Vox AC4TV output transformer replacement

[Tone Meister]

I have a Vox AC4TV with an OT that is arcing internally - peeled it back and don't think I'm gonna try to repair it. It doesn't appear that the original part is available either. Looks like it needs a 5K primary and a 3 ohm secondary. It's a single-ended amp with one EL84 and a 16-ohm speaker. Mounting center is 3 1/8"

I was looking at this one: CHAMP STYLE 5W Single-Ended

I reckon that one will work fine, but anyone know of a better option for an OT that will pretty much drop in with no mods? I don't mind drilling a mounting hole for an odd footprint, but that's about it.

Maybe a slightly beefier OT while we're at it?

[Mick Bailey]

I use the Hammond 125xSE series in SE amps as a generic fit. They all offer 5k primaries and usually perform better than the original. 8k will work but isn't the best match for this amp.

[Tone Meister]

Yeah, the 125CSE would definitely be a bolt-in upgrade to the stock OT. However, I was hoping to keep cost down to a minimum since it's just a $300 amp, which is why I was considering that TF103-48 from Triode. The power tube lost vacuum and also needs replacing, so it's gonna cost a little to get her going again anyway.

Anyone else have a suggestion or are these the best choices out there?

[Chuck H]

It's interesting that the schematic has the load circuit so cryptically labeled. It's also interesting that you mention a 3ohm secondary and a 16ohm speaker. Is it possible the amp was running into an improper load? Or that the "attenuator network" failed to do it's job causing the resulting failure? I think if a new OT must be retrofitted with extra effort I might see about these things. I would suspect things to break because of a problem outside the intended operating parameters before the notion of inferior parts used for it's design. Though I know that happens. Still... If there's a reason OTHER than a wimpy OT for the problem it should be addressed in the repair. Vox knew people were going to crank this amp. That's why it was designed with an attenuator.

[Tone Meister]

Chuck, I didn't mean to imply the original OT was wimpy because it really isn't. It just doesn't seem to be available domestically, unless I am missing something. I was able to determine visually that it was arcing under load and don't feel that I can repair it successfully. I don't have an opinion as to what caused it to start arcing, but I can clearly see it happen while playing a guitar through the input.

I got the transformer specs from an old AC4 schematic from the Jennings era, which show a 5K primary and a 3-ohm secondary into an 8-ohm speaker. That may or may not be accurate for this amp. Here is one that is purported to be a "drop in replacement" for the AC4: Vox AC4 output transformer

[Chuck H]

The specs for ac4tv say 4W into a 16ohm load. They also state an optional 16ohm jack? How much of this is relative to the "attenuator network" is anyone's guess without a complete schematic, but I would be inclined to default to a 16ohm secondary. The tf103-48 you linked in post #3 would present a primary impedance of 16k plugged into the little combo's stock speaker. Further, replacing the speaker may not be an ideal option because the "attenuator network" is almost certainly designed to work with the stock 16 ohm speaker. You should probably hold out for an OT that offers the 16ohm secondary you need.

[Tone Meister]

Where did you find those specs for the AC4TV? That Hammond 125CSE would be a good match given what we've learned, wouldn't it? Was hoping for something for about half the cost.



125CSE Specifications
Audio Watts: 8
Primary Impedance (Ohms): 2,500 to 10,000
Secondary Impedance (Ohms): 4 to 32
Max. DC Bias (mA): 60
Mounting centers: 3.13" u-bracket
Weight: 1.3 lbs.

-OR-

125BSE Specifications
Audio Watts: 5
Primary Impedance (Ohms): 2,500 to 10,000
Secondary Impedance (Ohms): 4 to 32
Max. DC Bias (mA): 45
Mounting centers: 2.81" u-bracket
Weight: 1.1 lbs.

[Chuck H]

I got the specs from one of the many vendors that set forth "features" in the sales lit. Since the price is almost the same I might go with the 125CSE rather than the BSE. The fact that the amp has an attenuator built in should explain why And I agree that it's too bad about the price. Such is often the case with DNR type amps. I haven't seen an original but the 125CSE is almost certainly an upgrade.

[Tone Meister]

Yep. The only other OT I've found that meets the specs is the ClassicTone 40-18031 which has the 5K primary and 16-ohm secondary. The mounting center would require that I drill one mounting hole but it is some $15 cheaper and is also very likely an upgrade over stock.

The cheapest I've found the Hammond 125CSE is $51.54 in the door at Angela Instruments ($44.19 + $7.35 shipping). The best I've found the ClassicTone 40-18031 is $42.51 at Amp Parts Direct ($30.66 + 11.85 shipping)

Anyone know of a better source for either?

[Tone Meister]

Just for the archive, here is another OT that will work from Musical Power Supplies, Inc in St. Louis, MO.

OT10SE USA STD - $28.95 on the website. It'll end up being about $35 in the door and the specs are spot on. I think I'm gonna give one a try and see how it does.

5K/7K primary
4/8/16 ohm output
3.125" mounting center

OT10SE datasheet and Musical Power Supplies price list attached.

[Tone Meister]

The Musical Power Supplies OT10SE ended up costing $34.25 in the door ($28.95 + $5.30). You can save about a dollar if you order from the website as opposed to buying through their eBay store.

Installed the OT and a NOS Sylvania blackplate 6BQ5 and the amp sounds killer. I don't know what it sounded like before, but this transformer sure didn't hurt anything. I learned a couple things about the AC4TV and the way it's wired, and it's easier to post the link rather than try to regurgitate it here: COOL AC4TV INFO

Bottom line is that I used the 8-ohm tap and the 5k primary tap and whoa! boy. The above link explains it better than I can. Anyway, here she is:

[Dave H]

...it's easier to post the link rather than try to regurgitate it here: COOL AC4TV INFO

Some of the posts in that link should have come with a BS warning.

[J M Fahey]

That's the crap that gets read, believed, referred to and copied/pasted ad infinitum all over the Net.

Oh well.

[Mick Bailey]

8-9 watts. Really? All that from a single EL84 in class A. Something to write home to Mother about.

And that's with transformer losses. So we need something like 20w dissipation.

[Tone Meister]

I intended to point out the way Vox loaded the secondary of the OT, only. Should have just copied and pasted what was germane.