the SE design, replace all signal-carrying wires by shielded, and checked and checked again all connections.
I even turn around the tubes with spare one to make sure it wasn't the cause. Also relocated the tubes to the far end opposite the power supply.
Result....
Still the noise of a galactic battle between old b27s and alien spaceships with a soindrack directly out of a F movie from Bollywood!

I checked voltages, and something strange happens.
On the second stage plate, the voltage reads slightly higher value than Guilhem's which makes sens because the. PT delivers more. The weird thing happens when I ground the shield of the wire going from pin 1 of the 12ax7 to the rest of the components on the board, the voltage drops to... 6 v.

Can someone explain this to me? How a shield grounding cand make a signal drop from 180 down to 6 v?

I have spent months trying to get sound out of this thing, I guess it's time to count my losses and go for any of the well documented ax84 instead. At least, the former can be found easily...

[Tillydog]

I checked voltages, and something strange happens.
On the second stage plate, the voltage reads slightly higher value than Guilhem's which makes sens because the. PT delivers more. The weird thing happens when I ground the shield of the wire going from pin 1 of the 12ax7 to the rest of the components on the board, the voltage drops to... 6 v.

Can someone explain this to me? How a shield grounding cand make a signal drop from 180 down to 6 v?

You must have a short between the core and shield on that cable (it is easily done by too much heat when soldering, or a stray whisker of shield wire). When you ground the shield you would also ground the cathode of the 12AX7 which shorts out the bias voltage across the cathode resistor, making the 12AX7 turn on full, so pulling the plate voltage right down.

[Thisch]

That was my first diagnostic. so I checked for any contact between core and shield: non visible, none according to my ohmmeter, and I even went as far as changing the wire and cutting a new one, checking it for contact core/shield, soldering it... same phenomenon.

weird weird weird.

[Tillydog]

Weird indeed...

Is the shield connected to anything at the other end?

[Thisch]

Just connected to the ground bus.