Adding 8 ohm output to Vox AC30TB6

[zjdrummond]

I have a "Korg era" 1993 Vox AC30TB6. I read on a forum that this amp's output transformer has an unused 8 ohm tap. Can anyone verify this? I was told the green wire terminating to the PCB is the 8 ohm wire (See photo). From the output transformer I see a black wires, one blue wire, and the green wire. There is also the thicker black wire running from the PBC to the 16 ohm jack. I assume this is a ground wire?

Does anyone know how I can add the 8 ohm tap or if this is possible? Would it be as simple as grounding the new 8 ohm output jack just as the exsisting 16 ohm one and running the green wire to the new 8 ohm jack the same way as the blue one is on the stock 16 ohm jack? I understand I could really screw up my amp if I did this wrong...

IMG_20190517_160721 by Zachary J Drummond, on Flickr

[The Dude]

If you can find transformer specs, that would be ideal. Otherwise, grab your meter and see what the DC resistance is compared to the other taps. It shouldn't be hard to figure out from there.

[Helmholtz]

Determining output impedance from DCR can be tricky. But it would tell if the green wire is a secondary tap.

I would connect a 16 Ohm dummy to the regular output, feed a signal and adjust the output voltage to say 20Vrms. The voltage between the green wire and ground should then measure 14.14 Vrms if it's an 8 Ohm tap.

Is this the appropriate schematic?

https://www.korguk.com/voxcircuits/c...s/ac3093pa.jpg

[The Dude]

Determining output impedance from DCR can be tricky. But it would tell if the green wire is a secondary tap......

I agree..... with no reference. In this case, we have a known 4 ohm tap and a known 16 ohm tap. If the mystery wire is an 8 ohm tap, the DCR ought to be somewhere in between the DCR of the other taps.

[Helmholtz]

we have a known 4 ohm tap

Not that I knew?

[The Dude]

Ah crap. I misread. My apologies. Carry on.

[zjdrummond]

Determining output impedance from DCR can be tricky. But it would tell if the green wire is a secondary tap.

I would connect a 16 Ohm dummy to the regular output, feed a signal and adjust the output voltage to say 20Vrms. The voltage between the green wire and ground should then measure 14.14 Vrms if it's an 8 Ohm tap.

Is this the appropriate schematic?

https://www.korguk.com/voxcircuits/c...s/ac3093pa.jpg

I believe that is the correct schematic. I'm not super electronics-literate. I'm mostly a player. It seems like I should just take this to a tech. I was hoping for a straightforward answer. I really don't want to ruin my beloved Vox though.

[nosaj]

Determining output impedance from DCR can be tricky. But it would tell if the green wire is a secondary tap.

I would connect a 16 Ohm dummy to the regular output, feed a signal and adjust the output voltage to say 20Vrms. The voltage between the green wire and ground should then measure 14.14 Vrms if it's an 8 Ohm tap.

Is this the appropriate schematic?

https://www.korguk.com/voxcircuits/c...s/ac3093pa.jpg

Expounding on this beautiful link provided look what it gives us.https://www.korguk.com/voxcircuits/Organized schematics

nosaj

[g1]

Here is a picture of the transformer that matches the schematic from post #3.
If yours has the same number on the sticker (TXMA 00013), you can proceed as you stated in post #1.

[zjdrummond]

Here is a picture of the transformer that matches the schematic from post #3.
If yours has the same number on the sticker (TXMA 00013), you can proceed as you stated in post #1.


[ATTACH=CONFIG]53651[/ATTACH]

My transformers do not have any stickers on them. I guess I'll need a tech to check it out with a meter. I wish I knew how to do this stuff myself.

[vintagekiki]

https://www.partsisparts.net/catalog/vox-ac30-output-transformer-0

In vintage AC30s, the two (blue or silver frame AlNiCo) speakers are 8 ohms each and connected in series for a total of 16 ohms.

Wires
Primaries Yellow – Red – White (center tapped, push pull)
4k Ohms center tapped

Secondaries
Ground Black
8 Ohms Green
16 Ohms Blue

8 / 16 ohm outputs for Vox AC30

[Helmholtz]

The transformer code in the Korg circuit seems to be a Drake (as used by Marshall) number: 784-413.
In principle the green tap could have been a 4 or 8 Ohm tap.

[vintagekiki]

https://www.etronic-parts.com/OUTLET/Output-transformer-Vox-AC30::1477.html
Output transformer for vox AC30, TXMA-00013. Drake PNr.: 784-413