Vintage Vox AC30 - Primary Wiring Question

[ebulakowski]

Hello all, A customer of mine came by with a 70's AC30 (SS rectifier). The amp still has a UK plug and the switch behind the panel is labeled 220v or 240v. There is a taped off lead on the transformer and a 115v tap shown but not connected on the schematic I found (attached)

My question is, does anyone have specific first hand knowledge about this. Or, is there a good way to confirm this or test this lead. I've only ever seen this switch before as 120v 240v. So I presume this particular amp was not intended as an export. The transformer itself just has a small paper label that says 'AC30 Power'

I know there is always step up transformer options. But I wanted to explore this first.
Any and all help is appreciated.

[Bieworm]

Hello all, A customer of mine came by with a 70's AC30 (SS rectifier). The amp still has a UK plug and the switch behind the panel is labeled 220v or 240v. There is a taped off lead on the transformer and a 115v tap shown but not connected on the schematic I found (attached)

My question is, does anyone have specific first hand knowledge about this. Or, is there a good way to confirm this or test this lead. I've only ever seen this switch before as 120v 240v. So I presume this particular amp was not intended as an export. The transformer itself just has a small paper label that says 'AC30 Power'

I know there is always step up transformer options. But I wanted to explore this first.
Any and all help is appreciated.

The 240VAC -220VAC switch is to use it all over Europe. There are countries that have closer to 220V from the wall and other countries that have wall power closer to 240V. Here in Belgium it's near 235V

[sluckey]

Here's an easy way to confirm that wire is actually a 115VAC tap. Leave the power cord disconnected. Now connect a 6.3VAC source to the filament string in the ac30. Now just measure the voltage between the primary common wire and that taped off brown wire.

The 6.3VAC source can even be from another amp sitting on the bench. Just use a couple gator clip leads.

[ebulakowski]

Here's an easy way to confirm that wire is actually a 115VAC tap. Leave the power cord disconnected. Now connect a 6.3VAC source to the filament string in the ac30. Now just measure the voltage between the primary common wire and that taped off brown wire.

The 6.3VAC source can even be from another amp sitting on the bench. Just use a couple gator clip leads.

This makes sense! Just to make sure I understand it correctly. Feed 6.3vac to the filaments and check the voltage on the primary side. (Similar to testing a OT). I presume it wouldn't harm anything else on the secondary side and I can just check the all leads on the primary side for 240v 220v and hopefully 120v

[sluckey]

Doing my experiment will also induce voltage into your B+ winding too. If you have any concerns just disconnect one of the HT leads. In fact, you may also want to disconnect your filament string so you're not trying to light up all those ac30 heaters. Should only take a few seconds to do this if you already have your dmm connected.

[ebulakowski]

Thanks so much. I'm happy to learn this trick and should be able to tackle this tomorrow.

[ebulakowski]

Doing my experiment will also induce voltage into your B+ winding too. If you have any concerns just disconnect one of the HT leads. In fact, you may also want to disconnect your filament string so you're not trying to light up all those ac30 heaters. Should only take a few seconds to do this if you already have your dmm connected.

Sluckey, thanks again. I was Was able to conduct the test today. I set my variac to 6.3vac and double it checked with my DMM. I disconnected the high voltage line and applied my 6.3vac to the heater line. The resulting voltages were 186, 170, and 90.
Any idea why I am not seeing the actual voltages? That said, they are the right ratio to one another for 240 220 and 115. I feel like that (along with the schematic) puts me in a place to assume this is a115v tap. I just figured i'd reach out in case there is anything else to learn, or be wary of.

[sluckey]

Did you check the 6.3v from your variac ***WHILE*** it was connected to your filament string? Maybe it dropped under load. Otherwise I don't understand the lower voltage readings.

However, I'm now very confident the taped off brown wire is the 110V tap. I would not be afraid to wire that tap for a 120V source. Plug into your variac set to 110V, leave the B+ leads disconnected and measure the 6.3VAC filament string.

[ebulakowski]

Great point, I checked it without a load. I just hooked up the new primary tap and 110v on the variac yields 6.5vac on the heater secondary. So I'd say we're there! Obviously now its just the typical modern high voltage wall stuff.

Thank you again so much for helping me through this and teaching me a new method for the bench!