1964 AC30 VOLUME ISSUE

[Aaron]

I bought my Vox about 20 years ago and it sounded great, plenty of volume. But after a while it sounded like it was getting quieter. Wasn't noticeable but then it got to the point where I was cranking the volume 3/4's. So it sat in my room for about 15 years, rarely played.

Fast forward to 2020 lockdown and I thought I'd have a good look at it now I've got a signal generator and scope, oh, and unlimited time!
So, 1kHz 100mV in Normal input, I get 22V at the PI coupling caps but only 3V at the speaker. Thought, that not right. Put the scope on the caps and the PI is in phase, the signal is the same on both sides, not 180deg reverse. Changed the caps same issue, I've replaced all the resistors in the signal path.

Any ideas?

Aaron

[sluckey]

So, 1kHz 100mV in Normal input, I get 22V at the PI coupling caps but only 3V at the speaker. Thought, that not right.

That's normal. A speaker is a current driven device. It needs lots of current but not much voltage.

Put the scope on the caps and the PI is in phase, the signal is the same on both sides, not 180deg reverse. Changed the caps same issue, I've replaced all the resistors in the signal path.

Most likely just a scope triggering issue. To see the phase difference you need to trigger the scope on one vert channel only.

[pompeiisneaks]

So, 1kHz 100mV in Normal input, I get 22V at the PI coupling caps but only 3V at the speaker. Thought, that not right.

That's normal. A speaker is a current driven device. It needs lots of current but not much voltage.

Put the scope on the caps and the PI is in phase, the signal is the same on both sides, not 180deg reverse. Changed the caps same issue, I've replaced all the resistors in the signal path.

Most likely just a scope triggering issue. To see the phase difference you need to trigger the scope on one vert channel only.

Or he could have one of the channels inverted?

~Phil

[Aaron]

So,update. User error it was inverted. It’s a push pull switch and a lead must have got caught and pulled it out. The Marshall I worked on earlier scoped fine.

Sad news though, thanks to Beasley helping me to diagnose it, output transformer had gone bad.

Aaron

[Stevem]

Well a nice Hammond replacement up grade cost a tad less then 100 Dollars and should take you less then 1 hour to cut the only one out and slap in the new one!

[Markusv]

I bought a Hammond for my AC 30 build and cannot be happier!!

Markus

[Aaron]

Hi Steve and Marcus,

Just so happened that I installed a Hammond replacement yesterday. And its back! I'd forgotten how loud it was.
It wasn't as direct a replacement as they claim, in mine the original lays on the side and the mounting bolts are in the middle. And the cover plate bolts that attach to the side didn't line up either. Will get the original rewound one day.

Thanks,
Aaron

[Markusv]

Hi Aaron

Sound wise- What are your thoughts??
Loud yes- but to my ears pretty musical sounding OT?

Markus

[Aaron]

Hi Aaron

Sound wise- What are your thoughts??
Loud yes- but to my ears pretty musical sounding OT?

Markus

Im a big fan of the Hammond Transformers. They're nice and big and have a lot of weight in them. I use them in most of my builds.
My VOX is the Treble model, so cuts through great and has that VOX chime. Nothing like the new ones, to me anyway

Thanks,
Aaron